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

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5 Results 5.2.3 Comparison with Microgravity Experiments on Droplet Arrays There are severe technical limitations on experimental setups regarding the realization of a representative single droplet combustion, combining droplet pre-vaporization and NO x formation. Thus, the experimental part of the study at hand was carried out to compare and, where possible, validate the numerical results on single droplets by resolving this issue on droplet arrays. Utilizing the environments of parabolic flight, drop tower, and sounding rocket flight, a significant number of experiments were conducted under microgravity conditions (see Tab. B.1). Figure 5.12 clarifies the difference in sequence between physical experiments and numerical simulation. Flame extinction and exhaust gas homogenization are not included in the simulation model, and thus production of thermal NO during this period is not accounted for. Experiment Prevaporization Ignition Droplet burning Extinction Homogenization Gas analysis Simulation Model Prevaporization Ignition Droplet burning No modeling Gas analysis Time t Figure 5.12: Combustion Sequence in Experiments and Numerical Simulation. This schematic is not to scale. The numerical simulations are stopped when the droplet diameter drops below the value 1/1000D 0 . A number of separate numerical simulations were conducted to include the heat loss of the combustion chamber induced by the insertion of the cold droplet array holder into the combustion chamber (see Chaps. 3.1.2 and 3.1.3) in the data post-processing. Their purpose is to relate the emission indices EI NOx | exp,T∞ =f (Ψ), as measured with the actual temperature history of the experiments (Fig. D.4), to one single preheating level. This temperature level in turn is set to the reference temperature of T ∞ = 500 K. The whole procedure helps to uncouple the effects of preheating on vaporization and NO x formation. The approach is similar to the one outlined in Baessler et al. [31, 32] but different in that single droplet simulations are used instead of calculations of laminar premixed flames. Equations (5.6) and (5.7) provide the necessary link via the correction factor f EINOx . The single droplet model (Chapter 4.5) is used to determine f EINOx from the full-scale droplet diameter D and the boundary 174
5.2 Combustion of Partially Pre-Vaporized Droplets conditions taken from the experiment runs. Finally, the experimental results on NO x formation are corrected by f EINOx , as far as applicable. As highlighted in Table 5.2, there is no linear correlation between pre-vaporization time t Ψ , gas phase temperature, and NO x formation. This effect stems from the spatial and, in particular, temporal overlap of the combustible gas region with the external heat supply. f EINOx = EI NO x | sim,T∞ =f (Ψ) EI NOx | sim,T∞ =500 K (5.6) EI NOx | exp,T∞ =500 K= EI NO x | exp,T∞ =f (Ψ) f EINOx (5.7) Table 5.2: Correction Factors for NO Formation as Affected by the Heat Loss of the Combustion Chamber. The measurement readings of Figure D.4 are taken as boundary conditions for the underlying numerical simulations. Pre-vaporization Correction factor f EINOx ref. to time t Ψ in s Reacting mass Droplet mass 0 1.0000 1.0000 5 0.9255 0.9210 10 0.9706 1.0092 15 0.9738 1.1284 18 0.6456 0.9404 Figure 5.13 illustrates the NO x and CO emissions as measured from the sounding rocket flight samples. In relation to Figure 2.5, the non-dimensional interdroplet distance S/D ign at ignition is in the range of 12 to 18. Since all precursor experiments included virtually no fuel pre-vaporization and, in addition, have a lower number S/D 0 , they are not plotted in Figure 5.13. The CO emissions are given in ppm, as obtained by FT-IR spectroscopy via Standard No. 1 and No. 2 (Tabs. 3.7 and 3.8). The pre-vaporization times investigated were t Ψ = 5, 10, and 18s, corresponding to pre-vaporization rates of Ψ = 0.1695, 0.2728, and 0.5378 (cf. Tabs. D.1 through D.3, Method No. 2), when accounting for the density decrease due to droplet heat-up during the initial phase of droplet vaporization (cf. Eq. (5.4)). The raw data of these experiments were 175
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TECHNISCHE UNIVERSITÄT MÜNCHEN In
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Vorwort Die vorliegende Arbeit ents
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Kurzfassung Die vorliegende Arbeit
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Contents List of Figures List of Ta
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CONTENTS 5 Results 155 5.1 Droplets
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LIST OF FIGURES xiv 3.3 Droplet Arr
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LIST OF FIGURES 5.19 Progression of
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LIST OF TABLES C.2 Overview of Tele
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NOMENCLATURE g Gravitational force
