Chemiluminescense for flame control
António Miguel Henriques
Dissertação para obtenção do grau:
Mestre em Engenharia Aeroespacial
Júri
Presidente: Prof. Doutor Fernando José Lau
Orientador: Prof. Doutor Edgar Caetano Fernandes
Vogal: Prof. Doutor Mário Manuel Costa
Dezembro 2011
Resumo
Apresente tese aborda o processo de desenho, construção, optimização e teste de
um sensor de quimiluminescência, no contexto do desenvolvimento de tecnolo-
gias de controlo de combustão. O protótipo desenvolvido neste trabalho adquire a luz
emitida pelos radicais excitados OH* e CH*, de forma a explorar a dependência da re-
lação OH∗/CH∗ = f(φ) com os sistemas de colecção de luz (na óptica de utilização não
intrusiva em motores turbofan) e com a deformação temporal da chama (imposta para
simular condições turbulentas). Para os testes utilizou-se uma chama de propano tipo
Bunsen, com razões de equivalência de φ = [0.86 1.1], e com a possibilidade de se defor-
mar acusticamente através de um pulso sinusoidal de amplitude variável (∆V = [0 0.045]
V a 50 Hz). Nestas condições, foi possível concluir que OH∗médio/CH
∗médio = f(φ) não é
afectada pela deformação e que a emissão de quimiluminescência local poderá depender
da deformação da chama. A evolução temporal de OH∗/CH∗ foi usada como entrada
num mecanismo simulado de controlo de combustão, do qual se mostrou que os modelos
de controlo baseados na quimiluminescência média poderão sobrestimar ou subestimar o
razão de equivalência da chama durante o seu período de actuação.
Palavras-chave: Quimiluminescência, combustão pobre, fotomultiplicadores, controlo
de chama, turbofan.
iii
Abstract
The aim of this thesis was to design, construct, optimize and test a chemilumines-
cence sensor, in the context of lean combustion control technology. The developed
prototype collects light from OH* and CH* species, in order to evaluate the dependence
of the OH∗/CH∗ = f(φ) relation with the optical collecting systems (in the context of
non-intrusive measurements in turbofan engines) and with the temporal flame deforma-
tion (imposed to simulate turbulent conditions). For the tests a propane Bunsen flame was
used, with equivalence ratios of φ = [0.86 1.1] and with the possibility to acoustically vary
the supplied mixture speed (sinusoid amplitude ∆V = [0 0.045] V at 50 Hz) for controlled
flame deformations. With this setup it was possible to conclude that OH∗mean/CH
∗mean
= f(φ) is not affected by flame deformation and that local chemiluminescence may be
dependent on flame deformation. OH∗/CH∗ time evolution was used in a simulated
combustion control mechanism, from which it was shown that control models, based in
mean chemiluminescence measurements, may overestimate or underestimate real flame
equivalence ratio.
Keywords: Chemiluminescence, lean combustion, photomultipliers, flame control, tur-
bofan.
v
Acknowledgements
I would like to express my utmost gratitude to Professor Edgar Fernandes. If I was
going to detail why I should thank him, I would have more pages of gratifications
than pages of work.
Then, it is important clarify one thing: I could have never walked through this journey
without my Friends. I believe that some how, the right friends, in the right place, at the
right time, will take your ideas to anywhere.
To Family, to whom I owe everything I am, I dedicate this work.
"Isto vai ser só uma caixa para apontar para a chama, e já está!"
vii
Contents
Resumo iii
Abstract v
Acknowledgements vii
List of tables xi
List of figures xvi
List of symbols xx
1 Introduction 1
1.1 Literature Review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
1.1.1 Chemiluminescence for combustion control . . . . . . . . . . . . 9
1.1.2 Chemiluminescence applications . . . . . . . . . . . . . . . . . . 16
1.2 Thesis Objective, Contribution and Outline . . . . . . . . . . . . . . . . 20
2 Experimental Methods 21
2.1 Chemiluminescence Sensor Design . . . . . . . . . . . . . . . . . . . . . 23
2.2 Data Acquisition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
ix
2.3 Prototype Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
2.3.1 Burning System . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
2.3.2 Measurement Accuracy . . . . . . . . . . . . . . . . . . . . . . 38
3 Case Study 49
3.1 Experimental analysis of steady flame . . . . . . . . . . . . . . . . . . . 51
3.2 Experimental analysis of unsteady flames . . . . . . . . . . . . . . . . . 55
3.3 Experimental analysis of unsteady flames . . . . . . . . . . . . . . . . . 64
4 Conclusions and Future Work 79
Bibliography 87
x
List of Tables
1.1 CFM56-3C1 and CFM56-7B24/3 characteristics. NOx emissions values
are for each kilogram of burned fuel. . . . . . . . . . . . . . . . . . . . . 2
1.2 Measuring Techniques. General configurations. Advantages and disad-
vantages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
1.3 Chemiluminescence application examples. . . . . . . . . . . . . . . . . . 19
2.1 Characteristics of photomultiplier and pre-amplifier socket. . . . . . . . . 33
2.2 Intersection area for two vertical setups. Iris diameter of diris = 0.7 cm. . 41
2.3 Power fitting equation for both species local intensity signals. . . . . . . . 43
2.4 Noise and signal analysis. . . . . . . . . . . . . . . . . . . . . . . . . . . 44
2.5 Mean chemiluminescence values for different optical probe iris diameters. 46
2.6 Power fitting equation for both measured intensity signals. . . . . . . . . 47
3.1 Exponential fitting equations for different collecting system configurations. 54
3.2 Exponential fitting equations of the ratio between mean intensity value
of OH* and CH*, OH∗mean/CH∗
mean, as a function of mixture ER, for
different excitation amplitude. . . . . . . . . . . . . . . . . . . . . . . . 63
xi
List of Figures
1.1 Turbine CTSFC, or SFC, along the years for the various configurations. . 2
1.2 CFM56-3C1 cut view . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.3 Newac Core Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.4 NOx formation and equivalence ratio as a function of swirl number. . . . 6
1.5 Temperature Sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.6 Bio-fuel example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
1.7 Reaction Steps Methane Flame . . . . . . . . . . . . . . . . . . . . . . . 10
1.8 Mathematically treated methane emission spectrum. . . . . . . . . . . . . 17
2.1 Prototype general scheme . . . . . . . . . . . . . . . . . . . . . . . . . . 21
2.2 Picture of prototype components. . . . . . . . . . . . . . . . . . . . . . . 22
2.3 Prototype overall scheme. Light collecting system configurations. . . . . 23
2.4 Optical fiber Transfer Function. . . . . . . . . . . . . . . . . . . . . . . . 24
2.5 Collimator probe mounting scheme. . . . . . . . . . . . . . . . . . . . . 25
2.6 First lens TF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
2.7 Light interface between optical probe plano convex lens and selected op-
tical fiber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
2.8 Lens and filtering system detail on the overall prototype scheme. . . . . . 26
2.9 The difference between the beam geometry on an ideal an real lens. . . . 27
xiii
2.10 UV lens TF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
2.11 Transfer Function of OH* and CH* filters. . . . . . . . . . . . . . . . . . 29
2.12 UV lens TF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
2.13 Optical path scheme detail. Optical path TF. . . . . . . . . . . . . . . . . 30
2.14 Scheme of PMT side-on working principle and picture of R3896 model. . 31
2.15 Picture of PMT quantum efficiency graphic. . . . . . . . . . . . . . . . . 32
2.16 Frequency response of built-in socket pre-amplifier. . . . . . . . . . . . . 33
2.17 Acquisition board interaction scheme. . . . . . . . . . . . . . . . . . . . 34
2.18 Diagram of the acquisition algorithm. . . . . . . . . . . . . . . . . . . . 35
2.19 Developed software for signal acquisition. . . . . . . . . . . . . . . . . . 36
2.20 Bunsen burner scheme. . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
2.21 Testrig description. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
2.22 Optical probe placement. . . . . . . . . . . . . . . . . . . . . . . . . . . 39
2.23 Light collecting cylinder intersecting the steady Bunsen flame, with the
aid of a laser. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
2.24 Representation of the collimated light beam solid cone. . . . . . . . . . . 40
2.25 Representation of probe viewing cone and flame surface intersection. . . . 40
2.26 Graphical representation of local intensity variation as a function of probe
position, for both species. . . . . . . . . . . . . . . . . . . . . . . . . . . 42
2.27 Influence of the vertical position on the distance between flame front and
collimator probe. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
2.28 Spectral distribution of Noise and signal. . . . . . . . . . . . . . . . . . . 45
2.29 Total intensity emitted by the flame for different iris diameters. . . . . . . 47
3.1 Turbofan engine active control scheme. . . . . . . . . . . . . . . . . . . 49
xiv
3.2 Combustor scheme. Respective mean chemiluminescence intensity ratio
evolution. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
3.3 Experimental setup for chemiluminescence and ER mixture model defi-
nition. Fiber head and optical probe viewing area scheme of the current
prototype. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
3.4 Laminar Bunsen flame pictures for different ER, power range P = [0.75
0.96] kW. Exposure time 1/8 s. . . . . . . . . . . . . . . . . . . . . . . . 52
3.5 Ratio between mean intensity value of OH* and CH*, as a function of
mixture ER. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
3.6 Schematic representation of steady and unsteady axissimetric flame. Ve-
locity balance for the flame front. . . . . . . . . . . . . . . . . . . . . . . 55
3.7 Flame front condition. Compression and stretching respectively. . . . . . 56
3.8 Mean local chemiluminescence intensity ratio as a function of strain rate
of the counterflow natural gas flames. . . . . . . . . . . . . . . . . . . . 57
3.9 Measuring a pulsed flame. . . . . . . . . . . . . . . . . . . . . . . . . . 58
3.10 Pictures of Bunsen flame, time exposure of 1/8 s. . . . . . . . . . . . . . 58
3.11 Ratio between mean intensity value of OH* and CH*, OH∗mean/CH∗
mean,
as a function of mixture ER, for different excitation amplitude and verti-
cal probe positions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
3.12 Flame geometry evolution for different amplitudes of excitation. φ = 0.86. 61
3.13 Flame geometry evolution for different amplitudes of excitation. φ = 1.1. . 62
3.14 Schematic representation of the experimental rig for a pulsed flame gen-
eration and image record. . . . . . . . . . . . . . . . . . . . . . . . . . . 64
3.15 Signal post-processing diagram. Phase average implementation. . . . . . 65
3.16 Sense area evolution and pictures of flame geometry for 11 pulse in-
stances. Representation for each light collecting configurations. . . . . . . 66
xv
3.17 Area evolution and chemiluminescence signals after phase average algo-
rithm, for each of the collecting probe configurations. . . . . . . . . . . . 67
3.18 Chemiluminescence evolution as a function of pulse amplitude and mix-
ture ER. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
3.19 Ratio between chemiluminescence signals and the viewing area, for each
of the collecting probe configurations. . . . . . . . . . . . . . . . . . . . 71
3.20 OH* local intensity for light collecting system z = 0.4 cm. Instant images. 72
3.21 Chemiluminescence ratio evolution as a function of pulse amplitude and
mixture ER. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
3.22 Chemiluminescence ratio time evolution and ER time evolution for each
optical configuration. Exponential fit equations. . . . . . . . . . . . . . . 76
xvi
List of Symbols
Acronyms
C∗2 Diatomic carbon excited state
C2 Diatomic carbon
C2H2 Acetylene
CH Hydrocarbon
CH∗ Hydrocarbon excited state
CO Carbon monoxide
CO∗2 Carbon dioxide excited state
CO2 Carbon dioxide
NH Nitrogen monohydride
NH∗ Nitrogen monohydride excited state
NO Nitric oxide
NO∗ Nitric oxide excited state
NOx Mono-nitrogen oxides
O2 Oxygen
xvii
OH Hydroxide
OH∗ Hydroxide excited state
A/D Analog-to-digital converter
AC Active core
ACARE Advising Council for Aeronautics Research in Europe
ADC Analog-to-digital Converter
ALFA-BIRD Alternative Fuels and Biofuels for Aircraft Development
BPF Band pass filter
CCD Charge couple device
ER Equivalence ratio
EU European Union
FA Focal area
FCC Flow control core
FL Focal length
HV High Voltage
IC Intercooled core
ICAO International Civil Aviation Organization
IEA International Energy Agency
IRC Intercooled recuperative core
LPP Lean premixed pre-vaporized
NA Numerical aperture
xviii
NASA National Aeronautics and Space Administration
NTP Normal temperature and pressure
OPR Overall Pressure Ratio
PMT Photomultiplier
RMS Root mean square
RTS Real time sreaming
SFC Specific Fuel Consumption
SLPM Standard liters per minute
SNR Signal-to-noise ratio
SWAFEA Sustainable Way for Alternative Fuels and Energy Aviation
TF Transfer function
UV Ultra-violet
Greek Symbols
∆V Pulse amplitude
λ Wavelenght
φ Equivalence ratio
Roman Symbols
f Focal length
Hz Hertz
K Stretch rate
kW Kilo Watt
xix
L Distance from burner to black target
V Volt
W Watt
Subscripts
θaberr Aberration solid angle
Aview Surface intersection area
CH∗mean Mean CH* chemiluminescence emission
Dexit Burner exit diameter
diris Optical probe iris diameter
dviewing viewing cylinder diameter
f# Focal number
Iemitted Emitted chemiluminescence light intensity
Ilocal Local chemiluminescence light intensity
OH∗mean Mean OH* chemiluminescence emission
SL Laminar flame speed
TFsystem Overall optical path transfer function
xfiber Longitudinal distance between optical fiber and burner
xprobe Longitudinal distance between optical probe and burner
xx
Chapter 1
Introduction
According to the study World Energy Outlook 2009 performed by International En-
ergy Agency [IEA, 2009], the aeronautic sector was responsible for around 3%
of global CO2 emission in 2007. Despite this low value, a recent survey done by the
Advisory Council for Aeronautics Research in Europe [ACARE, 2010], showed that this
sector is expected to grow an annual average rate of 5% in the next 20 years. Based on this
data ACARE imposed ambitious goals for the next generation of airplanes, a reduction
of 6% of CO2 and 16% of NOx emissions until the year 2025.
Even with improvements on airplane efficiency, turbine engines are still the biggest res-
ponsibles for pollutants emission. Since the first gas turbine designed to power an air-
plane in 1939, the engine core configuration remains based on the Brayton Cycle. As
a guideline, ideal Brayton Cycle thermal efficiency is mainly a function of the Overall
Pressure Ratio (OPR - ratio between the flow pressure exiting and entering the engine)
while its specific work also depends on the turbine entry temperature. Improvements on
the OPR are being introduced since 1960, but the main constraint is still the material
temperature limit at the turbine entry. Although big improvements ere done on Specific
Fuel Consumption (SFC) until 1992, the rate of technology evolution presented in figure
1.1 is no longer adequate to reach ACARE goals.
1
2 CHAPTER 1. INTRODUCTION
Figure 1.1: Turbine SFC along the years for the various configurations. Source [CAT, 1992].
Take the example of the engine CFM56-3C1, figure 1.2, presented in table 1.1. The first
entry into service of this model was in 1984, built to replace the low by-pass, 1:1, Pratt
& Whitney JT8D model that powered the Boeing 737 family. Its main innovation was
the 5:1 high by-pass ratio. Compared to the old JT8 model it was able to produce 80% of
thrust from the secondary flow, which dramatically decreased SFC and engine noise. In
1998 CFM56 introduced to service the CFM56-7B turbine engine, an improved version
of its predecessor CFM56-3C1.
Specification CFM56-3C1 CFM56-7B24/3 Comparing [%]
Maximum Thrust [KN] 104 108 + 4%
NOx emission [g/Kg] 20.7 18.93 - 9%
OPR 30.6 32.8 + 7%
Bypass Ratio secondarymass flowprimarymass flow
5 5.3 + 6%
Entry service year 1984 2009 -
Table 1.1: CFM56-3C1 and CFM56-7B24/3 characteristics. NOx emissions values are for each
kilogram of burned fuel, from ICAO Engine Exhaust Emissions Data Bank [CFM, 2010].
3
Figure 1.2: CFM56-3C1 cut view. Schematic of primary and secondary flow. Source [CFM,
1996].
The 2009 CFM56-7B24/3 model presented in table 1.1, an upgraded version of the 1998
CFM56-7B, shows that 25 years of technological evolution were necessary to reduce only
9% of NOx compared to its predecessor CFM56-3C1. This trend does not meet the re-
quirements of ACARE goals, and shows that non-conventional solutions on the research
for more environmental friendly engines are necessary. The problem that engineers need
to solve is simple to understand: for a specified thrust it is necessary to increase the ther-
mal efficiency and reduce the polluting products of combustion.
Increase Thermal Efficiency
The process of increasing thermal efficiency is defined as passive pollutants control be-
cause it is inherent to engine design and related to improvement of materials temperature
limit that constitute the combustor and turbine, as well as with the enhancement of cycle
specific work.
To encourage research for new unconventional designs programs such as NEWAC (New
Aero Engine Core Concepts [NEWAC, 2010]) are created. This program co-funded by
the European Union (EU) gathers the main turbines manufactures, such as MTU, Volvo
Aero, Snecma and Turbomeca, to work in 6 different projects: 4 new core concepts
(see figure 1.3), new combustors design and new engine integration philosophies on the
airplane.
4 CHAPTER 1. INTRODUCTION
T
s
Intercooler
Compressors
Regenerator
Combustor
1
23 4
5
6
7
89
8
1
7 6
5
432
Turbines
Figure1.3:
Therm
alEfficiency
ofthe
4N
EW
AC
coreconcepts,source
[NE
WA
C,2010].
Idealtemperature
-entropy
diagramof
IRC
improved
Brayton
cycle.
5
The projects presented in the previous figure are related to active systems development
(Active Core - AC), new flow control technologies (Flow Controlled Core - FCC), in-
tegration of an intercooler on the compression stage (Intercooled Core - IC) and the
inclusion of a exhaust gas heat exchanger (Intercooled Recuperative Core - IRC) which
is the the high thermal efficient core solution for a low OPR, [NEWAC, 2010].
In terms of thermodynamics, IRC is known as regenerative gas turbine with intercooling
and reheat design. It is widely used for ground power generating gas turbine since there is
no limitation in adding complexity and weight to the system. For the same turbine entry
temperature limit this solution increases the total network provided by the cycle, figure
1.3. The drawback of implementing this geometry on an airplane turbofan engine is that
the presence of the intercooler and heat exchanger implies a smaller core dimension,
which ends up giving more flexibility to the engine than the necessary. It is then required
to add more stiffening parts to the core, [NEWAC, 2010].
The NEWAC team responsible for the IRC combustion design took advantage of its low
OPR imposed by the circuit of air exiting the compressor, passing through the heat ex-
changer towards the combustor chamber, to implement the Lean Premixed Prevaporized
combustor (LPP). This new air path is not suitable for high pressures, meaning that pro-
blems of auto-ignition or flashback are less probable to occur. The LPP system designed
for the NEWAC combustor project is expected to reduce the NOx formation for 57%
relative to the environmental limits, [NEWAC, 2010].
