Combustion theory and modelling

combustion calculations theory worked examples and problems and internal combustion engine theory
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Published Date:23-07-2017
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Combustion Theory and Applications in CFD CEFRC Combustion Summer School 2014 Prof. Dr.-Ing. Heinz Pitsch Copyright ©2014 by Heinz Pitsch. This material is not to be sold, reproduced or distributed without prior written permission of the owner, Heinz Pitsch. What is Combustion? • What is the difference between combustion and fuel oxidation in a fuel cell? • In contrast to isothermal chemically reacting flows  Heat release induces temperature increase  Thereby combustion is self accelerating • Important  Each chemical or physical process has associated time scale • Interaction of flow (transport) and chemistry  Laminar and turbulent combustion  New dimensionless groups (similar to Reynolds number) • Damköhler number, Karlovitz number, … 2 Combustion Applications: Examples • Premixed combustion Example: SI-engine  Spark-ignition engine  Premixed • Non-premixed combustion Example: Aircraft engine  Diesel engine  Aircraft engine 3 Impact of Combustion Demand for energy: • Transport and electricity • Atmospheric pollution • Global warming 4 DOE’s International Energy Outlook 2011 15 World Energy Consumption 10 Btu • Increase in world wide energy consumption from 2008 until 2035: 53% • Fossil fuels: great share (80%) of the world wide used energy • Mineral oil remains dominating source of energy • Traffic and transport: Share of about 15 World Energy Consumption by Fuel 10 Btu 25% 5 DOE’s International Energy Outlook 2011 • Increase of renewable energy by a factor of 2 • Combustion of fossil fuels remains dominating source of energy • Nearly 80% of energy consumption covered by fossil fuels 6 Greenhouse Gas Emissions • 85% of Greenhouse gas emissions CO 2 EPA Inventory of US Greenhouse Gas Emissions, 2006 7 Sources of CO 2 Combustion of fossil fuels: World Energy-Related CO Emissions billion tons 2 • 95% of CO - emissions 2 • 80% of all greenhouse gas- emissions • Expected increase of CO 2 emissions: 43% from 2008 until 2035 Quelle: International Energy Outlook, 2011 8 Reduction of Greenhouse Gas Emissions Various approaches: • Hydrogen economy • CO -sequestration (Carbon Capture and Storage, CCS) 2 • Bio-fuels • … • Increase in efficiency Combustion Theory 9 New Technologies • Challenge of concurrent optimizing of efficiency, emissions and stability • Examples of new technologies • Aircraft turbines NASA Lean Direct Injection • Lean direct injection (LDI) Aircraft Engine Combustor • Automotive sector • Homogeneous charge compression ignition (HCCI, CAI) • Electricity generation • Oxy-coal combustion • Integrated gasification combined cycle (IGCC) • Flameless oxidation (FLOX) / MILD combustion • Progress in technology increasingly supported by numerical simulations • New technologies often lead to changes in operating range  New challenges in the field of combustion theory and modeling Aim of this Course • Develop understanding of combustion processes from physical and chemical perspectives • Fundamentals:  Thermodynamics  (Kinetics  see parallel course)  Fluid mechanics  Heat and mass transfer • Applications:  Reciprocating engines  Gas turbines  Furnaces 11 Course Overview Part I: Fundamentals and Laminar Flames Part II: Turbulent Combustion 12 Course Overview Part I: Fundamentals and Laminar Flames • Introduction • Fundamentals and mass balances of combustion systems • Thermodynamics, flame temperature, and equilibrium • Governing equations • Laminar premixed flames: Kinematics and Burning Velocity • Laminar premixed flames: Flame structure • Laminar diffusion flames 13 Course Overview Part II: Turbulent Combustion • Turbulence • Turbulent Premixed Combustion • Turbulent Non-Premixed Combustion • Modeling Turbulent Combustion • Applications 14 Fundamentals and Mass Balances of Combustion Systems Combustion Summer School 2014 Prof. Dr.-Ing. Heinz Pitsch Thermodynamics The final state (after very long time) of a homogeneous system is governed by the classical laws of thermodynamics Prerequisites: • Definitions of concentrations and thermodynamic variables • Mass and energy balances for multicomponent systems 16 Course Overview Part I: Fundamentals and Laminar Flames • Introduction • Fundamentals and mass • Definitions, Equation of State, Mass balances of combustion systems Balance • Thermodynamics, flame • Elementary and Global Reactions temperature, and equilibrium • Coupling Functions • Governing equations • Stoichiometry • Laminar premixed flames: • Mixture Fraction Kinematics and Burning Velocity • Laminar premixed flames: Flame structure • Laminar diffusion flames 17 Definitions, Equation of State, Mass Balance • In chemical reactions mass and chemical elements are conserved • Combustion always in (gas) mixtures The mole fraction • Multi-component system with k different chemical species 23 • Mole: 6.0236 ·10 molecules are defined as one mole  Avogadro number N A • Number of moles of species i: n i • Total number of moles: • Mole fraction of species i: 18 The mass fraction • Mass m of all molecules of species i is related to its number of moles by i where W is the molecular weight of species i i • Total mass of all molecules in the mixture: • Mass fraction of species i: • Mean molecular weight W: • Mass fraction and mole fraction: 19 The mass fraction of elements • Mass fractions of elements are very useful in combustion • Mass of the species changes due to chemical reactions, but mass of the elements is conserved • Number of atoms of element j in a molecule of species i: a ij • Mass of all atoms j in the system: where k is the total number of elements in the system, W is e j molecular weight of element j 20