Partially Premixed Combustion Tutorial

Partially Premixed Combustion in a Co-axial Combustor

Graham Goldin 2002 Fluent Users’ Group Meeting

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Problem u

A swirler at the center of the combustor introduces the lean methane/air mixture. u u u u u

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equivalence ratio=0.8 axial velocity = 30 m/s radial velocity = 30 m/s axial velocity of air at outer tube = 10 m/s major species involved in the combustion process are CH4, O2, CO2, CO, H2O, and N2

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Setup and Solution u u u u u u u

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Generate PDF look-up table using prePDF Read Grid Define Model Define Material Operating and Boundary Conditions 1st and 2nd Order Solutions Postprocessing Confidential

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Generate PDF look-up Table (1) u

Start prePDF and define the model type. Setup:Case… u Enable Partially Premixed Model u Retain the default settings for other parameters

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Generate PDF look-up Table (2) u

Define the chemical species in the system. u u u u

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Setup:Species:Define… Under Database Species, select the name Set the Species number Define the species: CH4, O2, CO2, CO, H2O, and N2

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Generate PDF look-up Table (3) u

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

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CH4 = 0.0453 O2 = 0.2264 CO2 = 0.7283

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Generate PDF look-up Table (4) u

Define oxidizer composition. u

Set Species Fraction: l

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O2 = 0. 233, N2 = 0.767

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Generate PDF look-up Table (5) • Define the system operating conditions. Setup:Operating Conditions… u Set the Inlet Temperature for Oxidiser to 650 and retain the default values. u u u

Retain the default PDF solution parameters Save the input file ch4-partialpremixed.inp Calculate the PDF table, and save the pdf file, ch4-partialpremixed.pdf

Calculate:PDF Table UGM 2002

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Generate PDF look-up Table (6) u

Examine temperature/mixture fraction, and species/mixture fraction relationship Display:Property Curves…:Plot Variable

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Generate PDF look-up Table (7) u

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

u

prePDF automatically fits a piecewise-linear function for the laminar flame speed for certain fuels and conditions u u u u

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H2, CH4, C2H2, C2H4, C2H6, C3H8 1atm < pressure < 40atm 300K < Tunburnt < 800K For other conditions, you must input the function

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Read Grid u u u u

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Start the 2D version of FLUENT Read the grid file, par-premixed.msh Scale the grid to inches Display the grid

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Define Model u

Define:Models:Solver

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u

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Define:Models:Viscous

Define Model u

Define:Models:Species

You will be prompted to read the ch4-partial-premixed.pdf file. When the file is read, the available material properties/methods will change to accomodate the partially premixed model. UGM 2002

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Material u

Define:Materials

Fluent will automatically select the material and other parameters. UGM 2002

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Operating Conditions u

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Retain default values.

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Boundary Conditions (1) Set boundary conditions for air inlet.

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Set boundary conditions for air-fuel inlet.

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Boundary Conditions (2) u

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Set boundary conditions for outlet.

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First Order Solutions (1) u

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Solve for Flow and Turbulence equation.

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First Order Solutions (2) u

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Enable the plotting of residuals.

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First Order Solutions (3) u

Initialize flow field and compute from all zones.

u

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

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First Order Solutions (4) u u u

Start the calculation (250 iterations). Define a region Adapt:Region… Patch a region close to fuel-air inlet.

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First Order Solutions (5) u

Solve for all equations

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Second Order Solutions (1) u

Change the discretization for the parameters: u u u u u u u

Pressure: Second Order Momentum: Second Order Upwind Turbulence Kinetic Energy: Second Order Upwind Turbulence Dissipation Rate: Second Order Upwind Progress Variable: Second Order Upwind Mean Mixture Fraction: Second Order Upwind Mixture Fraction Variance: Second Order Upwind

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Second Order Solutions (2) u u

Start the calculation (250 iterations). Save the data file par-premixed.dat.gz.

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Postprocessing (1) u

Velocity Vectors.

Contours of Steam Function. u

Set Scale Factor to 10 and Skip Value to 3

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Postprocessing (2) Filled contours of mean Progress Variable.

Filled contours of Static Temperature

u

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Postprocessing (3) u

Mass fractions of CH4

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Mass fractions of H2O

Postprocessing (4) u

Mass fractions of CO2

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Mass fractions of CO

Results u

The partially premixed model in FLUENT can be used to simulate problems with: u

u

u

A premixed stream and a non-premixed (or inert stream such as air) Equivalence ratio fluctuations in the premixed inlet stream Can be used in the limit of… l l

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

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3D Simulation of the IFRF Industrial Pulverized-Coal Furnace

Graham Goldin 2002 Fluent Users’ Group Meeting

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Overview u

u

u

The International Flame Research Foundation (IFRF) experimental facility is used to validate industrial coal combustion models. This tutorial is an extension of the 2dimensional simulation of this furnace by Peters and Weber. The mixture fraction/PDF model with the k-e turbulence model and P-1 radiation model has been used.

