On the Formation of Nitrogen Oxides During the Combustion of ...

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1 Introduction residence time of the combustion gases in the hot combustion zone and controls the availability of oxygen [51, 201, 240, 241, 251, 353, 367, 368]. Current approaches in technology development adopt staged combustion with a rich and a lean stage (RQL) and lean premixed pre-vaporized (LPP) combustion. The concept of LPP combustion, for instance, requires fine atomization and a careful placement of fuel so that a high degree of mixing can be achieved before the onset of ignition. The potential for NO x reduction depends on the homogeneity of the fuel-air mixture at the downstream end of the premixing zone [42, 69, 263, 264]. Where lean, fully premixed combustion at moderate temperatures cannot be achieved due to operational, safety, or reliability issues, partially premixed flames will continue to play a major role in future applications. In this context, the NO x formation of technical sprays was investigated by Cooper [86], Baessler et al. [32], and Beck et al. [39] regarding different degrees of fuel vaporization. However, as illustrated in Figure 1.1, the fundamental processes occur in the proximity of single droplets and droplet clouds. Hence, the present thesis has the goal of improving the understanding of the processes around single droplets and the most simple droplet clouds, namely linear onedimensional droplet arrays. The study attempts to break down the influence of the degree of droplet vaporization and the influence of ambient conditions on NO x formation. It aims to highlight correlations and dependencies of the most relevant parameters to the reader. Microgravity environment was selected for the execution of physical as well as numerical experiments, since it is important to provide well-defined and idealized conditions for such basic droplet setups. As depicted in Figure 1.2, microgravity allows for avoidance of the disturbing effects of buoyancy due to density gradients, flame shape deviations from a perfectly spherical shape, convective flow along the flame front, and flame stretching. Furthermore, it allows the simplification of modeling from two or three dimensions to 1D. From a practical point of view, microgravity is also necessary for the formation of equal vapor distribution around each droplet and safe ignition. These requirements are due to the fact that under normal gravity not only the hot combustion products would rise but also the vaporized, gaseous fuel would settle down, resulting in deviations of the drivers of droplet combustion from their nominal value in opposing directions [311]. 6
1.5 Thesis Overview Oxidizer Fuel Oxidizer Fuel Heat Droplet Droplet Flame front D flame Products Flame front Figure 1.2: Schematic of Droplet Envelope Flames. Left: droplet under normal gravity conditions, right: droplet under microgravity conditions [220, 225, 233, 235, 352]. 1.5 Thesis Overview Chapter 1 is devoted to introducing some technical background of this thesis by outlining current scientific understanding of thermodynamics and fluid mechanics, on the one hand. On the other hand, it positions the scope of the thesis – namely, the issue of NO x production in droplet and spray flames – within its economical, ecological, and social context. It also provides the reader with the motivation behind this work and the goals of this thesis. Chapter 2 explains the theory of and provides in-depth information on droplet and spray combustion, including all relevant combustion regimes. It establishes relationships and addresses details starting with general, wellunderstood combustion phenomena, such as premixed and nonpremixed flames, droplet vaporization, and hydrocarbon kinetics. A straightforward approach extends those formulations to account for the characteristics of droplet combustion. These include amongst others inhomogeneities, the transient nature of the process, and a comparison of reaction mechanisms including the selection of a suitable one for this study. Chapter 2 tries to provide all basics essential to this thesis, keeping the chapter as compact as possible. Additional information on the state of the art of particular details is given throughout all chapters in the course of this work. 7
Page 1: TECHNISCHE UNIVERSITÄT MÜNCHEN In
Page 5 and 6: Vorwort Die vorliegende Arbeit ents
Page 7 and 8: Kurzfassung Die vorliegende Arbeit
Page 9 and 10: Contents List of Figures List of Ta
Page 11: CONTENTS 5 Results 155 5.1 Droplets
Page 14 and 15: LIST OF FIGURES xiv 3.3 Droplet Arr
Page 16 and 17: LIST OF FIGURES 5.19 Progression of
Page 18 and 19: LIST OF TABLES C.2 Overview of Tele
Page 20 and 21: NOMENCLATURE g Gravitational force
Page 22 and 23: NOMENCLATURE σ S Surface tension N
Page 24 and 25: NOMENCLATURE () T Thermal, temperat
Page 26 and 27: NOMENCLATURE PAL PAN PDU PFA PFC PH
Page 28 and 29: 1 Introduction In thermal engines t
Page 30 and 31: 1 Introduction 1.3 Oxides of Nitrog
Page 34 and 35: 1 Introduction Chapter 3 describes
Page 36 and 37: 2 Combustion Theory 2.1.1 Premixed
Page 38 and 39: 2 Combustion Theory molecules by ea
Page 40 and 41: 2 Combustion Theory four types. Out
Page 42 and 43: 2 Combustion Theory ventional combu
Page 44 and 45: 2 Combustion Theory of partial pre-
Page 46 and 47: 2 Combustion Theory Formation of Fu
Page 48 and 49: 2 Combustion Theory Moreover, exper
Page 50 and 51: 2 Combustion Theory For the gas pha
Page 52 and 53: 2 Combustion Theory combustion, ext
Page 54 and 55: 2 Combustion Theory termed “natur
Page 56 and 57: 2 Combustion Theory thin layer of u
Page 58 and 59: 2 Combustion Theory nor generally c
Page 60 and 61: 2 Combustion Theory is assumed to o
Page 62 and 63: 2 Combustion Theory before it follo
Page 64 and 65: 2 Combustion Theory position, which
Page 66 and 67: 2 Combustion Theory 2.3 Kinetic Mod
Page 68 and 69: 2 Combustion Theory 1.2 m s −1 La
Page 70 and 71: 2 Combustion Theory Mole fraction o
Page 72 and 73: 2 Combustion Theory Reburn Miller a