Reduce polluting products of combustion
Lean combustion control means that chemical reactions occur with an excess of air with
significantly lower flame temperatures and lower NOx production. To achieve this condi-
tions it is necessary to homogeneously premix the fuel with air before it enters the com-
bustor module. The problem is that working under lean conditions has a strong effect on
the combustion stability, leading to thermo-acoustic instabilities and, if not controlled,
structural damages [Ditaranto, 2007]. A work developed by Anacleto et al. [2003], at
the Laboratory of Thermofluids and Combustion in Instituto Superior Técnico, showed
6 CHAPTER 1. INTRODUCTION
that the LPP system has a low range of high efficient work. The study subject was a LPP
injector with the objective to understand the influence of the swirl number on the NOx
and CO formation, for different equivalent ratio (ER or φ) setups. It was found that the
increase of swirl number decreases the formation of polluting products, both for liquid
jet-fuel or for gaseous propane. For high swirl numbers, the trend of NOx and CO forma-
tion is approximately the same, see figure 1.4(a), but the working range becomes a much
narrow band, bounded by the flashback and the extinction, see figure 1.4(b). The study
showed that it is possible to reduce pollutants from combustion for high swirl number
with an advanced level of control, and that it is possible to study a hydrocarbon gaseous
flame instead of using a Jet-A1 liquid fuel flame, expecting to have results with the same
behavior.
(a) NOx Formation (b) Equivalence ratio working range
Figure 1.4: NOx formation and equivalence ratio as a function of swirl number, [Anacleto et al.,
2003].
Aeronautic certification entities only accept LPP combustion with high level of control
that guarantees reliability on maintaining the flame in the optimal regime, as one can
see on figure 1.4(b). However, the actual turbine engine temperature sensor, usually a
thermocouple type K, is not suitable for this type of control because it has to be located far
from the combustion zone due to material limitations [CFM, 1996], and it has a slow time
response to temperature change. Figure 1.5 illustrates the exhaust gas temperature (EGT)
sensor of a CFM56-3C1 engine used as an indirect indicator of combustion temperature.
7
To keep the measuring system redundancy there are 9 of this sensors placed inside the
low pressure turbine second stage nozzles. A closer look shows that thermocouple head
is not in contact with the flow, which is guided through the nozzle, looses energy, and
then reaches the thermocouple tip. This kind of sensor is useless for LPP combustion
monitoring.
Figure 1.5: CFM56-3 EGT sensor. Detail view of thermocouple assembly, [CFM, 1996].
In the context of improving airplane efficiency Clean Sky appears as another example
of an European project, [CleanSky, 2011a]. Regarding engine projects, the last call for
proposal presented the research topic of a lean burn control system [CleanSky, 2011b],
which has to be designed, developed and installed in a conventional turbofan. The engine
will then undergo a range of experiments, including flight testing, to enable technology
validation across the engine operational envelope. The proposal contract states that this
testing procedures will occur 20 months after the beginning of the project, anticipating a
cleaner engine prototype earlier than 2025.
Alternative Fuels
Immediate harmful impact on the environment of Jet-A1 combustion products is not the
only problem that needs to be solved. It takes millions of years for the burned fuel on a
8 CHAPTER 1. INTRODUCTION
combustor chamber to be produced, a time scale that it is not suitable for the continuous
increase of finite fossil fuel consumption. On the other side the production time scale of
a bio-fuel cycle is significantly lower, since it is generated directly from the plants, which
is an enormous advantage in terms of guaranteeing energy reserves.
Two recent important examples of EU co-funded projects on alternative fuel area are
ALFA-BIRD (Alternative Fuels and Biofuels for Aircraft Development [Salvi, 2008])
and SWAFEA (Sustainable Way for Alternative Fuels and Energy Aviation [Blakey and
Novelli, 2010]), both created to investigate and develop alternative fuels for use in aero-
nautic engines. The results are new types of fuels, such as modified vegetable oils, Bio-
diesel, Bio-ethanol, Bio-methanol, liquid bio-fuel, biogas and recently syngas derived
from biomass, as shown by Gupta et al. [2010].
Figure 1.6: First commercial flight of an A320 with one CFM56 engine powered with 30% of
Jatropha bio-fuel, [Sucar-Hamel, 2011].
All the benefits of using bio-fuel increase when lean technology is used. This is why
the scientific community and the engine manufacturers are gathering efforts in order to
develop control technology for LPP combustion with alternative fuels.
1.1. LITERATURE REVIEW 9
1.1 Literature Review
1.1.1 Chemiluminescence for combustion control
According to what was seen in the previous section, the first step on the search for re-
liable control system is the development of accurate sensing technology. A closer look
on the work of Higgins et al. [2001], that studied the influence of ER and pressure on
light emission from a gas combustion, reveals the importance of chemical light emission
(chemiluminescence = chemical + luminescence) for combustion control. During his
work, Higgins showed under realistic conditions that chemiluminescence sensors could
be used within an electronic management system to control the combustion process to-
wards low pollutants emissions. This research was supported by project BRITE-EURAM
III, created to help several industrial areas with the same lean combustion problems. Later
in time, Muruganandam et al. [2003] showed the applicability of chemiluminescence for
turbine sensor design, since it could be measured avoiding the combustor harsh environ-
ment with non-intrusive optic methods.
The first important work on flame chemiluminescence was conducted by Gaydon and
published in his book, The Spectroscopy of Flames, in 1957. Since radiation can be ea-
sily measured, early researchers looked for a relationship between its intensity, frequency
and flame properties. Following Gaydon observations Clark attempted, in 1958, to find
correlations between measured electromagnetic radiation and flame variables, such as
mass flow rate or fuel type. Later in 1968, Price and Hurley took a great step on flame
analysis, by first showing that the chemiluminescence of C∗2 and CH* was proportional
to the heat release. This work is still the basis of all heat release studies. Gaydon, Clark,
Price and Hurley works are cited in Haber’s thesis, [Haber, 2000].
Models
The chemical equation of a reaction between hydrocarbons and air, combustion, repre-
sents a global mass balance. However, combustion is composed of several high speed
10 CHAPTER 1. INTRODUCTION
chemical reactions, around 500 on these flames (see for example figure 1.7), that lead
to the formation of the final products in the stoichiometric equation. The high rate of
these chemical reactions leads to formation of electronically excited species, from which
the most energetic are the radicals OH∗, CH∗ and C∗2 (the asterisk stands for excited
electronic state). Even with current technologies it is still difficult to understand the total
mechanism of excited species formation on hydrocarbon flames.
Figure 1.7: Example of a pathway for OH formation in a methane-air free flame [Najm, 1998].
The actually accepted model for OH* production path is:
CH +O2 ↔ CO +OH∗ (1.1)
first proposed by Gaydon and later experimentally demonstrated by the results of Broida’s
work, cited on Panoutsos work [Panoutsos, 2009]. Using this model Dandy and Vosen
[1992], performed the first numerical study of the hydroxyl radical. Panoutsos recently
presented a more accurate model, still based on Gaydon’s work.
For the radical CH*, work of Devriendt and Peeters [1996] presented unambiguous evi-
dence of the formation mechanism:
C2H +O ↔ CO + CH∗ (1.2)
1.1. LITERATURE REVIEW 11
For C∗2 formation, the accepted chemical reaction is:
C + C2H2 ↔ C∗2 +H2 (1.3)
This mechanism was first proposed by Homman and cited on Kojima’s work [Kojima
et al., 2005]. Kojima reinforced the applicability of this reaction by showing that theo-
retical results of this model were in agreement with experimental data.
These are the radicals that present the highest concentration during the reaction. Never-
theless, 500 steps of hydrocarbon combustion also generate other excited radical species,
such as CN∗, NO∗, CH2O∗, NH∗ as well as other minor species.
All mentioned species are in an electronic excited state. Return to ground state can occur
by three different ways [Sugden, 1962]:
1. Re-dissociation. Usually the less intense process, it is generally given by the equa-
tion:
A+B +X AB∗ +X (1.4)
where A, B and X are generic species, and it occurs on both ways. For the OH∗
radical Dandy and Vosen [1992] proved that this is not a significant mechanism.
2. Quenching. Due to the thermal agitation, the excited species may loose the excess
of energy by colliding with other species, in what is usually named a quenching
collision:
AB∗ +X → AB +X (1.5)
The efficiency of this process is highly dependent on the type of species reacting
and on the reaction temperature. Despite the fact that this process was known since
Gaydon’s work, the major contribution was given by Garland and Crosley [1986]
who estimated the quantity of quenching undergoing during the de-energizing pro-
cess.
12 CHAPTER 1. INTRODUCTION
3. Chemiluminescence: The species emit electromagnetic radiation while returning
to their ground energy state:
AB∗ → AB + hv (1.6)
where h is the Planck constant, and v the radiation frequency. This frequency is
a unique characteristic of each excited species. The complexity of the electro-
magnetic spectrum is directly related with the complexity of the emitting species.
Notice that measuring the intensity of emitted light from a certain specie is a qua-
litative analysis of the flame’s chemical reaction.
Chemiluminescence measuring techniques
To quantify the amount of radiation emitted by chemiluminescence it is necessary to use
optical measuring systems. These systems are usually composed by a light collecting
system, a frequency selector and a sensing device, that can be combined to acquire the
desired information.
The light collecting system defines the spatial resolution since it selects the control vol-
ume of the flame front that is being measured, see table 1.2. The probe usually presents
one of three configurations:
1. Collimator probe. Using a collimator only the radiation entering the cylinder vo-
lume will reach the sensor. This allows a specific area of the flame front to be
analyzed (see Leitão [2009], Fernandes [1998]);
2. Cassegrain probe. Using this lens system it is possible to collect light from a
significantly small control volume of the surface, ensuring a high spatial resolution
( [Hardalupas et al., 2004]);
3. No optical probe. The head of the optical fiber only receives radiation that goes
through a conical volume, enabling the acquisition from a bigger flame area (see
Venkata [2007], Leitão [2009]).
1.1. LITERATURE REVIEW 13
After being collected by any of these configurations, radiation is guided through the
optical fiber towards the frequency selector. Before entering this component it is possible
to split the beam, in order to sense more than one specie, by using Dichroic mirrors.
The frequency selector is responsible for defining the spectral resolution, see table 1.2.
It can be a Band Pass Filter (BPF) or diffraction grating. The BPF only allows the trans-
mission of a narrow band frequency. The more narrow and transmissible the BPF is, the
more precise and accurate the system becomes. Tinaut et al. [2010] measured the OH*
and CH* chemiluminescence emission on a combustion chamber with Dichroic mirrors,
to split the beam, and two BPF, to select the desired frequency. Zimmer et al. [2003] used
this technique to evaluate the CH* chemiluminescence emission and relate fluctuations
intensity with pressure variations recorded with a pressure probe. If the BPF is replaced
by a diffraction grating, it is possible to split radiation in several beams, each one with
a specific wavelength. The interval between two wavelengths depends on the quality of
the diffracting grating. Muruganandam et al. [2003] used this technique to evaluate the
chemiluminescence spectra of a flame for turbine sensor design.
A sensing device is responsible for determining the time resolution. Typically there are
two types of sensors: the photomultiplier (PMT) and the charge coupled device (CCD)
camera. The PMT is used when the objective is to track the intensity of radiation emitted
by a specie during the time of experience. It generates an analogic signal proportional to
the intensity of the incident light, see table 1.2. Due to its high sampling frequency PMT’s
are commonly used for active flame control, see the work of Higgins et al. [2001]. With
the CCD camera it is possible to sense each beam intensity, by split it with a diffracting
grate. The measuring systems which have this configuration are known as spectrometer,
since they are able to sense the intensity of chemiluminescence effects on a wide spec-
tral range. The interval between two wavelengths depends not only on the quality of
the diffracting grating but also on CCD’s resolution, see table 1.2. The drawback of this
technology is the time between two recorded samples, defined by the CCD frequency.
Companies such as Hamamatsu [2007] are continuously improving the CCD time res-
ponse and spatial resolution, because with faster CCD’s this technique could provide
14 CHAPTER 1. INTRODUCTION
information about the intensity of emitted light in all wavelength, with a good time reso-
lution. Since this technology is not yet available, researchers like Ferro [2008] investigate
new mathematical models for treating the data acquired with this devices.
When the objective is to visualize flame behavior or structure at a certain instant of time,
the collecting system is removed and the CCD camera acquires the flame’s image. De-
pending on the desired information, the frequency selector can be installed or absent.
After the image is acquired there are several algorithms that process it and gather infor-
mation related to scalar properties of the flame reaction, see table 1.2.
1.1. LITERATURE REVIEW 15
Tech
niqu
eR
esol
utio
nM
ount
ing
Sche
me
Col
lect
ing
Syst
emSe
nsin
gD
evic
eA
dvan
tage
sD
isad
vant
ages
Tim
eA
DC
fiber
prob
e
Cass
egra
in
PMT
•O
ptic
al
colli
mat
or
•C
asse
grai
nle
ns
•O
ptic
alfib
erhe
ad
PMT
•H
igh
freq
uenc
y
resp
onse
•C
asse
grai
nle
ns
prov
ides
high
spat
ial
reso
lutio
n
•Tw
oor
mor
e
spec
ies
data
acqu
isiti
on
•E
xpen
sive
PMT
•B
PFqu
ality
dire
ctly
affe
cts
the
accu
racy
ofth
e
mea
sure
men
t
Spec
tral
fiber
prob
e
Cass
egra
in
Spec
trop
hoto
met
er
•O
ptic
al
colli
mat
or
•C
asse
grai
nle
ns
•O
ptic
alfib
erhe
ad
CC
DC
amer
a
•H
igh
spec
tral
reso
lutio
n
•M
easu
res
all
wav
elen
gths
sim
ulta
neou
sly
•C
asse
grai
nle
ns
prov
ides
high
spat
ial
reso
lutio
n
•L
owfr
eque
ncy
resp
onse
Spat
ial
CCD
Inte
rfer
ence
filte
r
•Si
ngle
orm
ultip
le
inte
rfer
ence
filte
rC
CD
Cam
era
•Sp
atia
lres
olut
ion
•L
owfr
eque
ncy
resp
onse
Tabl
e1.
2:M
easu
ring
Tech
niqu
es.G
ener
alco
nfigu
ratio
ns.A
dvan
tage
san
ddi
sadv
anta
ges
16 CHAPTER 1. INTRODUCTION
1.1.2 Chemiluminescence applications
Aeronautic Industry is one of the main sponsors for creation of new sensors capable of
measuring chemiluminescence effects. These sensors are important for health monito-
ring of combustion chambers, because they withstand higher temperatures than thermo-
couples. Following the mentioned Higgins work, in 2003 Muruganandam et al. [2003]
presented their work focused on chemiluminescence sensors for turbine engines active
control and health monitoring. They showed that the ratio between the intensity of radi-
ation from CH* and OH*, CH*/OH*, increases monotonically with the E.R., for several
combustor configurations and several types of fuel. The work also included the capacity
of measuring radiation from chemiluminescence to detect precursors events of combus-
tion blow out. All this work was partially supported by NASA.
Recent work has been done by Venkata [2008], to better understand the capacity of
chemiluminescence measurements to sense heat release and ER. This work included nu-
merical modulation of OH*, CH* and CO∗2 chemiluminescence emission, and it showed
the importance of better understanding theCO∗2 emission spectrum that, opposite to other
species, emits on a continuous broadband, from 200 to 700 nm. The mentioned work
done by Ferro [2008] showed that the shape of the spectrum obtained by a normal spec-
trometer is due to the low spectral resolution. By proposing a mathematical treatment to
the signal it was possible to reproduce the flame spectrum without the influence of CO∗2
continuous light emission, and understand the contribution of other several minor species
to the spectrum, see figure 1.8.
1.1. LITERATURE REVIEW 17
51
300 350 400 450 500 5500
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
WAVELENGTH, nm
FLA
ME
SIG
NA
L, a
.u.
Original SignalEstimated SignalEstimated CO2 BroadbandRe-estimated Individual Gaussian Function
Figure 4.13 – Original and Estimated flame spectra from a laminar premixed methane-air flame (f=1.2, P=0.4 KW and d=10 mm).
For the propane-air flame it was obtained an overall intensity RMS,
RMS=0,002 and for the methane air flame RMS=0,004. For both flames
spectrum the approximation represents an error less than 5%.
In Figure 4.14 is observed the quasi linearity between the estimated
wavelengths and the theoretical ones (given by Gaydon [1]) for the
premixed propane and methane air flames.
Figure 1.8: Mathematically treated methane emission spectrum. Original and estimated flame
spectra from a laminar premixed methane-air flame (φ = 1.2 and P = 0.4 KW), by Ferro [2008].
Recently Guyot et al. [2010] presented an accurate model for correlation between chemi-
luminescence intensity of OH* and CH*, mass flow and ER of the premixed swirl-
stabilized flame. They gathered all available knowledge and measuring techniques, in-
cluding studies on the CO∗2 broadband emission, to obtain the best correlation.
Despite all this work, chemiluminescence is still not clearly understood. While working
on his accurate model, Guyot uncovered an important limitation: each correlation is
restricted to each combustor and flame configuration.
Understanding these problems is essential since all major work is done over laminar
flames but the final target are high turbulent flame environments.
18 CHAPTER 1. INTRODUCTION
Chemiluminescence technique for different purposes
The problems of auto-ignition on LPP combustion are also studied with this sensing
technology. With the objective of studying auto-ignition of liquid fuel sprays, Hinkeldey
et al. [2007] used the spatial resolution technique to identify and analyze points where
auto-ignition occurs.
Besides the auto-ignition issue, there is another lean combustion problem that is being
analyzed with this technology: when the ER is reduced the energy required for ignition
increases exponentially, and normal ignition systems are not efficient enough to start the
process. Ahmed [2006] presented in his work, related to laser ignition of turbulent flames,
the importance of recording with a PMT the chemiluminescence emission of OH* and
CH* in the spark zone.
Chemiluminescence is not only related to combustion. The presence of radical OH on
the troposphere is responsible for control and removal of many atmosphere pollutants, as
explained on the work of Wasselin-Trupin et al. [2000]. The research in this area is related
to formation of this radical in laboratory environment, where the process undergoing on
the atmosphere is simulated: an electromagnetic radiation incident a water molecule,
ionizing it to H+ and OH−. This happens for a very short period of time, and during the
following moments the radical looses its excess of energy, mostly by chemiluminescence.
The use of a PMT and a frequency selector allow tracking the emission of radiation and
relating it with OH* concentration.
Another area of application for these devices is Oncology. Kohno et al. [2008] recently
showed on their work that if a sample of cells is injected with a certain agent, intensity
of emitted radiation from the O2 molecules is linearly correlated to concentration of that
agent, which is only absorbed by cancer cells.
1.1. LITERATURE REVIEW 19
Aut
hor
App
licat
ion
/Obj
ectiv
esE
xper
imen
talS
etup
Tech
niqu
e
Mur
ugan
anda
met
al.[
2003
]
Sens
orde
velo
pmen
tfor
turb
ine
engi
neco
ntro
land
heal
th
mon
itori
ng.L
ocal
mon
itori
ngof
E.R
.and
chem
ilum
ines
-
cenc
eem
issi
onto
dete
ctpr
ecur
sore
vent
sto
blow
out.
