Chemiluminescense for flame control Ant³nio Miguel Henriques

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.


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


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


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


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)


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.


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]).


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


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.


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


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.


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.


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.


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

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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


(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.


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


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


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



Figure 2.8: Optical probe lens Transfer Function


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


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


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.


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



(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.


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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28 mm (1-1/8 Inch) Diameter, 9-Stage, Side-On Type

(a) PMT working principle.


R3896



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


LIGHT


GRIDGLASS BULB

ANODE


2nd DYNODE

1st DYNODE













525 µA/lm (Typ.)


30 % (Typ.)14 % (Typ.)

185 nm to 900 nm



(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,


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.


R3896



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


LIGHT


GRIDGLASS BULB

ANODE


2nd DYNODE

1st DYNODE













525 µA/lm (Typ.)


30 % (Typ.)14 % (Typ.)

185 nm to 900 nm



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


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.


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.


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.


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.


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


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


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


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.


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.


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.


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.


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.


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.


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.


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.


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.


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


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.


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.


filtering system Photomultiplier and pre-amplifier

Loudspeaker

Phantom V42


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.


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.


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.


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.


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.


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


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


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.


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


trigger

Figure 3.14: Schematic representation of the experimental rig for a pulsed flame generation and

image record.


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.


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.


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.


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.


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.


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.


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.


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.


Figure 3.20: OH* local intensity for light collecting system z = 0.4 cm. Instant images.


tude: 0.045 V.


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.


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.


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.


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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Chemiluminescense for flame control Ant³nio Miguel Henriques

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