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

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A Chemical Mechanisms only deduced to describe the high temperature oxidation of gaseous alkane hydrocarbons up to butane (C 4 H 10 ). Ennetta et al. [124] compare the four-step, quasi-global mechanism of Jones and Lindstedt [199] and a single-step kinetic of Duterque et al. [106] with the detailed GRI 3.0 mechanism [408] for methane combustion. The authors conclude that global mechanisms can provide results that are in good agreement with more detailed mechanisms with regard to particular engineering applications. However, the reduced mechanisms are incapable of predicting pollutant emissions. The major drawback of all global mechanisms is that they do not account for chain branching reactions involving radicals [381, 474]. These are essential for auto-ignition at low temperatures. Still, utilization of the above mechanisms may well be an option for high temperature applications, when the limiting factor for ignition is thermal feedback rather than reproduction of the chain branching reactions. In contrast, Sazhin [381] refers to the Shell model [167, 168, 385] as the most commonly employed mechanism accounting for auto-ignition in automotive applications. This model captures the essential features of the combustion process, such as chain branching and intermediate species, but it does not reproduce the actual physics and chemistry. It is an eight-step scheme incorporated into four processes, employing generic species with chemical reaction constants deduced from experiments. A.2 Concepts of Kinetics Reduction Apart from global kinetics presented in Chapter A.1, there are various concepts to obtain user-specific mechanisms consisting of a lower number of species and reactions. In comparison with detailed mechanisms, reduced mechanisms typically are free of the fastest timescales but still able to reproduce the most essential features of the problem under investigation. Conventional reduction methods require the selection and elimination of particular species and reactions by the user. Starting from detailed mechanisms, reduced kinetics are derived assuming reactions in partial equilibrium and species at quasi-steady state. Comparing the timescales of different reactions, the combustion process is limited by the slower reactions, while the fast ones are in partial equilibrium [149, 336, 443]. In order to save computational 202
A.2 Concepts of Kinetics Reduction power, the net reaction rates of the latter can be set to zero, leading to algebraic relations. The steady-state approximation searches for species whose production and consumption cancel out immediately. By doing so, many intermediate species can be eliminated from a detailed mechanism if their net production rates tend towards zero. In practice, species are regarded as quasisteady if their net production rate is small compared to their respective production and consumption rates [11, 43, 336]. This process of analyzing the timescales was successfully automated, resulting in the so-called computer assisted reduction method (CARM). It evaluates reaction rates and automatically eliminates species at quasi-steady state on the basis of results obtained from perfectly stirred reactor models, employing more or less detailed mechanisms [67, 90, 299]. The computational singular perturbation (CSP) method investigates the dynamics of the source term vector and tries to find the directions in which it will rapidly reach steady-state. Eigenvalues and eigenvectors of the Jacobian matrix of the source term represent chemical timescales and reaction groups, respectively [11, 43, 256]. The intrinsic low-dimensional manifold (ILDM) method was presented by Maas and Pope [262] in 1992. Similar to the CSP method, the ILDM method tries to find directions in state space in which the source term rapidly reaches steady-state. The method is based on the assumption that a combustion system follows certain trajectories in state space during the combustion process. However, these trajectories are not associated with any particular species or reaction. Since the steady-state assumption is applied locally, different reaction paths can be captured. The common part of all trajectories can be described as a parameterized curve of only a single progress variable. Assuming all but two processes to be in steady-state results in a surface parameterized by two variables and so on. This process generally leads to intrinsic lowdimensional manifolds, in which the number of dimensions represents the number of slow processes. However, the higher the number of dimensions, the more rapidly the trajectories through state space will approach the manifold. As time approaches infinity and independently of the initial conditions, all solution trajectories of stable systems will approach the same equilibrium point. Thus, movement along the ILDM corresponds to the evolution of the predefined slow processes, and the equilibrium point effectively would be a zero-dimensional manifold [191, 262, 399, 400]. 203
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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
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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
Page 177 and 178: 4.7 Scope and Limitations of Single
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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 200 and 201: 5 Results 5.2.3 Comparison with Mic
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 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
Page 250 and 251: D Raw Data of Microgravity Experime
Page 256 and 257: SUPERVISED THESES Student Sebastian
Page 259 and 260: References [1] J.A. Aardenne van, G
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