Partially Premixed Combustion Tutorial

1UGM 2002 Confidential

Partially PremixedPartially PremixedCombustion in a Co-axialCombustion in a Co-axial

CombustorCombustor

Graham Goldin

2002 Fluent Users’ Group Meeting


Problemu A swirler at the center of the combustor

introduces the lean methane/air mixture.u equivalence ratio=0.8u axial velocity = 30 m/su radial velocity = 30 m/su axial velocity of air at outer tube = 10 m/su major species involved in the combustion process

are CH4, O2, CO2, CO, H2O, and N2


Setup and Solution

u Generate PDF look-up table using prePDFu Read Gridu Define Modelu Define Materialu Operating and Boundary Conditionsu 1st and 2nd Order Solutionsu Postprocessing


Generate PDF look-up Table (1)

u Start prePDF and definethe model type.Setup:Case…u Enable Partially Premixed

Modelu Retain the default settings

for other parameters



u Define the chemical species in the system.u Setup:Species:Define…u Under Database Species, select the nameu Set the Species numberu Define the species: CH4, O2, CO2, CO, H2O,

and N2



u Define fuel composition.Setup:Species:Composition…u Set Species Fraction:

l CH4 = 0.0453l O2 = 0.2264l CO2 = 0.7283



u Define oxidizer composition.u Set Species Fraction:

l O2 = 0. 233, N2 = 0.767



• Define the system operating conditions.Setup:Operating Conditions…u Set the Inlet Temperature for Oxidiser to 650

and retain the default values.

u Retain the default PDF solution parametersu Save the input file ch4-partialpremixed.inpu Calculate the PDF table, and save the pdf file, ch4-partial-

premixed.pdfCalculate:PDF Table



u Examine temperature/mixture fraction, andspecies/mixture fraction relationshipDisplay:Property Curves…:Plot Variable



u prePDF automatically fits 3rd-order polynomialfunctions (of f ) for unburnt density, temperature,specific heat and thermal diffusivity.

u prePDF automatically fits a piecewise-linear function forthe laminar flame speed for certain fuels and conditionsu H2, CH4, C2H2, C2H4, C2H6, C3H8

u 1atm < pressure < 40atmu 300K < Tunburnt < 800Ku For other conditions, you must input the function


Read Grid

u Start the 2D version of FLUENTu Read the grid file, par-premixed.mshu Scale the grid to inchesu Display the grid


Define ModeluDefine:Models:Solver uDefine:Models:Viscous


Define Modelu Define:Models:Species

You will be prompted to read the ch4-partial-premixed.pdf file. Whenthe file is read, the available material properties/methods willchange to accomodate the partially premixed model.


Materialu Define:Materials

Fluent will automatically select the material and other parameters.


Operating Conditions

u Retain default values.


Boundary Conditions (1)Set boundary conditions forair inlet.

Set boundary conditions forair-fuel inlet.


Boundary Conditions (2)

u Set boundary conditions for outlet.


First Order Solutions (1)

u Solve for Flow and Turbulence equation.


First Order Solutions (2)u Enable the plotting of residuals.


First Order Solutions (3)u Initialize flow field and compute from all zones.

u Save the case file par-premixed.cas.gz.


First Order Solutions (4)u Start the calculation (250 iterations).u Define a region Adapt:Region…

u Patch a region close to fuel-airinlet.


First Order Solutions (5)u Solve for all equations


Second Order Solutions (1)

u Change the discretization for the parameters:u Pressure: Second Orderu Momentum: Second Order Upwindu Turbulence Kinetic Energy: Second Order Upwindu Turbulence Dissipation Rate: Second Order Upwindu Progress Variable: Second Order Upwindu Mean Mixture Fraction: Second Order Upwindu Mixture Fraction Variance: Second Order Upwind


Second Order Solutions (2)u Start the calculation (250 iterations).u Save the data file par-premixed.dat.gz.


Postprocessing (1)uVelocity Vectors.Set Scale Factor to 10 andSkip Value to 3

uContours of SteamFunction.


