Turbulent Combustion Modeling
(Introduction to Combustion Models in FLUENT, Star CCM+ and OpenFOAM)
Dr. Sharad N. Pachpute (Ph.D.)
Modeling of Multiphysics in Reactive Flows
Introduction to Combustion Modeling
- Combustion involves chemical reactions converting fuel and oxygen to combustion products with light and heat energy. It involves fluid flow, consumption and generation of species by chemical kinetics and heat transfer ( convection and radiation).
- Combustion becomes more complex because of phenomena of flame propagation, slow or fast ignition, explosions, slow ignition, correct fuel to oxidizer ratios, phase change for liquid fuel,and so forth depend in subtle ways on the conditions under which combustion takes place.
- Modeling Chemical Kinetics in Combustion, the user must be familiar with the problems of fast or slow chemistry problems
Basic subjects for Combustion Modeling
- Thermodynamics : Laws of thermodynamics action, thermodynamics states (T, u, V, h, s) and mixture properties ( mass or mole basis) in reactive flows, Heat release rate (HHV/LHV)
- Fluid mechanics: Governing equations in Laminar and turbulent flow
- Heat and mass transfer: Governing equations for conduction, convection and radiative heat transfer, Species transport equations.
- Chemical Kinetics: Reaction rate (forward and backward), consumption and production of species, chemical equilibrium
- Numerical Methods/Basic of CFD Modeling: Discretization method or scheme of governing equations for convection, diffusion and source terms. Pressure-velocity coupling and Solution Procedure
Application of Combustion Modeling
- Burners: LPG gas stoves, Furnaces, Boilers, Gasifier etc.
- Engine : Petrol and diesel engines
- Gas turbine combustor: Power plant steam turbine, air-craft gas turbine combustor etc.
- Safety: protection from fires, explosions or blast etc.
- Emission controls: Design and development of low NOx/SOx burner or combustor
- Examples of Combustion for premixed and non-premixed are given below
- IC Engine examples premixed and non-premixed are given below: the combustion mechanism is changed due to change in mixing and type of fuel
- Typical burner: major components of burners are given as
- Fuel (gas) inlet can me more for low NOx fuel staged burner
- Air inlet can be more for low NOx air staged burners
- Ignition tube (Pilot) is provided to provide ignition energy
- Combustion chamber
- Oil inlet if the burner is multiple fuel based
Governing Equations and Classification of Combustion Models
Governing Equations for combustion
- Mass transfer equation
- Momentum equation
- Energy ( enthalphy ) Equation
- Species transport equation
- Turbulent transport equations (RANS/LES)
- Turbulent chemistry interaction term (TCI)
- The Damköhler number (Da) is dimensionless numbers used in reactive flow to relate the chemical reaction timescale (reaction rate) to the transport phenomena rate (flow time scale) occurring in a system. This number is named after German chemist Gerhard Damköhler.
- Damkohler number, Da = Turbulent flow time/chemistry time
- Da ~ 1: Slow chemistry , Da >> 1 : Fast Chemistry
Example of Fast and Slow Chemistry
- Example of fast chemistry combustion from burner
- Example of slow chemistry combustion from the plume flow from chimney
Classification of Combustion Model based on Da
- Da is an important parameter for selection of reaction models and turbulent chemistry interaction (TCI) models
- As per type of combustion, species transport model is selected
Overview of Models for Combustion
- The overview of combustion models :
- Transport equations
- Turbulence models
- Chemistry of solution
- Type of chemistry: fast/slow chemistry models
- Multi-phase models; solid or liquid fuel
- Radiation models
- Pollutants models
Outline of Combustion Models in ANSYS FLUENT
- Before selecting the models , CFD user must study the basic theory of combustion and assumption for combustion models given in ANSYS FLUENT Theory Guide
Overview of Combustion Models in Star CCM+
- In star CCM+, based on chemistry and type of phase combustion models are selected
- The basic idea of CFD modeling is same irrespective of CFD solvers
- In Star CCM, the multi-component gas combustion is classified based on type of fuel and air mixing and emissions
Classification Based on Mixing Mechanism of Fuel and Oxidizer:
- In the mixing chamber, fuel and oxidizer (air) are already mixed at the molecular level before ignition or combustion
- Cold reactants propagate into hot products
- Rate of propagation (flame speed) depends on the internal flame structure
- Much more difficult to model than non-premixed combustion problems
- Turbulence distorts the laminar
- Examples: SI enginer, LPG gas stove etc.