2 A
mer
ican
Inst
itute
of A
eron
autic
s an
d A
stro
naut
ics
prov
ide
info
rmat
ion
on th
e pr
esen
ce a
nd st
reng
th o
f the
co
mbu
stio
n pr
oces
s in
a
spec
ific
regi
on
of
the
com
bust
or, m
akin
g it
wel
l-sui
ted
for h
ealth
mon
itorin
g an
d di
agno
stic
s. Si
nce
the
chem
ilum
ines
cenc
e in
tens
ities
rel
ate
to
the
spec
ies
that
ar
e pr
oduc
ed
durin
g th
e re
actio
n pr
oces
s, th
e in
tens
ity o
f lig
ht p
rodu
ced
is re
late
d to
the
rate
of p
rodu
ctio
n/de
plet
ion
of th
at s
peci
es. T
hese
rate
s va
ry w
ith r
eact
ion
path
way
s w
hich
is
a fu
nctio
n of
eq
uiva
lenc
e ra
tios,
mak
ing
it ea
sier
to
dedu
ce t
he
equi
vale
nce
ratio
fr
om
the
chem
ilum
ines
cenc
e in
tens
ities
. It h
as b
een
show
n th
at th
e ra
tios
of c
hem
i-lu
min
esce
nce
from
diff
eren
t mol
ecul
es, e
.g.,
OH
*/C
H*
and
C2*
/OH
*, v
ary
with
equ
ival
ence
rat
io i
n si
mpl
e fla
mes
.9-13
Mor
rell
et a
l.14 u
sed
the
C2*
/OH
* ra
tio t
o de
term
ine
the
axia
l evo
lutio
n of
the
loca
l equ
ival
ence
ra
tio i
n a
nonp
rem
ixed
, liq
uid-
fuel
ed c
ombu
stor
at
atm
osph
eric
con
ditio
ns.
Prac
tical
com
bust
ors
usua
lly o
pera
te a
t a ra
nge
of
pres
sure
s us
ually
hig
her
than
atm
osph
eric
pre
ssur
e.
Sinc
e pr
essu
re c
an c
hang
e th
e re
actio
n m
echa
nism
s, th
e nu
mbe
r de
nsity
of t
he r
adic
als
and
thus
the
chem
i-lu
min
esce
nce
sign
al w
ill b
e af
fect
ed b
y ch
ange
in
pres
sure
. H
iggi
ns e
t al
.11 a
nd I
keda
et
al.12
ha
ve
inve
stig
ated
th
e be
havi
or
of
OH
an
d C
H
chem
i-lu
min
esce
nce
at
high
pr
essu
re
in
sim
ple,
la
min
ar
met
hane
-air
flam
es. T
here
is li
ttle
data
in th
e lit
erat
ure
for
mor
e co
mpl
icat
ed c
ombu
stor
s (e
.g.,
turb
ulen
t or
sw
irl s
tabi
lized
), es
peci
ally
at e
leva
ted
pres
sure
s. Th
is
wor
k de
mon
strat
es
the
usef
ulne
ss
of
chem
ilum
ines
cenc
e as
a u
sefu
l di
agno
stic
too
l fo
r co
mbu
stio
n pr
oces
ses
in a
var
iety
of
com
bust
ors,
over
a
rang
e of
pre
ssur
es, a
nd fo
r a n
umbe
r of f
uels
. A
s no
ted
abov
e, s
ensi
ng o
f pr
oxim
ity t
o th
e bl
owou
t lim
it in
com
bust
ors
is a
lso
an im
porta
nt is
sue
for
com
bust
or h
ealth
mon
itorin
g. L
ean
blow
out
in
com
bust
ors
has
gain
ing
atte
ntio
n am
ong
rese
arch
ers
due
to t
he i
ncre
asin
g co
nstra
ints
on
NO
x em
issi
ons
cons
train
ts
in
the
turb
ine
engi
ne
indu
stry
. Fl
ame
stab
ility
has
bee
n st
udie
d cl
ose
to b
low
out b
y a
num
ber
of
rese
arch
ers.15
-23
The
focu
s w
as
mai
nly
on
unde
rsta
ndin
g th
e m
echa
nism
s th
at i
nflu
ence
the
los
s of
sta
bilit
y. T
his
is u
sual
ly a
ttrib
uted
to h
igh
heat
loss
an
d st
rain
rate
s.
A
hand
ful
of
rese
arch
ers
have
fo
cuse
d on
de
tect
ing
the
appr
oach
of s
tabi
lity
loss
. Som
e ob
serv
ed
sign
ifica
nt f
lam
e in
stab
ility
nea
r le
an b
low
out
and
note
d th
at t
here
was
a c
onsi
dera
ble
amou
nt o
f tim
e w
hen
ther
e w
as e
ssen
tially
no
flam
e pr
esen
t in
the
co
mbu
stor
.15,2
4-30
M
urug
anan
dam
et a
l.31 s
how
ed th
at
the
appr
oach
of L
BO
can
be
dete
cted
by
obse
rvin
g th
e O
H c
hem
ilum
ines
cenc
e fr
om s
peci
fic l
ocat
ions
in
a ga
s-fu
eled
, atm
osph
eric
pre
ssur
e, n
on-p
rehe
ated
, sw
irl-
stab
ilize
d co
mbu
stor
. Si
nce
this
wor
k fo
cuse
s on
the
us
eful
ness
of
chem
ilum
ines
cenc
e em
issi
ons
from
the
co
mbu
stor
, th
e pr
ecur
sor
even
ts t
o bl
owou
t of
the
com
bust
or w
ere
also
use
d to
giv
e in
form
atio
n ab
out t
he
prox
imity
of
the
blow
out
limit
from
the
ope
ratio
n co
nditi
ons.
Var
ious
com
bust
ors
have
bee
n us
ed t
o sh
ow
the
robu
stne
ss
and
appl
icab
ility
of
th
e te
chni
ques
.
EX
PER
IME
NT
SET
UP
C
HE
MIL
UM
INE
SCE
NC
E C
OL
LEC
TIO
N
Che
milu
min
esce
nce
is
colle
cted
us
ing
two
syst
ems.
The
first
mod
els
a pr
actic
al s
enso
r dev
ice
and
empl
oys
a 36
5 µm
fus
ed s
ilica
opt
ical
fib
er. T
he f
iber
ha
s a
num
eric
al a
pertu
re o
f 0.
22 w
hich
allo
ws
it to
co
llect
lig
ht f
rom
a c
onic
al r
egio
n of
hal
f an
gle
of
12.5
o . Th
e co
llect
ed l
ight
pas
ses
thro
ugh
appr
opria
te
inte
rfer
ence
filte
rs (3
08 n
m c
ente
r wav
elen
gth
for O
H*
and
430
nm f
or C
H*,
bot
h 10
nm
ful
l-wid
th-h
alf-
max
imum
) an
d is
det
ecte
d by
min
iatu
re,
met
al-c
an
phot
omul
tiplie
r tu
bes
(PM
T, H
amam
atsu
H57
84).
The
loca
tion
of th
e fib
er d
eter
min
es th
e re
gion
of t
he fl
ame
obse
rved
(th
is g
ives
the
fle
xibi
lity
of c
hoos
ing
the
view
ing
volu
me)
. Fo
r m
ost
of t
he m
easu
rem
ents
, th
e fib
er w
as u
sed
only
to d
emon
stra
te L
BO
det
ectio
n an
d th
us th
e fib
er c
olle
cted
ligh
t fro
m a
regi
on v
ery
clos
e to
th
e st
abili
zatio
n zo
ne o
r th
e br
ight
fla
me
zone
. Si
nce
the
PMT
has
a ve
ry f
ast
time
resp
onse
(20
kH
z ba
nd
wid
th) i
t can
be
used
to m
onito
r tem
pora
l var
iatio
ns o
f ch
emilu
min
esce
nce
in th
e co
mbu
stor
.
Fi
gure
1. S
chem
atic
of
the
imag
ing
spec
trom
eter
set
up
used
to o
btai
n th
e em
issi
on sp
ectra
from
the
com
bust
ors.
The
seco
nd d
etec
tion
syst
em e
mpl
oys
an im
agin
g sp
ectro
met
er (
300
groo
ve/m
m g
ratin
g) c
oupl
ed t
o a
1024
×256
int
ensi
fied
CC
D c
amer
a to
sim
ulta
neou
sly
capt
ure
the
ultra
viol
et a
nd v
isib
le o
ptic
al e
mis
sion
sp
ectru
m
(~28
0–55
0 nm
). In
or
der
to
mim
ic
the
antic
ipat
ed
reso
lutio
n of
pr
actic
al
engi
ne
optic
al
sens
ors,
the
reso
lutio
n of
the
spec
trom
eter
is ty
pica
lly
set
to ~
5-10
nm
thr
ough
con
trol
of t
he e
ntra
nce
slit
wid
th. W
ith t
his
setu
p, f
lam
e sp
ectra
can
be
acqu
ired
with
rela
tivel
y lo
w te
mpo
ral r
esol
utio
n (~
10 H
z).
CO
MB
UST
OR
S Ex
perim
ents
w
ere
cond
ucte
d in
va
rious
co
mbu
stor
s fr
om s
impl
e op
en f
lam
es t
o co
mbu
stor
s th
at e
mul
ate
man
y as
pect
s of
pra
ctic
al, t
urbi
ne e
ngin
e co
mbu
stor
s. B
oth
prem
ixed
and
non
-pre
mix
ed/p
artia
lly
prem
ixed
com
bust
ors w
ere
used
in th
is st
udy.
Spec
tral
and
time
reso
lutio
n.
PMT
and
aC
CD
cam
era
Ven
kata
[200
7]Sy
ngas
and
jetf
uela
naly
sis.
Influ
ence
ofte
mpe
ratu
rein
the
mea
sure
men
ts.
A
mer
ican
Inst
itute
of A
eron
autic
s and
Ast
rona
utic
s 4
The
char
acte
ristic
blu
e ch
emilu
min
esce
nce
obse
rved
in
hydr
ocar
bon
flam
es a
roun
d 43
0 nm
is
due
to C
H*
emis
sion
, sp
ecifi
cally
the
(0,
0)
band
of
the
CH
(A
2 ∆ -
X2 Π
) tra
nsiti
on.
Ther
e is
stil
l de
bate
on
the
impo
rtant
fo
rmat
ion
reac
tions
resp
onsi
ble
for C
H* . O
f the
man
y pl
ausi
ble
sour
ces,
rese
arch
has
focu
sed
on th
e fo
llow
ing.
C
2 + O
H →
CH
* + C
O
(R5)
C2H
+ O
→ C
H* +
CO
(R
6)
C2H
+ O
2 → C
H* +
CO
2 (R
7)
Gay
don26
sug
gest
ed th
e re
actio
n R
5; th
is w
as la
ter c
halle
nged
, firs
t by
Bre
nig27
and
late
r by
Gre
be a
nd H
oman
n.28
B
reni
g's
expe
rimen
ts s
ugge
sted
tha
t the
CH
* ra
dica
l mig
ht b
e pr
oduc
ed fr
om th
e re
actio
n of
gro
und
stat
e et
hyny
l ra
dica
ls w
ith O
ato
ms,
whi
ch h
ad b
een
earli
er p
ropo
sed
by G
lass
et a
l.29 R
enlu
nd e
t al.30
sugg
este
d th
e re
actio
n (R
7)
of C
2H w
ith O
2 ra
ther
than
ato
mic
oxy
gen.
Dev
riend
t et a
l.31 in
thei
r pu
lse
lase
r ph
otol
ysis
stu
dy a
t low
pre
ssur
e de
term
ined
the
tem
pera
ture
dep
ende
nce
of R
6 an
d co
nclu
ded
that
the
maj
ority
of C
H*
is p
rodu
ced
by th
at re
actio
n.
How
ever
in a
rec
ent s
tudy
by
Car
l et a
l. ba
sed
on f
lash
pho
toly
sis
of a
cety
lene
at l
ow p
ress
ure32
, the
tem
pera
ture
de
pend
ence
of R
7 w
as fo
und;
they
als
o su
gges
ted
that
R7
mig
ht c
ontri
bute
sig
nific
antly
to C
H*
chem
ilum
ines
cenc
e in
hot
flam
es a
nd e
spec
ially
und
er fu
el le
an c
ondi
tions
. So,
in th
is s
tudy
, a m
odel
bas
ed o
n R
6 an
d R
7 w
as u
sed
to
mod
el C
H*
in m
etha
ne a
nd J
et-A
flam
es. T
he c
ompl
ete
CH
* m
echa
nism
with
the
reac
tions
and
thei
r ass
ocia
ted
rate
pa
ram
eter
s ar
e gi
ven
in T
able
1. A
s in
the
OH
* an
alys
is, t
he p
hoto
n em
issi
on r
ate
i CH
* is
fou
nd f
rom
the
quas
i-st
eady
con
cent
ratio
n [C
H*]
usi
ng C
H* c
ollis
iona
l que
nchi
ng a
nd sp
onta
neou
s em
issi
on ra
tes.
III.
Expe
rim
enta
l Set
up
A.
Bur
ners
Che
milu
min
esce
nce
spec
tra w
ere
acqu
ired
in tw
o pr
emix
ed b
urne
rs: t
he s
ynga
s fu
els
used
a la
min
ar je
t fla
me
as
show
n in
Fig
ure
1a. T
his
burn
er w
as p
revi
ousl
y us
ed to
mea
sure
lam
inar
flam
e sp
eeds
for s
ynga
s m
ixtu
res,
and
its
deta
ils in
clud
ing
syng
as m
ixtu
re p
repa
ratio
n ca
n be
foun
d th
ere.
24 T
he b
urne
r is
a st
raig
ht c
ylin
dric
al s
tain
less
ste
el
tube
with
an
inne
r di
amet
er o
f 4.
5mm
, the
leng
th o
f w
hich
is e
nsur
ed to
mak
e th
e flo
w la
min
ar a
nd e
xit v
eloc
ity
prof
ile fu
lly d
evel
oped
. The
rota
met
ers u
sed
for f
low
rate
mea
sure
men
ts w
ere
calib
rate
d w
ith a
bub
ble
flow
met
er to
±1
% a
ccur
acy,
im
plyi
ng a
max
imum
err
or o
f 4%
in
equi
vale
nce
ratio
. To
stu
dy t
he e
ffec
ts o
f pr
ehea
ting,
the
re
acta
nts
can
be h
eate
d by
ele
ctric
al r
esis
tanc
e ta
pe w
ound
aro
und
the
burn
er t
ube.
The
dat
a pr
esen
ted
here
co
rres
pond
to
mix
ture
s w
ith H
2:CO
vol
umet
ric r
atio
s of
50:
50 a
nd 3
3:67
. A
dditi
onal
ly f
or t
he H
2:CO
=50
:50
mix
ture
, tw
o fu
rther
cas
es w
ere
stud
ied:
reac
tant
pre
heat
ing
to ~
500K
and
20%
dilu
tion
with
CO
2.
(a
)
(b
)
Figu
re 1
. Bur
ner s
chem
atic
s for
(a) l
amin
ar je
t fla
me
for s
ynga
s mea
sure
men
ts an
d (b
) swi
rl co
mbu
stor f
or m
etha
ne
studi
es.
L
12.5
o
4.4m
m
To S
pect
rom
eter
Fibe
r he
ad
Spec
tral
reso
lutio
n.C
CD
cam
era
Hin
keld
eyet
al.[
2007
]A
uto-
igni
tion
prob
lem
son
LPP
com
bust
ion
envi
ronm
ent.
Aut
o-ig
nitio
npo
ints
iden
tifica
tion
Pl
anar
LIF
and
Mie
scat
teri
ng
In t
he s
econ
d pa
rt of
the
inv
estig
atio
n, i
mag
es o
f pl
anar
LIF
of C
H2O
and
sim
ulta
neou
s M
ie s
catte
ring
of
drop
lets
in
the
cont
inuo
us f
low
wer
e ca
ptur
ed.
The
obje
ctiv
e of
th
is
mea
sure
men
ts
was
to
st
udy
the
inte
ract
ions
of
drop
lets
, ig
nitio
n an
d fla
me
in d
etai
l, w
ith r
espe
ct t
o th
e th
eore
tical
im
plic
atio
ns d
escr
ibed
ab
ove.
Tw
o m
easu
rem
ent
tech
niqu
es w
ere
com
bine
d as
fo
llow
s: T
he L
IF s
yste
m c
onsi
sts
of a
XeC
l-exc
imer
pu
mpe
d dy
e-la
ser
tune
d to
an
abso
rptio
n lin
e of
CH
2O
at 3
39,3
nm
and
a 1
6-bi
t IC
CD
cam
era
(Dyn
aMig
ht
(LaV
isio
n)).
Mie
sca
tterin
g fr
om t
he U
V-la
ser
was
bl
ocke
d by
an
UV
-filt
er. M
ie s
catte
ring
is e
xcite
d by
a
Nd:
YV
O4
lase
r at 5
32 n
m, f
ilter
ed b
y a
band
pas
s fil
ter
and
capt
ured
by
one
of t
he c
amer
as o
f th
e 4x
-IC
CD
sy
stem
(HSF
C-Pr
o; p
co).
Bot
h ca
mer
as w
ere
dire
cted
to
oppo
site
sid
es o
f th
e pl
enum
and
alig
ned
in o
rder
to
disp
lay
the
sam
e lig
ht-s
heet
sec
tion.
For
mal
dehy
de w
as
chos
en a
s a c
andi
date
for t
he L
IF te
chni
que
beca
use
it is
w
ell s
uite
d as
indi
cato
r for
the
auto
-igni
tion
/ coo
l-fla
me
prog
ress
[13
]. A
hig
h am
ount
of
CH
2O i
s ge
nera
ted
durin
g th
e fir
st r
eact
ions
lea
ding
to
igni
tion.
In
a tw
o st
age
igni
tion
proc
ess,
CH
2O*
chem
ilum
ines
cenc
e al
so
cont
ribut
es
to
the
cool
fla
me
lum
inos
ity
[4, 1
2].
Even
tual
ly, C
H2O
is c
onsu
med
in th
e fin
al s
tage
of
the
igni
tion.
Figu
re 5
: Sim
ulta
neou
s LIF
and
Mie
scat
terin
g w
ith
addi
tiona
l im
age
of e
mitt
ed c
hem
ilum
ines
cenc
e (3
6x36
mm
²; Je
t-A1,
pfu
el=2
.5 M
Pa)
Pres
ently
, th
is s
et u
p w
as u
sed
in a
sin
gle
shot
co
nfig
urat
ion.
How
ever
, it
is p
lann
ed t
o ex
tend
it
to
high
-spe
ed se
quen
ces o
f LIF
/Mie
pai
rs.
Figu
re 5
sho
ws
two
exam
ples
of
the
LIF/
Mie
twin
-sh
ots
(∆t=
1 µs
), w
ith
an
addi
tiona
l pi
ctur
e of
th
e em
itted
che
milu
min
esce
nce.
The
wid
th o
f the
lase
r lig
ht
shee
ts w
as o
nly
18 m
m. A
lthou
gh b
eing
not
cor
rect
ed
for
shee
t in
hom
ogen
eity
, th
e LI
F-si
gnal
of
C
H2O
di
spla
ys
pron
ounc
ed
area
s of
hi
gher
an
d lo
wer
flu
ores
cenc
e in
tens
ities
w
hich
ca
n be
at
tribu
ted
to
diff
eren
t le
vels
of
re
actio
n pr
ogre
ss
at
diff
eren
t lo
catio
ns. F
or in
terp
reta
tion,
how
ever
, one
has
to k
eep
in m
ind
that
inte
nsity
cor
rela
tes
with
rea
ctio
n pr
ogre
ss
as w
ell
as w
ith i
nher
ent
reac
tant
. Thi
s m
eans
, tha
t th
e LI
F-si
gnal
is
in
fluen
ced
by
conc
entra
tion
of
form
alde
hyde
whi
ch i
n tu
rn d
epen
ds o
n th
e re
actio
n pr
ogre
ss a
nd th
e lo
cal e
quiv
alen
ce ra
tio. A
dditi
onal
ly, i
t is
al
so
quen
ched
by
sp
ectro
scop
ical
in
fluen
ces.
