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fluent tutorial tut14

https://www.360docs.net/doc/639294228.html,ing the Non-Premixed Combustion Model

Introduction

A300KW BERL combustor simulation is modeled using the PDF mixture fraction model.

The reaction can be modeled using either the species transport model or the non-premixed combustion model.In this tutorial you will set up and solve a natural gas combustion problem using the non-premixed combustion model for the reaction chemistry.

This tutorial demonstrates how to do the following:

?De?ne inputs for modeling non-premixed combustion chemistry.

?Prepare a Probability Density Function(PDF)table in FLUENT.

?Solve a natural gas combustion simulation problem.

?Use the P-1radiation model for combustion applications.

?Use the k- turbulence model.

The non-premixed combustion model uses a modeling approach that solves transport equations for one or two conserved scalars and the mixture fractions.Multiple chemical species,including radicals and intermediate species,may be included in the problem de?nition.Their concentrations will be derived from the predicted mixture fraction distribution.

Property data for the species are accessed through a chemical database and turbulence-chemistry interaction is modeled using aβ-function for the PDF.See Chapter15of the User’s Guide for details on the non-premixed combustion modeling approach. Prerequisites

This tutorial assumes that you are familiar with the menu structure in FLUENT and that you have completed Tutorial1.Some steps in the setup and solution procedure will not be shown explicitly.

Using the Non-Premixed Combustion Model

Problem Description

The?ow considered is an unstaged natural gas?ame in a300kW swirl-stabilized burner.

The furnace is vertically-?red and of octagonal cross-section with a conical furnace hood and a cylindrical exhaust duct.The furnace walls are capable of being refractory-lined or water-cooled.The burner features24radial fuel ports and a blu?centerbody.Air is introduced through an annular inlet and movable swirl blocks are used to impart swirl.

The combustor dimensions are described in Figure14.1,and Figure14.2shows a close-up of the burner assuming2D axisymmetry.The boundary condition pro?les,velocity inlet boundary conditions of the gas,and temperature boundary conditions are based on experimental data[1].

Figure14.1:Problem Description

Using the Non-Premixed Combustion Model

195 mm

swirling

Do = 87 mm

Figure14.2:Close-Up of the Burner

Setup and Solution

Preparation

1.Download non_premix_combustion.zip from the Fluent https://www.360docs.net/doc/639294228.html,er Services Center

or copy it from the FLUENT documentation CD to your working folder(as described

in Tutorial1).

2.Unzip non_premix_combustion.zip.

berl.msh and berl.prof can be found in the non premix combustion folder,which

will be created after unzipping the?le.

The mesh?le,berl.msh is a quadrilateral mesh describing the system geometry

shown in Figures14.1and14.2.

3.Start the2D(2d)version of FLUENT.

Using the Non-Premixed Combustion Model

Step1:Grid

1.Read the mesh?le berl.msh.

File?→Read?→Case...

The FLUENT console will report that the mesh contains9784quadrilateral cells.A

warning will be generated informing you to consider making changes to the zone

type,or to change the problem de?nition to axisymmetric.You will change the

problem to axisymmetric swirl in Step2.

2.Check the grid.

Grid?→Check

FLUENT will perform various checks on the mesh and will reports the progress in

the console.Make sure that the minimum volume reported is a positive number.

3.Scale the grid.

Grid?→Scale...

(a)Select mm(millimeters)from the Grid Was Created In drop-down list in the

Unit Conversion group box.

(b)Click Change Length Units.

All dimensions will now be shown in millimeters.

(c)Click Scale to scale the grid.

(d)Close the Scale Grid panel.

Using the Non-Premixed Combustion Model

4.Display the grid(Figure14.3).

Display?→Grid...

(a)Retain the default settings.

(b)Click Display and close the Grid Display panel.

Figure14.3:2D BERL combustor Mesh Display

Due to the grid resolution and the size of the domain,you may?nd it more useful to display just the outline,or to zoom in on various portions of the grid display.

Using the Non-Premixed Combustion Model

Extra:You can use the mouse zoom button(middle button,by default)to zoom in to the display and the mouse probe button(right button,by default)to?nd

out the boundary zone labels.The zone labels will be displayed in the console.

5.Mirror the display about the symmetry plane.

Display?→Views...

(a)Select axis-2from the Mirror Planes list.

(b)Click Apply and close the Views panel.

The full geometry will be displayed,as shown in Figure14.4.

Using the Non-Premixed Combustion Model

Figure14.4:2D BERL Combustor Mesh Display Including the Symmetry Plane

Using the Non-Premixed Combustion Model

Step2:Models

1.Change the spatial de?nition to axisymmetric swirl.

De?ne?→Models?→Solver...

(a)Retain the default selection of Pressure Based in the Solver list.

The non-premixed combustion model is available only with the pressure-based

solver.

(b)Select Axisymmetric Swirl in the Space list.