- Schematic of premixed combustion
- Separate inlet/ streams for Fuel and oxidizer (air) to the combustion chamber
- Convection or diffusion of reactants from either side into a flame sheet
- Turbulent eddies distort the laminar flame shape and enhance mixing
- May be simplified to a mixing problem
- Examples: Burner for boiler /furnaces, coal combustion, candle flame, wood fire etc.
- Schematic of non-premixed combustion
Partially premixed combustion
- In some applications, primary air (oxidizer) is mixed with fuel to form the unburnt mixture before combustion such cases are treated as partially premixed combustion.
- Premixed fuel/oxidizer inlet streams
- Schematic of partially-premixed combustion
Modeling of Premixed Combustion Model
Premixed Flame Structure
- The structure of premixed combustion is given as below
- There are four major zones: unburnt mixture, prehated zones, reaction zones, burned mixture
- The flame is defined within the preheated and reaction zones
- Schematic of Premixed combustion Structure: Local velocity (without the effect wrinkles)
Turbulent Premixed Combustion Models
• In this case, a reaction progress variable is used to tracks the position of the flame front (e.g. Zimont model).
Applicability of Models
- Flow Regime: Turbulent flow (high Re)
- Chemistry: Fast Chemistry
- Configuration: Premixed only
Premixed CFD Models available in FLUENT
- Progress variable (C) equation (RANS based)
- Extended Coherent Flame Model (RANS based)
- cLevel Set (G) Equation (Not RANS based)
Limitations of Premixed CFD Models:
- Cannot realistically model phenomena which depend on detailed kinetics (such as ignition, extinction).
Progress Variable (C-equation)
- On the product side, the progress variable is 1
- On the reactant side, the progress variable (c) is equal to 0
- The flame has no thickness and is represented by the progress variable discontinuity
Governing Equation for Premixed Combustion
Concept of Progress Variable for Premixed flame flame front capturing
Turbulent Flame Speed Model for Premixed Combustion
- The important part of premixed combustion model is the prediction of turbulent flame speed (St),which is normal to the mean surface of the flame.
- The turbulent flame speed is influenced by the following:
- laminar flame speed, which is capitulated by the fuel concentration, temperature, and molecular diffusion properties,
- detailed chemical kinetics flame front wrinkling and stretching by large eddies, and flame thickening by small eddies
Summary of Turbulent flame Speed for Premixed Flames
- Depends upon the thermo-chemical state of the mixture
- Depends upon the turbulence intensity
- Depends upon the turbulence length scale
Zimont Flame Speed Model
- The turbulent flame speed is determined
- The turbulent flame speed can be written in terms of flow and chemistry time scales
Extended Coherent Flame Model (ECFM)
- The turbulent flame area (At) is of critical importance. The ECFM tracks an additional parameter, the flame area density (Σ) along with the progress variable C.
Applicability of ECFM:
- Model valid in the wrinkled regime
- Widely used in Internal Combustion Engine
- The range of applicability of the ECFM model is shown on the Borghi diagram in the following fifure, where the wrinkled flamelets regime is indicated below the line. Typical Internal Combustion (IC) engines typically operate in this wrinkled flamelet range.
- Borghi diagram for turbulent combustion
ECFM-3Z (Extended Coherent Flame Model – Three Zones) model:
- This model is widely used combustion model for in-cylinder simulations.