Nev
erth
eles
s, zo
nes
of c
ompl
ete
CH
2O c
onsu
mpt
ion
due
to e
xpan
ding
fla
me
stru
ctur
es c
an b
e di
stin
guis
hed
by
shar
p gr
adie
nts
in
com
paris
on
to
the
smoo
th
trans
ition
s be
twee
n zo
nes
of m
ore
and
less
CH
2O-L
IF
inte
nsity
. Th
e im
ages
sho
uld
ther
efor
e se
rve
rath
er t
o lo
calis
e th
e bo
unda
ries
of t
he h
ot f
lam
e zo
nes
than
to
show
a q
uant
itativ
e di
strib
utio
n of
the
CH
2O.
The
mat
ch o
f fla
me
stru
ctur
es o
f the
LIF
sho
t to
the
emis
sion
of
ch
emilu
min
esce
nce
is
quite
ob
viou
s, es
peci
ally
in
the
right
pic
ture
ser
ies,
whe
re t
he f
lam
e st
ruct
ures
cov
er a
maj
or p
art o
f the
ligh
t she
et p
lane
and
ar
e al
so v
isib
le o
n in
tens
ified
LIF
im
age
desp
ite t
he
very
sho
rt ex
posu
re t
ime.
In
addi
tion,
in
this
cas
e of
la
rge
flam
e di
sper
sion,
no
mor
e re
sidu
al d
ropl
ets
can
be
dete
cted
fro
m t
he M
ie s
igna
l in
con
trast
to t
he l
eft
serie
s.
Fi
gure
6: S
imul
tane
ous L
IF a
nd M
ie sc
atte
ring
(14.
7x18
.2m
m²;
Jet-A
1; p
fuel=3
MPa
)
4
Spat
ial
reso
lutio
n.16
-bit
ICC
Dca
mer
a
Ahm
ed[2
006]
Las
erig
nitio
non
turb
ulen
tflam
es.
2.4
PL
IFM
easu
rem
ents
Fig
ure
2.15
:Sch
emat
icdia
gram
ofth
eP
LIF
mea
sure
men
tsin
the
blu
ff-b
ody
flam
eex
per
imen
ts.
33
Spat
ial
and
time
reso
lutio
n.
16-b
itIC
CD
cam
era
and
PMT
Was
selin
-Tru
pin
etal
.[20
00]
New
met
hod
for
the
mea
sure
men
tof
low
conc
entr
atio
n
OH
*w
ithpu
lse
Rad
ioly
sis
tech
nolo
gy.
Inth
epr
esen
ceof
O 2,re
actio
ns7-
9m
ust
beco
nsid
ered
.
The
hydr
oxyl
radi
cal(
OH
)re
acts
with
the
depr
oton
ated
form
oflu
min
ol,
LH-
(pK
a)
6.3)
byre
actio
ns2
and
3.T
hesu
pero
xide
radi
cal
(O 2-)
reac
tsw
ithL-
byre
actio
n5
togi
ve3-
amin
opht
hala
te(3
-AP
)in
itsfir
stsi
ngle
texc
ited
stat
e,18
whi
chre
turn
sto
itsfu
ndam
enta
lsta
tew
ithem
issi
onof
aph
oton
(λm
ax)
420
nm,τ
)6
ns19
).T
hech
emilu
min
esce
nce
quan
tum
yiel
dof
lum
inol
isde
pend
ento
npH
,and
itsm
axim
umva
lue
isne
arpH
12.20
,21
Thi
sch
emilu
min
esce
nce
mec
hani
smre
quire
sth
epa
rtic
ipat
ion
oftw
opr
imar
yra
dica
lsof
wat
erra
diol
ysis
:O
Han
dO
2-.
Aco
nseq
uenc
eis
that
,ift
heco
ncen
trat
ion
ofon
eof
thes
era
dica
lsis
low
erth
anth
eot
her,
this
spec
ies
limits
the
form
atio
nof
3-A
P*
and
then
ofth
ein
tens
ityof
chem
ilum
ines
cenc
eby
eith
erre
actio
n2
orre
actio
n5.
Thu
s,th
em
easu
rem
ent
ofch
emilu
min
esce
nce
give
sth
eco
ncen
trat
ion
ofth
era
dica
lth
atha
sth
elo
wes
tco
ncen
trat
ion.
Bel
owan
LET
valu
eof
100
eV/n
m,t
hehy
drox
ylra
dica
lyi
eld
ishi
gher
than
the
supe
roxi
dera
dica
lyi
eld.
Ove
r10
0eV
/nm
,the
hydr
oxyl
radi
caly
ield
islo
wer
than
supe
roxi
dera
dica
lyi
eld.1
The
n,th
ism
etho
dal
low
sfo
rth
em
easu
rem
ent
ofth
esu
pero
xide
yiel
dat
low
LET
and
ofth
ehy
drox
ylyi
eld
athi
ghLE
T.
The
chem
ilum
ines
cenc
ene
eds
tobe
calib
rate
dby
mea
surin
gth
esi
gnal
prod
uced
bya
know
nco
ncen
trat
ion
ofra
dica
l.T
hen,
itis
nece
ssar
yto
know
the
quan
tity
oflig
htm
easu
red
from
agi
ven
conc
entr
atio
nof
supe
roxi
dera
dica
lin
the
solu
tion.
Inae
rate
dso
lutio
ns,
atlo
wLE
T(a
sin
the
case
ofan
elec
tron
beam
),th
esu
pero
xide
radi
cal
ispr
oduc
edby
the
reac
tion
ofO
2w
ithe-
aq(r
eact
ion
7)an
dH
(rea
ctio
n8)
.U
nder
thes
eco
nditi
ons,
the
radi
caly
ield
sar
ekn
own,
and
the
conc
entr
atio
nof
the
supe
roxi
dera
dica
lcan
beca
lcul
ated
.N
ote
that
the
mec
hani
smpr
esen
ted
inth
isar
ticle
isth
em
ain
mec
hani
smfo
rth
elu
min
olm
olec
ule.
Act
ually
,ano
ther
path
way
lead
ing
toth
eex
cite
dst
ate
mol
ecul
e(3
-AP
*)ex
ists
,bu
ton
lyin
the
pres
ence
ofO 2
and
exce
ssL-
(thu
s,ex
cess
ofO
H).
Inae
rate
dso
lutio
nsan
dun
der
cond
ition
sle
adin
gto
high
OH
yiel
ds,r
eact
ion
3ca
nin
dire
ctly
prod
uce
lum
ines
cenc
e.T
oav
oid
addi
tion
ofO
Hra
dica
lon
toth
eCd
Cbo
nd(r
eact
ion
3),
itis
po
ssib
leto
use
carb
on
ate
ion
sC
O32-
,w
itha
suff
icie
nt
conc
entr
atio
n,to
scav
enge
OH
radi
cals
(rea
ctio
n10
);th
isfo
rms
CO
3-ra
dica
ls,
whi
chre
act
with
LH-
togi
veL.-
(rea
ctio
n11
).23,2
4
Inth
eca
seof
high
-LE
Tirr
adia
tion
and,
then
,und
erco
nditi
ons
lead
ing
tolo
wO
Hyi
elds
,the
lum
ines
cenc
ein
itiat
edby
reac
tion
3ca
nbe
negl
ecte
d,an
dca
rbon
ate
solu
tions
beco
me
usel
ess.
Whe
nth
eso
lutio
nco
ntai
ning
lum
inol
and
CO
32-is
satu
rate
dby
N2O
/O2
mix
ture
s,m
ost
ofth
ee-aq
are
scav
enge
dby
N 2O(r
eact
ion
12)
togi
vead
ditio
nalO
Hra
dica
ls.I
nth
isca
seG(O
H)
>G
(O2-
),an
dth
ein
tens
ityof
chem
ilum
ines
cenc
eis
prop
or-
tiona
lto
the
conc
entr
atio
nof
O 2-pr
oduc
edby
the
reac
tion
of
O2
with
Han
dth
esm
allf
ract
ion
ofe-aq
that
isno
tsc
aven
ged
byN
2O(r
eact
ion
7).T
hus,
aca
libra
tion
can
bedo
ne,p
rovi
ding
that
the
cell
isirr
adia
ted
unifo
rmly
.
To
sum
mar
ize
this
prin
cipl
e,on
eca
nm
easu
reth
elo
wra
dica
lyi
eld
sa
sho
rttim
ea
fte
ra
pu
lse
dir
rad
iatio
nw
ithh
igh
-LE
Tpa
rtic
les
bym
easu
ring
chem
ilum
ines
cenc
e.
Exp
erim
enta
lMet
hod
Ion
Bea
m.
Pu
lse
dir
rad
iatio
ns
we
rep
erf
orm
ed
with
an
40A
r18+
ion
beam
ofen
ergy
95M
eV/n
ucle
on(w
hich
give
s3.
8G
eV/io
n)at
the
Gra
ndA
cce
´ler
ateu
rN
atio
nal
d’Io
nsLo
urds
(GA
NIL
)cy
clot
ron.
The
setu
pis
depi
cted
inF
igur
e1.
The
irrad
iatio
nce
llis
acy
lindr
ical
flow
cell,
mad
eof
poly
prop
ylen
eto
avoi
da
lum
ines
cenc
eba
ckgr
ound
from
quar
tz(m
ater
ial
com
mon
lyus
edin
such
expe
rimen
ts).
The
thic
knes
sof
the
poly
prop
ylen
eis
0.5
mm
,allo
win
gan
alm
ost-
cons
tant
LET
(280
(31
eV/n
m)
into
the
tota
lth
ickn
ess
ofth
esa
mpl
e(3
mm
).T
heLE
Tis
calc
ulat
edfr
omth
eT
RIM
com
pila
tion.25
The
sect
ion
geom
etry
ofth
eio
nbe
amis
defin
edby
aho
rizon
tals
litin
fron
toft
hece
ll.T
hebe
amin
tens
ityis
mea
sure
dw
itha
seco
ndar
yel
ectr
onde
tect
orlo
cate
din
the
beam
.It
cons
ists
ofa
thin
titan
ium
foil
plac
edbe
twee
ntw
oth
inal
umin
umfo
ils.A
pote
ntia
lof4
8V
appl
ied
toth
ese
alum
inum
foils
crea
tes
anel
ectr
icfie
ldth
atge
nera
tes
anea
sily
mea
sure
dcu
rren
t.T
his
inte
nsity
isca
libra
ted
befo
reth
eac
tual
irrad
iatio
nsw
itha
Far
aday
cup.
The
sam
ede
vice
has
been
used
inpr
evio
usex
perim
ents
.9T
hedo
sede
liver
edto
the
sam
ple
isth
enca
lcul
ated
byth
een
ergy
loss
ofth
eio
nin
the
wat
eran
dth
ein
tens
ityof
the
beam
.T
heio
nbe
amha
sin
trin
sica
llyon
epu
lse
of1
nsdu
ratio
nev
ery
100
ns.
How
ever
,a
puls
ege
nera
tor
isus
edto
mod
ulat
eth
ishi
ghfr
eque
ncy,
and
itge
nera
tes
abu
rsto
fpul
ses
havi
nga
dura
tion
ofbe
twee
n5µ
san
d2
ms
ata
freq
uenc
yof
20H
z.
e aq-
+O
2f
O2-
k 6)
1.9
×10
10M
-1
s-1
(7)
H+
O2
fH
O2
k 7)
1.2
×10
10M
-1
s-1
(8)
HO
2S
O2-
+H
+pK
a)
4.822
(9)
CO
32-+
OH
fC
O3-
+O
H- k 9
)3.
9×
108
M-
1s-
118
(10)
CO
3-+
LH-
fL
-+
HC
O3- k 1
0)
7.7
×10
8M
-1
s-1
(11)
Fig
ure
1.S
etup
sche
me
ofth
etim
e-re
solv
edch
emilu
min
esce
nce
expe
rimen
tw
ithio
nbe
am.
e aq-
+N
2O9
8H
2OO
H+
N2
+O
H-
k 12)
8.7
×10
9M
-1
s-1
(12)
8710
J.P
hys
.C
he
m.
A,
Vo
l.1
04
,N
o.
38
,2
00
0W
asse
lin-T
rupi
net
al.
Tim
ere
solu
tion.
PMT
to
acqu
ire
OH
*ch
emilu
min
es-
cenc
eem
issi
on.
Tabl
e1.
3:C
hem
ilum
ines
cenc
eap
plic
atio
nex
ampl
es.
20 CHAPTER 1. INTRODUCTION
1.2 Thesis Objective, Contribution and Outline
Thesis Objective and Contribution
The objective of this thesis is to design, develop, optimize and test a chemilumines-
cence sensing prototype to measure OH* and CH* light emission, under different ER
conditions, motivated by the current search for fast and reliable controllers of lean com-
bustion turbofan engines. By using the developed prototype it aims to study the relation
OH∗/CH∗ = f (φ, optical collecting system, flame surface deformation in time). It also
intents to use the obtained results to gather more information about the influence of the
optical collecting systems in the measured values, as well as analyze the influence of
flame deformation in the local chemiluminescence emission. Apart from this fundamen-
tal flame chemiluminescence analysis, the main practical achievement is presented at the
end of the case study, where the time evolution of the OH∗/CH∗ relation is used as
an input parameter in a simulated lean combustion control mechanism, and shows that
control models based in mean chemiluminescence measurements may overestimate or
underestimate flame equivalence ratio during its actuating period.
Thesis Outline
The remainder of this thesis is presented in three chapters. Chapter 2 presents the design,
construction and test optimization of the chemiluminescence sensing’s prototype. It also
includes the data acquisition process along with identification of the prototype sensing
limits. Chapter 3 presents a study case with analysis of an unsteady flame, on a time
basis. Chapter 4 summarizes the main conclusions/results of present work.
Chapter 2
Experimental Methods
Tis chapter’s objective is to present the design, construction and optimization pro-
cess of a chemiluminescence sensor prototype, schematically represented on fi-
gures 2.1 and 2.2. It also presents the data acquisition process and a section with testing
trials for prototype characterization.
Bunsen burner
optical probe
HV
Negative High Voltage Supply -1000 V
± 15V
A/D board
optical fiber
UV lens BPF Pre-amplifier Supply
Photomultiplier
optical probe and optical fiber lens + filtering system
Photomultiplier and pre-amplifier Optical path Signal Converter
Figure 2.1: Schematic overview of the prototype measuring system.
21
22 CHAPTER 2. EXPERIMENTAL METHODS
To Photomultiplier
To Photomultiplier
Bunsen burner
optical probe
optical fiber U
V Lens
Pre-amplifier
A/D
board
Photomultiplier
BPF
Figure2.2:Picture
ofprototypecom
ponents.
2.1. CHEMILUMINESCENCE SENSOR DESIGN 23
2.1 Chemiluminescence Sensor Design
The prototype design presented in this work was based on 4 important requirements: (1)
it has to collect the radiation from a certain specie emitting at the flame front, (2) convert
photons emitted from chemiluminescence into an analogue signal, (3) convert signal to
digital information and (4) allow time analysis.
Optical Probe and Optical Fiber
A prototype light collecting system, composed by an optical probe and an optical fiber,
defines local and spatial resolution of the collected signal. In figure 2.3 one can identify
the location of the light collecting system on the prototype as well as its two possible
probe configurations.
HV ± 15V
A/D board
optical probe and optical fiber lens and filtering system
Photomultiplier and pre-amplifier Optical path Signal Converter
(a) Prototype overall scheme.
26
q
Optical Fiber
Solid angle
Diaphragm CollimatorTo optical
fiber
Fig. 20 – a) Schematic of the fiber optic probe. b) Schematic diagram of cylindrical light
probe.
The entire tests were performed on the metallic plate burner with the steady state flame. The
total light intensity of the flame in the steady state 0I , was measured for several values of
steady heat release 0Q at a fixed to determine the optical configuration with maximum SNR
and with the highest and more linear dI dQ relation. Note that all the tests were performed
with the laboratory lights turned off and windows closed, however, was impossible to totally
isolate the experiment. The optical configurations tested in this work are summarized in Table
1.
a) b)
(b) Optical fiber probe.
26
q
Optical Fiber
Solid angle
Diaphragm CollimatorTo optical
fiber
Fig. 20 – a) Schematic of the fiber optic probe. b) Schematic diagram of cylindrical light
probe.
The entire tests were performed on the metallic plate burner with the steady state flame. The
total light intensity of the flame in the steady state 0I , was measured for several values of
steady heat release 0Q at a fixed to determine the optical configuration with maximum SNR
and with the highest and more linear dI dQ relation. Note that all the tests were performed
with the laboratory lights turned off and windows closed, however, was impossible to totally
isolate the experiment. The optical configurations tested in this work are summarized in Table
1.
a) b)
(c) Collimator probe.
Figure 2.3: Prototype overall scheme. Light collecting system configurations.
24 CHAPTER 2. EXPERIMENTAL METHODS
The probe presented in figure 2.3(b) simply consists of an optical fiber pointing to the
flame, usually characterized by its solid angle θ, that is related to the Numerical Aperture
(NA) by equation 2.1, where n is the relative index of refraction of the environment,
usually 1 for the air (see Hecht [2002]).
NA = n× sin θ2 (2.1)
The selected optical fiber is from Ocean Optics and presents a core diameter of 200 ±
4µm and a solid angle of 25◦. It is made of polymide, with an efficient transmission
wavelength ranging from 180 to 900 nm, as shown in figure 2.4. If the prototype is
sensing light emitted by the OH* specie at 308 nm, only 74% of it is transmitted through
the fiber. If the prototype is sensing light emitted by the CH* at 432 nm, the transmission
percentage is higher, 88%. This transmission percentage is defined as the ratio between
the light exiting and entering the fiber, also known as transfer function, TF.
200 300 400 500 600 700 800 900 10000
10
20
30
40
50
60
70
80
90
100
lambda [nm]
% o
f Tra
nsm
issi
on
Figure 2.4: Optical Fiber Transfer Function.
The second probe, figure 2.3(c), was designed to collect light integrated over a cylindrical
volume, as shown in figure 2.5. It includes built-in plano-convex lens with a diameter
of 20 mm mounted in a moving screw, so that its focal length (FL) of 50 mm can be
2.1. CHEMILUMINESCENCE SENSOR DESIGN 25
coincident with the distance that separates it from the optical fiber input. An adjustable
iris diaphragm was attached in front of the unit to limit the visible area, and control the
amount of light entering the fiber and the spatial resolution of the measuring system.
adjustable diaphragm plano-convex lens
adjustable distance - moving screw
optical fiber
Figure 2.5: Collimator probe mounting scheme.
Information about the lens TF is essential for the prototype optimizing process and for
post-processing. To obtain this information for all the optical components of the proto-
type that did not have this data provided by the manufacturer, a spectrophotometer from
Thermo Spectronic, model Helios α, was used. The covered spectrum was 200 nm to
1050 nm, with a scan speed of 240 nm per minute and a data interval of 0.2 nm. The
transmission graphic presented on figure 2.6 is from the collimator probe lens and shows
that for 308 nm the transmission is 72%, while for 432 nm the transmission is 90%.