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NOMENCLATURE σ S Surface tension N
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NOMENCLATURE () T Thermal, temperat
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NOMENCLATURE PAL PAN PDU PFA PFC PH
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1 Introduction In thermal engines t
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1 Introduction 1.3 Oxides of Nitrog
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1 Introduction residence time of th
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1 Introduction Chapter 3 describes
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2 Combustion Theory 2.1.1 Premixed
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2 Combustion Theory molecules by ea
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2 Combustion Theory four types. Out
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2 Combustion Theory ventional combu
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2 Combustion Theory of partial pre-
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2 Combustion Theory Formation of Fu
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2 Combustion Theory Moreover, exper
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2 Combustion Theory For the gas pha
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2 Combustion Theory combustion, ext
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2 Combustion Theory termed “natur
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2 Combustion Theory thin layer of u
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2 Combustion Theory nor generally c
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2 Combustion Theory is assumed to o
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2 Combustion Theory before it follo
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2 Combustion Theory position, which
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2 Combustion Theory 2.3 Kinetic Mod
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2 Combustion Theory 1.2 m s −1 La
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2 Combustion Theory Mole fraction o
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2 Combustion Theory Reburn Miller a
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2 Combustion Theory The results of
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2 Combustion Theory Temperature T 3
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2 Combustion Theory 0.0004 GRI 3.0
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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
Page 149 and 150: 4.2 Basics for Numerical Modeling n
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Page 155 and 156: 4.3 Modeling of Ignition In the end
Page 157 and 158: 4.3 Modeling of Ignition 10 W Time
Page 159 and 160: 4.3 Modeling of Ignition Hence, the
Page 161 and 162: 4.4 Modeling of Nitrogen Oxide Form
Page 163 and 164: 4.5 Simulation of Single Droplets t
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Page 167 and 168: 4.6 Model Validation with index i
Page 169 and 170: 4.6 Model Validation utilized model
Page 171 and 172: 4.6 Model Validation Figure 4.7 dep
Page 173 and 174: 4.6 Model Validation 340 K Ambient
Page 175 and 176: 4.6 Model Validation Table 4.1: Val
Page 177 and 178: 4.7 Scope and Limitations of Single
Page 179: 4.7 Scope and Limitations of Single
Page 182 and 183: 5 Results In total, it includes 99
Page 184 and 185: 5 Results atmosphere of Figure 5.1
Page 186 and 187: 5 Results 32 g kg −1 2400 K Emiss
Page 188 and 189: 5 Results 4.0 g kg −1 Emission in
Page 190 and 191: 5 Results uses constant spatial pos
Page 192 and 193: 5 Results 16 g kg −1 Emission ind
Page 194 and 195: 5 Results Maximum temperature Tmax
Page 196 and 197: 5 Results Even though the parameter
Page 198 and 199: 5 Results Flame stand-off ratio ζ
Page 202 and 203: 5 Results 0.50 5000 ppm Emission in
Page 204 and 205: 5 Results Ψ = 0.1695 (t Ψ = 5 s):
Page 206 and 207: 5 Results 6.0 g kg −1 Emission in
Page 208 and 209: 5 Results combustion in exhaust gas
Page 210 and 211: 5 Results Flame stand-off ratio ζ
Page 212 and 213: 5 Results D 0 being varied between
Page 214 and 215: 5 Results 0.40 g kg −1 Initial dr
Page 216 and 217: 5 Results A twofold trend was obser
Page 218 and 219: 5 Results 5.6 Recommendations and F
Page 221 and 222: 6 Summary and Conclusions In times
Page 223: APPENDIX
Page 226 and 227: A Chemical Mechanisms The earliest
Page 228 and 229: A Chemical Mechanisms only deduced
Page 230 and 231: A Chemical Mechanisms Another, very
Page 232 and 233: A Chemical Mechanisms the reactions
Page 234 and 235: B Investigated Conditions There are
Page 236 and 237: B Investigated Conditions Table B.1
Page 238 and 239: B Investigated Conditions high mech
Page 240 and 241: 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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