Chemiluminescence analysis of vitiated conditions forMethane and Propane flames
Nelson dos Santos Alves
Thesis to obtain the Master of Science Degree in
Mechanical Engineering
Supervisors: Prof. Edgar Caetano FernandesProf. Teodoro José Pereira Trindade
Examination Committee
Chairperson: Prof. Viriato Sérgio de Almeida SemiãoSupervisor: Prof. Edgar Caetano Fernandes
Member of the Committee: Prof. Patrícia de Carvalho Baptista
November 2016
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To my parents
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Acknowledgments
First and foremost I would like to thank Prof. Edgar C. Fernandes for the opportunity of working together
with him. It has been my pleasure to be his student and I’m sure his teachings will guide me throughout
the years. Without his guidance and support none of this would be possible. You have become like a
mentor to me, thank you.
I would like to extend by gratitude to Prof. Teodoro Trindade who have been always present in each
step of the way. His guidance was invaluable throughout this work. Thank you for your support and
wisdom, you have a special place in my heart.
I also would like to acknowledge the friends I made at IN+, your help and criticism was precious
during the experiments.
My deepest gratitude goes to Andre Marvao, my colleague throughout this past years. His help and
companionship were crucial to get this far. You began as a colleague but you have grown into a friend. I
am deeply honoured to have come to know you.
And finally, I would like to express my appreciation to my family, in particular to my parents who
always supported me in my decisions, I really hope I have made you proud of me. One final note to my
soul mate Sara who was always present when I needed comfort. Words cannot express my gratitude.
To all who make me who I am today, thank you. This is for you.
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Resumo
Nos ultimos anos tem existido um grande interesse no controlo do processo de combustao e nas
emissoes de NOx . Uma das tecnicas usadas para minimizar estas emissoes e a recirculacao de
gases de escape (EGR). A quimiluminescencia emergiu como uma tecnica promissora para controlo
de chamas e embora uma ligacao ja tenha sido estabelecida com o EGR, e necessario investigar a
influencia que algumas das consequencias do uso do EGR trazem a este fenomeno. Esta tese propoe-
se a estudar dois desses efeitos (temperatura e conteudo em CO2) nas emissoes de OH*, CH* e C∗2.
Com esse objectivo, foi projectada uma experiencia para estudar estes efeitos em chamas laminares de
pre-mistura de metano e propano (1000 ≤ Re ≤ 2000 ; 0.80 ≤ φ ≤ 1.30). A influencia da temperatura
e do CO2 e descrita e um modelo empırico e apresentado. Foi descoberto que embora o aumento
de temperatura leve a um aumento das emissoes de OH*, CH* e C∗2 (dependendo do radical pode
chegar a 50 % para ∆ T = 100 K), as variacoes observadas nos quocientes entre estes radicais e nas
fraccoes quimiluminescentes sao de modo geral desprezaveis. Por outro lado, o efeito de CO2 e mais
acentuado com as emissoes de radicais a diminuirem com a adicao de CO2 (ate 80% para o radical C∗2
para xCO2 = 0.30). Foram descobertas variacoes consideraveis nos quocientes entre radicais (ICH∗ /IC∗2
pode chegar a 5 vezes mais do que para a condicao de referencia). Os desvios observados para as
fraccoes quimiluminescentes nao excederam 74%.
Palavras-chave: Quimiluminescencia, Controlo da razao de equivalencia, Pre-aquecimento,
Adicao de CO2
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Abstract
Some of the concern put into combustion processes in the past years is related with combustion mon-
itoring and NOx emissions. One of the techniques used to deal with the later is the recirculation of
exhaust gases (EGR). Flame chemiluminescence emerged as a good technique to flame monitoring
and although a link with EGR was already established, investigations are still necessary regarding the
effects of EGR in flame chemiluminescence. This master thesis proposes the study of two of these
effects (temperature and CO2 content) on the emissions of OH*, CH* and C∗2. An experiment was de-
signed to study this effects on laminar premixed flames of methane and propane (1000 ≤ Re ≤ 2000
; 0.80 ≤ φ ≤ 1.30). The influence of temperature and CO2 content on flame chemiluminescence is
described and an empirical model to evaluate these effects is presented. It was found that although an
increase in temperature leads to an increase in the emissions of OH*, CH* and C∗2 (depending on the
radical can go up to 50 % for ∆ T = 100 K), the variations found for the ratios between this radicals and
the chemiluminescence fractions are in general negligible. On the other hand, the effect of CO2 is more
pronounced with the emissions decreasing with an increase in CO2 content (up to 80% for the radical C∗2
for xCO2 = 0.30). Considerable deviations (ICH∗ /IC∗2
can be 5 times higher than the value for its reference
condition) were found in the ratios between radicals. The deviations found for the chemiluminescence
fractions did not exceed 74%.
Keywords: Flame chemiluminescence, Equivalence ratio monitoring, Preheating, CO2 addition
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Contents
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v
Resumo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii
Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ix
List of Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xiii
List of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xv
Nomenclature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xvii
1 Introduction 1
1.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 State of the art . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.3 Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.4 Thesis Outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2 Experimental Setup 5
2.1 Burner system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2 Premixed control system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.3 Flame spectrum acquisition system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3 Methodology 13
3.1 Flame chemiluminescence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
3.2 Temperature effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
3.3 CO2 effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
4 Results 25
4.1 Morphological differences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
4.2 The effect of temperature in the flame spectrum . . . . . . . . . . . . . . . . . . . . . . . . 27
4.3 Analysis of temperature on chemiluminescence relations . . . . . . . . . . . . . . . . . . 31
4.4 The effect of CO2 in the flame spectrum . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
4.5 Analysis of CO2 on chemiluminescence relations . . . . . . . . . . . . . . . . . . . . . . . 38
4.6 Combined effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
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5 Conclusions 43
5.1 Achievements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
5.2 Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
References 45
A Conditions measured 49
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List of Tables
2.1 Range of gas flow rates used for all the conditions tested . . . . . . . . . . . . . . . . . . 7
2.2 Specifications of the gases used in the experiments. . . . . . . . . . . . . . . . . . . . . . 10
2.3 Spectrometer specification list . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.1 Wavelengths range considered to the spectrum integration. . . . . . . . . . . . . . . . . . 16
3.2 Reactions considered to the formation of the radicals OH*, CH* and C∗2. . . . . . . . . . . 18
4.1 Values to be used with Eq.4.1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
4.2 Coefficients for Eq.4.3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
4.3 Values of β for methane and propane flames . . . . . . . . . . . . . . . . . . . . . . . . . 37
A.1 Conditions measured . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
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List of Figures
2.1 Experimental rig schematic. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2 Internal contours of the burner, designed by Eq. 2.1. . . . . . . . . . . . . . . . . . . . . . 6
2.3 Partial view of the burner and burner in situ . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.4 Specimen of flow controller used to formulate the gas premixture. . . . . . . . . . . . . . . 8
2.5 Spectrometer QE65000 and optical fiber QP400-2-SR-BX. . . . . . . . . . . . . . . . . . 11
3.1 Spectrum from a typical propane and methane flame at φ = 1.3 . . . . . . . . . . . . . . . 14
3.2 Typical spectrum with identification of ICO∗2, Ii and an example of radiation intensity of
neighbor species . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
3.3 Spectrum of a propane flame at φ = 1.3 before and after the subtraction of ICO∗2
. . . . . . 15
3.4 Variation of flame temperature with the increase of xAr for a methane flame at φ = 1.0 . . 17
3.5 Temperature profiles of methane and propane flames at φ = 0.90 . . . . . . . . . . . . . . 18
3.6 Concentration profiles of radical OH for methane and propane flames at φ = 0.90 . . . . . 19
3.7 Concentration profiles of O2 for methane and propane flames at φ = 0.90 . . . . . . . . . 19
3.8 Concentration profiles of radical C for methane and propane flames at φ = 0.90 . . . . . . 20
3.9 Concentration profiles of radical C2H for methane and propane flames at φ = 0.90 . . . . 20
3.10 Concentration profiles of radical CH for methane and propane flames at φ = 0.90 . . . . . 21
3.11 Concentration profiles of radical CH2 for methane and propane flames at φ = 0.90 . . . . 21
3.12 Concentration profiles of radical H for methane and propane flames at φ = 0.90 . . . . . . 22
3.13 Concentration profiles of radical HCO for methane and propane flames at φ = 0.90 . . . . 22
3.14 Concentration profiles of radical O for methane and propane flames at φ = 0.90 . . . . . . 23
3.15 Variation of flame temperature with the increase of xCO2for a methane flame at φ = 1.0 . 24
4.1 Influence of temperature in the flame height for methane flames at φ = 0.90. . . . . . . . 25
4.2 Influence of temperature in the flame height for propane flames at φ = 0.90. . . . . . . . 26
4.3 Influence of CO2 content in the flame height for methane flames at φ = 1.20. . . . . . . . 26
4.4 Influence of CO2 content in the flame height for propane flames at φ = 1.20. . . . . . . . . 26
4.5 Dependence of flame height regarding temperature for a propane flame at φ = 0.9 and
CO2 content for methane flame at φ = 1.2 . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
4.6 Influence of gas preheating and CO2 addition on the flame emission spectrum (CH4/air φ
= 1.30) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
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4.7 Normalized Intensity of OH*, CH*, C∗2 and CO∗
2 radicals emission in C3H8/N2/O2/Ar flames
at φ = 1.10 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
4.8 Dependency of parameter α of Eq.4.2 with φ for both propane and methane flames . . . . 30
4.9 Data validation between experimental and model predictions by Eq. 4.2 for methane and
propane flames . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
4.10 Effect of temperature in the radiation intensity of the radicals OH*, CH* and C∗2 for methane
and propane flames . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
4.11 Ternary diagram of methane and propane and the effect of temperature . . . . . . . . . . 33
4.12 Influence of temperature in the ratio of radicals for methane and propane flames . . . . . 33
4.13 Influence of temperature in the chemiluminescence fraction for methane and propane
flames . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
4.14 Emission intensity of OH*,CH*, C∗2 and CO∗
2 radicals of a propane flame (φ = 1.20) . . . . 35
4.15 Variation of parameter β of Eq.4.5 with the equivalence ratio for propane and methane
flames . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
4.16 Data validation of CO2 effect on flame chemiluminescence facing predictions by Eq.4.5
for methane and propane flames . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
4.17 Effect of CO2 addition in the radiation intensity of the radicals OH*, CH* and C∗2 for
methane and propane flames . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
4.18 Effect of CO2 addition on the ternary diagram for propane and methane flames . . . . . . 40
4.19 Effect of CO2 addition on the ratio of radicals for methane and propane flames . . . . . . 40
4.20 Influence of the addition of CO2 in the chemiluminescence fraction for methane and
propane flames . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
4.21 Data validation of temperature and CO2 combined effects on flame chemiluminescence
facing predictions by Eq.4.6 for methane and propane flames . . . . . . . . . . . . . . . . 42
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Nomenclature
Greek symbols
∆ Difference.
λ Wavelength.
φ Equivalence ratio.
Roman symbols
a, b, c... Fitting constants.
c0 Light velocity.
ei Uncertainty in quantity i.
fi Chemiluminescence fraction of species i.
h Planck constant.
I∗ Radiation intensity from excited species.
Iλ Signal in population units.
Ii Radiation intensity of species i.
IBB Radiation from black body emissions.
ni Quantity of species i.
Qmax Maximum capacity of flow meter.
Sλ Signal in energetic units.
Tad Adiabatic flame temperature.
v Frequency.
xi Fraction of species i.
Qi Flow rate of species i.
� Diameter.