200 300 400 500 600 700 800 900 10000
10
20
30
40
50
60
70
80
90
100
lambda [nm]
% o
f Tra
nsm
issi
on
Figure 2.6: Optical probe lens Transfer Function
26 CHAPTER 2. EXPERIMENTAL METHODS
The assembling process is done by screwing the probe to the optical fiber. Note that
this probe was first designed to collect light into a multimode optical fiber, with a core
diameter of 0.5 mm and a NA of 0.15, figure 2.7(a). When the optical fiber, previously
presented on figure 2.3(b), is placed on the same position of the multimode fiber, not all
the light coming from the collimator lens enters the fiber, due to the smaller core size and
different solid cone angle, see figure 2.7(b).
θmultimode
Optical multimode fiberNA=0.15Core diameter = 0.5 mm
(a) Assembling NA=0.15
Optical fiberNA=0.22Core diameter = 0.2 mm
θmultimode θfiber
(b) Assembling NA=0.22
Figure 2.7: Light interface between optical probe plano convex lens and selected optical fiber
Lens and filtering system
The lens and filtering system are responsible for light guidance and selection towards the
photomultiplier, as it exits the optical fiber with a specified NA, see figure 2.8.
HV ± 15V
A/D board
optical probe and optical fiber lens + filtering system
Photomultiplier and pre-amplifier Optical path Signal Converter
Figure 2.8: Optical probe lens Transfer Function
2.1. CHEMILUMINESCENCE SENSOR DESIGN 27
The use of plano-convex lens to collimate light is always associated with chromatic and
focal aberration, because it is impossible to have lens with a focal point where all the
wavelengths merge. On a real lens, the light beams merge into a focal area (FA) instead of
a focal point, as sketched in figure 2.9. Therefore, to guarantee that all light is collected,
it is necessary to choose a lens that has a FA diameter equal or smaller than the optical
fiber core area diameter.
Focal Length optical fiber
(a) Ideal lens.
Focal Length
Focal Area
optical fiber
(b) Real lens.
Figure 2.9: The difference between the beam geometry on an ideal an real lens.
Lens FA diameter is given by equation 2.2, a relation between focal length (FL or f ) and
focal number, f#.
FAdiameter = 0.067× ff 3
#(2.2)
Focal number is known as a function of the lens NA, equation 2.3, and is usually defined
by the ratio between f and clear aperture, CA, equation 2.4.
NA = 12× f#
(2.3)
f# = f
CA(2.4)
By using equations 2.2 and 2.3, the 0.2 mm core diameter and 0.22 NA of the previously
presented optical fiber imposed a lens with a FL of 35 mm. Since the CA is around 90%
of the lens diameter, equation 2.4 returns a lens diameter of at least 18 mm. Based on this
28 CHAPTER 2. EXPERIMENTAL METHODS
calculus, the choice was a ultra violet (UV) coated lens from Thorlabs, with a diameter
of 25 mm and FL of 35 mm.
Another trial was performed using the spectrophotometer to determine the UV lens TF.
Results presented in figure 2.10 show that for 308 nm, the percentage of transmission is
95%, while for 432 nm the percentage of transmission is 92%.
200 300 400 500 600 700 800 900 10000
10
20
30
40
50
60
70
80
90
100
lambda [nm]
% o
f Tra
nsm
issi
on
Figure 2.10: Thorlabs UV lens TF. FL: 35 mm.
The beam exiting the plano-convex lens has the intensity from all the wavelength emitted
by the flame. To sense only the light emitted by a certain specie it is necessary to force
it through a specific Band Pass Filter (BPF). Ideally only the photons emitted on the
wavelength of the BPF should pass it, but a real BPF has a TF with a Gaussian distribution
over all the wavelength instead of the desired Dirac distribution. Consequently, signal
quality depends on width and on transmission peak of each BPF. The more narrow and
higher the band of transmission is, the more accurate the prototype becomes.
Figure 2.11 shows the TF obtained with the spectrophotometer for each of the Melles
Griot used BPF. As one can see, the OH* filter has a transmission maximum of 14%
at 310 nm, while the CH* filter has a maximum of 45% at 429 nm. It means that the
wavelenght at which maximum percentage of transmission occurs is not coincident with
the wavelenght of emitting species. In fact at 308 nm, the OH* filter has a transmission
2.1. CHEMILUMINESCENCE SENSOR DESIGN 29
percentage of 12%, while at 432 nm, the CH* filter has a transmission percentage of
41%.
200 300 400 500 600 700 800 900 10000
5
10
15
20
25
30
35
40
45
50
lambda [nm]
% o
f Tra
nsm
issi
on
(a) OH* filter.
200 300 400 500 600 700 800 900 10000
5
10
15
20
25
30
35
40
45
50
lambda [nm]
% o
f Tra
nsm
issi
on
(b) CH* filter.
Figure 2.11: Transfer Function of OH* and CH* filters.
After the beam pass through the respective BPF it is necessary to use a collimating plano-
convex lens to merge it into the PMT photocathode. The choice was another Thorlabs
UV coated lens, with a diameter of 25 mm and a FL of 50 mm. Its TF is presented in
figure 2.12, showing a transmission of 97% for 308 nm wavelength and 92% for 432 nm
wavelengths.
200 300 400 500 600 700 800 900 10000
10
20
30
40
50
60
70
80
90
100
lambda [nm]
% o
f Tra
nsm
issi
on
Figure 2.12: Thorlabs UV lens TF. FL: 50 mm.
30 CHAPTER 2. EXPERIMENTAL METHODS
Overall Optical System Transfer Function
Knowing the TF of each optical component aloows estimation of the entire optical sys-
tem’s TF, by multiplying each TF according to the light path. Figure 2.13 evidences the
components of the optical path and presents the graphic of the overall system TF, where
one can see that only 7% of the light emitted by OH* achieves the PMT, while 28% of
the light emitted by the CH* species achieves the PMT.
HV ± 15V
A/D board
optical probe and optical fiber lens + filtering system
Photomultiplier and pre-amplifier Optical path Signal Converter
(a) Optical path.
200 300 400 500 600 700 800 900 10000
5
10
15
20
25
30
35
lambda [nm]
% o
f Tra
nsm
issi
on
CH PathOH Path
(b) Overall system TF.
Figure 2.13: Optical path scheme detail. Optical path TF.
2.1. CHEMILUMINESCENCE SENSOR DESIGN 31
Note that if the chemiluminescence measured values are divided by the respective TF,
the obtained result will be an approximation of the value that the PMT would sense if
it was measuring light directly from OH* and CH* species. All results presented in the
following sections are already divided by the respective TF, unless stated otherwise.
Photomultiplier and Pre-amplifier
The use of high sampling sensors accounts for a good time resolution. According to
Nyquist theorem, sampling frequency of a sensor has to be at least two times the fre-
quency of the process that it is being analyzed (see the work of Bendat and Piersol
[1971]). PMT’s are characterized by high speed response, due to the fact that its wor-
king principles are based in interaction between photons and electrons enhanced by high
potential differences, figure 2.14(a). PMT time response is also known as transient time,
since it is the interval between the incidence of a photon on the photocathode and the
exiting of the generated electrons.
PHOTOMULTIPLIER TUBE
R3896
Figure 2: Typical Spectral Response
Figure 1: Electro Optical Structure
TPMSB0049EB
TPMSC0024EA
100
10
1
0.1
0.01100 200 300 400 500 600 700 800 900 1000
WAVELENGTH (nm)
CA
TH
OD
E R
AD
IAN
T S
EN
SIT
IVIT
Y (
mA
/W)
QU
AN
TU
M E
FF
ICIE
NC
Y (
%)
R928
R3896
QUANTUM EFFICIENCY
CATHODE RADIANTSENSITIVITY
LIGHT
PHOTOELECTRONTRAJECTORIES
GRIDGLASS BULB
ANODE
9th DYNODEPHOTOCATHODE
2nd DYNODE
1st DYNODE
Information furnished by HAMAMATSU is believed to be reliable. However, no responsibility is assumed for possible inaccuracies or omissions. Specifications aresubject to change without notice. No patent rights are granted to any of the circuits described herein. ©2006 Hamamatsu Photonics K.K.
Subject to local technical requirements and regulations, availability of products included in this promotional material may vary. Please consult with our sales office.
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PHOTOMULTIPLIER TUBE
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Figure 2: Typical Spectral Response
Figure 1: Electro Optical Structure
TPMSB0049EB
TPMSC0024EA
100
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0.1
0.01100 200 300 400 500 600 700 800 900 1000
WAVELENGTH (nm)
CA
TH
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2nd DYNODE
1st DYNODE
Information furnished by HAMAMATSU is believed to be reliable. However, no responsibility is assumed for possible inaccuracies or omissions. Specifications aresubject to change without notice. No patent rights are granted to any of the circuits described herein. ©2006 Hamamatsu Photonics K.K.
Subject to local technical requirements and regulations, availability of products included in this promotional material may vary. Please consult with our sales office.
FEATURES�High Sensitivity
Luminous ................................................Radiant
at 450nm (peak wavelength) .............at 633nm .............................................
Quantum Efficiencyat 260nm (peak wavelength) ....................at 633nm ....................................................
�Wide Spectral Response .........................�High Signal to Noise Ratio�Newly Designed Electro Optical Structure
APPLICATIONS�Biomedical Analysis
Blood Analyzer, Flow Cytometer, DNA Sequencer�Environmental Monitoring
NOx Analyzer�Spectroscopy
Fluorescence Spectrometer, Raman Spectrometer,UV–VIS Spectrometer
�Semiconductor IndustryWafer Inspection, Particle Counter
525 µA/lm (Typ.)
90 mA/W (Typ.)73 mA/W (Typ.)
30 % (Typ.)14 % (Typ.)
185 nm to 900 nm
High QE Multialkali PhotocathodeNew Electro–Optical Design
28 mm (1-1/8 Inch) Diameter, 9-Stage, Side-On Type
(b) R3896 model.
Figure 2.14: Scheme of PMT side-on working principle and picture of R3896 model.
The choice for this prototype was a PMT of type R3896 from Hamamatsu [2006], de-
picted in figure 2.14(b). According to the manufacturer when the applied potential dif-
ference is -1000 V, the transient time is 23 × 10−8 s, which means a frequency response
of 43.5MHz, for wavelengths between 185 nm and 900 nm. Aside from high frequency,
32 CHAPTER 2. EXPERIMENTAL METHODS
efficiency of a PMT is not constant in all its spectral range, since one of the working prin-
ciples is related to emission of electrons as a consequence of incidence of photons with
a specific energy. This efficiency is defined as the ratio between the number of electrons
released, also known as photoelectrons, and the intensity of incident light at a specified
wavelength. For the desired wavelengths, 308 nm and 432 nm, efficiency is respectively
29% and 27%, as one can see in figure 2.15.
PHOTOMULTIPLIER TUBE
R3896
Figure 2: Typical Spectral Response
Figure 1: Electro Optical Structure
TPMSB0049EB
TPMSC0024EA
100
10
1
0.1
0.01100 200 300 400 500 600 700 800 900 1000
WAVELENGTH (nm)
CA
TH
OD
E R
AD
IAN
T S
EN
SIT
IVIT
Y (
mA
/W)
QU
AN
TU
M E
FF
ICIE
NC
Y (
%)
R928
R3896
QUANTUM EFFICIENCY
CATHODE RADIANTSENSITIVITY
LIGHT
PHOTOELECTRONTRAJECTORIES
GRIDGLASS BULB
ANODE
9th DYNODEPHOTOCATHODE
2nd DYNODE
1st DYNODE
Information furnished by HAMAMATSU is believed to be reliable. However, no responsibility is assumed for possible inaccuracies or omissions. Specifications aresubject to change without notice. No patent rights are granted to any of the circuits described herein. ©2006 Hamamatsu Photonics K.K.
Subject to local technical requirements and regulations, availability of products included in this promotional material may vary. Please consult with our sales office.
FEATURES�High Sensitivity
Luminous ................................................Radiant
at 450nm (peak wavelength) .............at 633nm .............................................
Quantum Efficiencyat 260nm (peak wavelength) ....................at 633nm ....................................................
�Wide Spectral Response .........................�High Signal to Noise Ratio�Newly Designed Electro Optical Structure
APPLICATIONS�Biomedical Analysis
Blood Analyzer, Flow Cytometer, DNA Sequencer�Environmental Monitoring
NOx Analyzer�Spectroscopy
Fluorescence Spectrometer, Raman Spectrometer,UV–VIS Spectrometer
�Semiconductor IndustryWafer Inspection, Particle Counter
525 µA/lm (Typ.)
90 mA/W (Typ.)73 mA/W (Typ.)
30 % (Typ.)14 % (Typ.)
185 nm to 900 nm
High QE Multialkali PhotocathodeNew Electro–Optical Design
28 mm (1-1/8 Inch) Diameter, 9-Stage, Side-On Type
Figure 2.15: Picture of PMT quantum efficiency graphic provided by Hamamatsu [2006].
This performance indication is also known as quantum efficiency, equation 2.5.
Quantumefficiency@λ = η% = number of photoelectrons
incident radiant flux (W ) at specified λ (2.5)
Knowing efficiency (also known as the PMT TF) and generated output current it is pos-
sible to estimate incident light intensity on the sensor at a specified wavelength. Due to
the fact that the PMT output signal is a low current analogue signal (see table 2.1) and
that acquisition systems usually sense potential differences, the manufacturer provides a
socket which includes a current to voltage converter and a pre-amplifier with a manually
2.1. CHEMILUMINESCENCE SENSOR DESIGN 33
tuned gain.
Photomultiplier Amplifying socket
Model R3896 C7246-01
Voltage Supply Minimum limit: -1250V Amplifier supply: ± 15 V
Output Values Maximum current: 0.1 mA Conversion factor: 0.3 VµA
Gain 9.5× 106 107 @ 25 turns
Table 2.1: Characteristics of photomultiplier and pre-amplifier socket. Source Hamamatsu [1999,
2006].
This device allows a simple PMT integration and signal analysis with a regular data
acquisitions board. Nevertheless, this device has an important drawback, it works as a
cut-off filter for frequencies higher than 10 KHz, see figure 2.16.
DA-TYPE SOCKET ASSEMBLIES C7246 SERIES
TACCB0045EB
Figure 3. Frequency Response of Built-in Amplifier
TACCB0046EB
Figure 2. DC Linearity of PMT Output
TACCB0048EB
In the case of C7246 with R374
In the case of C7246-01 with R928
* ”PMT OUTPUT CURRENT” IS EQUIVALENT TO ANODE CURRENT AT THE INPUT OF INCLUDED AMPLIFIER.
1 100010-50
20
30
-10
0
10
-20
-40
-30
40
50
100
DE
VIA
TIO
N (
%)
PMT OUTPUT CURRENT (µA)*
+2%–2%
HV = -900V (VR = MIN.)
HV = -900V (VR = MAX.)
HV = -1200V (VR = MIN.)
HV = -1200V (VR = MAX.)
1 100010-50
20
30
-10
0
10
-20
-40
-30
40
50
100
DE
VIA
TIO
N (
%)
PMT OUTPUT CURRENT (µA)*
+2%–2%
HV = -700V (VR = MIN.)
HV = -700V (VR = MAX.)
HV = -1000V (VR = MIN.)
HV = -1000V (VR = MAX.)
0.1 10001-20
5
-3dB
-5
0
-10
-15
FREQUENCY (kHz)
RE
LAT
IVE
GA
IN (
dB)
10
10 100
Figure 2.16: Frequency response of built-in socket pre-amplifier. Source Hamamatsu [1999].
This restriction does not influence the study case presented on this work because sam-
pling frequency chosen for signal acquisition is lower than 10 kHz. The gain influence
was tested by recording signal variation with change of gain, for the same light input
condition. The most stable and constant values were obtained with maximum gain, set to
all measurements of the present work.
34 CHAPTER 2. EXPERIMENTAL METHODS
2.2 Data Acquisition
The signal acquisition was done with the aid of an USB data acquisition module, DT9841
- SB, controlled automatically by the software program developed in the Measure Foundry
Professional 5.1 (MFP), see DataTranslation [2010].
Ideally the study of the chemiluminescence emitted by two different species should be
performed simultaneously, by using two PMT’s and a Dichroic mirror that could split
the beam towards each of the sensors. The prototype designed in this work only has one
PMT, and for each experimental trial the BPF needs to be manually changed to acquire
data from both species. When the prototype is used for analyzing steady flames, time
information is not included, since it is assumed that the chemiluminescence remains ap-
proximately constant during the trial. However, when the prototype is used for analyzing
a pulsed flame, information about the instance of time when data was saved is needed,
so information from both species can be synchronized. This is achieved by recording the
periodic signal that is generated to pulse the flame. To better understand this relations,
figure 2.17 presents a simple interaction scheme.
photomultiplier loudspeaker
acquisition generation
Figure 2.17: Acquisition board interaction scheme.
2.2. DATA ACQUISITION 35
Regarding these objectives, the developed program was designed to acquire data from
the PMT sensor, record the generated sinusoidal signal for pulsed measurements syn-
chronization, average the recorded values by a quantity specified by the user, to reduce
the noise and increase the stability of the recorded signal. Then it performs an average
and a root mean square (RMS) before saving all the results to a binary file for post pro-
cessing. The sampling rate, controlled by the real time stream (RTS) component, was set
to 8192 Hz due to the 10 kHz pre-amplifier cutoff frequency. All the data was acquired
to a 4096 points stabilized buffer size and then appended to a vector. This process is
repeated 8 times to obtain a data vector with 32768 points, which guarantees a spectral
resolution of 0.25 Hz. Figure 2.18 presents the diagram of the implemented algorithm of
this process.
time
V
RTS - 8192 Hz
4096 points buffer
……….
Append to vector
Buffer #1
32768 points vector
Perform mathematical operations
Achieved the number of averages?
No
Yes
Buffer #2 Buffer #8
Display and Write to file
Write to file
Figure 2.18: Diagram of the acquisition algorithm.
The overview of the final program is shown on figure 2.19. The error introduced by the
signal digitalization is 1.2 µV in absolute amplitude.
36 CHAPTER 2. EXPERIMENTAL METHODS
Channel 0 Frequency signal analysis
Channel 0 Signal processing – average and RM
S Channel 0 Tim
e signal analysis
Channel 2 Frequency signal analysis
Channel 2 Signal processing – average and RM
S Channel 2 Tim
e signal analysis
Generation
Figure2.19:D
evelopedsoftw
areforsignalacquisition.
2.3. PROTOTYPE TESTING 37
2.3 Prototype Testing
2.3.1 Burning System
The burning system used for prototype testing and for the case study is represented in
figure 2.20. It consists of a convergent cylindrical nozzle (Bunsen burner type) and two
computer controlled electronic flowmeters.
Flowmeter 2: C3H8
Flowmeter 1: Air
porousdiscs
Dexit
C3H8 + Air
plenum
metallicmesh
Figure 2.20: Diagram of the experimental set-up showing the Bunsen burner. Assembling scheme.
The burner was designed and built by Leandro [2006], with the objective to stabilize
laminar premixed flames of propane and air with a range of power between 0.9 and 3 kW.
