Lecture 1: Introduction to Reacting Flows 15.0 Release
Advanced Combustion Training
Outline •
Introduction to combustion
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Combustion applications
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CFD modeling
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Types of flames
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Role of turbulence
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Fast vs. slow chemistry
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Reacting flow models in Fluent 15
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Modeling examples at a glance
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Non-dimensional numbers
Combustion •
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One of the most important aspects of human life Important technological innovation during the Paleolithic period (2.5 to 2 M years ago) Early usage of combustion (fire) – To provide a source of light and heat – To protect early humans from wild animals – To cook food
It fostered a sense of community for for the groups of people gathered around it
Modern Use of Combustion Boilers Rockett Propulsion Rocke Gas Turbine Combustors Gas Flares IC Engines
Cement Kilns Steel Making
Several Other Applications!!
Other Important Reacting Flows Thought, combustion is the dominant form of the reacting flows, other forms of reacting flows are also of importance! Biochemical and Biomedical
CVD
Electro-Chemicall Reactions Electro-Chemica
Battery
Micro-reactors
Anode off-gas
Pollution Control Fire and Fire Protection
Reacting Flows: CFD Modeling •
Devices are very complex – Complex geometry – Complex boundary conditions – Complex physics: Turbulence; Multi-phase; Chemistry; Radiation,…..
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CFD modeling to gain insight and understanding – Flow field and mixing characteristics – Temperature Temperature field – Species concentrations – Particulates and pollutants
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Better insight helps to reduce redu ce expensive experiments
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Eventually better designs!
Types of Flames Diffusion
Premixed
Close
Open
Diffusion flames • •
Separate streams for fuel and oxidizer Convection or diffusion of reactants from either side into a flame sheet
Premixed flames •
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Fuel and oxidizer are already mixed at the molecular level prior to ignition Flame propagation from hot products to cold reactants Rate of propagation (flame speed) depends on the internal flame structure
Air Hole Opening
Fuel Oxidizer Fuel + Oxidizer
Diffusion flame
Premixed flame
Why Reacting Flow Modeling is Complex? •
Turbulence – Most industrial flows are turbulent – Full resolution is not possible: wide range of time and length scales involved
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Chemistry sin gle or – Realistic chemical mechanisms cannot be described by a single two reaction equations •
Tens of species, hundreds of reactions
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Known in detail for only a limited number of fuels
– Stiff kinetics: Wide range of reacting time scales •
Turbulence-chemistry interaction Reaction rates sensitive to local changes due to enhanced en hanced mixing
Role of Turbulence in Reacting System • •
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Reactions and turbulence affect each other Turbulence is modified by flames – Through flow acceleration, modified kinematic viscosity Modified turbulence alters the flame structure – Enhanced mixing and chemical reactions (through temp and species/radicals fluctuations)
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Mixing time scale ( F ) relative to chemical time scale ( chem) – Important parameter to decide whether the reaction is mixing limited or chemically limited
– Mixing time scale in turbulent flows (for RANS models) = – Damkohlar Number ( Da) = •
Da > 1
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Da ≤ 1
Fast chemistry (thin flame structure) Finite rate chemistry
Fast vs. Slow Chemistry Fast Chemistry • •
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Da >> 1 Reactions limited by turbulent mixing Selection of turbulence closure model is important Combustion in • • • • •
Furnaces Boilers Gas Turbines Gasifiers, Incinerators Flares, etc.
Slow Chemistry • •
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Da ~ 1 Reactions limited by chemistry and turbulence interactions interactions Turbulence/chemistry interactions are important Selection of reaction mechanism is important Reactions associated with • • • •
Pollutants formation Ignition and Extinction Chemical Vapor Deposition (CVD) Non-Equilibrium Non-Equilibrium Phenomenon
Overview of Combustion Modeling Solid or Liquid Fuels (DPM)
Transport Equations • • • •
Mass Momentum Energy Turbulence
Fast Chemistry Models •
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Turbulence Models •
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RANS: k-e, k-w, RSM….. LES, DES, SAS ….
Chemistry Solution • •
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Species OR Mixture fraction OR Progress variable OR ……
Combusting Flow Solution
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Eddy Dissipation model Premixed model Equilibrium model Steady Laminar Flamelet model Flamelet Generated Generated Manifold model Partially premixed model
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Radiation Models •
Finite Rate (Slow) Chemistry Models • • •
Laminar Finite rate model EDC Composition PDF
Droplet/particle dynamics DEM collisions Evaporation Devolatilization Heterogeneous reaction
P1, DO (Gray/Non-gray)
Real Gas Effects •
SRK, ARK, RK, PR, UDRGM
Pollutant Models •
Combustion Models
NOx, Soot, SOx
Models for Additional Physics
Reacting Flow Models in Fluent 15.0 Flow Configuration Premixed Combustion
Non-Premixed Combustion
Partially Premixed Combustion
Finite Rate/Eddy Dissipation Model (Species Tr Transport) ansport)
y r t s i m e h C
Fast Chemistry Closures
Premixed Combustion Premixed Model
Non-Premixed Equilibrium Non-Premixed Model
Reaction Progress Variable
Mixture Fraction
Partially Premixed Model Reaction Progress Variable + Mixture Fraction
Steady Laminar Flamelet Model
Finite Chemistry Closures
Flamelet Generated Generated Manifold Model (Premixe (Premixed/Diffusion) d/Diffusion)
Finite Rate
Laminar Finite Rate Model
Chemistry Models
Eddy-Dissipation Concept (EDC) Model
Unsteady Laminar Flamelet Model
Composition PDF Transport Model
Additional Distinctive Capabilities • • •
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Materials database Robust and accurate solver Solution-adaptive mesh refinement – Conformal and hanging-node Industry-leading parallel performance Several chemistry acceleration tools for modeling detailed chemistry ch emistry User-friendly GUI, post-processing and reporting Highly customizable through User Defined Functions Zone-based definition of volumetric and surface reaction mechanisms – Reactions can be turned off/on in different fluid zones – Allow different reaction mechanisms in different fluid zones Reacting Channel Model – 1D model for modeling reactions within a bundle of tubes without resolving the mesh inside them
Modeling Examples: Fast Chemistry Models Gasifier
Furnace Gas Turbine
CO mass fraction
Modeling Examples: Slow Chemistry Models Reacting Rocket Plume
CVD
Hydrogen auto-ignition Velocity Velocity (m/s) R e e n t r y p a c k a g e
Non Dimensional Numbers •
Reynolds Number =
=
are characteristic density, velocity, length and dynamic viscosity, respectively
– , U, L,
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e.g. Inlet conditions
– Turbulence models are valid at high Re
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Damkohler Number
= ~ ~
– Adiabatic flame density – Slowest reaction rate at and stoichiometric concentrations
– Gas phase turbulent combustion models valid at high
Non Dimensional Numbers (cont…) •
Mach Number =
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=
– Mixture fraction based models are valid at Ma < 0.3 •
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Karlovitz number 0
s L
/
= –
( ) ~ Stefan-Boltzman constant 5.672 x 10-8 W/m2K4
e.g. Incompressible flows
Ka
Boltzman Number
~
chemical time scale Kolmogorov Kolmogorov time scale scale
- Impact of the turbulence on the flame structure
– Assumes convection overwhelms conduction – Radiation is important at Bo < 10