- The model is provide a sub-grid description of the mixing and combustion processes, where the turbulence timescale enters the equations describing mixing and combustion, and that is important when modeling turbulent combustion in engines.
- Applicability of ECFM-3Z model
- Flow Regime: Turbulent flow (high Re)
- Chemistry: Fast Chemistry
- Configuration: Premixed and Non-premixed
Use CFD Solver for Combustion Modeling:
a) ANSYS FLUENT for Premixed Flame Models:
Modeling of Non-Premixed Combustion Model
Mixture fraction for non-premixed flames
Applicability of Non_premixed Combustion Models
- Flow Regime: Turbulent flow (high Re)
- Chemistry: Equilibrium or moderately non-equilibrium (flamelet)
- Configuration: Non-Premixed
Applications of Diffusion Flame Models:
- Gas reaction (furnaces, burners). This is usually the model of choice if assumptions are valid for gas phase combustion problems.
- Accurate tracking of intermediate species concentration and dissociation effects without requiring knowledge of detailed reaction rates (equilibrium).
Limitations of Models
• Unreliable when mixing and kinetic time scales are comparable
• Cannot realistically model phenomena which depend on detailed kinetics (such as ignition, extinction).
- A turbulent flame brush as an ensemble of discrete, steady laminar flames, called flamelets.
- The individual flamelets are assumed to have the same structure as laminar flames in simple configurations, and are obtained by experiments or calculations.
Diffusion Flame Models
- Use of mixture fraction to decouple the chemical kinetics from the flow dynamics and reduces the burden of solving a large number of species transport equations.
- Under certain assumptions for non-premixed combustion, the thermochemistry can be reduced to a single parameter: the mixture fraction (f or Z)
- The mixture fraction is the mass fraction that originated from the fuel stream
- It is a conserved scalar quantity. Hence, its transport equation does not have a source term
- Combustion is simplified to a mixing problem, and the difficulties due to the closing non-linear mean reaction rates are avoided.
- Once fuel and oxidizer are mixed, the chemistry can be modeled as being in chemical equilibrium with the Equilibrium (chemistry) mode,
- Being near chemical equilibrium with the Steady Laminar Flamelet model, or significantly departing from chemical equilibrium with the Unsteady Laminar Flamelet model can be used
Mixture fraction approach for diffusion flame modeling:
Pre-flamelet computing and look up table for diffusion flame modeling
- Computing the scalar in terms of mean mixture fraction and variance mixture fraction
Modeling of Chemical Kinetics in Diffusion Flames
- The CHEMKIN® software is developed to model the chemically reacting flow configurations due to low computational cost for modeling chemical kinetics
- CHEMKIN GRI Mech-3 is widely used for detailed chemical kinetics for diffusion flame modeling due to low computational for maximum number of reaction steps
- Use of CHEMKIN GRI-Mech 3 of turbulent diffusion flame modeling;
How to get more details of CHEMKIN:
ANSYS FLUENT- Non-premixed Combustion Modeling
Star CCM+ Non-premixed Combustion Modeling
OpenFOAM Non-premixed Combustion Modeling
- Thierry Poinsot, Denis Veynate,Theoretical and Numerical Combustion, 2nd Edn., R T Edwards Inc., Click here to Theoretical_and_Numerical_Combustion_PDF
- De, S., Agarwal, A.K., Chaudhuri, S., Sen, S, Modeling and Simulation of Turbulent Combustion, Springer Publication, Click here: Modeling_and_simulation_combustion_PDF
- Echekki, Tarek, Mastorakos, Epaminondas (Eds.), Turbulent Combustion Modeling, Advances,New Trends and Perspectives, Springer Pub.,2011
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- Masayuki Taniguchi, Fundamental Experiments of Coal Ignition for Engineering Design of Coal Power Plants,
- Zhao F. Tian, Peter J. Witt, Mark P. Schwarz, and William Yang, Numerical_Modelling_of_Pulverised_Coal_Combustion ,Springer (2016)