The exit has a diameter of Dexit = 18 mm, identified in figure 2.20, with a contraction
area rate of 6.53. To guarantee a uniform cap type flow exiting the burner, the mixture
is forced to pass through 3 equally spaced glass spheres porous discs with a width of 5
mm, positioned inside a plenum that has an approximate volume of 285 cm3. A metallic
mesh was placed between the plenum and the convergent area to prevent flame flashback.
The air and propane flow were measured in standard NTP conditions (p = 105 Pa and T
= 298.15K) using two Alicat Scientific flow meters, with a capacity of V = 50 and V = 5
standard liters per minute (SLPM), respectively. The measuring error was ±0.008 V +
0.002 Vmax.
38 CHAPTER 2. EXPERIMENTAL METHODS
2.3.2 Measurement Accuracy
Spatial localization
The optimization process of a measuring device which has optical components requires
a sequence of tests to define the best configuration possible. As shown by Leitão [2009],
the position of the light collecting system has a strong influence on the strength and
quality of the flame collected signal. Therefore, to test this influence the optical probe
was mounted on a position mechanism that allowed continuous movements in the x, y
and z directions, with a spatial accuracy of 0.5 mm (see figure 2.21).
Collimator Probe
Bunsen burner
black target
3D positioning mechanism
Figure 2.21: Picture with the details of the experimental setup.
The accuracy tests were conducted with the collimator probe, and with reference axis
origin set coincident to the burner exit center. Note that all values regarding the probe
position presented on this work are related to this reference. To minimize the influence
of environment light, a black target was placed opposite to the probe at a distance of L
= 210 mm from the burner vertical plane of symmetry. All these details are sketched in
figures 2.21 and 2.22.
2.3. PROTOTYPE TESTING 39
To PMT
z
x
L = 210 mm
Bunsen burner
black target
collimator probe
Figure 2.22: Scheme with the details of the experimental setup and optical probe placement.
The next step in fine tuning the optical path is the collimator lens position adjustment,
to obtain a light collecting cylinder. This was achieved by observing the shape of a light
beam generated by a laser, introduced through the optical fiber towards the collimator
probe. Figure 2.23 shows the light collecting cylinder and its intersection with the flame,
for a specified flame mixture setup and iris diameter, diris.
Bunsen burner
black target
laser
LASER LASER
Collimator Probe
Bunsen burner
Figure 2.23: Light collecting cylinder intersecting the steady Bunsen flame, with the aid of a laser.
During these tests it was noticed that due to lens aberration and to the assembly of a
different optical fiber to the collimator probe (figure 2.7), beam geometry was a cone
40 CHAPTER 2. EXPERIMENTAL METHODS
with a solid angle of θaberr = 0.6◦, with a viewing diameter of dviewing, instead of the
ideal cylinder with iris diameter, as sketched in figure 2.24. Consequently, the higher the
distance of the probe to the flame, the higher the viewing area.
θaberr
aberration
collimator lens
optical fiber
diris – ideal viewing area dviewing – real viewing area
Figure 2.24: Representation of the collimated light beam solid cone.
It was also seen that the true flame viewing area was the intersection surface between
probe viewing cone and flame surface, and for this reason, it is affected by the vertical
position at which the intersection occurs. To better understand this influence, a mathe-
matical model was generated, and its graphical representation presented in figure 2.25.
flame representation
probe viewing cone
flame front sensed area
Figure 2.25: Representation of probe viewing cone and flame surface intersection.
By changing the ideal light viewing cylinder’s vertical position, the mathematical model
retrieves the area of intersection, with results of two possible configurations presented in
2.3. PROTOTYPE TESTING 41
table 2.2 for a iris diameter of diris = 0.7 cm.
z[cm] Intersection representation Intersection Area, Aview [cm2]
0.4 43× 10−2
1.2 49× 10−2
Table 2.2: Intersection area for two vertical setups. Iris diameter of diris = 0.7 cm.
All these details are intrinsically related to prototype sensing accuracy. It is then neces-
sary to know how light emission occurs on the flame front correspondent to the intersec-
tion surface. Current mathematical definitions states that intensity of light emitted by a
certain area of a steady flame front is given by:
Iemitted =∫Ilocal dA (2.6)
When light is emitted by a steady flame, it is usually assumed that local intensity of
chemiluminescence, Ilocal, is independent of the emitting area. If light is being measured
with a prototype with a known optical path TF, Imeasured equal to Iemitted, it is possible
to obtain an estimation of the local intensity, by means of:
Ilocal[V
cm2 ] = Imeasured[V ]TFsystem × Aview[cm2] (2.7)
It is important to estimate local intensity since light theory is commonly associated to
local emission. For instance, the inverse square law theory states that strength of the
light emitted by a point source is proportional to the inverse square distance to it. Based
on this information it is important to know the probe localization that defines the best
signal output with the lowest temperature structural damages. Therefore, for the two
previous vertical positions, z = 0.4 cm and z = 1.2 cm, mean local intensity evolution was
analyzed as a function of collimator longitudinal placement. Results presented on figure
42 CHAPTER 2. EXPERIMENTAL METHODS
2.26 account for the light cylinder aberration phenomena previously identified. Please
refer to figure 2.22 for a better understand about longitudinal movement.
2 4 6 8 10 12
0.015
0.02
0.025
0.03
0.035
0.04
0.045
x [cm]
I loca
l − m
ean
valu
es [V
/mm
2 ]
OH* z = 0.4 cm CH* z = 0.4 cm OH* z = 1.2 cm CH* z = 1.2 cmPower fit − OH* z = 0.4 cmPower fit − CH* z = 0.4 cm Power fit − OH* z = 1.2 cmPower fit − CH* z = 1.2 cm
Figure 2.26: Graphical representation of local intensity variation as a function of probe position,
for both species.
Working conditions: z = 0.4 cm and z = 1.2 cm, diris = 0.7 cm, φ = 0.86 and P = 0.75 kW.
From this figure one can to observe that strength of the local intensity decreased as dis-
tance increased. The power equations that best fit the obtained values are graphically
represented in figure 2.26 and described in table 2.3. Analysis shows that power equa-
tions present a goodness of fit higher than 0.89. Note that none of the equations is an
inverse square evolution, but the overall trend has a behavior similar to theory. Due to
all mentioned aberration restrictions, to light probe assembling fitness and to the fact that
light emission from the flame front is not from a mathematical source point as in theory,
it is experimentally difficult to have a perfect fit of an inverse square law.
2.3. PROTOTYPE TESTING 43
Configuration Power equation R-square
z = 0.4 cmIlocal OH∗ = 0.0591× x−0.2024 0.9655
Ilocal CH∗ = 0.0187× x−0.1982 0.8944
z = 1.2 cmIlocal OH∗ = 0.05313× x−0.2079 0.9346
Ilocal CH∗ = 0.01699× x−0.1768 0.9199
Table 2.3: Power fitting equation for both species local intensity signals.
Working conditions: z = 0.4 cm and z = 1.2 cm, diris = 0.7 cm, φ = 0.86 and P = 0.75 kW.
The graphic also allows to note that vertical position has an influence on local intensity.
The scheme presented in figure 2.27 was designed to show the influence of flame ge-
ometry on the measurements: for the same collimator longitudinal position, if vertical
position is increased, the distance the distance between flame front and collimator probe
increases too. According to the inverse square law theory, the closer the flame front is to
the collimator, the higher the signal intensity is. It is then suggested that higher values
obtained for lower vertical position are due to the higher proximity of collimator to the
flame front.
Z=0.4 cm
Z=1.2 cm
x + x’
x
Figure 2.27: Influence of the vertical position on the distance between flame front and collimator
probe.
Considering all the tests related to the collimator probe position, as well as its structural
integrity, the longitudinal position for the best signal is set to x = 5 cm, for a flame setup
of φ = 0.86 and P = 0.75 kW.
44 CHAPTER 2. EXPERIMENTAL METHODS
Electronic noise
The electronic noise of a system that uses PMT is usually known as dark current, due
to the fact that even in dark conditions the PMT releases current that is added to the
usual electronic noise. Performing the signal-to-noise ratio (SNR) is a simple method
of quantifying dark current the influence on measurements accuracy. For the first SNR
quantification, measures were recorded for 3 different conditions: (1) closing the probe
iris diaphragm to impose a dark condition (diris = 0 cm) in order to obtain the electronic
noise, (2) setting up the collimator to a vertical position of z = 0.6 cm and a diameter
of diris = 1.2 cm in order to sense a flame with 308 nm BPF and (3) with 432 nm BPF.
The results of this 3 conditions are presented in table 2.4, along with the respective SNR
values. Regard that the CH* signal has a higher SNR than the OH* signal.
# diris [cm] Condition Mean Amplitude of Imeasured [V] SNR
1 0 electronic noise 8× 10−5 -
2 1.2 OH* - 308 nm BPF 5.15× 10−3 64
3 1.2 CH* - 432 nm BPF 5.46× 10−3 68
Table 2.4: Noise and signal analysis.
Working conditions: x = 5 cm and z = 0.6 cm, φ = 0.86 and P = 0.75 kW.
If the device is used for pulsed flame it is necessary to understand the amplitude distribu-
tion of electronic noise and signal along the spectral resolution. Figure 2.28 presents the
spectral analysis of the OH* and CH* measurements along with the spectral analysis of
the electronic noise. It shows that electronic noise level intersects the OH* signal spectra
at a frequency close to 200 Hz, which means that the current prototype is not accurate
for sampling OH* signals on frequencies higher than 200 Hz. For the CH* signal the
intersection occurs for a lower frequency of around 100 Hz. This spectral information is
achieved by using a post processing program that performs a Fast Fourier Transform to
the recorded data vector.
2.3. PROTOTYPE TESTING 45
10−1 100 101 102 103 10410−9
10−8
10−7
10−6
10−5
10−4
Frequency [Hz]
Am
plitu
de [V
]
NoiseOH*
(a) OH* signal
10−1 100 101 102 103 10410−9
10−8
10−7
10−6
10−5
10−4
Frequency [Hz]
Am
plitu
de [V
]
NoiseCH*
(b) CH* signal
Figure 2.28: Spectral distribution of Noise and signal.
Working conditions: x = 5 cm and z = 0.6 cm, diris = 0.7 cm, φ = 0.86 and P = 0.75 kW.
46 CHAPTER 2. EXPERIMENTAL METHODS
Viewing area
The prototype final test was done to understand how the optical probe geometry influ-
ences the signal, by varying its iris diaphragm diameter. Special care was taken to guar-
antee that the vertical position of the collimator probe allowed the high rate of viewing
area variation inside the flame geometry. Once again, with the aid of a laser (figure 2.23),
the best position was found to be z = 0.6 cm with a iris diameter variation of diris = [0
1.1] cm. Table 2.5 presents the obtained intensity values during the experiment, for both
species, along with the respective SNR value.
diris [cm] Imeasured [V] OH* SNROH∗ Imeasured [V] CH* SNRCH∗
1.1 5.77× 10−3 52 6.41× 10−3 58
1.0 4.12× 10−3 37 4.28× 10−3 40
0.9 3.03× 10−3 28 3.22× 10−3 29
0.8 1.81× 10−3 16 1.89× 10−3 17
0.7 1.49× 10−3 14 1.41× 10−3 13
0.5 0.90× 10−3 8 0.96× 10−3 9
0.4 0.53× 10−3 5 0.53× 10−3 5
0.3 0.25× 10−3 2 0.23× 10−3 2
0 0.11× 10−3 - 0.11× 10−3 -
Table 2.5: Imeasured mean values for different optical probe iris diameters.
Working conditions: x = 5 cm and z = 0.6 cm, φ= 0.86 and P = 0.75 kW.
From the analysis of table 2.5 it is possible to notice that lowering iris diameter has a
strong influence on PMT output signal amplitude. Considering that a reliable measure has
a SNR higher than 10, for this particular flame setup, the results for iris diameter lower
than diris = 0.7 cm should not be used. Figure 2.29 shows the graphical representation of
table 2.5.
2.3. PROTOTYPE TESTING 47
0.4 0.6 0.8 1
1
2
3
4
5x 10−3
Iris diameter [cm]
I mea
n [V]
OH*CH*Power fit − OH* Power fit − CH*
Figure 2.29: Total intensity emitted by the flame for different iris diameters.
Working conditions: x = 5 cm and z = 0.6 cm, φ = 0.86 and P = 0.75 kW.
The power equations that best fit the results are also represented in the graphic, with the
respective mathematical equation in table 2.6. It is possible to see that the best fit is closer
to a square evolution, which suggest that the measured intensity trend is related to probe
viewing area.
Specie Power equation R-square
OH* ImeasuredOH∗ = 0.003559× d2.813[V ] 0.9759
CH* ImeasuredCH∗ = 0.003910× d2.813[V ] 0.9887
Table 2.6: Power fitting equation for both measured intensity signals.
Working conditions: x = 5 cm and z = 0.6 cm, φ = 0.86 and P = 0.75 kW.
Despite the fact that experimental results are coherent with the presented theories, it
is important not to forget that: (1) the interface between selected optical fiber and the
optical probe lens was not optimized, with a consequent lost of radiation (figure 2.7(b)),
(2) the setup of the iris diaphragm is done manually for each measure, with consequent
introduction of reading error of 1 mm, (3) the flame is not a steady geometric cone and
(4) for high surface areas of intersection the assumption of local intensity independent
of the area may lose validity. Even knowing that higher values of iris diameter present
higher SNR, see table 2.5, these notes show that higher areas introduce higher errors on
the measurements.
Chapter 3
Case Study
As stated in the first chapter of the present work, the search of new sensors for com-
bustion monitoring is mainly supported by Aerospace Industry due to the fact that
reliability of new turbofan combustion control systems directly depends on the quality
of sensing devices. A simple turbofan engine active control scheme can be divided into
three main parts: the sensors, the actuators and the controller, as one can see in figure
3.1.
compressor turbinesfan
fuel
controller
sensoractuator combustion chamber
Figure 3.1: Turbofan engine active control scheme.
49
50 CHAPTER 3. CASE STUDY
Active control model definition is based in a deep understanding of combustion reac-
tion. However, inside a combustion chamber high turbulence environment increases the
complexity of model definition process. To overcome this problem researchers started
to study simple phenomena in a steady flame and then try to use the learned principles
to modulate unsteady turbulent flames. As suggested by Anacleto et al. [2003], this can
be done with a propane flame, not only because it behaves like jet-A fuel, but also be-
cause it is possible to actuate it, in order to simulate unsteady controlled conditions, as
an approximation to instant deformation in a turbulent flame.
The present chapter aims to show the obtained results of several tests performed in or-
der to evaluate chemiluminescence emissions of a Bunsen flame under different working
conditions. The first part will introduce steady flame chemiluminescence analysis and in-
fluence of prototype setup in the measurements, as well as comparison between obtained
results and other studies. The second part will focus on the main objective of the present
work, chemiluminescence analysis of an unsteady flame for active control.
3.1. EXPERIMENTAL ANALYSIS OF STEADY FLAME 51
3.1 Experimental analysis of steady flame
When studying chemiluminescence emission flame processes, it is essential to analyze
the work developed by Muruganandam et al. [2003]. As one of the predecessors of sensor
design for active combustion control, his studies included chemiluminescence analysis of
several premixed and non-premixed combustors from different turbine engines, in various
operating conditions. Figure 3.2 presents the experimental setup of an optical fiber light
collecting system mounted on a premixed swirl dumped air-methane combustor. It also
shows the respective intensity ratio evolution between mean measured values of OH* and
CH* as a function of ER, OH∗mean/CH∗
mean = f(φ), with a PMT sensor and an optical
system similar to the one developed in this work.
3 American Institute of Aeronautics and Astronautics
PREMIXED COMBUSTORS Three types of premixed combustors were
employed: 1) a ring-pilot stabilized, unconfined jet flame, 2) an atmospheric pressure, swirl stabilized, dump combustor with circular cross-section and 3) a high pressure, gas turbine combustor simulator, with square cross-section.
The ring-pilot stabilized, jet flame used both methane and natural gas as fuels. The jet inner diameter was 0.91” with a step on the outside of the lip of the burner, where the pilot mixture (usually slightly fuel rich) was injected through 25 holes. The pilot flow rates were very small compared to the main flow rates. The main jet exit velocity was ~5m/s, with a Reynolds number based on jet diameter of ~5000. The spectrometer collected the optical emission from a 1×15 mm region centered on the axis of the flame, far enough downstream where the effect of the pilot flame was negligible.
45o swirl
30o swirlCH4/air mixture
Optical fiber
Quartz tube
Flame shape
127
mm
70 mm Dia.
45o swirl
30o swirlCH4/air mixture
Optical fiber
Quartz tube
Flame shape
45o swirl45o swirl
30o swirl30o swirlCH4/air mixtureCH4/air mixture
Optical fiber
Optical fiber
Quartz tube
Flame shape
127
mm
70 mm Dia.
127
mm
70 mm Dia.
127
mm
70 mm Dia.
127
mm
70 mm Dia.
Figure 2. Schematic of the Swirl stabilized dump combustor used in this work. The location of the fiber, its collection volume and flame shape are also shown.
A schematic of the second combustor used is shown in Figure 2. The overall combustor configuration was chosen as a simplified model of a lean, premixed, gas turbine combustor that includes a swirling inlet section. Premixed gas, consisting of gaseous fuel (methane or natural gas) and air flows through swirl vanes housed in a 23 mm i.d. tube. The swirler consists of two sets of vanes, 30o followed by 45o causing the exit flow to have a (theoretical) swirl number of 0.66.32 The quartz wall permits uncooled operation of the combustor and facilitates detection of ultraviolet (UV) radiation. The data presented here correspond to a bulk average axial velocity of around 4 m/s in the combustor under cold conditions. Assuming complete combustion, the average axial velocity of the product gases would be ~20 m/s. Also shown in figure are the nominal flame shape and the location of the optical fiber used to collect optical emissions.
The third premixed combustor (Figure 3) is a high pressure, gas turbine combustor simulator.33 The facility consists of modular inlet, combustor and exhaust sections. High-pressure air and natural gas are supplied through building lines capable of providing 720 and 1000 psi pressures, respectively. Both the air and fuel flow rates are measured through the use of calibrated critical orifices. The equivalence ratio was kept constant by double choking of both air and fuel flow paths. The fuel-air mixture enters the circular 4.75cm diameter, 60cm long inlet section and passes through a 45o swirler prior to entering the combustor. Combustion occurs in the 51 cm long, 5×5cm square combustor downstream of the conical flame holder, and the combustion products then flow through a circular 7.6cm diameter, 195cm long exhaust section before leaving the system. A separate high-pressure air stream cools the combustor walls, and is then injected through a tube into the exhaust section where it mixes with the combustion products. Tests were performed at pressures ranging 1.5 to 10 atm, over equivalence ratios ranging from lean blow out to near stoichiometric with mass flowrates of 10-50 g/sec.
Figure 3. Picture of the high pressure gas turbine combustor simulator.
NON-PREMIXED COMBUSTORS Two non-premixed, liquid fueled, atmospheric
pressure, swirling combustors were used in this study: 1) an axisymmetric, air-staged combustor, and 2) a single cup, annular section model of a CFM56 combustor.
The first non-premixed combustor34 used n-heptane (C7H16) fuel, which is supplied to the bottom of the combustor through a modified pressure-atomizer. One-third of the air premixes with the fuel spray in a central diverging tube, and the remaining air mixes with the central flow downstream of the diverging tube. The combustor walls are made of 42 mm diameter quartz tube. The maximum airflow rate is ~15 g/s and the maximum fuel flow rate is ~1 g/s. The lean stability limit of the flame without air preheating is at an overall equivalence ratio of ~0.6.