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Problem u

To simulate a realistic industrial pulverisedcoal furnace and compare with the measured data. u

u

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3D analysis of 2.4 MW Swirling, Pulverized Coal Flame Furnace One quarter periodic model of furnace (shown in fig)

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Setup and Solution u u u u u u u u u u u

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Select a Combustion Model Generate PDF look-up table using prePDF Read Grid Define Model Define Materials Define Operating Conditions Compile UDF Define Boundary Conditions Define Injections Solve for non reacting and reacting flows Postprocessing Confidential

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Select a Combustion Model u

Assumptions u u

u

Combustion Model selected u

u

Chemical equilibrium Modeling the devolatization and char off-gases as a single mixture Mixture Fraction Model

Coal Specifications u u

u

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Name: Saar Gottelborn hvBb High Temperature yield (mole, dry) volatiles 55%, char 36.7%, and ash 8.3% Ultimate analysis (mole, dry-ash-free (daf)) C 53%, H 40%, O 6%, and N 1%

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Generate PDF look-up Table (1) u

Start prePDF and define a case. Setup:Case… u Enable Non-Adiabatic Heat transfer options u Enable Fuel stream for Empirically Defined Streams u Retain the default settings for other parameters

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Generate PDF look-up Table (2) u

Define the chemical species in the system. Setup:Species:Define… u Under Database Species, select the name u Set the Species number u Define the species: C, H, O, N, C(S), O , CO , 2 2 CO, H2O, N2 , OH, and H2

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Generate PDF look-up Table (3) u

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

u u

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

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

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Generate PDF look-up Table (4) u

Define oxidizer composition. u

Set Species Fraction: l l

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O2 = 0. 21 N2 = 0.79

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Generate PDF look-up Table (5) Define the system operating conditions. Setup:Operating Conditions… u

u u u

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

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Fuel = 373 Oxidiser = 573

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Generate PDF look-up Table (6) u

Define the solution parameters. u

u

u

u

u

Non-Adiabatic Model: Enthalpy Points = 20 Fuel Mixture Fraction Points = 32 Mixture Fraction Variance Points = 16 Disable Automatic Distribution Distribution Center Point = 0.2

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u

u

Calculate the pdf table and view it with the graphics routines. Save the pdf file (ifrf.pdf).

Grid u u

u

Start the 3D version of FLUENT Read the grid file, ifrf.msh Check and display the grid

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Define Models (1) u

Define:Models:Solver

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u

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Define:Models:Viscous

Define Models (2) u

Define:Models:Species

u

When prompted read the ifrf.pdf file. When the file is read, the available material properties /methods will change to accomodate the model.

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Define:Models:Radiation To choose an appropriate radiation model, calculate optical thickness = mean beam length (about 2m) x absorption coefficient (around 1 /m for hydrocarbon combustion) Since this optical thickness is greater than unity, the P1 model is appropriate.

Define Models (3) u

Define:Models:Discrete Phase Model u

u

u

Set the Max. Number Of Steps to 25000 Deactivate Specify Length Scale Set Step Length Factor to 20

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Materials u

Define:Materials u

u

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Set Absorption Coefficient = wsggmcell-based Set Scattering Coefficient = 0.15

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Operating Conditions u

Retain default values.

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Compile Interpreted UDFs u

u

u

Create a working directory and save the C functions. Start Fluent from the working directory and read the case file. Compile the UDF using the Interpreted UDFs panel Enter name of the C function (ifrf.c) under Source File Name u Specify the C preprocessor under CPP Command Name field u Retain the default Stack Size u Click Compile u Close the panel when compilation is over u

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Boundary Conditions (1) Set boundary conditions for v-1 zone.

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Set boundary conditions for v-2 zone.

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

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Set boundary conditions for periodic zone .

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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 Zone Name

Temperature

Internal Emissivity

w-1

343

0.6

w-2

573

0.6

w-3

873

0.6

w-4

1273

0.6

w-5

udf-wall5temp

1

w-5

udf-wall6temp

1

w-7

udf-wall7temp

1

w-8

1323

0.5

w-9

1073

0.5

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Define Injections (1) u

Create Injections Define:Injections… u

u

Click Create in the Injections panel

Set Injection properties u

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

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Define Injections (2) u

Under Point Properties, set the following values: Parameter

Value

Z-Velocity

23.11

Temperature

343

Total Flow Rate

0.01826

Min. Diameter

1e-06

Max. Diameter

0.003

Mean Diameter

4.5e-05

Spread Parameter

1.36

Number Of Diameters

6

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Define Injections (3) u

Modify the properties for the combusting particle. u u

Name: gottelborn-hy Set Properties as per table

Parameter

Value

Density

1000

Cp

1100

Latent Heat

0

Vaporization Temperature

300

Volatile Component Fraction

55.02

Binary Diffusivity

3e-05

Combustible Fraction

36.7

Combustion Model

kinetics/diffusionlimited

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

Solution (1) u

Solve for Non reacting flow u

u

u

Initialize the solution u u

u u u

Disable Energy, P1 and Pdf for equations Set pressure discretization to PRESTO! Compute from all-zones Set the initial value for temperature to 2000

