CFD Modeling of natural gas burner Using ANSYS FLUENT
Introduction to Natural Gas
Composition of Natural Gas
 Natural gas is a type of petroleum product that commonly associates with crude oil
 Natural gas is one of important fuel which widely used for in thermal power plant for electricity generation, heating of metals in steel industries, domestic cooking and IC engines (https://justenergy.com/blog/whatisnaturalgasneedtoknow/)
 Natural gaseous fuels are an essential clean source of energy for reducing harmful pollutants (like CO, NO)
 Characteristics of Natural gas: Colourless highly flammable gaseous
 The natural gas primarily consist of pure hydrocarbon like methane and ethane. These hydrocarbons are in gaseous phase for atmospheric condition
 It may contain other hydrocarbon like propane, butane, pentane, and hexane. Butane and propane are stored as liquefied petroleum gas
Natural Gas Burner
 Natural gas burner can be premixed or nonpremixed
 In premixed natural gas burner, air and fuel mixes prior to combustion
 In nonpremixed or partially premixed burner, air and fuel have separate inlet in the burners
 Selection of burner depends on follwing factors:
 Basics of combsution : types of fuels, fuel composition, turnulent or laminar flame, premixed or nonpremixed flames, chemical kinetics
 Pollution controling technqiues: excess air ratio, air or fuel staged burners used in idnustries, adibatic flame temperature
 Application of Burners: natural gas burner s popular in fired heaters, coal fired burners in boilers, coal gas burners in furnaces or cement kiln
 Flames generated from natural gas burner is bluish in colour. The flame does not produce pollution. Combustion modeling of natural gas burner is easier compared to heavy hydrocarbon fuels.
CFD Model of Natural Gas Burner
 CFD analysis has been carried out for a simple burner model to study the effect different combustion models
 Velocity distribution in the various sections of the duct
 Pressure distribution in various sections of the duct
 Mixing of fuel and air
 CFD modelig of gas burners depends on follwing subjects
Geometry of Natural gas Burner
 A 2D Geometry created in ANSYS ICEM CFD. The burner consist of simple air and fuel inlets.
 Burner is placed at the bottom of cylindrical heater.
 Mesh Model of Burner
 Structured mesh genereted in ANSYS ICEM CFD with hexahedral elements
 High mesh density used near fuel ports and high velocity gradient regions
Governing equations
Modelings of Turbulent Combustion
 The laminar – flamelet combustion model used to simulate nonpremixed (diffusion) flames
 This model consider laminar that a turbulent diffusion flame consists of diffusion flamelets
 Using the mixture fraction (Z) and flamelet equation, the modelling of turbulence chemistry interaction becomes easier by decoupling of finiterate chemical kinetics
 For a general hydrocarbon–oxygen reaction,
 A mixture fraction is defined as
 In above equation subscript 1 id for fuel streams and 2 for oxygen streams
 ξ_{c}, ξ_{H}, and ξ_{o} presents the mass fractions for C, H, and O elements respectively. M_{c}, M_{H}, and M_{o} denotes their atom weights, respectively, ν_{o2} is the stoichiometric coefficient
 The concept of instantaneous laminar flamelets is illustrated as below
 The value of Z denotes the local position of the flamelet for given time and space
 For Reynolds averaged equations for transport equation of mean mixture fraction considering Favre averaging
 Equation for the variance of mean mixture fraction
The last tern is modeled as the mean scalar dissipation rate
Enthalpy transport equation for combustion
 The first term on the right side of above equation describes temporal mean pressure changes. This term is important for the combustion modeling of internal combustion engines for nonpremixed flames (Diesel engine). It is neglected for other applications
 The mean volumetric heat exchange term is radiative heat exchange which has an effect on the local enthalpy balance. Radiative heat transfer is significant in many industrial application
Equation for Turbulent Jet Diffusion Flame
 In many industrial applications fuel enters into the combustion chamber as a turbulent jet, with or without swirling effect
 To understand, the basic properties of jet diffusion flames, we can consider here the simplest 2D the axissymmetric jet diffusion flame without buoyancy force
 In this case, we can determine the flame length based on CFD simulation. The flame length is defined as the axial distance from the nozzle exit to the point on the centerline of the flame where the stoichiometric mean mixture fraction (Zst) exist there
 Mass conservation equation
 Momentum equation
 Mean mixture fraction
 For mean combustible flow, the round turbulent jet flame looks as shown in the following figure
 The scalar dissipation rate for turbulent flame speed is calculated as function of diffusivity and gradients of mixture fraction
 To model premixed turbulent model, a progress variable is defined as
 The Favreaveraged transport equation for progress variable c after neglecting the molecular transport
Where, double bar over variables denote the Favreaveraged values. Variables with two primes symbols are the fluctuation values in the Favre averaging
 The reaction rate (ω) is determined as
Where,
l_{0} denotes the local strain (flame stretch) factor based on laminar flame velocity.