Page 74 and 75: 2 Combustion Theory The results of
Page 76 and 77: 2 Combustion Theory Temperature T 3
Page 78 and 79: 2 Combustion Theory 0.0004 GRI 3.0
Page 80 and 81: 2 Combustion Theory In conclusion,
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3 Experiments on Droplet Array Comb
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3.1 Droplet Combustion Facility and
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3.1 Droplet Combustion Facility C D
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3.1 Droplet Combustion Facility ous
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3.1 Droplet Combustion Facility for
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3.1 Droplet Combustion Facility par
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3.1 Droplet Combustion Facility The
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3.1 Droplet Combustion Facility a c
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3.1 Droplet Combustion Facility •
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3.1 Droplet Combustion Facility Tab
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3.1 Droplet Combustion Facility Fig
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3.2 Measurement Techniques and Data
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3.3 Numerical Study of the Fluid Dy
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4 Numerical Modeling and Simulation
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4.2 Basics for Numerical Modeling 4
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4.2 Basics for Numerical Modeling 4
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4.2 Basics for Numerical Modeling n
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4.2 Basics for Numerical Modeling P
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4.2 Basics for Numerical Modeling E
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4.3 Modeling of Ignition In the end
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4.3 Modeling of Ignition 10 W Time
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4.3 Modeling of Ignition Hence, the
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4.4 Modeling of Nitrogen Oxide Form
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4.5 Simulation of Single Droplets t
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4.5 Simulation of Single Droplets e
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4.6 Model Validation with index i
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4.6 Model Validation utilized model
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4.6 Model Validation Figure 4.7 dep
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4.6 Model Validation 340 K Ambient
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4.6 Model Validation Table 4.1: Val
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4.7 Scope and Limitations of Single
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4.7 Scope and Limitations of Single
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5 Results In total, it includes 99
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5 Results atmosphere of Figure 5.1
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5 Results 32 g kg −1 2400 K Emiss
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5 Results 4.0 g kg −1 Emission in
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5 Results uses constant spatial pos
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5 Results 16 g kg −1 Emission ind
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5 Results Maximum temperature Tmax
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5 Results Even though the parameter
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5 Results Flame stand-off ratio ζ
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5 Results 5.2.3 Comparison with Mic
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5 Results 0.50 5000 ppm Emission in
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5 Results Ψ = 0.1695 (t Ψ = 5 s):
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5 Results 6.0 g kg −1 Emission in
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5 Results combustion in exhaust gas
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5 Results Flame stand-off ratio ζ
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5 Results D 0 being varied between
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5 Results 0.40 g kg −1 Initial dr
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5 Results A twofold trend was obser
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5 Results 5.6 Recommendations and F
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6 Summary and Conclusions In times
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APPENDIX
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A Chemical Mechanisms The earliest
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A Chemical Mechanisms only deduced
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A Chemical Mechanisms Another, very
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A Chemical Mechanisms the reactions
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B Investigated Conditions There are
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B Investigated Conditions Table B.1
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B Investigated Conditions high mech
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C Design Details of Experiment Equi
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D Raw Data of Microgravity Experime
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SUPERVISED THESES Student Sebastian
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References [1] J.A. Aardenne van, G
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REFERENCES [20] K. Annamalai and W.
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REFERENCES [39] C.H. Beck, R. Koch,
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