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Chapter 1
Introduction
In the past few years non-intrusive techniques to combustion monitoring such as the use of flame chemi-
luminescence have been developed. This technology allows to monitor combustion characteristics such
as the equivalence ratio in a simple, cheaper way than most traditional technologies.
This chapter is organized as follows. A brief motivation on the investigated topic is presented in
Section 1.1. Section 1.2 provides a review of some of the work done in the past years concerning this
technology. In Section 1.3 the objectives for this master thesis are presented. Section 1.4 ends this
chapter presenting the thesis outline.
1.1 Motivation
Since its first use, combustion has remained throughout the years as the most important controllable
energy source for mankind. Electrical power generation, industry and domestic applications, transporta-
tion, all of them make use of combustion processes to convert chemical energy in thermal energy or
propulsive force. Both methane and propane are widely used in this applications. For instance, the main
fuel for industrial purposes is natural gas which is a blend mainly constituted by CH4 and in smaller
quantities by high-order hydrocarbons such as C2H6. On the other hand, one of the main components
of LPG (liquefied petroleum gas) is propane. The importance of this fuels for everyday applications is
well established. These fuels are also important for research purposes. CH4 is the smallest hydrocar-
bon molecule thus representing the simplest alkane to study. On the other hand, C3H8 has the lightest
weight of the simple hydrocarbons that start to exhibit general features of chain processes. The study of
C3H8 can then give insight to more heavier, complicated molecules.
In the past few years a lot of concern was put in the environmental impact of combustion processes.
This concern lead to an interest in new and advanced technologies to achieve lower pollutants emissions
and higher energetic efficiency.
One of the main concerns of the past years and that remains nowadays is the emissions of NOx .
Some success was achieved with the introduction of techniques like the exhaust or flue gas recirculation
(EGR or FGR). This techniques lead to the preheating of the premixture, changes in O2 concentration
1
and the addition of gases like CO2.
One of the aspects to have in mind to achieve a more efficient combustion is the control of combustion
systems. Conventionally this control rely on monitoring the CO2 and/or O2 content of the exhaust gases,
having inherent time-lag efficiency limitations [1]. Other technologies are available such as the optical
exploitation of the electromagnetic spectrum namely the flame chemiluminescence phenomena [2]. This
techniques are non-intrusive so they may operate away from the extreme combustion conditions. Since
flame chemiluminescence is an inherent process to combustion it also provides instantaneous informa-
tion. A connection between EGR and flame chemiluminescence was already reported in the literature
[3] nevertheless more research has to be done in this field. The purpose of this master thesis is then to
explore the relation that some of the consequences of EGR (preheating and the addition of CO2) have
in the flame chemiluminescence monitor techniques for methane and propane flames.
1.2 State of the art
In the middle of the 20th century a work by Gaydon [4] linked light emission with the concentration of
specific radical species. Several radicals were further studied by Clark [5] who investigated the spectral
emission of the species : OH*, CH*, C∗2 and CO∗
2 on C3H8/air flames. As usual in combustion literature
the superscript (*) denotes an excited chemical species. In his work, he found a relation between the
emission ratio C∗2/CH* and fuel/air ratio being one of the first works relating flame chemiluminescence
to equivalence ratio monitoring. Orain and Hardalupas [6, 7] studied flame chemiluminescence on flat
flames for natural gas and propane flames. It was reported that the emission ratio OH*/CH* decreased
with the equivalence ratio being appointed as the elected parameter to control burning conditions. Other
work [8] reported the same result but for acetylene/oxygen flames. Several studies [9, 10, 5] have also
suggested intensity ratios of two chemiluminescence radicals as robust parameter for equivalence ratio
monitoring. The most used ratios are OH*/CH*, CH*/C∗2 and C∗
2/OH*. The robustness of these ratios
is related to their claimed independence on optical and geometrical system parameters [11], strain rate
[12] and fuel consumption rate [10].
A link between flame chemiluminescence and EGR applications was established by Gupta et al. [3].
Measures were made at different values of EGR using CO∗2 chemiluminescence in natural gas fired re-
ciprocating engines to estimate important engines parameters such as in-cylinder bulk gas temperature
and heat release rate. Since the use of EGR technology leads to differences in the combustion process
it is of importance to investigate the impact of those differences in flames. The effects of preheating
and CO2 dilution in flames has been widely studied. Zhen et al. [13] studied the effects of air preheat
on the combustion and heat transfer characteristics of premixed flames diluted by CO2 and N2. It was
reported that the dilution generally deteriorates flame stability, on the other hand the preheating of the
reactants showed a favorable effect in lean mixtures expanding the stability limit. It was suggested that
the adiabatic flame temperature increases linearly with the initial temperature of reactants.
Some works studied the effects of other conditions in flame chemiluminescence. Higgins et al.
studied the influence of pressure (0.25 − 2.5 MPa), equivalence ratio (0.66 − 0.86 ) and mass flow rate
2
on OH* [14] and CH* [15] chemiluminescence for CH4/air premixed laminar flames. It was reported
that the OH* and CH* chemiluminescence increases with the equivalence ratio and decreases with the
pressure. It was also showed that the equivalence ratio can be deducted knowing the airflow rate and the
pressure. Nori et al. [16] examined CH*, OH* and CO∗2 chemiluminescence in methane and mixtures
of H2/CO. It was found that the ratio CO∗2/OH* is weakly dependent on temperature and fuel dilution
for H2/CO blends while the ratio CH*/OH* is only a weak function of reactants preheating for methane
flames. The effect of local properties on chemiluminescence stoichiometry measurement was studied
by Armingon et al. [17]. It was showed that the ratio OH*/CH* has a non-negligible variation along
turbulent flames which suggested that the local properties of the flame may have an effect on flame
chemiluminescence. The newly found interest in syngas lead to studies regarding the application of
chemiluminescence techniques in flame monitoring for these type of fuels. A recent work by Armingol et
al. [18] studied the effect of fuel composition in flame chemiluminescence for CH4/CO2/H2/CO premixed
flames. It was reported that for high hydrogen content no CH* emission peak could be detected. Thus,
only the emissions from OH* and CO∗2 can be used for monitoring purposes in these kind of fuels. In
particular, the ratio OH*/CO∗2 was found to be a good alternative for equivalence ratio monitoring in a
limited range.
A particular problem of the use of emission ratios for flame monitoring is the necessity of using several
ratios when a wide range of equivalence ratios is used. A recent work [1] defined a new parameter for
flame monitoring. This new parameter, called chemiluminescence fraction, was reported to achieve a
broader application range than the emission ratios.
Flame monitoring assumed a great importance in the past few years due to the concerns about
sustainability and efficiency. Flame chemiluminescence has already show potential to be a dominant
technique in flame monitoring due to its simplicity, robustness and price. However, further investigation
is needed particularly in the effects that may disrupt the parameters that are traditionally used.
1.3 Objectives
From the literature reviewed, one can recognize that flame chemiluminescence was been widely studied
in the past few years. A link between flame chemiluminescence and EGR has already been established.
However, only a few works studied the effects of temperature and CO2 addition in flame chemilumi-
nescence and even those works are generally more related to the describing of the effects than the
quantification of them.
Subsequently, to fill this gap, the objective of this master thesis is the study and quantification of pre-
heating and CO2 addition effects on flame chemiluminescence. In order to do that, special attention was
paid to the parameters used in flame monitoring namely the use of emission ratios and chemilumines-
cence fractions. Besides, a model that allow to describe the effects of preheating and CO2 was pursued.
Additionally, the validity of the use of emission ratios and chemiluminescence fractions regarding these
effects was investigated.
3
1.4 Thesis Outline
This master thesis is organized as follows:
Chapter 2 describes the experimental setup used in the experiments. The equipment used is pre-
sented as well as the conditions in which the measures were made. Several pictures are showed to
ensure clarity.
Chapter 3 describes briefly the methodology followed in this work. Some concepts are explained
and a brief explanation of the chemiluminescence phenomena is presented. Some insight is given to
the techniques used as well as the validation for these techniques.
Chapter 4 is the bulk of this work and consists in the presentation and discussion of the results
obtained in the experiments. The chapter is divided in two main sections: The effect of temperature
and the effect of CO2 in flame spectrum. A photographic documentation of some of the flames used is
shown. A simple model to predict the effects of preheating and CO2 addition is presented. Validation
between the experimental results and the model is also presented. Each part of this chapter ends with
the quantification of the effects studied in the emission ratios and chemiluminescence fractions. A brief
overview in the ternary diagrams of methane and propane flames is also shown.
The thesis ends in Chapter 5 with an overview of the main conclusions as well as some suggestions
on future work.
4
Chapter 2
Experimental Setup
The experimental results presented in this master thesis were obtained from the experimental rig schematic
shown in Figure 2.1. The rig is composed by essentially three parts: the burner system, the premixed
control system and the flame spectrum acquisition system. The burner system comprises not only the
physical equipment necessary to stabilize a flame but also the conditions at which the gaseous premix-
ture arrives to the burner nozzle (flow uniformity, gas temperature, etc). This is described in Section 2.1.
Section 2.2 describes the gas premixture control system which corresponds to the characteristics of
the fuel/air mixture but also to the equipment necessary to control the premixture. The flame spectrum
acquisition system is described in Section 2.3 and comprises not only the spectrometer used and the
optical fiber but also the optical probe positioning.
Figure 2.1: Experimental rig schematic.
2.1 Burner system
The Bunsen burner assembly was designed to ensure a steady and fully developed gas flow on laminar
regime at the nozzle level. The burner is a circular open nozzle of 20 mm in diameter at the exit, with
a high area contraction ratio of Aratio = 25 between the entrance and exit section areas. Its aim is to
uniformly accelerate the flow in order to reduce the flow turbulence and non uniformities. The contraction
of the burner was designed based on the fifth polynomial equation given by [19]:
5
y = (−10ξ3 + 15ξ4 − 6ξ5)(yi − y0) + yi (2.1)
where ξ = z/L and z is the axial burner coordinate and L the length of the nozzle. y is the height at
position z, yi is the height of the contraction wall from the center line at inlet and y0 is the height of the
contraction wall from the center line at outlet. These can be seen in Figure 2.2.
Figure 2.2: Internal contours of the burner, designed by Eq. 2.1.
The burner assembly is entirely made of stainless steel 304 and comprises three sections of identical
length. Between each section there is a distribution plate made of low porosity sintered glass (ROBU �
100 mm×5 mm, ε ≤ 100 µm) with the aim of straightening the flow, breaking the gas radial and angular
velocity profiles. The bottom section has four tangential gas inlets equally distributed throughout the
chamber wall, the interior of which is filled with Raschig rings (� 5 mm) acting as a jet breaking and
spreading evenly the flow. The intermediate section is an open cylindrical chamber used to equalize
gas pressure all over the second sintered plate area. The outer section is the contraction zone, which is
responsible for the velocity equalization at the burner exit. Figure 2.3 show a partial view of the burner
and the burner in situ respectively.
Experiments were conducted controlling: fuel type (CH4/C3H8), premixture composition (N2/O2/Ar/CO2)
and equivalence ratio (φ). The tested fuel power ranged from 0.75 kW to 1.60 kW while the range of
Reynolds number was 1000 ≤ Re ≤ 2000. Due to limitations of the burner stability range it was not
possible to fix the fuel power for each fuel type thus, for each fuel, two fuel powers were used. One for
the leanest mixture and one to all other mixtures. The unburned gas mixture was formulated as com-
binations of CH4/C3H8/O2/N2/Ar/CO2 depending on the experiment with methane and propane altering.