(c)Click OK to close the Solver panel.

2.Enable the Energy Equation.

De?ne?→Models?→Energy...

Since heat transfer occurs in the system considered here,you will have to solve the

energy equation.

Using the Non-Premixed Combustion Model

3.Select the standard k-epsilon turbulence model.

De?ne?→Models?→Viscous...

(a)Select k-epsilon(2eqn)from the Model list.

For axisymmetric swirling?ow,the RNG k-epsilon model can also be used.

(b)Retain all other default settings.

(c)Click OK to close the Viscous Model panel.

Using the Non-Premixed Combustion Model

4.Select the P1radiation model.

De?ne?→Models?→Radiation...

(a)Select P1from the Model list.

(b)Click OK to close the Radiation Model panel.

The FLUENT console will list the properties that are required for the model you

have enabled.An Information dialog box will open,reminding you to con?rm

the property values.

(c)Click OK to close the Information dialog box.

The DO radiation model produces a more accurate solution than the P1radiation

model but it can be CPU intensive.The P1model will produce a quick,acceptable

solution for this problem.

See Chapter13of the User’s Guide for details on the di?erent radiation models

available in FLUENT.

Using the Non-Premixed Combustion Model

5.Select the Non-Premixed Combustion model.

De?ne?→Models?→Species?→Transport&Reaction...

(a)Select Non-Premixed Combustion from the Model list.

The panel will expand to show the related inputs.You will use this panel to create the PDF table.

When you use the non-premixed combustion model,you need to create a PDF table.

This table contains information on the thermo-chemistry and its interaction with turbulence.FLUENT interpolates the PDF during the solution of the non-premixed combustion model.

Using the Non-Premixed Combustion Model

Step3:Non Adiabatic PDF Table

1.Enable the Create Table option,in the PDF Options group box of the Species Model

panel.

This will update the panel to display the inputs for creating the PDF table.The Inlet

Di?usion option enables the mixture fraction to di?use out of the domain through

inlets and outlets.

2.Click the Chemistry tab to de?ne chemistry models.

(a)Retain the default selection of Equilibrium and Non-Adiabatic.

In most non-premixed combustion simulations,the Equilibrium chemistry model

is recommended.The Steady Flamelets option can model local chemical non-

equilibrium due to turbulent strain.

(b)Retain the default value for Operating Pressure.

(c)Enter0.064for Fuel Stream in the Rich Flammability Limit box.

For combustion cases,a value larger than10%–50%of the stoichiometric

mixture fraction can be used for the rich?ammability limit of the fuel stream.

In this case,the stoichiometric fraction is0.058,therefore a value that is10%

greater is0.064.

Using the Non-Premixed Combustion Model The Fuel Rich Flammability Limit allows you to perform a“partial equilibrium”

calculation,suspending equilibrium calculations when the mixture fraction ex-ceeds the speci?ed rich limit.This increases the e?ciency of the PDF cal-culation,allowing you to bypass the complex equilibrium calculations in the fuel-rich region.This is also more physically realistic than the assumption of full equilibrium.

3.Click the Boundary tab to add and de?ne the boundary species.

(a)Add c2h6,c3h8,c4h10,and co2.

i.Enter c2h6in the Boundary Species text-entry?eld and click Add.

ii.Similarly,add c3h8,c4h10,and co2.

All four species will appear in the table.

(b)Select Mole Fraction from the Species Units list.

(c)Retain default values for n2and o2under Oxid.

The oxidizer(air)consists of21%O2and79%N2by volume.

(d)Specify the fuel composition by entering the following values under Fuel:

The fuel composition is entered in mole fractions of the species,c2h6,c3h8, c4h10,and co2.

Using the Non-Premixed Combustion Model

Species Mole Fraction

ch40.965

n20.013

c2h60.017

c3h80.001

c4h100.001

co20.003

Hint:Scroll down to see all the species.

Note:All boundary species with a mass or mole fractions of zero will be ig-nored.

(e)Enter315for Fuel and Oxid each in the Temperature group box.

(f)Click Apply.

4.Click the Control tab and retain default species to be excluded from the equilibrium

calculation.

5.Click the Table tab to specify the table parameters and calculate the PDF table.

(a)Retain the default values for all the paremeters in the Table Parameters group

box.

(b)Click Apply.

The maximum number of species determines the number of most preponderant

species to consider after the equilibrium calculation is performed.

Using the Non-Premixed Combustion Model

(c)Click Calculate PDF Table to compute the non-adiabatic PDF table.

panel.

(d)Click the Display PDF Table...button to open the PDF Table

i.Retain the default parameters and click Display(Figure14.5).

ii.Close the PDF Table panel.

Figure14.5:Non-Adiabatic Temperature Look-Up Table on the Adiabatic Enthalpy Slice The3D look-up tables are reviewed on a slice-by-slice basis.By default,the slice selected is that corresponding to the adiabatic enthalpy values.You can select other slices of constant enthalpy for display,as well.