(a) Swirl stabilized dump com-
bustor.
0.65 0.7 0.75 0.8 0.85 0.9 0.95 1
3
3.5
4
4.5
5
5.5
Equivalence Ratio
OH
* mea
n/CH
* mea
n
Swirl / dump stabilized combustor
(b) OH∗mean/CH∗
mean evolution
Figure 3.2: Combustor scheme. Respective mean chemiluminescence intensity ratio evolution,
by Muruganandam et al. [2003].
ER and viewing area influence
The main difference between the developed prototype and Murunganandam sensing de-
vice is the light collecting system, designed to use several configurations for the analysis
of the viewing area influence on chemiluminescence measurements, see figure 3.3.
52 CHAPTER 3. CASE STUDY
Acquisition board
Prototype
fiber
Z=0.4 cm
Z=1.4 cm
xfiber
xprobe
Figure 3.3: Experimental setup for chemiluminescence and ER mixture model definition. Fiber
head and optical probe viewing area scheme of the current prototype.
To perform this study on the propane Bunsen flame presented in the previous chapter it
was first necessary to understand the influence of the ER on the flame geometry. As one
can see in figure 3.4, an increase in the flame ER leads to a shape reduction as well as an
increase in light emission.
(a) φ = 0.86 (b) φ = 0.90 (c) φ = 1.00 (d) φ = 1.10
Figure 3.4: Laminar Bunsen flame pictures for different ER, power range P = [0.75 0.96] kW.
Exposure time 1/8 s.
This analysis was essential to verify if the prototype viewing area intersected the flame
surface in all operating conditions: for the optical fiber the head was placed at a distance
3.1. EXPERIMENTAL ANALYSIS OF STEADY FLAME 53
of xfiber = 45 cm from the flame, as shown in figure 3.3; for the optical probe position
was set to a distance of xprobe = 5 cm, for two vertical positions, z = 0.4 and z = 1.4 cm.
According to signal analysis done in the previous chapter, iris diameter was set to diris =
0.7 cm.
The literature review of this work showed that the research community does not fre-
quently studies propane Bunsen flames. It was only found that Ikeda’s object of study, see
[Ikeda et al., 2004], was a propane Bunsen flame, to which he analyzed theOH∗mean/CH∗
mean
evolution as a function of ER, φ = [0.9 1.5]. Moreover, he found that the mathematical
equation that best fit the evolution OH∗mean/CH∗
mean = f(φ) is exponential.
Based in this data, for each collecting system of previously presented experimental setup,
the mean chemiluminescence values were measured, post-processed and the ratio calcu-
lated to each ER condition (ER range of φ = [0.86 1.1]. The graphical representation
in figure 3.5 includes the obtained evolution points along wit the lines of the exponen-
tial fitted equations. The correspondent mathematical equations are summarized in table
3.1. Note that the chemiluminescence ratio is now defined as mean values, since it is the
output of the average signals during the measuring interval (equal to Ikeda et al. [2004]).
0.8 0.9 1 1.1 1.2 1.3
1
1.5
2
2.5
3
3.5
Equivalence Ratio
OH
* mea
n/CH
* mea
n
fiberz = 0.4 cm z = 1.4 cm IkedaExponential fit − fiberExponential fit − z = 0.4 cm Exponential fit − z = 1.4 cm Exponential fit − Ikeda
Figure 3.5: Ratio between mean intensity value of OH* and CH*, as a function of mixture ER.
Working conditions: xfiber = 45 cm, xprobe = 5 cm, ER range φ = [0.86 1.1] and power range P
= [0.75 0.96] kW.
54 CHAPTER 3. CASE STUDY
Configuration Exponential equation R-square
Ikeda et al. [2004] OH∗mean/CH∗
mean = 6.80× e−1.563×φ -
fiber OH∗mean/CH∗
mean = 13.97× e−1.596×φ 0.990
z = 0.4 cm OH∗mean/CH∗
mean = 31.27× e−2.551×φ 0.986
z = 1.4 cm OH∗mean/CH∗
mean = 25.25× e−2.877×φ 0.997
Table 3.1: Exponential fitting equations for different collecting system configurations.
Working conditions: xfiber = 45 cm, xprobe = 5 cm and diris = 0.7 cm, ER range φ = [0.86 1.1]
and power range P = [0.75 0.96] kW.
The analysis of figure 3.5 and the fitting equations in table 3.1 show that: (1) obtained
equations are in agreement with Ikeda’s results, presenting a high goodness of fit (R-
square > 0.95), (2) the light collecting system resolution influences the slope of the
exponential fitting line and (3) for the same resolution, vertical position influences the
ratio magnitude.
Based on this observations it is suggested that the light collecting system setup has a
strong influence on measured values and that the developed prototype is suitable for
steady Bunsen flame control model definition.
3.2. EXPERIMENTAL ANALYSIS OF UNSTEADY FLAMES 55
3.2 Experimental analysis of unsteady flames
The experimental study of unsteady flames is driven by the search of simple models to
replicate turbulent flames occurrences. In this context, it is important to understand how
flame deformation affects the Bunsen flame light emission, since it influences the reaction
mechanisms and SL through the stretch rate.
The laminar flame speed, SL, is defined as the speed at which the premixed reagents
are being consumed by the flame front reaction. When reagents are delivered to the
reaction with an absolute velocity component equal to its SL, the flame front achieves
a macroscopic steady condition, defining its conical geometry, figure 3.6. Since it is a
steady condition, SL is considered constant along the time.
n
Steadyflame Pulsed
flame
S L nFlamefront
αUmixture
Umixture
S L n
Umixtureα
Flamefront
Figure 3.6: Schematic representation of steady and unsteady axissimetric flame. Velocity balance
for the flame front.
However, if the supplied mixture is actuated, the variation will affect flame curvature/stretch
rate, which will consequently close the loop by changing SL. Note that SL is a function
of mixture ER, fuel and stretch rate.
In 1975 Williams proposed to define the fractional time rate of modification of a flame
surface element of area A as stretch rate, K (equation 3.1). His studies, cited in Marley
and Roberts [2005] work, considered that K is a consequence of aerodynamic forces and
56 CHAPTER 3. CASE STUDY
flame curvature variations.
K = 1A
dA
dt(3.1)
For laminar axissimetric Bunsen flames, with steady conditions of geometry and mixture
velocity supply, equation 3.1 can be defined as 3.2, where V0 is the mixture velocity, α is
the flame cone angle (figure 3.6) and R the flame cone radius at a certain high.
K = −V0 × sin(2× α)2×R (3.2)
The negative signal on equation 3.2 indicates that the flame is under compression (ne-
gative stretch, see figure 3.7). This geometry promotes a local pre-heat of the reagents
achieving combustion, which affects the flame maximum temperature.
Negative stretch
Positive stretch
Figure 3.7: Flame front condition. Compression and stretching respectively.
In fact, work developed by Yokomori and Mizomoto [2003] showed that measured tem-
perature of a pulsed flame varied as an effect of flame stretch modification. Moreover,
Hardalupas and Orain [2004] experimentally evaluated the influence of K on the emis-
sion of chemiluminescence from species OH*, CH* and C∗2 for a methane flame, with
Cassegrain lens and PMT sensors. The results, presented in figure 3.8 showed that chemi-
luminescence intensity ratio OH∗mean/CH∗
mean remained constant while K was varying,
for different ER, and that chemiluminescence intensity ratio C∗2mean/CH∗
mean was af-
fected by K for rich mixtures.
3.2. EXPERIMENTAL ANALYSIS OF UNSTEADY FLAMES 57
202 Y. Hardalupas, M. Orain / Combustion and Flame 139 (2004) 188–207
Fig. 12. Mean local chemiluminescence intensity ratio OH·/CH· as a function of (a) equivalence ratio and (b) strain rate of thecounterflow natural gas flames.
Fig. 13. Mean local chemiluminescence intensity ratio C·2/CH· as a function of (a) equivalence ratio and (b) strain rate of the
counterflow natural gas flames.
from 0.7 to 1.3 (Fig. 12a) and remained independentof the rate of strain (Fig. 12b). The dependence of theratio of the intensities OH·/CH· onΦ can be approx-imated by the following curve fit:
(3)OH·CH· = 0.497+ 2.107exp
(−(Φ − 0.7)/0.260).
It should be noted that Eq.(3) corresponds to the cal-ibration for rate of strain associated withV0 = 3 m/s.
For all flow conditions and for a given value ofΦ,the ratio of intensities OH·/CH· lies within 5% of thevalue given by Eq.(3), which leads to an uncertaintyof approximately 0.05 in the determination of flameequivalence ratio forΦ � 1.0. The ratio of intensi-ties OH·/CH· is sensitive to flame equivalence ratioonly below unity, because the gradient of the curveof Fig. 12is maximum in that region, but shows littlesensitivity to values of equivalence ratio larger thanstoichiometry. ForΦ > 1.0, the technique has uncer-tainties of around 0.2 up toΦ = 1.3. Fig. 12suggeststhat the ratio of intensities OH·/CH· could be used
to infer the local flame equivalence ratio in a burneroperating with natural gas at atmospheric pressure.However, the technique has smaller uncertainty forlean and stoichiometric flames than for rich ones.
Fig. 13 shows the ratio of intensities C·2/CH· asa function of equivalence ratio and strain rate. Thisratio exhibits nonmonotonic dependence on equiv-alence ratio (Fig. 13a). It is almost independent ofequivalence ratio for lean and stoichiometric flamesbut increases strongly with equivalence ratio for richflames (Φ > 1.1). Its dependence on rate of strain isstrong for rich flames (Fig. 13b). Therefore, ratio ofintensities C·2/CH· cannot be used for measurementof flame equivalence ratio, because of its insensitiv-ity to Φ for lean and stoichiometric flames and itsdependence upon strain rate for rich flames. This isin contradiction to the suggestions of[29], perhapsbecause their conclusions were drawn from measure-ments in Bunsen burner flames in which the effectsof strain rate on the intensity ratio could not be estab-lished. The ratio of intensities C·2/CH· could, how-
(a) OH∗mean/CH∗
mean
202 Y. Hardalupas, M. Orain / Combustion and Flame 139 (2004) 188–207
Fig. 12. Mean local chemiluminescence intensity ratio OH·/CH· as a function of (a) equivalence ratio and (b) strain rate of thecounterflow natural gas flames.
Fig. 13. Mean local chemiluminescence intensity ratio C·2/CH· as a function of (a) equivalence ratio and (b) strain rate of the
counterflow natural gas flames.
from 0.7 to 1.3 (Fig. 12a) and remained independentof the rate of strain (Fig. 12b). The dependence of theratio of the intensities OH·/CH· onΦ can be approx-imated by the following curve fit:
(3)OH·CH· = 0.497+ 2.107exp
(−(Φ − 0.7)/0.260).
It should be noted that Eq.(3) corresponds to the cal-ibration for rate of strain associated withV0 = 3 m/s.
For all flow conditions and for a given value ofΦ,the ratio of intensities OH·/CH· lies within 5% of thevalue given by Eq.(3), which leads to an uncertaintyof approximately 0.05 in the determination of flameequivalence ratio forΦ � 1.0. The ratio of intensi-ties OH·/CH· is sensitive to flame equivalence ratioonly below unity, because the gradient of the curveof Fig. 12is maximum in that region, but shows littlesensitivity to values of equivalence ratio larger thanstoichiometry. ForΦ > 1.0, the technique has uncer-tainties of around 0.2 up toΦ = 1.3. Fig. 12suggeststhat the ratio of intensities OH·/CH· could be used
to infer the local flame equivalence ratio in a burneroperating with natural gas at atmospheric pressure.However, the technique has smaller uncertainty forlean and stoichiometric flames than for rich ones.
Fig. 13 shows the ratio of intensities C·2/CH· asa function of equivalence ratio and strain rate. Thisratio exhibits nonmonotonic dependence on equiv-alence ratio (Fig. 13a). It is almost independent ofequivalence ratio for lean and stoichiometric flamesbut increases strongly with equivalence ratio for richflames (Φ > 1.1). Its dependence on rate of strain isstrong for rich flames (Fig. 13b). Therefore, ratio ofintensities C·2/CH· cannot be used for measurementof flame equivalence ratio, because of its insensitiv-ity to Φ for lean and stoichiometric flames and itsdependence upon strain rate for rich flames. This isin contradiction to the suggestions of[29], perhapsbecause their conclusions were drawn from measure-ments in Bunsen burner flames in which the effectsof strain rate on the intensity ratio could not be estab-lished. The ratio of intensities C·2/CH· could, how-
(b) C∗2 mean/CH
∗mean
Figure 3.8: Mean local chemiluminescence intensity ratio as a function of strain rate of the coun-
terflow natural gas flames, [Hardalupas and Orain, 2004].
All these researches suggest chemiluminescent emission of a flame at a certain instant
of time may be different from averaged values. The present section aims to analyze this
possibility on an unsteady propane Bunsen Flame.
Experimental setup
One of the methods to obtain an unsteady Bunsen flame is based on supplied mixture
velocity variation. If pressure inside the plenum of a Bunsen burner varies by means of
loudspeaker excitation, the velocity mixture at the burner exit will vary to, proportional
to it. Therefore, for certain limits, if the generated function is periodic, SL variation will
be periodic to.
Based on this knowledge, the experimental setup previously presented in figure 2.1 was
modified to include a loudspeaker, actuated by a sinusoid generated by the DT9841 - SB
board, as schematically shown in figure 3.9.
58 CHAPTER 3. CASE STUDY
filtering system Photomultiplier and pre-amplifier
Loudspeaker
Phantom V42
acquisition generation
chemiluminescence signal
generated sinusoid signal
Figure 3.9: Schematic representation for measuring a pulsed flame. Acquisition and generation
path.
The first tests with a pulsed flame showed influence of the mixture velocity variation in
the flame stability. Aside from low mixture ER, the possibility of flame extinction now
depends on the amplitude and frequency of the generated pressure waves that change
the mixture velocity. For the same frequency, the higher the ER, the higher the pressure
amplitude variation the flame supports. As frequency is increased, pulse signal ampli-
tude needs to be incremented to achieve notorious flame front deformations. Figure 3.10
presents two Bunsen flame pictures for steady and unsteady conditions, where is notori-
ous the influence of the mixture velocity variation in the overall shape.
(a) Pulse amplitude: 0 V (b) Pulse amplitude: 0.045 V
Figure 3.10: Pictures of Bunsen flame, time exposure of 1/8 s.
Working conditions: φ = 0.86 and P = 0.75 kW. Excitation frequency: 50 Hz.
3.2. EXPERIMENTAL ANALYSIS OF UNSTEADY FLAMES 59
Regarding lean combustion control as one of the main objectives of Aerospace Industry,
several trials were performed to define the leanest flame that could withstand high rates
of deformation. For an excitation amplitude range of ∆V = [0 0.045] V at 50 Hz, the
lowest ER possible was found to be φ = 0.86. The highest ER was selected to φ = 1.1, as
an example of rich mixture.
Based on these tests the experimental procedure was set to measure chemiluminescence
emission as a function of ER, for each rang amplitude. The process was repeated on each
optical collecting system configurations.
Parallel to these process, a high speed camera was prepared to record the flame shape
during the period of measurement. The selected device was a Phantom V42-7059 with a
sample rate of 2500 fps, which allows the acquisition of 50 flame images, for each period
of oscillation.
Figure 3.11 presents the obtained mean intensity ratio for each optical configurations,
OH∗mean/CH∗
mean = f(φ,∆V ). Figures 3.12 and 3.13 present pictures of the flame
during chemiluminescence measurements, for the lowest and highest ER conditions, res-
pectively.
60 CHAPTER 3. CASE STUDY
fiber
Z = 0.4 cm
Z = 1.4 cm
Figure 3.11: Ratio between mean intensity value of OH* and CH*, OH∗mean/CH∗
mean, as a
function of mixture ER, for different excitation amplitude and vertical probe positions.
Working conditions: xfiber, z = 0.4 cm and z = 1.4 cm at xprobe = 5 cm, ER range φ = [0.86 1.1]
and power range P = [0.75 0.96] kW. Excitation frequency: 50Hz.
3.2. EXPERIMENTAL ANALYSIS OF UNSTEADY FLAMES 61
0 0.005 0.015 0.025 0.035 0.045
Excitation amplitude [V]
Ti
me
evol
utio
n [s
]
0.02
00
0.01
78
0.0
156
0
.013
3
0.01
11
0.0
089
0.
0067
0
.004
4
0.00
22
0.0
00
Figure 3.12: Flame geometry evolution for different amplitudes of excitation.
Working conditions: φ = 0.86 and P = 0.75 kW. Excitation frequency: 50Hz.
62 CHAPTER 3. CASE STUDY
0 0.005 0.015 0.025 0.035 0.045
Excitation amplitude [V]
Tim
e ev
olut
ion
[s]
0.02
00
0.01
78
0.0
156
0
.013
3
0.01
11
0.0
089
0.
0067
0
.004
4
0.00
22
0.0
00
Figure 3.13: Flame geometry evolution for different amplitudes of excitation.
Working conditions: φ = 1.1 and P = 0.96 kW. Excitation frequency: 50Hz.
3.2. EXPERIMENTAL ANALYSIS OF UNSTEADY FLAMES 63
Note that the graphics presented in figure 3.11 also include lines of exponential fit for
the two boundary cases, ∆V = 0 V and ∆V = 0.045 V. The respective mathematical
equations are presented in table 3.2.
Configuration Amplitude [V] Exponential fit R-square
fiber0 OH∗
mean/CH∗mean = 13.97× e−1.596×φ 0.9899
0.045 OH∗mean/CH∗
mean = 15.38× e−1.676×φ 0.9787
z = 0.4 cm0 OH∗
mean/CH∗mean = 31.27× e−2.551×φ 0.9858
0.045 OH∗mean/CH∗
mean = 32.19× e−2.593×φ 0.999
z = 1.4 cm0 OH∗
mean/CH∗mean = 25.25× e−2.439×φ 0.9970
0.045 OH∗mean/CH∗
mean = 19.17× e−2.135×φ 0.9974
Table 3.2: Exponential fitting equations of the ratio between mean intensity value of OH* and
CH*, OH∗mean/CH∗
mean, as a function of mixture ER, for different excitation amplitude.
When analyzing data from the experiment an important information is derived: if the
study of the unsteady flame chemiluminescence emission is done only based on the in-
formation provided by the mean ratio, graphics show that pulse amplitude does not signif-
icantly affect evolution magnitude or slope. In fact, for the collimator probe configuration
of z = 0.4 cm, obtained fit are almost identical for steady (∆V = 0 V) and unsteady (∆V =
0.045 V) conditions, as one can see in table 3.2. However, looking at the respective high
speed pictures in figure 3.12, it is possible to verify that the flame front undergoes high
rates of deformation, varying from negative to positive stretch. If the chemiluminescence
emission mechanism of a propane flame is affected by the stretch rate, as it was shown
for methane flames by Hardalupas and Orain [2004], it is being aliased by mean values.
The obtained pictures also show the influence of optical probe vertical placement in mea-
sured values. If the optical probe is set to a high vertical position, there is a period during
oscillation, when the intersection surface is significantly lower, and consequently, magni-
tude of the mean values obtained during unsteady conditions are lower than steady ones.