The air was formulated in a volumetric base as 21 % O2 and 79 % N2. This mixture was used instead of
normal atmospheric air since the methodology used in this work required the control of the composition
of the inert gas (more details can be found in Chapter 3). The equivalence ratio was varied between
6
(a) View of the burner (b) Burner in situ
Figure 2.3: Partial view of the burner (a) and burner in situ (b).
φ = 0.8 and 1.3 with increments of 0.1. When CO2 was involved, the tests began at φ = 0.90. Data
repeatability was ensured by a set of measurements made in different days (typically half of the amount
of conditions measured), achieving a 5.7% of maximum signal span. Table A.1 presents the range of
conditions and the number of measurements (#). More details can be found in Appendix A.
Table 2.1: Range of gas flow rates in SLPM (Standard liters per minute, 298 K, 101.3 kPa) used for allthe conditions tested.
Fuel φ Power(kW) # Qfuel QN2QO2
QAr QCO2
0.80 0.75 10 1.234 [11.65 - 7.15] 3.095 [0 - 4.50] 00.90 1.25 11 2.056 [17.25 - 12.25] 4.586 [0 - 5.00] 01.00 1.25 8 2.056 [15.52 - 12.22] 4.127 [0 - 3.30] 01.10 1.25 9 2.056 [14.12 - 10.12] 3.753 [0 - 4.00] 01.20 1.25 10 2.056 [12.94 - 8.44] 3.440 [0 - 4.50] 0
CH4 1.30 1.25 10 2.056 [11.95 - 7.45] 3.175 [0 - 4.50] 0
0.90 1.25 11 2.056 [17.25 - 12.51] 4.586 [0 - 3.015] [0 - 1.725]1.00 1.25 11 2.056 [15.52 - 6.47] 4.127 [0 - 5.987] [0 - 3.074]1.10 1.25 9 2.056 [14.12 - 4.69] 3.753 [0 - 6.000] [0 - 3.431]1.20 1.25 11 2.056 [12.90 - 3.16] 3.440 [0 - 5.960] [0 - 3.818]1.30 1.25 12 2.056 [11.95 - 2.03] 3.175 [0 - 5.992] [0 - 3.918]
1.30 1.25 4 2.056 [4.5 - 1.5] 3.175 [4.581-7.581] 2.867
0.80 1.00 11 0.650 [15.57 - 10.57] 4.140 [0 - 5.00] 00.90 1.60 11 1.060 [22.52 - 17.52] 5.980 [0 - 5.00] 01.00 1.60 11 1.060 [20.27 - 15.27] 5.380 [0 - 5.00] 01.10 1.60 11 1.060 [18.42 - 13.42] 4.900 [0 - 5.00] 01.20 1.60 9 1.060 [16.89 - 12.89] 4.490 [0 - 4.00] 0
C3H8 1.30 1.60 7 1.060 [15.59 - 12.59] 4.140 [0 - 3.00] 0
0.90 1.60 8 1.060 [22.52 - 18.04] 5.980 [0 - 2.883] [0 - 1.575]1.00 1.60 7 1.060 [20.27 - 12.83] 5.380 [0 - 4.993] [0 - 2.419]1.10 1.60 9 1.060 [18.42 - 9.94] 4.900 [0 - 5.541] [0 - 2.949]1.20 1.60 11 1.060 [16.89 - 7.98] 4.490 [0 - 5.537] [0 - 3.378]1.30 1.60 11 1.060 [15.59 - 11.42] 4.140 [0 - 2.596] [0 - 1.557]
1.20 1.60 4 1.060 [12.37 - 9.37] 4.490 [2.829-5.829] 1.689
7
2.2 Premixed control system
The burner system requires the use of a gas flow control system to ensure stationary conditions in the
flame. Precision gas flow controllers (Alicat Scientific, Series 16) of maximum capacity of 20, 5 and
1 SLPM were used. In Figure 2.4 an example of one of this controllers can be seen. The controllers
were operated with FlowVision software package which enables not only the monitoring of the gas flow
rate but also the temperature and pressure of it.
Figure 2.4: Specimen of flow controller used to formulate the gas premixture.
The uncertainty on gas flow rate eQi , which is indicated by the manufacturer, is related with the actual
flow rate Qi (L/min) and the maximum device capacity Qmax by the following equation:
eQi = 0.008Qi + 0.002Qmax (2.2)
where the subscript i indicates the type of gas used. Since the equivalence ratio is a very important
parameter in this work, the evaluation of its uncertainty is crucial.
The equivalence ratio φ is defined as the ratio between quantities of fuel nfuel and air nair at actual
and stoichiometric proportions as expressed by Eq. 2.3.
φ =(nfuel/nair)
(nfuel/nair)st(2.3)
When the amount of air is the necessary for, theoretically, burn all the fuel it is said that the mixture
is stoichiometric and the value of φ is unitary. For values higher than one it is said that the mixture is a
rich mixture. On the other hand, values lower than one indicate the presence of a lean mixture.
In practical terms, the equivalence ratio is a function of proportions between flow rates of gases in
the mixture.
8
φ = f(Qfuel, QO2 , QN2 , ..., Qn) (2.4)
The uncertainty in the value of the equivalence ratio can then be derived by:
e2φ = (
dφ
dQfuel)2e2
fuel + (dφ
dQO2
)2e2O2
+ (dφ
dQN2
)2e2N2
+ ...+ (dφ
dQn)2e2
n (2.5)
where the subscript n represents the amount of other species considered in the formulation of the
air. The value of the uncertainties for the gases measured by flowmeters can be obtained by Eq. 2.2.
Only the analysis of the derivative terms is left to do. Assuming the same value for the densities at actual
and stoichiometric conditions, Eq. 2.3 can be written in the following way:
φ =Qfuel/Qair
(Qfuel/Qair)st(2.6)
The value of Qfuel/Qair for stoichiometric conditions is a constant Ki that depend on the fuel used
(0.1050 for methane and 0.0420 for propane). Expanding the flow rate of air one can obtain:
φ =1
Ki
QfuelQO2 +QN2 + ...+Qn
(2.7)
Two derivatives can then be obtained, one for the fuel and the other for the various species that
formulate the air (N2, O2, Ar, CO2, ...). The equations are given below:
dφ
dQfuel=
1
Ki
1
QN2+QO2
+ ...+Qn(2.8)
dφ
dQi= −Qfuel
Ki
1
(QN2 +QO2 + ...+Qn)2(2.9)
The value for the uncertainty in φ can then be obtained combining the previous equations. It follows
then:
eφ =
√(
1
Ki
1
QN2+QO2
+ ...+Qn)2e2
fuel + ...+ (−QfuelKi
1
(QN2+QO2
+ ...+Qn)2)2e2
n (2.10)
As an example, for a methane flame, φ = 0.8 , QCH4= 1.234 , QN2
= 11.65 and QO2= 3.095 SLPM
and the appropriated flowmeters one can obtain eφ = 0.0149. Considering all the experimentally tested
conditions, the uncertainty in the value of the equivalence ratio didn’t exceed 2.5 %.
All the gases used were bottled (Air Liquide). Their specifications can be found on Table 2.2
9
Table 2.2: Specifications of the gases used in the experiments.Gas Code Molecular weight (g/mol) Purity(%)
CH4 UN 1971 16.04 ≥ 99.995C3H8 UN 1978 44.10 ≥ 99.95
N2 UN 1066 28.01 ≥ 99.8O2 UN 1072 32.00 ≥ 99.5Ar UN 1006 39.95 ≥ 99.999
CO2 UN 1013 44.01 ≥ 99.7
2.3 Flame spectrum acquisition system
The flame spectrum acquisition system is comprised of a optical fiber and a spectrometer.
The optical fiber (Ocean Optics, QP400-2-SR-BX) is made in fused silica, has a length of 2 m and a
core diameter of 400 µm. The fiber yields an average acceptance angle of 25.4◦ and produces a conical
field of view. In order to have the flame the most possible inside the field of view having at the same time
a signal with low noise a compromise in its position was made. The fiber was placed radially at 20 cm of
the burner axis and at a height of 17 cm above the burner exit.
The optical fiber was connected to a high-sensitivity spectrometer by an entrance slit of 100 µm wide.
A summary of the specifications of the spectrometer (Ocean Optics, QE65000) is presented in Table
2.3.
Table 2.3: Spectrometer specification listFeature Spectrometer
Manufacturer Ocean OpticsBase part number QE65000 Pro
Detector Hamamatsu S7031-1006Array type Matrix CCD array
Pixels 1024×58Grating HC1 composite 300 lines/mm
Wavelength range 200-1100 nmEntrance slit 100 µm
Optical resolution < 3 nm
During the experiments the spectrometer was controlled by Ocean Optics SpectraSuit software. The
average flame signal of each test was acquired for 100 exposures and an integration time of 1 second
was selected in order to obtain an high signal to noise ratio. Electronic dark noise was removed from
every spectrum aquired. The average and standard deviation of each exposure was computed using
a Matlab code. The background signal (signal obtained in the absence of flame) was subtracted from
the average flame signal. The result corresponds to the spectral signal of the measuring condition. The
units of this signal are µJ/s/nm/cm2. However, since the photons have different energy depending on the
wavelength, during the course of this thesis the results will be usually presented in photons/s/nm/cm2
since there is a direct correspondence between the number of photons measured and the number of
radicals that are emitters. Denoting the first signal by Sλ and the second by Iλ the conversion can be
10
made by the following equation:
Iλ =λ
hc0Sλ (2.11)
where h is the Planck Constant, c0 the light velocity and λ the radiation wavelength. Figure 2.5 shows
the acquisition system comprised of the optical fiber and spectrometer.
Figure 2.5: Spectrometer QE65000 and optical fiber QP400-2-SR-BX.
11
12
Chapter 3
Methodology
The objective of this work is to quantify the flame chemiluminescence response to the increase in tem-
perature and to the addition of CO2. The methodology followed will be explained in this Chapter. Section
3.1 presents a brief overview of the chemiluminescence phenomena as well as the steps necessary to
access the desired information. Sections 3.2 and 3.3 describe the details in these experiments.
3.1 Flame chemiluminescence
Flame chemiluminescence is related to the emission of electronically excited species produced by chem-
ical reactions such as A + B C + D*. Species D* may then be destroyed by spontaneous emission (D*
→ D + hv) or collisional quenching (D* + M→ D + M) [11]. The second reaction is the one responsible
by the mission of light thus being the one important to this work. Flame monitoring using flame chemi-
luminescence is concerned with the behavior of several species that may emit light through the reaction
described before. In flame monitoring the species that are usually used are OH*, CH*, C∗2 and CO∗
2 [18].
The information that these species provide may be accessed through the flame spectrum. Figure 3.1
shows an example of a typical spectrum for both methane and propane flames.
The spectra present distinct features which is a indication that flame spectra may be used to access
information regarding the combustion process. The steps necessary to acquire a flame spectrum were
presented in Chapter 2. As one can see, the spectrum present three major peaks. The peaks corre-
spond to the emission by the radicals OH* (309 nm), CH* (430 nm) and C∗2 (515 nm). To each radical
corresponds a band system which consists in the range of wavelengths where the emission of light is
performed. All major chemiluminescence species for flame monitoring are located in the range from
225 nm to 575 nm thus the analysis will only focus on this wavelength range. Lower wavelengths are
affected by molecular oxygen absorption while in higher wavelengths the black body emissions can’t be
neglected [1]. Consider Figure 3.2 where an example of a typical spectrum is shown.
The narrow band radical emissions are superimposed to a wide band continuum (dashed line in
Figure 3.2). This emission is attributed to CO∗2 emitters [20]. This continuum emission was reported [1]
to extend roughly from 250 nm to 650 nm thus being important in the wavelength range considered. A
13
Figure 3.1: Spectrum from a typical propane (dashed line) and methane flame (solid line) at φ = 1.3.
