Co Combustor

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Tutorial: Partially Premixed Combustion in a Co-axial

Combustor

Purpose

The purpose of this tutorial is to provide guidelines and recommendations for setting upand solving a reacting flow using the partially premixed combustion model.

Prerequisites

This tutorial assumes that you are familiar with the FLUENT interface and have a goodunderstanding of basic setup and solution procedures. Some steps will not be shown explic-itly.

In this tutorial, you will use the partially premixed combustion model. The partially pre-mixed model is based on both non-premixed (mixture-fraction based) and premixed (reac-tion progress variable based) combustion models. If you have not used these models before,it would be helpful to first refer to sections 15 and 16 ofFLUENT 6.2 Users Guide. Also, re-fer to Tutorial 13: Using the Non-Premixed Combustion Model in the FLUENT 6.2 Tutorial

Guide.

Problem Description

The coaxial combustor considered is shown in the following figure. A swirler at the centerof the combustor introduces the lean methane/air mixture (equivalence ratio=0.8) with anaxial velocity of 50 m/s and swirl velocity of 30 m/s. Pure air at an axial velocity of 10 m/sis introduced from the outer tube to stabilize the flame. The major species involved in thecombustion process are CH4, O2, CO2, CO, H2O, and N2.

Preparation

1. Copy the mesh file, par-premixed.msh.gz to the working directory.

2. Start FLUENT 2D.

c Fluent Inc. November 7, 2005 1
http://www.fluentusers.com/fluent/doc/ori/html/tg/main_pre.htmhttp://www.fluentusers.com/fluent/doc/ori/html/tg/main_pre.htmhttp://www.fluentusers.com/fluent/doc/ori/html/tg/node193.htmhttp://www.fluentusers.com/fluent/doc/ori/html/ug/main_pre.htmhttp://www.fluentusers.com/fluent/doc/ori/html/ug/node595.htmhttp://www.fluentusers.com/fluent/doc/ori/html/ug/node557.htm


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Tutorial: Partially Premixed Combustion in a Co-axial Combustor

Figure 1: Schematic Figure

Setup and Solution

Step 1: Grid

1. Read the mesh file, par-premixed.msh.gz.

2. Check the grid.

Grid Check

3. Scale the grid to inches.

Grid Scale

4. Display the grid (Figure 2).

Display Grid...

5. Reorder the grid to speed up the computations.

Grid Reorder Domain

Do this step twice to get a bandwidth reduction in the order of 1.00.

Step 2: Models

1. Define Segregated solver for Axisymmetric Swirl space and Steady time condition.

Define Models Solver...

2. Enable the standard k-epsilon (2 eqn) turbulence model.

Define Models Viscous...

3. Select Partially Premixed Combustion as the Species Model.

Define Models Species Transport & Reaction...

(a) Enable Create Table option.

You can choose to skip the next step and read the pdf file, par-premixed.pdf.gz.

2 c Fluent Inc. November 7, 2005


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Z

Y

X

GridFLUENT 6.2 (axi, segregated, pdf7, ske)

Mar 03, 2005

Figure 2: Grid

Step 3: Pdf Table

1. Under the Chemistry tab, do the following settings:

(a) Ensure that Equilibrium and Adiabatic options are selected.

(b) Set the Rich Flammability Limit to 1.

2. Under the Boundary tab, do the following settings:

(a) Specify the value ofTemperature for Fuel and Oxid as 300 K and 650 K respec-tively.

(b) Set the Species Units as Mass Fraction.

(c) Set the composition of Fuel and Oxid for the Species as shown in the followingtable:

Species Fuel Oxid

ch4 0.0453 0

n2 0.7283 0.767

o2 0.2264 0.233

3. Under the Table tab, set retain the default settings and click Calculate PDF Table.

4. Under Premixed tab, examine the properties of unburnt mixture and laminar flamespeed.

5. Examine the relationship between Mean Temperature and Mean Mixture Fraction.

Display PDF Tables/Curves...



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6. Write the pdf file, par-premixed.pdf.gz.

File Write PDF...

Step 4: Materials

FLUENT will automatically select pdf-mixture under Mixture Materials, and the mixturespecies as defined in the pdf. The Density will be set as pdf and the Laminar Flame Speedas prepdf-polynomial.