Using the Non-Premixed Combustion Model

The maximum and minimum values for mean temperature and the corresponding

mean mixture fraction will also be reported in the console.The maximum mean

temperature is reported as2246K at a mean mixture fraction of0.058.

6.Save the PDF output?le(berl.pdf).

File?→Write?→PDF...

(a)Enter berl.pdf for the PDF File name.

(b)Click OK to write the?le.

By default,the?le will be saved as formatted(ASCII,or text).To save a

binary(unformatted)?le,enable the Write Binary Files option in the Select File

dialog box.

7.Click OK to close the Species Model panel.

Step4:Materials

1.Specify the continuous phase(pdf-mixture)material.

De?ne?→Materials...

Using the Non-Premixed Combustion Model All thermodynamic data for the continuous phase,including density,speci?c heat, and formation enthalpies are extracted from the chemical database when the non-premixed combustion model is used.These properties are transferred as the pdf-mixture material,for which only transport properties,such as viscosity and thermal conductivity,need to be de?ned.

(a)Select wsggm-domain-based from the Absorption Coe?cient drop-down list.

Hint:Scroll down to view the Absorption Coe?cient option.

This speci?es a composition-dependent absorption coe?cient,using the weighted-

sum-of-gray-gases model.WSGGM-domain-based is a variable coe?cient that

uses a length scale,based on the geometry of the model.Note that WSGGM-

cell-based uses a characteristic cell length and can be more grid dependent.

See Section13.3.8of the User’s Guide for more details.

(b)Click Change/Create and close the Materials panel.

You can click the View...button next to Mixture Species to view the species included in the pdf-mixture material.These are the species included during the system chem-istry setup.The Density and Cp laws cannot be altered:these properties are stored in the non-premixed combustion look-up tables.

FLUENT uses the gas law to compute the mixture density and a mass-weighted mixing law to compute the mixture c p.When the non-premixed combustion model is used,do not alter the properties of the individual species.This will create an inconsistency with the PDF look-up table.

Step5:Operating Conditions

1.Keep the default operating conditions.

De?ne?→Operating Conditions...

The Operating Pressure was already set in the PDF table generation in Step3.

Using the Non-Premixed Combustion Model

Step6:Boundary Conditions

1.Read the boundary conditions pro?le?le.

File?→Read?→Pro?le...

(a)Select berl.prof from the Select File dialog box.

(b)Click OK.

The CFD solution for reacting?ows can be sensitive to the boundary conditions,in

particular the incoming velocity?eld and the heat transfer through the walls.Here,

you will use pro?les to specify the velocity at air-inlet-4,and the wall temperature

for wall-9.The latter approach of?xing the wall temperature to measurements is

common in furnace simulations,to avoid modeling the wall convective and radia-

tive heat transfer.The data used for the boundary conditions was obtained from

experimental data[1].

2.De?ne the boundary conditions for the zones.

De?ne?→Boundary Conditions...

Using the Non-Premixed Combustion Model 3.Set the boundary conditions for pressure outlet(poutlet-3).

(a)Click the Momentum tab.

(b)Select Intensity and Hydraulic Diameter from the Speci?cation Method drop-

down list in the Turbulence group box.

(c)Enter5%for Back?ow Turbulent Intensity.

(d)Enter600mm for Back?ow Hydraulic Diameter.

(e)Click the Thermal tab and enter1300for the Back?ow Total Temperature.

(f)Click OK to close the Pressure Outlet panel.

The exit gauge pressure of zero de?nes the system pressure at the exit to be the operating pressure.The back?ow conditions for scalars(temperature,mixture frac-tion,turbulence parameters)will be used only if?ow is entrained into the domain through the exit.It is a good idea to use reasonable values in case?ow reversal occurs at the exit at some point during the solution process.

Using the Non-Premixed Combustion Model

4.Set the boundary conditions for the velocity inlet(air-inlet-4).

(a)Select Components from the Velocity Speci?cation Method drop-down list.

(b)Select vel-prof u from the Axial-Velocity(m/s)drop-down list.

(c)Select vel-prof w from the Swirl-Velocity(m/s)drop-down list.

(d)Select Intensity and Hydraulic Diameter from the Speci?cation Method drop-

down list in the Turbulence group box.

(e)Enter17%for Turbulent Intensity.

(f)Enter29mm for Hydraulic Diameter.

(g)Click the Thermal tab and enter312for Temperature.

Turbulence parameters are de?ned based on intensity and length scale.The

relatively large turbulence intensity of17%may be typical for combustion air

?ows.

For the non-premixed combustion calculation,you have to de?ne the inlet Mean

Mixture Fraction and Mixture Fraction Variance in the Species tab.In this case,

the gas phase air inlet has a zero mixture fraction.Therefore,you can accept

the zero default settings.

(h)Click OK to close the Velocity Inlet panel.

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