Sara McAllister • Jyh-Yuan Chen A. Carlos Fernandez-Pello

Fundamentals of Combustion Processes

Spri inger

Contents

1 Fuels 1

1.1 Types of Fuel 1 1.2 Fuel Usage 5 1.3 Basic Considerations of the Choice of Fuels 6 1.4 Classification of Fuels by Phase at Ambient Conditions 8 1.5 Identification of Fuel by Molecular Structure: International

Union of Pure and Applied Chemistry (IUPAC) 9 1.6 Some Related Properties of Liquid Fuels 12 Exercises 13 References 13

2 Thermodynamics of Combustion 15

2.1 Properties of Mixtures 15 2.2 Combustion Stoichiometry 17

2.2.1 Methods of Quantifying Fuel and Air Content of Combustible Mixtures 19

2.3 Heating Values 23 2.3.1 Determination of HHV for Combustion Processes

at Constant Pressure 24 2.3.2 Determination of HHV for Combustion Processes

from a Constant-Volume Reactor 27 2.3.3 Representative HHV Values 29

2.4 Adiabatic Flame Temperature 31 2.4.1 Constant-Pressure Combustion Processes 31 2.4.2 Comparison of Adiabatic Flame Temperature

Calculation Methods 36 2.5 Chapter Summary 40 Exercises 44

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xii Contents

3 Chemical Kinetics 49

3.1 The Nature of Combustion Chemistry 50 3.1.1 Elementary Reactions: Chain Initiation 51 3.1.2 Elementary Reactions: Chain Branching 51 3.1.3 Elementary Reactions: Chain Terminating

or Recombination 52 3.1.4 Elementary Reactions: Chain Propagating 52

3.2 Elementary Reaction Rate 52 3.2.1 Forward Reaction Rate and Rate Constants 52 3.2.2 Equilibrium Constants 54

3.3 Simplified Model of Combustion Chemistry 55 3.3.1 Global One-Step Reaction 55 3.3.2 Pressure Dependence of Rate of Progress 61 3.3.3 Heat Release Rate (HRR) 61 3.3.4 Modeling of Chemical Kinetics

with Detailed Description 61 3.3.5 Partial Equilibrium 65 3.3.6 Quasi-Steady State 65

Exercises 69 References 73

4 Review of Transport Equations and Properties 75

4.1 Overview of Heat and Mass Transfer 75 4.2 Conservation of Mass and Species 78 4.3 Conservation of Momentum 80 4.4 Conservation of Energy 80

4.4.1 Terms in the Conservation of Energy Equation 80 4.4.2 Derivation of a 1-D Conservation of Energy Equation 82

4.5 Normalization of the Conservation Equations 84 4.6 Viscosity, Conductivity and Diffusivity 87 References 88

5 Ignition Phenomena 89

5.1 Autoignition (Self-ignition, Spontaneous Ignition) Based on Thermal Theory 89

5.2 Effect of Pressure on the Autoignition Temperature 93 5.3 Piloted Ignition 95 5.4 Condensed Fuel Ignition 98

5.4.1 Fuel Vaporization 98 5.4.2 Important Physiochemical Properties 99 5.4.3 Characteristic Times in Condensed Fuel Ignition 100 5.4.4 Critical Heat Flux for Ignition 106

Exercises 108 References 109

Contents xiii

6 Premixed Flames I l l

6.1 Physical Processes in a Premixed Flame I l l 6.1.1 Derivation of Flame Speed and Thickness 113 6.1.2 Measurements of the Flame Speed 117 6.1.3 Structure of Premixed Flames 119 6.1.4 Dependence of Flame Speed on Equivalence Ratio,

Temperature and Pressure 121 6.1.5 Dependence of Flame Thickness on Equivalence Ratio,

Temperature and Pressure 125 6.2 Flammability Limits 125

6.2.1 Effects of Temperature and Pressure on Flammability Limits 127

6.3 Flame Quenching 127 6.4 Minimum Energy for Sustained Ignition and Flame

Propagation 130 6.5 Turbulent Premixed Flames 133

6.5.1 Eddy Diffusivity 133 6.5.2 Turbulent Flame Speed 134

6.6 Summary 135 Exercises 136 References 137

7 Non-premixed Flames (Diffusion Flames) 139

7.1 Description of a Candle Flame 139 7.2 Structure of Non-premixed Laminar Free Jet Flames 140 7.3 Laminar Jet Flame Height (Lf) 142 7.4 Empirical Correlations for Laminar Flame Height 145 7.5 Burke-Schumann Jet Diffusion Flame 147 7.6 Turbulent Jet Flames 149

7.6.1 Lift-Off Height (h) and Blowout Limit 151 7.7 Condensed Fuel Fires 152 Exercises 153 References 154

8 Droplet Evaporation and Combustion 155

8.1 Droplet Vaporization in Quiescent Air 155 8.1.1 Droplet Vaporization in Convective Flow 159

8.2 Droplet Combustion 162 8.3 Initial Heating of a Droplet 164

8.3.1 Effect of Air Temperature and Pressure 166 8.4 Droplet Distribution 171 Exercises 174 Reference 175

Contents

Emissions 177

9.1 Negative Effects of Combustion Products 177 9.2 Pollution Formation 178

9.2.1 Parameters Controlling Formation of Pollutants 179 9.2.2 CO Oxidation 182 9.2.3 Mechanisms for NO Formation 183 9.2.4 Controlling NO Formation 189 9.2.5 Soot Formation 190 9.2.6 Relation Between NOx and Soot Formation 191 9.2.7 Oxides of Sulfur (SOx) 191

9.3 Quantification of Emissions 193 Exercises 196 References 198

Premixed Piston IC Engines 199

10.1 Principles of SI Engines 199 10.2 Thermodynamic Analysis 201 10.3 Relationship between Pressure Trace and Heat Release 206 10.4 Octane Number 207

10.4.1 Definition of Octane Rating 207 10.4.2 Measurement Methods 208

10.5 Fuel Preparation 210 10.6 Ignition Timing 213 10.7 Flame Propagation in SI Engines 214 10.8 Modeling of Combustion Processes in IC Engines 215

10.8.1 A Simplified Two-Zone Model of Engine Combustion 216

10.9 Emissions and Their Control 219 10.9.1 Three-Way Catalyst 220

10.10 Gasoline Direct Injection (GDI) Engines 221 Exercises 224 References 226

Diesel Engines 227

11.1 Overall Comparisons to SI Engines 227 11.1.1 Advantages of Diesel Engines as Compared

to SI Engines 228 11.1.2 Disadvantages of Diesel Engines as Compared

to SI Engines 228 11.2 Thermodynamics of Diesel Engines 229 11.3 Diesel Spray and Combustion 230 11.4 Cetane Number 235

Contents xv

11.5 Diesel Emissions 237 11.6 Homogeneous Charge Compression Ignition (HCCI) 238

11.6.1 HCCI Overview 238 11.6.2 HCCI Emissions 238 11.6.3 Challenges with HCCI 240

References 241

Appendices 243

Index 299