Figure 3.2: Typical spectrum with identification of ICO∗2
(dashed line), Ii (gray area) and an example ofradiation intensity of neighbor species (forward slash).
raw spectral intensity I (photons/s/nm/cm2) can then be seen as roughly the sum of three contributions:
I = I∗ + ICO∗2
+ IBB (3.1)
where I∗ is the radiation intensity from excited radicals (not only OH*, CH* and C∗2 but all excited
14
species that emit light in the wavelength considered), ICO∗2
the radiation intensity from excited CO2*
molecules and IBB is the radiation from black body emissions. Since the latter, described by Planck’s
law, has a growing effect towards longer wavelengths particularly for the infrared region and the analysis
performed was limited at 575 nm this radiation is neglected, thus:
I ≈ I∗ + ICO∗2
(3.2)
ICO∗2
can be outlined using the equation [21]:
ICO∗2
= ξ. exp[− exp(−λ+ λ0
w)− (
λ− λ0
w) + 1] (3.3)
where ξ a scaling factor, λ the wavelength, λ0 the band head wavelength and w and extent parameter
related to the wide band length.
I∗ can then be accessed subtracting the CO∗2 contribution from the raw spectral signal. Figure 3.3
shows the differences in the flame spectrum before and after this subtraction was carried out.
Figure 3.3: Spectrum of a propane flame at φ = 1.3 before (solid line) and after (dashed line) thesubtraction of ICO∗
2.
The narrow bands chemiluminescence can be accessed by the spectrum integration over a certain
wavelength range. Table 3.1 shows the wavelength intervals used in this work. To access the radiation
intensity of the radicals (Ii), besides the subtraction of the CO∗2 wide band one has also to take into
account the radiation intensity of neighbor species (forward slash in Figure 3.2). Both constitute what is
called background contamination. To estimate the CO∗2 radiation Eq. 3.3 can be used. To estimate the
remaining of the background contamination a smooth line connecting the upper and lower limits of the
interval used is required (see Figure 3.2). Ii (gray area in Figure 3.2) can then be obtained subtracting
the neighbor radiation from I∗ in a certain wavelength range. For more details in this method, the reader
15
can see other works [22, 23].
Table 3.1: Wavelengths range considered to the spectrum integration.Band Wavelength range [nm]
OH* [275 300] , [300-335]
CH* [375-405] , [410-445]
C∗2 [455-480] , [490-525] , [525-570]
It is important to note that several of the results shown in this work are normalized by a value which
corresponds to the ”reference condition”. These conditions are explained when needed. This works
focus on relative changes instead of absolute ones in order to minimize the effect that the experimental
setup can have in the measures.
Since all the flames studied are conical laminar flames a good representation of each flame is given
by its height. In order to evaluate that, a Matlab program was created. The program computed the
height of the flame in each picture taken. Since several pictures of the same condition were taken, the
characteristic height of the flame is given by the average of the heights.
3.2 Temperature effect
In order to increase the temperature of the flame, argon was added to the gas premixture. Since the
specific heat capacity of argon is lower than the one of nitrogen, the same energy release by fuel com-
bustion will result in an higher flame temperature. Argon was injected and nitrogen withdrawn in the
same amount to ensure an equivalent air flow pattern to the flame and the same fuel-oxidizer concen-
tration. The relation between the argon concentration and the increase in adiabatic flame temperature
is described by a linear relation of the type:
∆Tad = kxAr (3.4)
where ∆Tad is the increase in flame temperature (K), xAr is a measure of Ar content in the N2/Ar
mixture (expressed in mol fraction) and k is a proportionality constant (K/mol fraction). An example of
the effect of argon addition can be seen in Figure 3.4.
Therefore, the higher the value of xAr in the mixture (being xAr = 0 the original mixture with 21 %
O2 and 79 % N2) the higher the flame temperature. The adiabatic flame temperatures were obtained
by numerical simulations using the Cantera solver package under adiabatic and equilibrium conditions
[24]. The GRI-Mech 3.0 detailed kinetics was used in the calculations, which consist of 325 elementary
chemical reactions with associated reaction rate coefficients and thermochemical parameters for the
53 species involved [25]. The numerical results obtained for CH4/air and C3H8/air flames are in close
agreements with published data [26]. A majorant of the uncertainty for the temperature was obtained. It
was considered the maximum and the minimum value of xAr. The maximum and the minimum values
for ∆T were then computed using Eq. 3.4. The maximum and minimum value of xAr are given by the
16
Figure 3.4: Variation of flame temperature with the increase of xAr for a methane flame at φ = 1.0obtained by numerical simulations using GRI-Mech 3.0.
following equations:
xAr,max =QAr + eAr
QN2,ini − eN2,ini(3.5)
xAr,min =QAr − eAr
QN2,ini + eN2,ini(3.6)
where Qi is the actual flow rate of species i, ei is the uncertainty in the flow rate of species i and the
subscript ini means the ”reference condition” that is the mixture with xAr = 0.
Despite being an inert gas, it is of relevance to ensure that the flame temperature alteration caused by
the substitution of nitrogen by argon leads to the same effect that of a preheating of the gas mixture. To
that analysis, concentration profiles of related chemical species throughout the flame front were used for
comparison. Investigations [1, 27, 28] reported that OH, O, CH, H, HCO, O2, C2H, C and CH2 among
others, are key species in the combustion process and also in the formation of chemiluminescence
species. The reactions involving these species are shown in Table 3.2.
The species profile were obtained through kinetics simulations in a one-dimensional flame of Can-
tera’s burner-stabilized code running over GRI-Mech 3.0. Two flames were tested: one with the air
mixture formulated as O2/N2/Ar and other as O2/N2 but with an higher initial temperature. The increase
in the initial temperature corresponds to the necessary temperature to ensure that the flame temperature
is equivalent to the one that its observed when argon is added to the mixture. Figure 3.5 show the tem-
perature profiles for both methane and propane flames while Figures 3.6–3.14 show the concentration
profiles of previously mentioned radicals for the same flames.
17
Table 3.2: Reactions considered to the formation of the radicals OH*, CH* and C∗2.
Number Reaction Ref
R1 O + H + M OH* + M [28]R2 CH + O2 OH* + CO [28]R3 HCO + O OH* + CO [1]
R4 C2 + OH CH* + CO [27]R5 C2H + O CH* + CO [27]R6 C2H + O2 CH* + CO2 [27]R7 C + H + M CH* + M [27]
R8 CH2 + C C∗2 + H2 [1]
(a) Methane (b) Propane
Figure 3.5: Temperature profiles of methane (a) and propane (b) flames at φ = 0.90 obtained throughGRI-Mech 3.0 simulations. Solid lines correspond to a mixture of Fuel/N2/O2 while dashed lines corre-sponds to Fuel/N2/O2/Ar.
18
(a) Methane (b) Propane
Figure 3.6: Concentration profiles of radical OH for methane (a) and propane (b) flames at φ = 0.90obtained through GRI-Mech 3.0 simulations. Solid lines correspond to a mixture of Fuel/N2/O2 whiledashed lines corresponds to Fuel/N2/O2/Ar.
(a) Methane (b) Propane
Figure 3.7: Concentration profiles of O2 for methane (a) and propane (b) flames at φ = 0.90 obtainedthrough GRI-Mech 3.0 simulations. Solid lines correspond to a mixture of Fuel/N2/O2 while dashed linescorresponds to Fuel/N2/O2/Ar.
19
(a) Methane (b) Propane
Figure 3.8: Concentration profiles of radical C for methane (a) and propane (b) flames at φ = 0.90obtained through GRI-Mech 3.0 simulations. Solid lines correspond to a mixture of Fuel/N2/O2 whiledashed lines corresponds to Fuel/N2/O2/Ar.
(a) Methane (b) Propane
Figure 3.9: Concentration profiles of radical C2H for methane (a) and propane (b) flames at φ = 0.90obtained through GRI-Mech 3.0 simulations. Solid lines correspond to a mixture of Fuel/N2/O2 whiledashed lines corresponds to Fuel/N2/O2/Ar.
20
(a) Methane (b) Propane
Figure 3.10: Concentration profiles of radical CH for methane (a) and propane (b) flames at φ = 0.90obtained through GRI-Mech 3.0 simulations. Solid lines correspond to a mixture of Fuel/N2/O2 whiledashed lines corresponds to Fuel/N2/O2/Ar.
(a) Methane (b) Propane
Figure 3.11: Concentration profiles of radical CH2 for methane (a) and propane (b) flames at φ = 0.90obtained through GRI-Mech 3.0 simulations. Solid lines correspond to a mixture of Fuel/N2/O2 whiledashed lines corresponds to Fuel/N2/O2/Ar.
21
(a) Methane (b) Propane
Figure 3.12: Concentration profiles of radical H for methane (a) and propane (b) flames at φ = 0.90obtained through GRI-Mech 3.0 simulations. Solid lines correspond to a mixture of Fuel/N2/O2 whiledashed lines corresponds to Fuel/N2/O2/Ar.
(a) Methane (b) Propane
Figure 3.13: Concentration profiles of radical HCO for methane (a) and propane (b) flames at φ = 0.90obtained through GRI-Mech 3.0 simulations. Solid lines correspond to a mixture of Fuel/N2/O2 whiledashed lines corresponds to Fuel/N2/O2/Ar.
22
(a) Methane (b) Propane
Figure 3.14: Concentration profiles of radical O for methane (a) and propane (b) flames at φ = 0.90obtained through GRI-Mech 3.0 simulations. Solid lines correspond to a mixture of Fuel/N2/O2 whiledashed lines corresponds to Fuel/N2/O2/Ar.
The flame temperature profile is roughly the same between the preheated and the Ar addition situ-
ation. Despite the different initial temperature, the flame temperature assumes the same value in both
cases, for methane and for propane flames. Regarding the concentration profiles, the concentration
integral through the flame was obtained. The deviations obtained (εi) were: εOH = 1.7 %, εO = 5.2 %,
εCH = 11.3 %, εH = 7.9 %, εHCO = 9.1 %, εO2= 0.2 %, εC2H = 19.1 %, εC = 15.2 % and εCH2
= 8.8 %
for methane flames. The values obtained for propane flames were: εOH = 1.9 %, εO = 4.5 %, εCH =
10.7 %10.7%, εH = 7.0 %, εHCO = 7.0 %, εO2 = 0.7 %, εC2H = 12.7 %, εC = 13.9 % and εCH2 = 8.2 %.
This may be attributed to the differences in the methods followed. The Ar addition situation requires
the substitution of N2 for Ar, however this is a volumetric operation, the differences in molecular weight
observed between N2 and Ar may help to explain the small deviations observed in the profiles. Other
possible explanation may reside in the differences at the preheating zone of the flame. Both cases as-
sume the same flame temperature, however due to the preheating observed in one of the flames, the
thermal gradients are different. It should be noted that the value of the preheating doesn’t correspond
to the same increase in flame temperature altough both are related by a linear relation [13, 29]. Never-
theless, the deviations found for all the radicals were very small. Since the concentration of species like
OH, O, H and CH that play a role as precursors of the excited radicals exhibit changes of few percent
stands to reason that in terms of flame chemiluminescence it is equivalent the addition of argon to the
mixture or making a preheating.
3.3 CO2 effect
The CO2 effect over flame chemiluminescence was studied in a similar way of the temperature effect
(replacing in equal amount the N2 by CO2). However, in this case there are two distinct effects. Besides
23
the alterations induced in the combustion kinetics, the addition of CO2 to the unburned gas mixture leads
to a decrease in the flame temperature following the relation:
∆Tad = ax2CO2
+ bxCO2(3.7)
where ∆Tad is the variation in the adiabatic temperature (K) above normal conditions, a (K/mol fraction2)
and b (K/mol fraction) are functions of the equivalence ratio and xCO2is a measure of the CO2 concen-
tration in the premixture and is defined as the amount of CO2 that replaces N2. An example of the
decrease in temperature with the addition of CO2 can be seen in Figure 3.15.