Postprocessing (2)uFilled contours of meanProgress Variable.

uFilled contours of StaticTemperature


Postprocessing (3)

uMass fractions of CH4uMass fractions of H2O


Postprocessing (4)

uMass fractions of CO2 uMass fractions of CO


Results

u The partially premixed model in FLUENT canbe used to simulate problems with:u A premixed stream and a non-premixed (or inert

stream such as air)u Equivalence ratio fluctuations in the premixed inlet

streamu Can be used in the limit of…

l Perfectly premixed (automatic calculation of props)l Non-premixed (can study mixed and unburnt flows)


3D Simulation of the IFRF3D Simulation of the IFRFIndustrial Pulverized-CoalIndustrial Pulverized-Coal

FurnaceFurnace

Graham Goldin

2002 Fluent Users’ Group Meeting


Overviewu The International Flame Research Foundation

(IFRF) experimental facility is used to validateindustrial coal combustion models.

u This tutorial is an extension of the 2-dimensional simulation of this furnace byPeters and Weber.

u The mixture fraction/PDF model with the k-eturbulence model and P-1 radiation model hasbeen used.


Problemu To simulate a realistic industrial pulverised-

coal furnace and compare with the measureddata.u 3D analysis of 2.4 MW Swirling,

Pulverized Coal FlameFurnace

u One quarter periodicmodel of furnace(shown in fig)


Setup and Solutionu Select a Combustion Modelu Generate PDF look-up table using prePDFu Read Gridu Define Modelu Define Materialsu Define Operating Conditionsu Compile UDFu Define Boundary Conditionsu Define Injectionsu Solve for non reacting and reacting flowsu Postprocessing


Select a Combustion Modelu Assumptions

u Chemical equilibriumu Modeling the devolatization and char off-gases as a

single mixture

u Combustion Model selectedu Mixture Fraction Model

u Coal Specificationsu Name: Saar Gottelborn hvBbu High Temperature yield (mole, dry) volatiles 55%, char

36.7%, and ash 8.3%u Ultimate analysis (mole, dry-ash-free (daf)) C 53%, H

40%, O 6%, and N 1%


Generate PDF look-up Table (1)u Start prePDF and define

a case.Setup:Case…u Enable Non-Adiabatic

Heat transfer optionsu Enable Fuel stream for

Empirically DefinedStreams

u Retain the default settingsfor other parameters


Generate PDF look-up Table (2)u Define the chemical species in the system.

Setup:Species:Define…u Under Database Species, select the nameu Set the Species numberu Define the species: C, H, O, N, C(S), O2 , CO2,

CO, H2O, N2 , OH, and H2


Generate PDF look-up Table (3)u Define fuel composition.

Setup:Species:Composition…u Set Species Fraction:

l C = 0.53l H = 0.40l O = 0.06l N = 0.01

u Lower Caloric Value = 3.232e+07u Specific Heat = 1100



u Define oxidizer composition.u Set Species Fraction:

l O2 = 0. 21l N2 = 0.79


Generate PDF look-up Table (5)u Define the system operating

conditions.Setup:Operating Conditions…

u Min. Temperature = 370u Max. Temperature = 2600u Set the Inlet Temperature

l Fuel = 373l Oxidiser = 573


Generate PDF look-up Table (6)u Define the solution

parameters.u Non-Adiabatic Model:

Enthalpy Points = 20u Fuel Mixture Fraction

Points = 32u Mixture Fraction Variance

Points = 16u Disable Automatic

Distributionu Distribution Center

Point = 0.2

u Calculate the pdf tableand view it with thegraphics routines.

u Save the pdf file(ifrf.pdf).


Grid

u Start the 3D version of FLUENTu Read the grid

file, ifrf.mshu Check and

display the grid


Define Models (1)u Define:Models:Solver u Define:Models:Viscous


Define Models (2)u Define:Models:Species

When prompted read the ifrf.pdffile. When the file is read, theavailable material properties/methods will change toaccomodate the model.

u Define:Models:RadiationTo choose an appropriateradiation model, calculate opticalthickness = mean beam length(about 2m) x absorption co-efficient (around 1 /m forhydrocarbon combustion)Since this optical thickness isgreater than unity, the P1 model isappropriate.