S_{L0 }presents the upstretched laminar flame velocity.
∑ presents the flame surface density (flame area per volume)
Premixed Combustion Models
In premixed combustion model, temperature depends is adiabatic or nonadiabatic condition of combustion chamber
 Adiabatic Combustion
 Adiabatic Temperature of Burnt gas Products (Tad ) is the maximum temperature of the burnt products under adiabatic conditions. This temperature need to be specified for adiabatic combustion model
 To calculate the adiabatic temperature of premixed flame , the linear variation of temperature is assumed as : T = (1 − c) × Tu + c × Tad
 Nonadiabatic combustion
 For nonadiabatic combustion models, the heat of generation per unit mass of fuel (heating value) and fuel composition need to be specified either mole or mass fraction basis
 The solver uses the values of heating value and fuel composition determine the heat losses or gains due to combustion. Governing equation are coupled and considers heat loss or gains in the energy equation which determine the temperature of flue gases
CFD Modeling of Chemical Kinetics
Two step airmethane reaction
 During combustion, the reaction of hydrocarbons with oxygen occurs quickly but the oxidation of CO to CO_{2} is slow.
 Considering all the reactions, the CO oxidation cannot accurately describe what actually occurs soon after ignition
 Twostep mechanism for methaneair:
CH_{4} + 1.5O_{2} = CO + 2H_{2}O
CO + 0.5O_{2} = CO_{2}
 Conservation of mass and species has to be enforced in all combustion models
Combustion Mechanism in ANSYS FLUENT
 Combustion mechanism or formation of flames depend on mixing of reactants (fuel and oxidizer) and rate of reactions due to chemical kinetics. Thus, the relative speed of flow mixing and chemical reaction to mixing is important to characterize the phenomena
 In combustion, Damköhler Number (Da) is one of the important dimensionless numbers to decide the speed of chemical reactions relative turbulent mixing time.
Da = Mixing Time Scale / Chemical Time Scale.
 Fast Reactions (Da >> 1):
 For, fast reaction in combustion, turbulent mixing time is much higher than chemical reaction time of reactants
 Chemical reaction rates are fast and mixing time limits the progress of combustion
 Eddy Dissipation Model (EDM) are used in some of the commercial programs in this case
 The eddydissipation model calculates the rate of reaction considering fast chemical kinetics compared to the mixing rate of reactants due to turbulent fluctuations (eddies).
 In eddy dissipation model, mixture is burnt approximately and complex chemical kinetics can be neglected.
 Slow Reactions (Da < 1):
 Chemical reaction rates are slow compared to flow time
 chemical kinetics limit reaction rate compared to mixing rate of fuel and oxidizer and also limits progress of combustion
 To model slow combustion, Finite Rate Chemistry Model (FRC) is more suitable
Combustion of homogeneous fuelair mixture
 PDF Mixture Fraction model represents nonpremixed combustion models.