Figure 3.15: Variation of flame temperature with the increase of xCO2 for a methane flame at φ = 1.0obtained by numerical simulations using GRI-Mech 3.0.
A majorant of the uncertainty for xCO2 was obtained. Similarly with the temperature effect it was
considered the maximum and the minimum value of xCO2 . The maximum and minimum value of xCO2
are given by the following equations:
xCO2,max =QCO2
+ eCO2
QN2,ini − eN2,ini(3.8)
xCO2,min =QCO2
− eCO2
QN2,ini + eN2,ini(3.9)
where Qi is the actual flow rate of species i, ei is the uncertainty in the flow rate of species i and the
subscript ini means the ”reference condition” that is the mixture with xCO2 = 0.
In order to isolate the effect of CO2 over combustion kinetics it was necessary to correct the flame
temperature by the addition of argon. This was possible due to the addition of argon and the corre-
sponding withdrawn of nitrogen (see Eq. 3.4).
24
Chapter 4
Results
Previous chapters presented the details of the experimental rig and the methodology followed in order
to study the effects previously mentioned. The results obtained will be presented in this Chapter. The
Chapter is organized as follow: Section 4.1 presents the morphological differences observed in the
flame when its temperature is increased and when the CO2 concentration is increased. Several pictures
are presented for clarity. An empirical expression relating temperature and CO2 concentration with the
height of the flame is presented. The effect of temperature in the flame spectrum is presented in Section
4.2. The main results are presented and an empirical model is showed. The agreement between the
model and the experimental data is investigated. The effects of temperature in the chemiluminescence
relations are presented in Section 4.3. Sections 4.4 and 4.5 are very similar with Sections 4.2 and 4.3,
being organized in the same way with the difference that the effect studied is not the temperature but the
CO2 concentration. This chapter ends with Section 4.6 where the models presented in previous sections
are combined and its agreement with experimental data regarding the combined effects of temperature
and CO2 concentration is investigated.
4.1 Morphological differences
The increase in temperature and the addition of CO2 lead to several differences in the flame spectrum
but also changes in the aspect of the flame. To quantify these changes, several pictures of different
flames were taken. Figures 4.1– 4.4 show some of these differences.
Figure 4.1: Influence of temperature in the flame height for methane flames at φ = 0.90.
25
Figure 4.2: Influence of temperature in the flame height for propane flames at φ = 0.90.
Figure 4.3: Influence of CO2 content in the flame height for methane flames at φ = 1.20.
Figure 4.4: Influence of CO2 content in the flame height for propane flames at φ = 1.20.
It can be seen that an increase in temperature leads to a decrease in flame height which is related
with the increase in the flame speed. Another effect observed is the increase in the luminosity emitted
by the flame. On the other hand, the increase in CO2 content lead to an increase in flame height which
can be explained by the decrease in the flame speed. It is also possible to see that when the content
of CO2 is increased, the flame gradually changes its shade of blue. An example of the dependence
between the flame height and the effects studied in this work is shown in Figure 4.5.
26
(a) Temperature effect (b) CO2 effect
Figure 4.5: Dependence of flame height regarding temperature (a) for a propane flame at φ = 0.9 andCO2 content (b) for methane flame at φ = 1.2.
The influence of temperature and CO2 in the flame height can be described by the generic linear
equation:
h = mx+ b (4.1)
where h is the flame height in milimeters, m and b are fitting parameters and x can be the value
of the increment in temperature (K) or the mol fraction of CO2 in the premixture. Table 4.1 shows the
parameters for the set of flames studied in this work.
As can be seen, the slope is not the same for all conditions and varies with the equivalence ratio. The
value of b represent the height of the flame at the reference condition of ∆T = 0 and xCO2 = 0. Although
the flame height is supposed to be constant for a given condition, in practice the flame oscillates a
bit which can be attributed to resonance in the burner. Since the flame height was computed through
the analysis of pictures, the value obtained depend of the position that the flame was occupying in the
moment the picture was taken. Thus, the values for the correlations presented should not be seen as
absolute values but as characteristic ones.
4.2 The effect of temperature in the flame spectrum
Figure 4.6b) shows an example of the emission spectrum of a typical CH4/O2/N2/ CO2/Ar flame as well
as the radicals more used in flame chemiluminescence monitoring [18].
In Figure 4.6c) one can see the effect of temperature on the flame chemiluminescence (it should be
noted that the contribution from CO∗2 is already subtracted). The temperature has an upward effect in
the flame emission. However, a distinct behavior has been noticed by the chemiluminescence radicals.
27
Table 4.1: Values to be used with Eq.4.1.Effect Fuel φ m b
0.8 -0.071 29.30.9 -0.072 35.9
CH4 1.0 -0.080 30.51.1 -0.069 28.71.2 -0.077 32.0
Temperature 1.3 -0.106 41.2
0.8 -0.047 30.80.9 -0.066 39.2
C3H8 1.0 -0.056 32.91.1 -0.056 31.21.2 -0.062 30.71.3 -0.071 35.5
0.9 0.738 35.91.0 0.419 30.5
CH4 1.1 0.422 28.71.2 0.360 32.0
CO2 1.3 0.249 41.2
0.9 0.884 39.21.0 0.692 32.9
C3H8 1.1 0.366 31.21.2 0.301 30.71.3 0.274 35.5
Figure 4.6: Influence of gas preheating and CO2 addition on the flame emission spectrum (CH4/air φ =1.30). The ∆T is the increase above normal flame temperature and xCO2 is the fraction of N2 substitutedby CO2.
28
These behavior can be seen in Figure 4.7 that shows the emission intensity of radicals OH*, CH* and
C∗2 as well as the radical CO∗
2 normalized by Ii at ∆T = 0 K. It should be noted that the uncertainty
associated with the radiation intensity is very small in all measures thus not being showed since its
representation in size is similar to the symbols presented in the figures.
Figure 4.7: Normalized Intensity of OH*, CH*, C∗2 and CO∗
2 radicals emission in C3H8/N2/O2/Ar flamesat φ = 1.10. ∆T corresponds to the increase in the flame temperature when compared with the corre-spondent temperature of a C3H8/N2/O2 flame.
Identical behavior occurs for both methane and propane flames along the tested equivalence ratios
following for the radicals the empirical expression:
IiIi,0
= eαi∆T (4.2)
in which Ii is the emission intensity of the radical i, Ii,0 is the reference value measured at ∆T = 0 K,
αi is a characteristic constant (K−1) and ∆T is the increase in the flame temperature (K). This model can
be used to describe the emissions of the radicals OH*, CH*, C∗2 and even CO∗
2 when there is an increase
in flame temperature. The characteristic constant αi can be seen as a measure of the proportionality
between an increment in temperature and the response in chemiluminescence radiation and translates
in a single value how the chemiluminescence phenomena is affected by the temperature. Figure 4.8
shows the variation of the characteristic constant α with the equivalence ratio for methane and propane
flames. For clarity, the error bar was omitted in the figures.
The effect of temperature is similar in the radicals CH* and C∗2 with the effect being maximum near φ
= 0.90 while for the radical OH* the effect is maximum near stoichiometry. The radical CO∗2 presents a
maximum near φ = 1.0 for methane flames and φ = 1.10 for propane flames. This was expected since
an increase in temperature leads to an increase in the reaction rate of the radicals [17, 1]. The radical
29
Figure 4.8: Dependency of parameter α of Eq.4.2 with φ for both propane (white symbols) and methaneflames (black symbols).
OH* seems to be more sensible to temperature than the others since the value of its characteristic
constant has a greater variation along the range of equivalence ratios studied. There seems to be little
difference in the behavior of the radicals between methane and propane flames. Since the focus of this
work is the chemiluminescence of OH*, CH* and C∗2 no further development will be made concerning
the CO∗2 emissions. An adjust of the values of α with the equivalence ratio was made. The relation can
be described by the following equation.
α = exp(a+ bφ+ cφ2 + dφ3) (4.3)
In which a, b, c and d are empirical coefficients which were determined (see Table 4.2) for both
methane and propane flames but also for the three radicals considered in this work.
Table 4.2: Coefficients for Eq.4.3 with α expressed in K−1.Fuel Radical a b c d R2
OH* -55.0319 126.6722 -105.6683 28.5915 0.9983CH4 CH* -48.2879 126.1876 -123.3160 39.3663 0.9522
C∗2 -138.2150 383.5241 -364.0750 112.7917 0.9997
OH* -42.7255 88.3156 -66.7524 15.7561 0.9855C3H8 CH* -38.3690 94.4864 -90.1900 28.1390 0.8317
C∗2 -28.1360 63.6768 -59.8683 17.9765 0.8017
The behavior of IOH∗, ICH∗ and IC∗2
when there is a preheating of the mixture can be predicted using
a simple model combining Eqs. 4.2 and 4.3. Figure 4.9 shows the agreement between the experimental
data (all equivalence ratios and both fuels) and the values predicted by this simple model.
The agreement is quite good for both methane and propane flames flames which shows that the
30
Figure 4.9: Data validation between experimental and model predictions by Eq. 4.2 for methane (blacksymbols) and propane (white symbols) flames ( OH*, CH*, C2*)
model proposed by Eqs. 4.2 and 4.3 can be used with good results for both propane and methane
flames in the range of conditions studied in this work.
Figure 4.10 summarize the effect of preheating in the radiation intensity of the radicals considered.
As one can see, the increase in temperature leads to a shift in the base curve (∆T = 0) for all radicals.
The intensity increase rate due to the temperature is not the same for all radicals. IOH∗ presents a
maximum for φ around stoichiometry for both methane and propane flames. ICH∗ presents a maximum
beyond φ = 1.3 for methane flames while propane has his maximum around 1.2. For IC∗2
there is no
data in this work for equivalence ratios higher than 1.3, however a previous work [1] suggested that its
maximum is around φ = 1.3 for methane flames and φ = 1.4 for propane flames. The data shows that the
radical OH* is the most sensible to temperature followed by the radical CH*. Propane seems also to be
more sensible to temperature than methane. In fact, for a typical equivalence ratio φ = 1.0 it was found
that for methane flames ∆IOH∗/∆T is around 1.1× 108 while ∆ICH∗/∆T is around 0.048× 108 and
∆IC∗2/∆T = 0.024× 108 photons/s/cm2/K. On the other hand, for propane flames the values obtained
were 2.5× 108, 0.54× 108 and 0.32× 108 photons/s/cm2/K for IOH∗ , ICH∗ and IC∗2
respectively. This
behavior shows agreement with the meaning of the characteristic constant α since the radicals that
have a higher value for α are also the ones where the increase in the radiation intensity is higher.
4.3 Analysis of temperature on chemiluminescence relations
It has been showed [1] the existence of a characteristic relation between the emissions of OH*, CH* and
C∗2 radicals, which is related with the hydrocarbon fuel composition. In Figure 4.11 is possible to see
31
(a) OH* (b) CH*
(c) C2*
Figure 4.10: Effect of temperature in the radiation intensity of the radicals OH* (a), CH* (b) and C∗2 (c)
for methane and propane flames
the ternary diagram that corresponds to the fuels that were studied in this work. The ternary diagram
is a tool that represents relative dependencies between variables. One can see that in the range of
temperatures considered (∆T = 100 K) there seems to be no influence in the baseline of each fuel thus
indicating that it is still possible to identify methane and propane flames by their ternary diagram even if
there is a preheating of the mixture.