Define Models (3)u Define:Models:Discrete

Phase Modelu Set the Max. Number Of

Steps to 25000u Deactivate Specify

Length Scaleu Set Step Length Factor

to 20


Materialsu Define:Materials

u Set AbsorptionCoefficient = wsggm-cell-based

u Set ScatteringCoefficient = 0.15


Operating Conditions

u Retain default values.


Compile Interpreted UDFsu Create a working directory and save the C

functions.u Start Fluent from the working directory and read

the case file.u Compile the UDF using the Interpreted UDFs

panelu Enter name of the C function (ifrf.c) under Source File Nameu Specify the C preprocessor under CPP

Command Name fieldu Retain the default Stack Sizeu Click Compileu Close the panel when compilation is over


Boundary Conditions (1)Set boundary conditions forv-1 zone.

Set boundary conditions forv-2 zone.


Boundary Conditions (2)Set boundary conditions forp-1 zone.

Set boundary conditions forperiodic zone.


Boundary Conditions (3) Set boundary conditions for wall zones w-1, w-2, w-3, w-4, w-5, w-6, w-7, w-8, and w-9 as per the table

0.51073w-9

0.51323w-8

1udf-wall7tempw-7

1udf-wall6tempw-5

1udf-wall5tempw-5

0.61273w-4

0.6873w-3

0.6573w-2

0.6343w-1

InternalEmissivity

TemperatureZoneName


Define Injections (1)u Create Injections

Define:Injections…u Click Create in the Injections panel

u Set Injection propertiesu Injection Type: Surface

u Release From Surfaces: v1

u Particle Type: Combusting

u Diameter Distribution: rosin-rammler

u Turbulent Dispersion: Stochastic Model

u Number Of Tries: 3


Define Injections (2)

6Number Of Diameters

1.36Spread Parameter

4.5e-05Mean Diameter

0.003Max. Diameter

1e-06Min. Diameter

0.01826Total Flow Rate

343Temperature

23.11Z-Velocity

ValueParameter

u Under Point Properties, set thefollowing values:


u Modify the properties for the combustingparticle.u Name: gottelborn-hyu Set Properties as per table

Define Injections (3)

ValueParameter

kinetics/diffusion-limited

Combustion Model

36.7Combustible Fraction

3e-05Binary Diffusivity

55.02Volatile Component Fraction

300Vaporization Temperature

0Latent Heat

1100Cp

1000Density

Kinetics Limited Rate Pre-exponentialFactor = 6.7Kinetics Limited Rate Activation Energy =1.1382e+08`


Solution (1)u Solve for Non reacting flow

u Disable Energy, P1 andPdf for equations

u Set pressure discretizationto PRESTO!

u Initialize the solutionu Compute from all-zonesu Set the initial value for

temperature to 2000

u Plot residuals during calculationsu Request 99 iterationsu Save the data file (ifrf1.dat.gz)


Solution (2)u Solve for Reacting flow

u Enable Interaction with ContinuousPhasel Set Number of Continuous Phase

Iterations per DPM Iteration to 20u Enable Energy, P1 and Pdf equationsu Set the under-relaxation factors

u Request another 20 iterationsu Save the data file (ifrf2.dat.gz)

ValueParameter

0.25Discrete Phase Sources

0.975P1

0.5Momentum

0.5Pressure


Solution (3)u Modify the properties of the combusting particle

u Request for an additional 200iterations

u Save the data file (ifrf3.dat.gz)

ValueParameter

Activation Energy= 7.4e+07

Pre-exponentialFactor = 2e+05 W

single-rateDevolatilization Model

773Vaporization Temperature


Solution (4)u Set the discretization to Second Order Upwind for:

u Momentumu Turbulence Kinetic Energyu Turbulence Dissipation Rateu Mean Mixture Fractionu Mixture Fraction Varianceu Energy

u Request for an additional 500iterations

u Save the data file (ifrf4.dat.gz)