 Instead of solving species transport equation transport of species, two equations of average mixture fraction and variance of the average mixture fraction models nonpremixed combustion
Species Transport Model for methane and air
 The following figure shows the set up for mixture of methane and air with two step reaction
 Select volumetric reaction and eddy dissipation model as turbulent chemistry reaction model
 Eddy dissipation model is suitable for current natural gas combustion
 Set for chemical kinetics of two reactions in ANSYS FLUENT with correct stoichiometric coefficients and constants of Arrhenius rates
Partially Premixed Flame Model in FLUENT
 In partially premixed combustion systems consists of premixed flames with nonuniform fueloxidizer mixtures
 Partially premixed flames comprise premixed jets discharging into a quiescent atmosphere, lean premixed combustors with diffusion flames and imperfectly mixed inlets
 The progress variable defines unburnt and burnt region either side of flame front. It is value is zero for unburnt mixture and one for burnt mixture
 The species mass fractions, temperature and density are calculated from the progress variables
 Within the flame region (0 < c < 1), a linear combination of both unburnt and burnt mixtures determines the flame speed
 Selection of Partially Premixed Flame Model
 Generation of flamelet model in ANSYS FLUENT
 3D isosurface of flamelet is a function of three variables temperature
Premixed combustion Model in FLUENT
 Select suitable premixed model
 Progress variable
 Select flame speed models: Zimont or peter flame speed model
 Extended coherent flame speed model
Radiation Model
 In combustion of boiler or furnaces, majority of heat transfer occurs by thermal radiation
 Radiative heater transfer determines the correct flame temperature. Hence, CFD users need to select a suitable ration model
 the Discrete Ordinate Model (DOM) is commonly used robust radiation model for widest range of optical thickness
 This model considers the effect of the radiatively participating media (flue gases containing triatomic gases water vapor and CO_{2}).
 The emissivity of gas medium is calculated using the weighted sum of gray gases model (WSGGM)
Numerical Procedures in FLUENT
Boundary Conditions
 Inlet: Inlet specified with mass flow rate
 Outlet: specified with negative gauge pressure
 Wall: adiabatic condition
 Flow rate and temperature for inlet conditions
 Heat release rate per burner (MW): 1.0
 % Excess Air ratio(%):1.15
 Air Temperature: 24°C
 Air flow rate (kgs/s): 0.431
 Fuel temperature: 20°C
 Fuel flow rate per burner (kg/s): 0.022
 Equivalent ratio: 0.87
Fuel Composition
 Natural gas comprised mainly methane, ethane and propane based on volume fraction:
 CH4 (92%)
 C2H6 (8%)
Selected Combustion Model
 Nonpremixed Combustion: Chemistry Equilibrium, PDF
 Partiallypremixed Combustion: Flamelet PDF, Premixed C Eqn, Zimmot flame speed
 Partiallypremixed Combustion: Flamelet PDF, Premixed C Eqn, Peter flame speed
 Partiallypremixed Combustion: Flame let PDF, Premixed ECFM
 Species Transport (methane –air 2 step): Eddy Dissipation TCI Model
Solver setting
 Pressure based steady state solver
 Turbulenr Model: realizable k e model
 Pressure Velocity coupling – SIMPLE algorithm
 Discretization – Second order for momentum, energy, DO, other variables
CFD Results
Nonpremixed combustion Model (Chemical Equilibrium)
 The follwoing figures hows the velocity contours obtained from CFD simulation
 Velocity vectors in burner
 Stream lines colored by Velocity Magnitude
 Pressure Contours
 Temperature Contours
 Species concentration for reactants is given below
 The concentration of methane is high in the Centre of burner just above the fuel port
 The concentration of oxygen is high on either side of burner of burner above the burner
Partially Premixed Combustion
 Species concentration by Partially Premixed
 Nonpremix type: PDF Flamelet
 Premixed type: ECFM
Flame Height Predicted by different Combustion Model
 Methane Air Two Step Model
 Chemical Equilibrium Model

 Partially Premixed Combustion Model: PDF Framelet, Premixed ECFM


Partially Premixed: PDF Framelet, with peter Flame speed model

Conclusion
 CFD Modeling of natural gas burner has been carried out using different combustion models in ANSYS LFUENT
 Flame height predicted by Nonpremixed and partially premixed model is lower
 For partially premixed combustion, the flow pattern predicted by Extended Coherent Flamelet Model (ECFM) flame speed model is different from than that for Zimont flame speed model
 Flame height predicted by Species transport with eddy dissipation model is higher compared to other models
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