One application of flame chemiluminescence is the estimation of the equivalence ratio using the ratio
of radical radiation intensities as been showed by previous works [30, 31, 6, 32]. The ratios IOH∗/ICH∗
and ICH∗/IC∗2
are usually used for lean mixtures while the ratio IC∗2/IOH∗ is more suitable for rich mixtures
[1, 32]. In this work, the focus is the differences that an increase in temperature will cause. To evaluate
that effect Figure 4.12 is shown below.
This figure show what happens to the value of the ratio between radicals when an increase in temper-
ature of 100 degrees is observed (white symbols). One can see that the general tendency is maintained.
32
Figure 4.11: Ternary diagram of methane and propane and the effect of temperature. Black symbolscorresponds to the reference condition while white symbols correspond to an increase of 100 K in theflame temperature.
(a) Methane flames (b) Propane flames
Figure 4.12: Influence of temperature in the ratio of radicals for methane (a) and propane (b) flames.The white symbols correspond to a variation of 100 K in the flame temperature.
For methane flames, the ratio IOH∗/ICH∗ has an higher variation with the temperature than the other two
with a maximum deviation of 25%. The ratios ICH∗/IC∗2
and IC∗2/IOH∗ are roughly invariant with the
temperature.
A similar analysis can be made for propane flames. Once again, the ratio with higher variation
with the temperature is IOH∗/ICH∗ with a maximum deviation of 27%. Similarly with methane flames,
the ratios ICH∗/IC∗2
and IC∗2/IOH∗ seem to be less affected by the temperature. These results are in
33
agreement with the previous analysis of the characteristic constant α since the ratio with higher devi-
ation correspond to the one involving the radicals OH* and CH* which are the ones more sensible to
temperature according with the data showed in Figure 4.8.
Since the objective is to find a ratio that is less sensible to temperature variations it follows that for
both methane and propane flames the ratio ICH∗/IC∗2
is more suitable for equivalence ratio evaluation in
lean mixtures while the ratio IC∗2/IOH∗ continues as the most adequate ratio for rich mixtures.
The use of the ratio of radicals is not the only way to monitor the equivalence ratio using flame
chemiluminescence. A previous work [1] introduced the concept of chemiluminescence fraction fi as
the ratio between an individual radical emission and the sum of all chemiluminescence. In the particular
case of i = OH∗,CH∗ and C∗2 the fi is defined as:
fi =I∗i
IOH∗ + ICH∗ + IC∗2
(4.4)
It was reported that the equivalence ratio can be monitored with high accuracy between φ = 0.80
and φ = 1.3 using the fractions fOH∗ and fC∗2. The influence of temperature was studied in the values of
these two fractions. The results are presented in Figure 4.13.
(a) Methane flames (b) Propane flames
Figure 4.13: Influence of temperature in the chemiluminescence fraction for methane (a) and propaneflames (b). The white symbols correspond to a variation of 100 K in the flame temperature.
It was found that for fOH∗ and an increment of 100 K the maximum deviation was 0.049 for methane
flames and 0.072 for propane flames. For fC∗2
the maximum deviation observed was 0.039 for methane
flames and 0.052 for propane flames. The main advantage of using a chemiluminescence fraction in-
stead of a ratio of radicals its the broader application range of the first. Since the deviations observed for
the fractions are in general smaller than the ones observed for the ratios, chemiluminescence fractions
are preferable to use to monitor the equivalence ratio when the flame is preheated. These conclusion
stands for both methane and propane flames.
34
4.4 The effect of CO2 in the flame spectrum
Figure 4.6a) shows the effect in the flame spectrum when CO2 is added in the mixture. The ”reference
condition” defined as xCO2= 0 corresponds to a gas premixture without the CO2 addition while xCO2
= 1
correspond to a blend without N2, having instead the same volumetric flow in CO2. As one can see, the
increase in the CO2 content leads to differences in the emission spectrum. Observing the peaks of the
radicals OH*, CH*, C∗2 it is visible that a decrease in the radiation intensity occurs when the concentration
of CO2 increases in the flame. The integration along the wavelengths described in Table 3.1 was carried
out for all the equivalence ratios studied and for both methane and propane flames. Figure 4.14 shows
the behavior of the radiation intensity of the three radicals focused in this work, as well as the radical
CO∗2, with the increase in CO2 concentration for a particular flame. The values are normalized by the
reference condition (xCO2 = 0).
Figure 4.14: Emission intensity of OH*,CH*, C∗2 and CO∗
2 radicals of a propane flame (φ = 1.20) withdifferent CO2 content in gas premixture.
The radical CO∗2 presents an increase in its intensity which is different from the behavior of the other
radicals. The addition of CO2 seems to increase the concentration of precursor species (like CO) in the
formation of CO∗2 which may explain the increasing in the radiation intensity observed. However, since
the focus of this work is in the radicals OH*,CH* and C∗2 no further development will be made concerning
CO∗2 emissions. There is a clear reduction of the radiation intensities of the three radicals studied. This
reduction is higher for the radical C∗2 than it is for the radicals OH* and CH*. This behavior was observed
for both methane and propane flames but also for all the equivalence ratios tested which was similar to
what was found before for the temperature effect. In order to investigate this phenomenon numerical
simulations were carried out involving a detailed combustion mechanism.
The influence of CO2 was investigated in the rate of three reactions usually indicated as responsible
35
for the formation of the species OH*, CH* and C∗2. The reaction considered for OH* was HCO + O
OH* + CO. It was found that an increase of 5% in CO2 content in gas premixture at φ = 0.90 causes
a decrease of 13% (propane flames) and 15% (methane flames) in the reaction extent. For the radical
CH* one of the the reactions usually indicated to its formation is C2H + O CH* + CO, the decrease
observed was 15% for propane flames and almost three orders of magnitude less for methane flames
(both at φ = 0.9). The formation of C∗2 can be attributed to the reaction CH2 + C C∗
2 + H2, it was found
that the reaction speed of this reaction decreases by 26% with the addition of 5% of CO2 for propane
flames and 32% for methane flames (φ = 0.9). It should be noted that the reactions presented aren’t the
only ones that produce the excited radicals thus, for instance, the greater decrease observed for ICH∗
in methane flames don’t necessary mean that the effect of CO2 in the emissions of this radical is higher
than in propane flames. It only means that for this particular reaction and for this equivalence ratio it is.
The reduction in the radicals emission observed seems associated with the decrease in concen-
tration of several species necessary to the chemiluminescence reactions. The chemical effect of CO2
addition was studied in previous works [33, 34]. Their main conclusion was that the addition of CO2
exercises its influence mainly through the reaction CO2 + H CO + OH which leads to a decrease of
the concentration of the radical H. Liu et al. [34] indicated in their work that the most important chain
branching reaction is H + O2 O + OH which leads to a decrease in the concentrations of the radicals
O and OH. Another explanation was proposed by Cong and Dagout [35]. These authors suggested that
the decrease in concentration of the radical H leads to a decrease in the fuel consumption rate via the
competition with the H-abstraction reaction CH4 + H CH3 + H2. This inhibition disrupts the combustion
since all the reactions that follow are also inhibited. It follows then that the addition of CO2 to the gas
premixture leads to a decrease in concentrations of three key radicals to chemiluminescence reactions
which leads to the observed reductions in the emissions of radicals OH* and CH*, as reported in Figure
4.14. The reduction in the rate of reaction that leads to C∗2 formation, seems to justify the decrease in
C∗2 emissions although no additional information was found in the literature.
It was then investigated the possibility of finding a single law (similarly to the one found for the
temperature effect) to model de data obtained. It was found that the radiation intensity of the radicals
follows the same empirical expression given by Eq. 4.2 having instead of the increment in temperature,
the value of xCO2. The equation is then:
IiIi,0
= eβixCO2 (4.5)
in which Ii is the radiation intensity of the radical i, Ii,0 is the reference value of that radical measured
at xCO2= 0, βi is a constant (mol fraction−1) and xCO2
is a measure of the CO2 content (mol fraction).
This model can be used to describe the emissions of the radicals OH*, CH* and C∗2 regarding the effect
of CO2. The parameter β assumes an analog meaning of α, describing the proportionality between the
addition of CO2 and the decrease in radiation intensity. The higher its absolute value the higher the
decrease in the radiation intensity. Figure 4.15 shows the value of β for several equivalence ratios and
for both methane and propane flames.
36
Figure 4.15: Variation of parameter β of Eq.4.5 with the equivalence ratio for propane (white symbols)and methane (white symbols) flames.
It can be seen in flames of both fuels a similar behavior of the parameter β being roughly independent
on the equivalence ratio. The emissions of radicals OH*, CH* are aligned having and identical β while the
one for C∗2 emissions has a much higher absolute value. Since the behavior of β is roughly independent
on the equivalence ratio, a characteristic value for each type of radiation is proposed. These values are
presented in Table 4.3.
Table 4.3: Values of β expressed in mol fraction−1 for methane and propane flames.Fuel Radical β
OH* -1.294CH4 CH* -0.810
C∗2 -6.354
OH* -1.235C3H8 CH* -0.967
C∗2 -5.285
The effect of an increase in CO2 concentration in the radiation intensity of the radicals OH*, CH*
and C∗2 can then be predicted recurring to Eq. 4.5 and the values of Table 4.3. Figure 4.16 faces the
experimental flame emission data and the predictions by the model (Eq. 4.5) for both propane and
methane flames.
The agreement is remarkable indicating that Eq. 4.5 describes numerically the phenomena. Fig-
ure 4.17 summarize the effect of the addition of CO2 representing three levels of CO2 content in the
premixture.
There is a clear reduction of the radiation intensity with the increase of CO2 concentration, how-
37
Figure 4.16: Data validation of CO2 effect on flame chemiluminescence facing predictions by Eq.4.5 formethane (black symbols) and propane (white symbols) flames ( OH*, CH*, C2*).
ever this decrease is independent on the equivalence ratio which is characterized by the constant value
of β. Despite the increase in CO2 concentration, the φ at which the intensity is maximum is unal-
tered. For stoichiometric conditions it was found that ∆IOH∗/∆xCO2= −2.2× 108, ∆ICH∗/∆xCO2
=
−0.13× 108 and ∆IC∗2/∆xCO2 = −0.24× 108 photons/s/cm2/mol fraction for methane flames. The val-
ues for propane are : ∆IOH∗/∆xCO2= −4.4× 108, ∆ICH∗/∆xCO2
= −1.5× 108 and ∆IC∗2/∆xCO2
=
−4.4× 108 photons/s/cm2/mol fraction. The data shows that, in general, the radical C∗2 is the most sen-
sible to the addition of CO2. The effect is greater for propane flames than methane flames which is in
agreement with the value of its characteristic β.
4.5 Analysis of CO2 on chemiluminescence relations
A ternary diagram facing the chemiluminescence of OH*, CH* and C∗2 under the effect of CO2 addition
is shown in Figure 4.18.
The increasing CO2 concentration in the flame leads to a shift toward higher CH* fraction of the refer-
ence line at xCO2 = 0. However, contrary to what happened with the effect of temperature, the expected
shift is pronounced enough to render useless the identification of the fuel by the ternary diagram since
mixtures of fuel with CO2 are in fact a different fuel.
It is of relevance to verify if the ratio of radicals chemiluminescence can still be used to estimate
the equivalence ratio. The effect of CO2 content on the ratio of radicals for both methane and propane
flames is shown in Figure 4.19.