Solution (5)u Define the NOx Model Define:Models:Pollutants:NOx...

u Enable the models Thermal NOand Fuel NO

u Under Turbulence Interaction:l PDFMode = Mixture Fractionl Beta PDF Points to 25

u Under Fuel NO Parameters:l Fuel Type = Solidl Volatile N Mass Fraction = 0.01015l Char N Mass Fraction = 0.00435l BET Surface Area = 25000


Solution (6)u For discrete phase model, set

Number of Continuous PhaseIterations per DPM Iteration = 0

u Set Solution parameters:u Disable all the equations except NO

and HCNu Under-relaxation factors for NO and

HCN to 1u Discretization scheme as Second

Order Upwindu Convergence Criterion for NO and

HCN = 1e-06

u Request for 20 iterationsu Save the data file (ifrf5.dat.gz)


Postprocessing (1)u Check the net in and out fluxes balance.

u Compute gas phase mass fluxesthrough all boundariesl Boundaries : Select all zonesl Click Compute

u Calculate the net mass transfer to thegas phase from the discrete phase coalparticles.l Options: Suml Cell Zones: fluidl Field Variable : Discrete Phase Model...

and DPM Mass Sourcel Click Compute


Postprocessing (2)u Compute the gas phase energy fluxes through

all the boundariesl Options : Total Heat Transfer Ratel Boundaries : Select all zonesl Click Compute

u Calculate the net mass transfer to thegas phase from the discrete phase coalparticles.l Options: Suml Cell Zones: fluidl Field Variable : Discrete Phase

Model... and DPM Enthalpy Sourcel Click Compute


Postprocessing (3)

u Static Temperature u Turbulent Viscosity

Display contours of flow variables of interest


Postprocessing (4)

u Mass fractions of CO2 u Particle Tracks


Resultsu The radial profiles and axial plots of time

averaged flow field values at 0.25m and 0.85mfrom the quarl end of the combustor werecollected and can be downloaded from the fileslisted in the table.

u Comparison of the experimental data and theCFD simulation data show an agreementwhich can be considered typical.


Center-line (z axis) parts-per-million (dry)radial-NO.xy

Center-line (z axis) carbon-dioxide volume percentage (dry)radial-CO2.xy

Center-line (z axis) parts-per-million (dry)radial-CO.xy

Center-line (z axis) temperature (K)radial-T.xy

Center-line (z axis) oxygen volume percentage (dry)radial-O2.xy

Tangential velocity (m/s) at z=0.25mradial-V-1.xy

Tangential velocity (m/s) at z=0.85mradial-V-2.xy

Axial velocity (m/s) at z=0.25mradial-U-1.xy

Axial velocity (m/s) at z=0.85mradial-U-2.xy

NO parts-per-million (dry) at z=0.25mradial-NO-1.xy

NO parts-per-million (dry) at z=0.85mradial-NO-2.xy

Carbon-monoxide parts-per-million (dry) at z=0.25mradial-CO-1.xy

Carbon-monoxide parts-per-million (dry) at z=0.85mradial-CO-2.xy

Carbon-dioxide volume percentage (dry) at z=0.25mradial-CO2-1.xy

Carbon-dioxide volume percentage (dry) at z=0.25mradial-CO2-2.xy

Oxygen volume percentage (dry) at z=0.85mradial-O2-2.xy

Oxygen volume percentage (dry) at z=0.25mradial-O2-1.xy

Temperature (K) at z=0.85mradial-T-2.xy

Temperature (K) at z=0.25mradial-T-1.xy

DescriptionFileExperimentalData :Files of radialprofiles andaxial plots oftime averagedflow fieldvalues.

Reference :Peters, A.F. and Weber,R. (1997), MathematicalModeling of a 2.4 MWSwirling, PulverizedCoal Flame, CombustionScience andTechnology, 122, 131.

Partially Premixed Combustion Tutorial

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