Computational Fluid Dynamics of Turbulent Combustion
The aim of this Best Practice Guidelines (BPG) is to provide guidelines to CFD users in a wide range of application areas where combustion is an essential process. Its overall structure is as follows: Chapters 1-3 summarise the key issues in model formulation, Chapter 4 is addressing validation of modeling using available experimental databases. Then, two application areas are elaborated in separate chapters: Chapter 5 on Internal Combustion Engines, and Chapter 6 on Gas Turbines. Best practice guidelines by the nature of technology development are always temporary. New insights and approaches will take over after some time. Therefore this BPG ends with a Chapter 7 on Emerging Methods, providing a preview of approaches so far only useful for simulating canonical configurations or requiring further developments.
A comprehensive CFD approach to turbulent combustion modeling relies on appropriate sub-models for flow, turbulence, chemistry and radiation, and their interactions. In the framework of this BPG, knowledge of turbulent flow modeling is pre-requisite and only briefly explained. Instead the discussion on models is divided in three parts: turbulence-chemistry interaction (Chapter 1), chemistry (Chapter 2) and radiative heat transfer (Chapter 3). Many of the models introduced in the first three chapters will reappear in the discussion in Chapter 4 to 6 and comments on challenges, advantages and disadvantages are formulated in all chapters.
Those looking for the immediate advice to tackle a specific application may want to proceed immediately to the application chapters (IC engines in Chapter 5 an Gas Turbines in Chapter 6) and return to the basic chapters when necessary. But everyone not finding in these chapters an immediate answer to the basic question: "What is the best model for my specific application?" should certainly spend some time on Chapter 4, because it addresses the mandatory preliminary steps that have to be considered to validate a simulation involving any sort of turbulent flames.
We hope that many readers will find this BPG useful. Feedback on possible improvement is welcome. Instructions on how to provide such a feedback is available on the ERCOFTAC webpages. When appropriate, the information received will be included in the courses promoting the use of this BPG and in future additions or updates.
Dirk Roekaerts and Luc Vervisch, July 2015.
1. Introduction to turbulent combustion modeling 1.1 Background and challenges 1.2 Major length and time scales 1.3 Generic scalar balance equation 1.3.1 Eddy viscosity – RANS & LES 1.4 Modeling review 16 1.4.1 Fixed flame structure 1.4.2 Micro-mixing modeling 1.4.3 LES-specialized turbulent combustion modeling 1.5 Conclusion 26
2. Combustion Chemistry 2.1 Introduction 2.2 Chemical Source Terms 2.2.1 Need for source terms 2.2.2 Rate laws 2.2.3 Rate laws for elementary reactions 2.3 Combustion Kinetics 2.3.1 Formation of Radicals 2.3.2 Fuel oxidation 2.4 Time Scales of Chemical kinetics 2.5 Detailed Reaction Mechanisms 2.5.1 State of the art 2.5.2 Different strategies for the mechanism development 2.5.3 Different strategies for optimization 2.5.4 Available Detailed Mechanisms 2.5.5 Implementation 2.5.6 General remarks 2.6 Skeleton Mechanisms 2.6.1 Construction of skeleton mechanisms based in intuition 2.6.2 Mathematical analysis 2.6.3 General remarks 2.6.4 Adaptive Chemistry 2.7 Principles of Reduced Kinetic Models 2.8 Knowledge Based Model Reduction 2.8.1 Methods for Analysis 2.8.2 Implementation of steady state and partial equilibrium assumptions 2.9 Manifold Methods 2.9.1 Principles of Manifold Methods 2.9.2 Identification of Low-Dimensional Manifolds 2.10 Global Mechanisms 2.11 Tabulation Strategies 2.12 Implementation of reduced schemes 2.13 Conclusions 63
