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/what-is-natural-gas-need-to-know/)
• 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 non-premixed
• In premixed natural gas burner, air and fuel mixes prior to combustion
• In non-premixed 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 non-premixed 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
  • Basic of combsustion
  • Premixed or non-premixed combsustion models
  • Turbulent chemistry interaction model

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 non-premixed (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 finite-rate 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 non-premixed 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 axis-symmetric 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 Favre-averaged transport equation for progress variable c after neglecting the molecular transport

 Where, double bar over variables denote the Favre-averaged values. Variables with two primes symbols are the fluctuation values in the Favre averaging

• The reaction rate (ω) is determined as

Where,

l₀ 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 non-adiabatic 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
• Non-adiabatic combustion
  • For non-adiabatic  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 air-methane reaction

• During combustion, the reaction of hydrocarbons with oxygen occurs quickly but the oxidation of CO to CO₂ is slow.
• Considering all the reactions, the CO oxidation cannot accurately describe what actually occurs soon after ignition
• Two-step mechanism for methane-air:

CH₄ + 1.5O₂ = CO + 2H₂O

CO + 0.5O₂ = CO₂

• 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 eddy-dissipation 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 fuel-air mixture

• PDF Mixture Fraction model represents non-premixed 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 non-premixed 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 non-uniform fuel-oxidizer 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 iso-surface 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 CO₂).
• 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  

• Non-premixed Combustion: Chemistry Equilibrium, PDF
• Partially-premixed Combustion: Flamelet PDF, Premixed C Eqn, Zimmot flame speed
• Partially-premixed Combustion: Flamelet PDF, Premixed C Eqn, Peter flame speed
• Partially-premixed 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

Non-premixed 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
• Non-premix 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 Non-premixed 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