This can be verified by comparing the fits for z = 1.4 cm in table 3.2.
64 CHAPTER 3. CASE STUDY
3.3 Experimental analysis of unsteady flames
The main purpose of developing a high speed chemiluminescence sensor is to verify the
validity of using the ratio between mean intensity values of OH* and CH* for active con-
trol systems when applied to unsteady or turbulent flames. The experiments performed
in previous sections were done to understand if the prototype was providing expected
results for known setups, but at the same time it retrieved valuable informations such as
influence of the light collecting system’s vertical position on the values magnitude and
evidence of high rates of flame front deformation during unsteady pulse.
To try to verify if high rates of deformation influence local chemiluminescence emission,
the experimental setup was updated to synchronously record images and chemilumines-
cence emission from the flame. The acquisition board was set to generate a pulse trigger
that starts the camera at the same instance it starts recording chemiluminescence signal,
as it is schematically represented in figure 3.14. By using the developed algorithm for
surface intersection area calculation (see chapter 2), the obtained images provide an esti-
mation of the area that the light collecting system is viewing.
filtering system Photomultiplier and pre-amplifier
Loudspeaker
Phantom V42
acquisition generation
trigger
Figure 3.14: Schematic representation of the experimental rig for a pulsed flame generation and
image record.
3.3. EXPERIMENTAL ANALYSIS OF UNSTEADY FLAMES 65
Experimental procedures
According to the flowchart for signal digitalization presented in chapter 2, figure 2.18,
digitalizing 32768 instances of analogue signal takes 4 seconds. When the signal has
a characteristic frequency, such as the generated sinusoid and the chemiluminescence
signal, it is possible to post process the data and merge information from all digitalized
periods into a single period, see figure 3.15. This technique, known as phase average, is
usually applied when the digitalized signal has a characteristic frequency and low ampli-
tude, such as the signal from a PMT sensing a pulsed lean flame.
acquired chemiluminescence signal
Divide by the optical path TF
Figure 3.15: Signal post-processing diagram. Phase average implementation.
The first analyzed setup was an ER of φ = 0.86 pulsed with an amplitude of ∆V =
0.045V at 50Hz, since it presented the highest rate of flame front deformation. The
acquired images were post processed to obtain the viewing area time evolution for each
different system configurations. Figure 3.16 presents a scheme for better understanding
the relation between viewing area evolution and flame geometry.
66 CHAPTER 3. CASE STUDY
fiber
Z=0.4 cm
Z=1.4 cm
1 - 0.0004 s 2 - 0.002 s 3 - 0.004 s 4 - 0.006 s 5 - 0.008 s 6 - 0.01 s
7 - 0.012 s 8 - 0.014 s 9 - 0.016 s 10 - 0.018 s 11 - 0.02 s
Fiber viewing area [mm2] Probe viewing area [mm2] Probe viewing area [mm2]
2
3
4 5
6 7
8
9
10 11
2
3
4 5
6 7
8
9
10
11
2
3
4
5 6
7
8
9
10
11
1
1 1
1 - 0.0004 s 2 - 0.002 s 3 - 0.004 s 4 - 0.006 s 5 - 0.008 s 6 - 0.01 s
7 - 0.012 s 8 - 0.014 s 9 - 0.016 s 10 - 0.018 s 11 - 0.02 s
1 - 0.0004 s 2 - 0.002 s 3 - 0.004 s 4 - 0.006 s 5 - 0.008 s 6 - 0.01 s
7 - 0.012 s 8 - 0.014 s 9 - 0.016 s 10 - 0.018 s 11 - 0.02 s
Collecting system
Collecting system sensed im
ages time
evolution Collecting system
sensed area time
evolution
Figure3.16:Sensed
areaevolution
andpictures
offlame
geometry
for11pulse
instances.Representation
foreachlightcollecting
configurations.
Working
conditions:E
Rφ
=0.86
andP
=0.75
kW.E
xcitationfrequency:50
Hz.Pulse
amplitude:∆
V=
0.045V.
3.3. EXPERIMENTAL ANALYSIS OF UNSTEADY FLAMES 67
The sensed area evolution lines presented in the graphics are the result of a fitting process,
since the area was calculated based on 16 of the 50 images taken by a high speed camera
during a period of oscillation. The fitting type that best suited the obtained values was a
sum of sines and cosines, which for the vertical set z = 0.4 cm and for the fiber set reveals
an evolution shape closer to the generated signal. The dashed line presented in the fiber
viewing area graphic, 347mm2, corresponds to the area calculated for a steady condition
(see ∆V = 0 V in figure 3.12) and is useful to show that during the time interval from
0.002 s to 0.015 s, unsteady surface area is lower than steady one, which suggests that,
for this interval, the flame is under compression.
Figure 3.17 presents the respective chemiluminescence signals for each collecting system
configuration, synchronously started at the same instance of the high speed pictures.
Fibe
r vie
win
g ar
ea [
mm
2 ]
fiber Z=0.4 cm Z=1.4 cm
Prob
e vi
ewin
g ar
ea [
mm
2 ]
Prob
e vi
ewin
g ar
ea [
mm
2 ]
Are
a
OH
*
CH*
A
mpl
itud
e [V
] A
mpl
itud
e [V
]
Figure 3.17: Area evolution and chemiluminescence signals after phase average algorithm, for
each of the collecting probe configurations.
Working conditions: ER φ = 0.86 and P = 0.75 kW. Excitation frequency: 50 Hz. Pulse ampli-
tude: 0.045 V.
68 CHAPTER 3. CASE STUDY
The OH* and CH* chemiluminescence emission graphics in figure 3.17 present, with
a gray color, all points obtained during the 4 seconds acquisition merged into a single
period of oscillation by phased average technique. Each graphic also includes chemi-
luminescence average values and respective standard deviation presented in black lines.
Presenting deviation values for each chemiluminescence evolution shows there are no
significant variations between them, which increases confidence in the obtained results.
When comparing chemiluminescence average values and the evolution, it is possible to
see that respective shapes are quite similar, being vertical set z = 0.4 cm, the most notori-
ous example. A closer look shows that the highest chemiluminescent values correspond
to the highest area values, which is in agreement with the previously presented theorem
(see equation 2.6 in chapter 2), where measured values are related to the viewing area.
Also note that chemiluminescence time evolution shape is similar for both species, which
may suggest that mechanisms of light emission from OH* and CH* are related.
For further analysis of this topic, the use Cassegrain lens is more accurate, even knowing
that instant area and chemiluminescence values allow a good estimation of respective
local intensity. If Cassegrain lens are designed based on current prototype optical fiber
characteristics, no special prototype modifications are necessary to obtain chemilumines-
cence values with higher resolution.
The experimental process was repeated for different flame setups, varying ER and pulse
amplitude. For each collecting system vertical configuration, results are presented in
figure 3.18. Note that all measured values presented were post processed to remove
optical path influence.
3.3. EXPERIMENTAL ANALYSIS OF UNSTEADY FLAMES 69
Ф =
0.8
6
Ф
= 0
.90
Ф
= 1
.00
Ф
= 1
.10
fiber
Z=0.4 cm Z=1.4 cm
Figure 3.18: Chemiluminescence evolution as a function of pulse amplitude and mixture ER.
Working conditions: Excitation frequency: 50 Hz.
70 CHAPTER 3. CASE STUDY
The scheme presented in figure 3.18 allows a look into the influence of mixture para-
meters in measured intensity. As one can see in all light collecting configurations, for
the same ER, an increase of pulse amplitude lead to an increase of intensity range. No-
tice that chemiluminescence emission of OH* species is more affected than on CH*,
which suggest that the OH* mechanism of light emission is more sensible to the supplied
mixture velocity variation.
As occurred in previous experiments, for the same light collecting system configuration,
an increase of ER leads to an increase in the intensity magnitude of both species. This can
easily be seen by looking at the intensity evolution of both species, for steady conditions
(∆V = 0 V).
Another important observation is related to the shape evolution of the optical fiber con-
figuration. It is possible to see that the transition between low excitation amplitude values
and higher ones leads to an evolution inversion, as one can see in the transition of ∆V
= 0.015 V to ∆V = 0.025 V for the ER of φ = 0.86. This phenomena is expected to be
more notorious in the chemiluminescence ratio analysis.
Local chemiluminescence intensity time analysis
In order to study flame local chemiluminescence emission, when operating under the
hardest stretching conditions (ER of φ = 0.86 and pulse amplitude of ∆V = 0.045 V),
corresponding chemiluminescence and area data presented in figure 3.18 was post pro-
cessed and results graphically represented in figure 3.19, for each light collecting systems
configuration. The dashed line in the graphics corresponds to local intensity estimated
with steady mean values, which helps to quantify the magnitude of data variation during
an oscillation period.
3.3. EXPERIMENTAL ANALYSIS OF UNSTEADY FLAMES 71
OH
/Are
a CH
/Are
a fiber
Z=0.4 cm Z=1.4 cm
Figure 3.19: Ratio between chemiluminescence signals and the viewing area, for each of the col-
lecting probe configurations.
Working conditions: ER φ = 0.86 and P = 0.75 kW. Excitation frequency: 50 Hz. Pulse ampli-
tude: 0.045 V.
As it is possible to see in figure 3.19, local intensity of both emitting species varies du-
ring the period of oscillation. This suggests that the mechanisms of chemiluminescence
emission are influenced by the supplied mixture velocity, which ends up affecting the
flame front stretching condition.
To analyze this assumption, previously presented graphics of chemiluminescence, area
and local chemiluminescence evolution for OH*, at z = 0.4 cm vertical configuration,
were included in the same scheme as the obtained high speed pictures. As one can see in
figure 3.20, special care was taken to guarantee that time scale of all the data was coinci-
dent. It is expected that this scheme helps relating each inserted picture with respective
local chemiluminescence emission value.
72 CHAPTER 3. CASE STUDY
Figure 3.20: OH* local intensity for light collecting system z = 0.4 cm. Instant images.
Working conditions: ER φ = 0.86 and P = 0.75 kW. Excitation frequency: 50 Hz. Pulse ampli-
tude: 0.045 V.
3.3. EXPERIMENTAL ANALYSIS OF UNSTEADY FLAMES 73
As can be seen, maximum and minimum values of OH* chemiluminescence emission are
apparently coincident with hard stretching conditions. Considering the stretching defini-
tion presented in the previous figure 3.7, the highest local intensity values correspond to
positive stretch conditions, while the lowest values correspond to negative ones.
Another important observation that is possible to do from the analysis of figure 3.20 is
that the instance at which the maximum value of local intensity occurs is not the same
instance as the maximum values of area and chemiluminescence light emission. This evi-
dences an important drawback of using chemiluminescence values from an optical probe
without accounting for emitting area effects. For fundamental flame studies it is essential
to use high spatial resolution optical probes or have a precise knowledge about current
viewing area. Note that this occurrence was also found in CH* light emission.
OH*/CH* time analysis
Several studies performed in the Aerospace context showed that the ratio between mean
chemiluminescence emission of OH* and CH* is essential for flame control. Never-
theless, earlier in this section figure 3.18 presented chemiluminescence evolution as a
function of pulse amplitude and ER, for OH* and CH* species, where it was noticed that
using mean values aliases intensity variations. In order to understand the magnitude of
these variations, figure 3.21 presents chemiluminescence ratioOH∗/CH∗, based in data
from figure 3.18. The presence of OH ∗ /CH∗ evolution, for steady conditions (∆V = 0
V), helps to quantify how different from mean values the ratio becomes.
74 CHAPTER 3. CASE STUDYФ
= 0
.86
Ф
= 0
.90
Ф
= 1
.00
Ф
= 1
.10
fiber Z=0.4 cm Z=1.4 cm
Figure 3.21: Chemiluminescence ratio evolution as a function of pulse amplitude and mixture
ER.
Working conditions: Excitation frequency: 50 Hz.
3.3. EXPERIMENTAL ANALYSIS OF UNSTEADY FLAMES 75
As expected from the analysis of chemiluminescence emission of OH* and CH*, figure
3.21 shows that for optical fiber setup, transition between the amplitude of ∆V = 0.015
V and ∆V = 0.025 V presents an inversion of the ratio evolution shape. Note that this
phenomena is more evident in the leanest conditions, which end up to be the most desire
conditions to work with.
A closer look at the ratio values presented in figure 3.21 shows magnitude of values
variation during the sensing period is significantly higher for low ER setups. In fact,
for the optical fiber configuration, the ratio values of φ = 0.86 and a pulse amplitude
of ∆V = 0.045 V oscillate between a maximum of OH*/CH* = 4.26 and a minimum
of OH*/CH* = 3.16, while values of φ = 1.10 and ∆V = 0.045 V oscillates between a
maximum of OH*/CH* = 2.69 and a minimum of OH*/CH* = 2.13. Note also that the
highest amplitude of oscillation mostly corresponds to the maximum pulse amplitude,
∆V = 0.045 V.
To better understand the ratio variation influence during the oscillation period consider
the work developed Higgins et al. [2001], where a simple flame control system was imple-
mented with average values of OH* chemiluminescence emission working as the feed-
back parameter. A desired ER could be fixed for an airflow variation, and the control
model would compensate the change with fuel flow variations. This simple proportional
feedback control system was based in fitting equations obtained during the study of ER
influence in chemiluminescence measured values.
Suppose the same process is applied to the Bunsen flame, working at φ = 0.86 and pulsed
with an amplitude of ∆V = 0.045 V. For each optical collecting system configuration,
consider there are two possible control models based in the exponential ratio evolutions
presented in table 3.2 for steady (∆V = 0 V) and unsteady (∆V = 0.045 V) conditions.
The OH*/CH* evolution presented in figure 3.22 corresponds to values sensed by the
prototype during the oscillation period, and are the input feedback parameter to each
control models. Based on instant ratio value of the input feedback parameter, the control
model estimates which is that flame’s the ER at that time, as it is graphically represented
in figure 3.22.
76 CHAPTER 3. CASE STUDY
fiber Z=0.4 cm
Z=1.4 cm
)596
.1exp(
97.
13* *
0
φ×−
×=
=∆
VV
mean
mean
CH
OH
)597.1
exp(38.
15* *
045.0
φ×−
×=
=∆
VV
mean
mean
CH
OH
)551.2
exp(27.
31* *
0
φ×−
×=
=∆
VV
mean
mean
CH
OH
)593.2
exp(19
.32
* *045.0
φ×−
×=
=∆
VV
mean
mean
CH
OH
)439
.2exp(
25.
25* *
0
φ×−
×=
=∆
VV
mean
mean
CH
OH
)135.2
exp(17
.19
* *045.0
φ×−
×=
=∆
VV
mean
mean
CH
OH
Figure3.22:C
hemilum
inescenceratio
time
evolutionand
ER
time
evolutionforeach
opticalconfiguration.Exponentialfitequations.
Working
conditions:E
xcitationfrequency:50
Hz,pulse
amplitude
∆V
=0.045
V.
3.3. EXPERIMENTAL ANALYSIS OF UNSTEADY FLAMES 77
The dashed line in the ER evolution graphic represents the real flame ER, φ = 0.86,
and it was included to quantify how different from it the control model assumes that the
Bunsen flame is. As it is possible to see, optical fiber configurations ends up introducing
the highest errors in flame estimation ER: the control model based in the unsteady fit
introduces the highest error, when it considers that for a Bunsen flame working at a
constant ER of φ = 0.86 is overestimated to a maximum of φ = 0.97 and underestimated
to a minimum of φ = 0.80.
From this simulation it is clear that mean chemiluminescence models, working with opti-
cal fiber configurations, are not suitable for high speed unsteady flame control systems.
Chapter 4
Conclusions and Future Work
The aeronautic sector was responsible for around 3% of global CO2 emissions in
2007. It is known that even with the design of more efficient airplanes, engines
are still main responsibles for these values. In search for cleaner engines, the international
research project ACARE [2010] showed that the use of active control lean combustion
systems will lead to a reduction in pollutants emissions. However this control technology
is dependent on the development of new high speed and accurate combustion sensors,
along with a better understanding of lean combustion mechanisms. This thesis presents
the design, construction, optimization and test of a chemiluminescence prototype sensor
for measuring the emission of OH* and CH* species, as it was shown by the research
community (Higgins et al. [2001], Muruganandam et al. [2003], Hardalupas and Orain
[2004] and Guyot et al. [2010]) that it can be used for active control purpose, in the
context of lean combustion technology.
The prototype was designed in order to: (1) collect the radiation from a certain specie
emitting at the flame front, with the aid of a band pass filter and an optical path composed
by an optical fiber and plano-convex lens, (2) convert photons emitted from chemilumi-
nescence into an analogue signal with a photomultiplier sensor, (3) transform the ana-
logue signal to digital information by using an acquisition board and (4) permit chemi-
luminescence time analysis. A propane Bunsen flame was used as a study case to test
79
80 CHAPTER 4. CONCLUSIONS AND FUTURE WORK
the prototype sensing capabilities, due to the possibility to vary its equivalence ratio (φ =
[0.86 1.1]) and cyclically pulse its mixture speed (acoustic perturbations based in a sinu-
soid with an amplitude of ∆V = [0 0.045] V at a fixed frequency of 50Hz) to simulate,
in a controlled form, the influence of flame front deformation in light emission. All to-
gether, the data was processed to obtain the relation OH∗/CH∗ = f (φ, optical collecting
system, flame surface deformation in time) for the presented flame setup, from which it
was found that:
− the light collecting system placement and configuration (global view with the opti-
cal fiber or local view with the optical probe) have a strong influence in the chemi-
luminescence measured values of OH* and CH*;
− for steady flames, the OH∗mean/CH
∗mean = f(φ) relation can be mathematically re-
produced with a exponential fitting equation, different for each light collecting
system configurations;
− for cyclic unsteady pulsed flames:
i) average values of OH∗mean/CH
∗mean = f(φ) evolution are not significantly af-
fected when the flame is acoustically deformed, irrespective of any light col-
lecting system configuration;
ii) the chemiluminescence time analysis of OH∗ and CH∗ shows that increase
of the equivalence ratio and/or the increase of the pulse amplitude lead to an
amplitude increase of the measured OH∗/CH∗(t) ratio ;
iii) the local chemiluminescence time emission, OH∗local(t) and CH∗
local(t), may
be dependent on the flame front deformation;
Using the experimental results of OH∗/CH∗ (t) evolution as input data to a simulated
lean combustion active control mechanism, it was shown that control models based in
mean chemiluminescence measurements, OH∗mean/CH
∗mean = f(φ), may overestimate or
underestimate the flame equivalence ratio during its sensing period. These errors are
81
more significant when using the optical fiber configuration, which is the most commonly
used light collecting setup.
However, it is important to note that some results, such as the local chemilumines-
cence emission, were based in viewing area estimation from high speed pictures post-
processing.
Therefore, it is suggested that future work should rely on the development of more pre-
cise algorithms for image based viewing area estimation, as well as on the design and
test of new light collecting systems, such as Cassegrain lens and new collimating probes,
to promote accurate chemiluminescence measurements. Work in a construction of a new
prototype that simultaneously measures the chemiluminescence emission from two dif-
ferent species, which will allow to sense unsteady random flame deformations, is also
advised. It is expected that with these improvements, testing different excitation ampli-
tudes and frequencies will be possible, to better contribute in the search for new lean
combustion turbine sensors design.
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