The effect of CO2 concentration is more pronounced than the effect of temperature. Contrary to what
38
(a) OH* (b) CH*
(c) C∗2
Figure 4.17: Effect of CO2 addition in the radiation intensity of the radicals OH* (a), CH* (b) and C∗2 (c)
for methane and propane flames
was shown with the latest, here the reference line shifts greatly in particular for IC∗2/IOH∗ and ICH∗/IC∗
2.
Since the decrease in the C∗2 intensity is higher than the other radicals, it was expected that the ratios
involving the radical C∗2 were more affected. However, due to the similarity in the curves, the monitoring
of the flame equivalence ratio is still possible, as long as the necessary correction is made in the transfer
function used. It was found that the ratio IOH∗/ICH∗ decreases with the addition of CO2 and presents
a deviation of 14% for methane flames and 8% for propane flames being the ratio with less deviation
relatively to the reference condition. The ratio IC∗2/IOH∗ decreases with the addition of CO2 and presents
a deviation from the reference condition of 78% for methane and 70% for propane flames. The value of
the ratio ICH∗/IC∗2
increases with the addition of CO2 and its value is about five times larger than the one
at the reference condition for methane flames and about 4 times larger for propane flames. It follows
that for monitoring purposes the transfer function I∗i = f(φ) has to be corrected when CO2 exists in the
39
Figure 4.18: Effect of CO2 addition on the ternary diagram for propane and methane flames. Blacksymbols correspond to the reference condition while white symbols correspond to an increase in CO2 of30 %.
(a) Methane flames (b) Propane flames
Figure 4.19: Effect of CO2 addition on the ratio of radicals for methane (a) and propane (b) flames.
premixture.
An analysis of the addition of CO2 in the chemiluminescence fractions was performed. The results
are shown in Figure 4.20 .
The maximum deviation found for the fraction fOH∗ was 0.19 for methane flames and 0.13 for propane
flames with the effect of CO2 addition being higher in rich flames and similar in both methane and
propane flames. The fraction fC∗2
presents a maximum shift of 0.29 for methane flames along the range
40
(a) Methane flames (b) Propane flames
Figure 4.20: Influence of the addition of CO2 in the chemiluminescence fraction for methane (a) andpropane (b) flames.
of equivalence ratio studied and 0.30 for propane flames. Once again, the presence of CO2 causes
great deviations in the reference flame chemiluminescence. This leads to the necessity of correcting the
intensity signals for equivalence ratio monitoring. However, comparing the intensity signals deviations
when chemiluminescence fractions and ratios are used one can conclude that the latter is preferable, at
least in lean flames, since the ratio IOH∗/ICH∗ has a lower shift than any of the fractions presented in
this work. Despite this, chemiluminescence fractions have a broader application range thus the choice
depends on the application.
4.6 Combined effects
A separated view on the effects of temperature and CO2 concentration on the flame emission spectrum
was already presented. In a generic combustion situation, both effects may be simultaneously present.
Assuming that both effects are independent the model describing the effect of temperature can be added
with the one for CO2 addition, obtaining a combined formulation:
IiIi,0
= eαi∆T+βixCO2 (4.6)
Figure 4.21 shows the agreement between the experimental data obtained from an independent set
of measures including both effects and the numerical values predicted by the combined model.
As one can see, the full model gives fair results for the OH* and CH* emissions. The results for
the radical C∗2 have a higher deviation. The kinetics of C∗
2 is the less studied of these three radicals,
some unknown effect which were not considered in this work may be the cause for the poor agreement
between the model and the experimental data obtained. The results indicate that the temperature effect
41
Figure 4.21: Data validation of temperature and CO2 combined effects on flame chemiluminescencefacing predictions by Eq.4.6 for methane (black symbols) and propane (white symbols) flames ( OH*,
CH*, C∗2).
and the addition of CO2 are not strictly independent effects thus it is necessary to introduce into the
formulation the interactions between the temperature effect and the addition of CO2. Future work is
necessary to continue to validate the model and to enhance it particularly for predictions of C∗2 radiation.
42
Chapter 5
Conclusions
The work performed throughout this master thesis lead to several conclusions regarding the application
of flame chemiluminescence in flame monitoring. However, there is still room to improve. This Chapter
is organized in two sections. An overview of the main achievements of this thesis is presented in Section
5.1. The chapter ends with Section 5.2 where some ideas for future work are pointed out.
5.1 Achievements
In this thesis, the effects of temperature and CO2 content on flame chemiluminescence were investigated
for both methane and propane flames.
The work was divided in two main branches: the effect of temperature and the effect of CO2 content.
An evaluation of the morphological differences on the flame was presented. It was showed that the flame
height decreases linearly with the flame temperature ( with a maximum of 0.1 mm/K ) and increases
linearly with the CO2 content (maximum of 0.9 mm/mol fraction).
The analysis proceeded with the effects on flame chemiluminescence. It was showed that while the
radiation intensity of the radicals OH*, CH* and C∗2 increase with increasing flame temperature the effect
is the opposite when CO2 is added to the mixture. As an example, for a methane flame (φ = 1.0) the
emissions of the radical OH* increase 54% with an increase of 100 K while for a content of 30% CO2
the emissions decrease 32%. An empirical model for the temperature effect (Ii/Ii,0 = exp(α∆T )) and
the effect of CO2 (Ii/Ii,0 = exp(βxCO2)) and its agreement with the experimental data was showed. The
model propose the definition of a characteristic constant α, β which is a measure of the proportionality
between the effect considered and the radiation intensity. It was found that the value of α is a function
of the equivalence ratio and a possible equation to define the dependence was presented. On the other
hand, β was found to be roughly invariant with the equivalence ratio.
The analysis was extended to chemiluminescence relations, namely ternary diagrams, ratios be-
tween radicals and chemiluminescence fractions. An analysis of the ternary diagram showed that, for
the range of temperatures considered the effect of temperature is negligible while the addition of CO2
leads to a pronounced shift of the reference line. Observing the ratios between radicals, it was con-
43
cluded that the ratios ICH∗/IC∗2
and IC∗2/IOH∗ don’t appear to be sensible to temperature variations
while the ratio IOH∗/ICH∗ presents a considerable variation (maximum of 25%). It follows then that the
ratio ICH∗/IC∗2
is preferable to equivalence ratio evaluation in lean mixtures while the ratio IC∗2/IOH∗
remained as the most adequate for rich mixtures. The addition of CO2 lead to high deviations from
the reference condition in the ratios IC∗2/IOH∗ and ICH∗/IC∗
2. Although the deviations observed were
high (almost 5 times higher for ICH∗/IC∗2
), the reference line seems to only shift from its initial position
which seems to mean that as long as the transfer function I∗i = f(φ) is corrected, an identification of the
equivalence ratio is still possible.
An analysis of the chemiluminescence fractions was also performed with the results indicating a low
dependence on temperature following that chemiluminescence fractions should be preferred over ratios
since the former have a broader range of application. On the other hand, these fractions seem to be
considerably affected by the addition of CO2, having been found a maximum deviation of 48%. Due
to the broader range of application of the chemiluminescence fractions, in the case of CO2 addition
the choice of what parameter to use for equivalence ratio evaluation should depend on the application.
Nevertheless it was found that for the case of CO2 addition the ratio IOH∗/ICH∗ seem to be more reliable,
at least for lean flames, that any of the chemiluminescence fractions presented.
The final part of the thesis was the validation of the full model for the effects of temperature and
CO2 simultaneously. The results showed that the model presented gives fair predictions although some
improvement should be made particularly for C∗2 emissions.
5.2 Future Work
In this master thesis some of the effects of EGR techniques were studied in flame chemilumescence.
Although the results obtained were satisfactory, further improvements could be made in order to enhance
the effectiveness of flame chemiluminescence in flame monitoring. Some suggestions are presented
next.
• The analysis should be extended to a broader range of equivalence ratios particularly for more
lean mixtures. The increasing energetic needs of nowadays society lead to the need of obtaining
more efficient processes from which combustion is an important one hence the interest in lean
flames. The extended analysis should complement the work already done and give more tools to
the proliferation of chemiluminescence techniques to flame monitoring.
• Investigations could be performed in biogas to validate the results concerning the addition of CO2.
Additionally, adjustments should be made to the model proposed to enhance its capabilities to
predict C∗2 emissions in flames with the addition of CO2.
44
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48
Appendix A
Conditions measured
The complete list of conditions measured during the experiments is listed below.