3. Thermal Radiation 3.1 Radiative Heat Transfer in Combustion Systems 3.1.1 Radiative transfer equation 3.1.2 Absorptivity, emissivity, transmissivity 3.1.3 Radiative heat flux and radiative source term 3.2 Solution Methods for Thermal Radiation 3.2.1 Optically thin approximation 3.2.2 Optically thick approximation 3.2.3 Spherical harmonics method 3.2.4 Discrete transfer method (DTM) 3.2.5 Discrete ordinates method (DOM) 3.2.6 Finite volume method (FVM) 3.2.7 Monte Carlo method 3.2.8 Other solution methods 3.3 Radiative Properties of Gases 3.3.1 Line-by-line model 3.3.2 Narrow band models 3.3.3 Wide band models 3.3.4 Correlated k-distribution method 3.3.5 Global models 3.3.6 Total gas radiative properties 3.3.7 Other gas property models 3.4 Radiative Properties of Particles 3.4.1 Radiative properties of soot particles 3.4.2 Radiative properties of coal particles 3.4.3 Radiative properties of biomass particles 3.4.4 Radiative properties of droplets 3.4.5 Radiative properties of porous media 3.5 Turbulence-Radiation Interaction 3.6 Computational Implementation of Radiation Models 3.7 Summary of Guidelines 3.7.1 Selection of the radiation model and RTE solution method 3.7.2 Selection of the radiative properties model 3.7.3 Computational Implementation 3.8 Examples of Validation Studies and Applications 3.8.1 Example 1: Comparison of radiative transfer models 3.8.2 Example 2: Comparison of spectral models 3.8.3 Example 3: Application to a laboratory combustor (BERL furnace) 3.8.4 Example 4: Application to MILD combustion 127
4. RANS and LES validation 4.1 Reacting versus non-reacting CFD validation 4.1.1 Why validate a CFD code? 4.1.2 Validation of Combustion CFD codes 4.1.3 Steps in a CCFD validation simulations 4.1.4 Analysis of CCFD validation simulations 4.1.5 Differences in CCFD validation for RANS and LES codes 4.2 Non premixed validation flames 4.2.1 Combustion physics 4.2.2 Overview of some available validation flames 4.2.3 Piloted Jet Flames 4.2.4 Bluff-Body Flame 4.2.5 Conclusions and best practice guidelines non premixed flames 4.3 Fully premixed validation flames 4.3.1 Combustion Physics and Remarks for Usability as Validation Flames 4.3.2 Premixed Turbulent Validation Flames 4.3.3 Atmospheric 48 mm Bunsen Flames from Erlangen 4.3.4 High Pressure Bunsen Flames from Orleans 4.3.5 Large Sudden Expansion Combustor from PSI 4.3.6 Conclusions and best practice guidelines for premixed turbulent flames 4.4 Partially Premixed / stratified validation flames 4.4.1 Combustion physics 4.4.2 Overview over validation flames 4.4.3 Swirl burner (TECFLAM) 4.4.4 Stratified burner 4.4.5 Conclusions and best practice guidelines for partially premixed flames 4.5 General conclusions
5. Internal Combustion Engine 5.1 Introduction 5.1.1 Generalities 5.1.2 Engine Typologies and Technology Trends 5.1.3 Structure of the Chapter 5.2 On Turbulent Flow Modelling in IC Engines 5.2.1 Large Eddy Simulation for ICE 5.2.2 Hybrid URANS/LES and their Applications in ICE Engines 5.2.3 Numerical Method, Quality Assessment and Postprocessing 5.2.4 Best Practice Guidelines 5.3 Spray Description, Atomization and Modelling 5.3.1 Generalities 5.3.2 Spray Description 5.3.3 Spray Atomization Models 5.3.4 Single Component Spray Modeling 5.3.5 Best Practice Guidelines 5.4 Multicomponent Droplet Evaporation 5.4.1 Introduction and Outline 5.4.2 Review of Modeling Approaches 5.4.3 Model Formulation 5.4.4 Application Examples 5.4.5 Best Practice Guidelines 5.5 Ignition Process and Combustion Models 5.5.1 Ignition process 5.5.2 Combustion Models 5.5.3 Best Practice Guidelines 5.6 Best Practice for Combustion LES and HLR 5.6.1 Choice of Models 5.6.2 Choice of Mesh 5.6.3 Choice of Discretisation Scheme 5.6.4 Choice of Time Step 5.6.5 Statistics and Required Number of Engine Cycles 5.7 Validation and Concluding Remarks
6. Gas-Turbines 6.1 Introduction 6.2 Modeling and computational issues 6.2.1 Combustor components 6.2.2 Turbulent combustion modeling 6.2.3 Modeling versus computing power 6.3 Pre-requisites for efficient CFD case studies 6.3.1 Setup phase 6.3.2 Run monitoring 6.3.3 Validation and exploitation 6.3.4 Recommendations to new CFD users 6.4 Conclusions and perspectives
7. Emerging methods 7.1 Introduction: why should you read this chapter? 7.2 Present problems and (future) solutions 7.3 The no-model approach: Direct Numerical Simulation 7.4 Developing, testing, and calibrating models 7.5 Solving computational limitations 7.6 Advanced and alternative numerical techniques 7.7 Uncertainty Quantification 7.8 Conclusions
ISBN - 978-0-9955779-0-9
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