Plot residuals during calculations Request 99 iterations Save the data file (ifrf1.dat.gz)

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Solution (2) u

Solve for Reacting flow u

Enable Interaction with Continuous Phase l

u u

u u

Set Number of Continuous Phase Iterations per DPM Iteration to 20

Enable Energy, P1 and Pdf equations Set the under-relaxation factors Parameter

Value

Pressure

0.5

Momentum

0.5

P1

0.975

Discrete Phase Sources

0.25

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

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Solution (3) u

Modify the properties of the combusting particle Parameter

Value

Vaporization Temperature

773

Devolatilization Model

single-rate Pre-exponential Factor = 2e+05 W Activation Energy = 7.4e+07

u

u

Request for an additional 200 iterations Save the data file (ifrf3.dat.gz)

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Solution (4) u

u

u

Set the discretization to Second Order Upwind for: u Momentum u Turbulence Kinetic Energy u Turbulence Dissipation Rate u Mean Mixture Fraction u Mixture Fraction Variance u Energy Request for an additional 500 iterations Save the data file (ifrf4.dat.gz)

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Solution (5) u

Define the NOx Model Define:Models:Pollutants:NOx... u

u

Enable the models Thermal NO and Fuel NO Under Turbulence Interaction: l l

u

PDFMode = Mixture Fraction Beta PDF Points to 25

Under Fuel NO Parameters: l l l l

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Fuel Type = Solid Volatile N Mass Fraction = 0.01015 Char N Mass Fraction = 0.00435 BET Surface Area = 25000

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Solution (6) u

u

For discrete phase model, set Number of Continuous Phase Iterations per DPM Iteration = 0 Set Solution parameters: u

u

u

u

u u

Disable all the equations except NO and HCN Under-relaxation factors for NO and HCN to 1 Discretization scheme as Second Order Upwind Convergence Criterion for NO and HCN = 1e-06

Request for 20 iterations Save the data file (ifrf5.dat.gz) UGM 2002

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Postprocessing (1) u

Check the net in and out fluxes balance. u

Compute gas phase mass fluxes through all boundaries l l

u

Boundaries : Select all zones Click Compute

Calculate the net mass transfer to the gas phase from the discrete phase coal particles. l l l

l

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Options: Sum Cell Zones: fluid Field Variable : Discrete Phase Model... and DPM Mass Source Click Compute Confidential

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Postprocessing (2) u

Compute the gas phase energy fluxes through all the boundaries l l l

u

Options : Total Heat Transfer Rate Boundaries : Select all zones Click Compute

Calculate the net mass transfer to the gas phase from the discrete phase coal particles. l l l

l

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Options: Sum Cell Zones: fluid Field Variable : Discrete Phase Model... and DPM Enthalpy Source Click Compute

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Postprocessing (3) Display contours of flow variables of interest u

Static Temperature

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Turbulent Viscosity

Postprocessing (4) u

Mass fractions of CO2

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Particle Tracks

Results u

u

The radial profiles and axial plots of time averaged flow field values at 0.25m and 0.85m from the quarl end of the combustor were collected and can be downloaded from the files listed in the table. Comparison of the experimental data and the CFD simulation data show an agreement which can be considered typical.

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Experimental Data : Files of radial profiles and axial plots of time averaged flow field values. Reference : Peters, A.F. and Weber, R. (1997), Mathematical Modeling of a 2.4 MW Swirling, Pulverized Coal Flame, Combustion Science and Technology, 122, 131.

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File

Description

radial-T-1.xy

Temperature (K) at z=0.25m

radial-T-2.xy

Temperature (K) at z=0.85m

radial-O2-1.xy

Oxygen volume percentage (dry) at z=0.25m

radial-O2-2.xy

Oxygen volume percentage (dry) at z=0.85m

radial-CO2-1.xy

Carbon-dioxide volume percentage (dry) at z=0.25m

radial-CO2-2.xy

Carbon-dioxide volume percentage (dry) at z=0.25m

radial-CO-1.xy

Carbon-monoxide parts-per-million (dry) at z=0.25m

radial-CO-2.xy

Carbon-monoxide parts-per-million (dry) at z=0.85m

radial-NO-1.xy

NO parts-per-million (dry) at z=0.25m

radial-NO-2.xy

NO parts-per-million (dry) at z=0.85m

radial-U-1.xy

Axial velocity (m/s) at z=0.25m

radial-U-2.xy

Axial velocity (m/s) at z=0.85m

radial-V-1.xy

Tangential velocity (m/s) at z=0.25m

radial-V-2.xy

Tangential velocity (m/s) at z=0.85m

radial-T.xy

Center-line (z axis) temperature (K)

radial-O2.xy

Center-line (z axis) oxygen volume percentage (dry)

radial-CO2.xy

Center-line (z axis) carbon-dioxide volume percentage (dry)

radial-CO.xy

Center-line (z axis) parts-per-million (dry)

radial-NO.xy

Center-line (z axis) parts-per-million (dry) 65