Table A.1: Conditions measured
Fuel φ Power(kW) Qfuel QN2QO2
QAr QCO2
0.80 0.75 1.234 11.65 3.095 0 0
0.80 0.75 1.234 11.15 3.095 0.5 0
0.80 0.75 1.234 10.65 3.095 1.0 0
0.80 0.75 1.234 10.15 3.095 1.5 0
0.80 0.75 1.234 9.65 3.095 2.0 0
0.80 0.75 1.234 9.15 3.095 2.5 0
0.80 0.75 1.234 8.65 3.095 3.0 0
0.80 0.75 1.234 8.15 3.095 3.5 0
0.80 0.75 1.234 7.65 3.095 4.0 0
0.80 0.75 1.234 7.15 3.095 4.5 0
0.90 1.25 2.056 17.25 4.586 0 0
CH4 0.90 1.25 2.056 16.75 4.586 0.5 0
0.90 1.25 2.056 16.25 4.586 1.0 0
0.90 1.25 2.056 15.75 4.586 1.5 0
0.90 1.25 2.056 15.25 4.586 2.0 0
0.90 1.25 2.056 14.75 4.586 2.5 0
0.90 1.25 2.056 14.25 4.586 3.0 0
0.90 1.25 2.056 13.75 4.586 3.5 0
0.90 1.25 2.056 13.25 4.586 4.0 0
0.90 1.25 2.056 12.75 4.586 4.5 0
0.90 1.25 2.056 12.25 4.586 5.0 0
1.00 1.25 2.056 15.52 4.127 0 0
1.00 1.25 2.056 15.02 4.127 0.5 0
49
1.00 1.25 2.056 14.52 4.127 1.0 0
1.00 1.25 2.056 14.02 4.127 1.5 0
1.00 1.25 2.056 13.52 4.127 2.0 0
1.00 1.25 2.056 13.02 4.127 2.5 0
1.00 1.25 2.056 12.52 4.127 3.0 0
1.00 1.25 2.056 12.22 4.127 3.3 0
1.10 1.25 2.056 14.12 3.753 0 0
1.10 1.25 2.056 13.62 3.753 0.5 0
1.10 1.25 2.056 13.12 3.753 1.0 0
1.10 1.25 2.056 12.62 3.753 1.5 0
1.10 1.25 2.056 12.12 3.753 2.0 0
1.10 1.25 2.056 11.62 3.753 2.5 0
1.10 1.25 2.056 11.12 3.753 3.0 0
1.10 1.25 2.056 10.62 3.753 3.5 0
1.10 1.25 2.056 10.12 3.753 4.0 0
1.20 1.25 2.056 12.94 3.440 0 0
1.20 1.25 2.056 12.44 3.440 0.5 0
CH4 1.20 1.25 2.056 11.94 3.440 1.0 0
1.20 1.25 2.056 11.44 3.440 1.5 0
1.20 1.25 2.056 10.94 3.440 2.0 0
1.20 1.25 2.056 10.44 3.440 2.5 0
1.20 1.25 2.056 9.94 3.440 3.0 0
1.20 1.25 2.056 9.44 3.440 3.5 0
1.20 1.25 2.056 8.94 3.440 4.0 0
1.20 1.25 2.056 8.44 3.440 4.5 0
1.30 1.25 2.056 11.95 3.175 0 0
1.30 1.25 2.056 11.45 3.175 0.5 0
1.30 1.25 2.056 10.95 3.175 1.0 0
1.30 1.25 2.056 10.45 3.175 1.5 0
1.30 1.25 2.056 9.95 3.175 2.0 0
1.30 1.25 2.056 9.45 3.175 2.5 0
1.30 1.25 2.056 8.95 3.175 3.0 0
1.30 1.25 2.056 8.45 3.175 3.5 0
1.30 1.25 2.056 7.95 3.175 4.0 0
1.30 1.25 2.056 7.45 3.175 4.5 0
0.90 1.25 2.056 17.25 4.586 0 0
0.90 1.25 2.056 16.77 4.586 0.309 0.173
0.90 1.25 2.056 16.29 4.586 0.617 0.345
0.90 1.25 2.056 15.81 4.586 0.922 0.518
50
0.90 1.25 2.056 15.34 4.586 1.226 0.690
0.90 1.25 2.056 14.86 4.586 1.529 0.863
0.90 1.25 2.056 14.39 4.586 1.830 1.035
0.90 1.25 2.056 13.92 4.586 2.129 1.208
0.90 1.25 2.056 13.45 4.586 2.426 1.380
0.90 1.25 2.056 12.98 4.586 2.721 1.553
0.90 1.25 2.056 12.51 4.586 3.015 1.725
1.00 1.25 2.056 15.52 4.127 0 0
1.00 1.25 2.056 14.57 4.127 0.642 0.311
1.00 1.25 2.056 13.63 4.127 1.276 0.621
1.00 1.25 2.056 12.69 4.127 1.901 0.932
1.00 1.25 2.056 11.77 4.127 2.518 1.242
1.00 1.25 2.056 10.85 4.127 3.126 1.553
1.00 1.25 2.056 9.94 4.127 3.726 1.863
1.00 1.25 2.056 9.03 4.127 4.318 2.174
1.00 1.25 2.056 8.14 4.127 4.901 2.484
1.00 1.25 2.056 7.26 4.127 5.476 2.795
CH4 1.00 1.25 2.056 6.47 4.127 5.987 3.074
1.10 1.25 2.056 14.12 3.753 0 0
1.10 1.25 2.056 12.93 3.753 0.767 0.424
1.10 1.25 2.056 11.74 3.753 1.527 0.847
1.10 1.25 2.056 10.57 3.753 2.280 1.271
1.10 1.25 2.056 9.40 3.753 3.025 1.694
1.10 1.25 2.056 8.24 3.753 3.763 2.118
1.10 1.25 2.056 7.09 3.753 4.493 2.541
1.10 1.25 2.056 5.94 3.753 5.214 2.965
1.10 1.25 2.056 4.69 3.753 6.000 3.431
1.20 1.25 2.056 12.90 3.440 0 0
1.20 1.25 2.056 11.90 3.440 0.66 0.388
1.20 1.25 2.056 10.90 3.440 1.308 0.776
1.20 1.25 2.056 9.83 3.440 1.944 1.165
1.20 1.25 2.056 8.82 3.440 2.567 1.553
1.20 1.25 2.056 7.82 3.440 3.178 1.941
1.20 1.25 2.056 6.84 3.440 3.776 2.329
1.20 1.25 2.056 5.86 3.440 4.363 2.718
1.20 1.25 2.056 4.90 3.440 4.938 3.106
1.20 1.25 2.056 3.95 3.440 5.50 3.494
1.20 1.25 2.056 3.16 3.440 5.96 3.818
1.30 1.25 2.056 11.95 3.175 0 0
51
1.30 1.25 2.056 10.90 3.175 0.639 0.358
1.30 1.25 2.056 9.97 3.175 1.257 0.717
1.30 1.25 2.056 9.01 3.175 1.856 1.075
1.30 1.25 2.056 8.07 3.175 2.436 1.433
1.30 1.25 2.056 7.15 3.175 2.998 1.792
1.30 1.25 2.056 6.25 3.175 3.542 2.150
1.30 1.25 2.056 5.37 3.175 4.070 2.508
1.30 1.25 2.056 4.50 3.175 4.581 2.867
1.30 1.25 2.056 3.64 3.175 5.077 3.225
1.30 1.25 2.056 2.80 3.175 5.557 3.583
1.30 1.25 2.056 2.03 3.175 5.992 3.918
1.30 1.25 2.056 4.5 3.175 4.581 2.867
CH4 1.30 1.25 2.056 3.5 3.175 5.581 2.867
1.30 1.25 2.056 2.5 3.175 6.581 2.867
1.30 1.25 2.056 1.5 3.175 7.581 2.867
0.80 1.00 0.650 15.57 4.140 0 0
0.80 1.00 0.650 15.07 4.140 0.5 0
0.80 1.00 0.650 14.57 4.140 1.0 0
0.80 1.00 0.650 14.07 4.140 1.5 0
0.80 1.00 0.650 13.57 4.140 2.0 0
0.80 1.00 0.650 13.07 4.140 2.5 0
0.80 1.00 0.650 12.57 4.140 3.0 0
0.80 1.00 0.650 12.07 4.140 3.5 0
0.80 1.00 0.650 11.57 4.140 4.0 0
0.80 1.00 0.650 11.07 4.140 4.5 0
0.80 1.00 0.650 10.57 4.140 5.0 0
0.90 1.60 1.060 22.52 5.980 0 0
C3H8 0.90 1.60 1.060 22.02 5.980 0.5 0
0.90 1.60 1.060 21.52 5.980 1.0 0
0.90 1.60 1.060 21.02 5.980 1.5 0
0.90 1.60 1.060 20.52 5.980 2.0 0
0.90 1.60 1.060 20.02 5.980 2.5 0
0.90 1.60 1.060 19.52 5.980 3.0 0
0.90 1.60 1.060 19.02 5.980 3.5 0
0.90 1.60 1.060 18.52 5.980 4.0 0
0.90 1.60 1.060 18.02 5.980 4.5 0
0.90 1.60 1.060 17.52 5.980 5.0 0
1.00 1.60 1.060 20.27 5.380 0 0
52
1.00 1.60 1.060 19.77 5.380 0.5 0
1.00 1.60 1.060 19.27 5.380 1.0 0
1.00 1.60 1.060 18.77 5.380 1.5 0
1.00 1.60 1.060 18.27 5.380 2.0 0
1.00 1.60 1.060 17.77 5.380 2.5 0
1.00 1.60 1.060 17.27 5.380 3.0 0
1.00 1.60 1.060 16.77 5.380 3.5 0
1.00 1.60 1.060 16.27 5.380 4.0 0
1.00 1.60 1.060 15.77 5.380 4.5 0
1.00 1.60 1.060 15.27 5.380 5.0 0
1.10 1.60 1.060 18.42 4.900 0 0
1.10 1.60 1.060 17.92 4.900 0.5 0
1.10 1.60 1.060 17.42 4.900 1.0 0
1.10 1.60 1.060 16.92 4.900 1.5 0
1.10 1.60 1.060 16.42 4.900 2.0 0
1.10 1.60 1.060 15.92 4.900 2.5 0
1.10 1.60 1.060 15.42 4.900 3.0 0
1.10 1.60 1.060 14.92 4.900 3.5 0
C3H8 1.10 1.60 1.060 14.42 4.900 4.0 0
1.10 1.60 1.060 13.92 4.900 4.5 0
1.10 1.60 1.060 13.42 4.900 5.0 0
1.20 1.60 1.060 16.89 4.490 0 0
1.20 1.60 1.060 16.39 4.490 0.5 0
1.20 1.60 1.060 15.89 4.490 1.0 0
1.20 1.60 1.060 15.39 4.490 1.5 0
1.20 1.60 1.060 14.89 4.490 2.0 0
1.20 1.60 1.060 14.39 4.490 2.5 0
1.20 1.60 1.060 13.89 4.490 3.0 0
1.20 1.60 1.060 13.39 4.490 3.5 0
1.20 1.60 1.060 12.89 4.490 4.0 0
1.30 1.60 1.060 15.59 4.140 0 0
1.30 1.60 1.060 15.09 4.140 0.5 0
1.30 1.60 1.060 14.59 4.140 1.0 0
1.30 1.60 1.060 14.09 4.140 1.5 0
1.30 1.60 1.060 13.59 4.140 2.0 0
1.30 1.60 1.060 13.09 4.140 2.5 0
1.30 1.60 1.060 12.59 4.140 3.0 0
0.90 1.60 1.060 22.52 5.980 0 0
0.90 1.60 1.060 19.94 5.980 1.66 0.9
53
0.90 1.60 1.060 19.62 5.980 1.865 1.012
0.90 1.60 1.060 19.30 5.980 2.070 1.125
0.90 1.60 1.060 18.99 5.980 2.274 1.237
0.90 1.60 1.060 18.67 5.980 2.477 1.350
0.90 1.60 1.060 18.35 5.980 2.680 1.462
0.90 1.60 1.060 18.04 5.980 2.883 1.575
1.00 1.60 1.060 20.27 5.380 0 0
1.00 1.60 1.060 18.97 5.380 0.865 0.405
1.00 1.60 1.060 17.71 5.380 1.717 0.810
1.00 1.60 1.060 16.47 5.380 2.558 1.214
1.00 1.60 1.060 15.23 5.380 3.388 1.619
1.00 1.60 1.060 14.01 5.380 4.206 2.024
1.00 1.60 1.060 12.83 5.380 4.993 2.419
1.10 1.60 1.060 18.42 4.900 0 0
1.10 1.60 1.060 17.36 4.900 0.707 0.369
1.10 1.60 1.060 16.29 4.900 1.410 0.737
1.10 1.60 1.060 15.22 4.900 2.109 1.106
1.10 1.60 1.060 14.15 4.900 2.804 1.475
1.10 1.60 1.060 13.10 4.900 3.494 1.843
1.10 1.60 1.060 12.04 4.900 4.181 2.212
C3H8 1.10 1.60 1.060 10.99 4.900 4.863 2.581
1.10 1.60 1.060 9.94 4.900 5.541 2.949
1.20 1.60 1.060 16.89 4.490 0 0
1.20 1.60 1.060 15.98 4.490 0.575 0.338
1.20 1.60 1.060 15.07 4.490 1.146 0.676
1.20 1.60 1.060 14.17 4.490 1.712 1.013
1.20 1.60 1.060 13.27 4.490 2.273 1.351
1.20 1.60 1.060 12.37 4.490 2.829 1.689
1.20 1.60 1.060 11.48 4.490 3.381 2.027
1.20 1.60 1.060 10.60 4.490 3.927 2.365
1.20 1.60 1.060 9.72 4.490 4.469 2.703
1.20 1.60 1.060 8.85 4.490 5.005 3.040
1.20 1.60 1.060 7.98 4.490 5.537 3.378
1.30 1.60 1.060 15.59 4.140 0 0
1.30 1.60 1.060 15.15 4.140 0.270 0.156
1.30 1.60 1.060 14.73 4.140 0.537 0.311
1.30 1.60 1.060 14.31 4.140 0.802 0.467
1.30 1.60 1.060 13.89 4.140 1.065 0.623
1.30 1.60 1.060 13.47 4.140 1.326 0.779
54
1.30 1.60 1.060 13.06 4.140 1.584 0.934
1.30 1.60 1.060 12.64 4.140 1.840 1.090
1.30 1.60 1.060 12.23 4.140 2.095 1.246
1.30 1.60 1.060 11.83 4.140 2.347 1.402
1.30 1.60 1.060 11.42 4.140 2.596 1.557
1.20 1.60 1.060 12.37 4.490 2.829 1.689
C3H8 1.20 1.60 1.060 11.37 4.490 3.829 1.689
1.20 1.60 1.060 10.37 4.490 4.829 1.689
1.20 1.60 1.060 9.37 4.490 5.829 1.689
55
56