1. Retain the default values for the other parameters.

Step 5: Operating Conditions

1. Retain the default operating conditions.

Step 6: Boundary Conditions

1. Set the boundary conditions for air inlet.

(a) Specify the Velocity Specification Method as Components and set the Axial-Velocityto 10.

(b) Specify the Turbulence Specification Method as Intensity and Hydraulic Diameterand set Hydraulic Diameter to 0.0254.

(c) Retain the default values for the other parameters.

Note: The Progress Variable (c) = 0 specifies reactant mixture, but since youhave specified the Mean Mixture Fraction (f) as 0, this will be treated as a

non-combustible mixture. In fact, you can use either values of c; 0 or 1, andthe air inlet results should not vary.

2. Set the boundary conditions for air-fuel inlet.

(a) Set the Axial-Velocity to 50 m/s and Swirl-Velocity to 30 m/s.

(b) Set the Hydraulic Diameter and Mean Mixture Fraction to 0.0254 and 1 respec-tively.

(c) Retain the default values for the other parameters.

3. Set the boundary conditions for the outlet zone.

(a) Set the Backflow Hydraulic Diameter and Backflow Progress Variable to 0.13 and1 respectively.

(b) Retain the default values for the other parameters.



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Step 7: First Order Solution

1. Solve for Flow, Swirl Velocity, and Turbulence equations.

(a) Keep the default values for Under-Relaxation Factors and Discretization scheme.

(b) Enable the plotting of residuals during calculation.

(c) Initialize the flow field and compute from all zones. Save the case file par-premixed.cas.g

(d) Start the calculation by requesting 1000 iterations.

(e) Save the data file par-premixed.dat.gz

Scaled ResidualsFLUENT 6.2 (axi, swirl, segregated, ske)

Oct 26, 2005

Iterations

2252001751501251007550250

1e+00

1e-01

1e-02

1e-03

1e-04

epsilonkswirly-velocityx-velocitycontinuityResiduals

Figure 3: Scaled Residuals

(f) Define a region with the following values:

Adapt Region...

Input Coordinates Value

Xminimum (m) 0.10

Xmaximum (m) 0.14

Yminimum (m) 0

Ymaximum (m) 0.03

(g) Patch a region close to fuel-air inlet.

i. Under Registers To Patch, select the marked region hexahedron-r0.

ii. Select Progress Variable under Variable.

iii. Retain the default Value of 0 and click Patch.



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2. Solve for all the equations.

(a) Request for an additional 500 iterations.

(b) Save the case and data files, par-premixed-1st.cas.gz.

Scaled ResidualsFLUENT 6.2 (axi, swirl, segregated, pdf14, ske)

Oct 26, 2005

Iterations

6005004003002001000

1e+00

1e-01

1e-02

1e-03

1e-04

1e-05

1e-06

fvarfmeanepsilonkpremixcswirly-velocityx-velocitycontinuityResiduals


Step 8: Second Order Solution

1. Change the discretization for the following parameters.

Parameter Value

Pressure Second Order

Momentum Second Order Upwind

Swirl Velocity 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

2. Request for an additional 500 iterations.

3. Save the case and data file, par-premixed-2nd.cas.gz.



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Scaled ResidualsFLUENT 6.2 (axi, swirl, segregated, pdf14, ske)

Oct 26, 2005

Iterations

7006005004003002001000

1e+00

1e-01

1e-02

1e-03

1e-04

1e-05

1e-06

fvarfmeanepsilonkpremixcswirly-velocityx-velocitycontinuityResiduals


Step 9: Postprocessing

Plot vectors and contours for various parameters as described below:

1. Display Velocity Vectors (Figure 6).

Set the Scale Factor to 10 and Skip Value to 3.

2. Display Contours of Stream Function (Figure 7).

Select Velocity... and Stream Function under Contours Of.

3. Display filled Contours of mean Progress Variable (Figure 8).

Select Premixed Combustion... and Progress Variable under Contours Of.

4. Display filled Contours of Static Temperature (Figure 9).



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Velocity Vectors Colored By Velocity Magnitude (m/s)FLUENT 6.2 (axi, swirl, segregated, ske)

Oct 26, 2005

5.85e+015.57e+01

5.28e+01

4.99e+01

4.70e+01

4.41e+01

4.12e+01

3.83e+01

3.54e+01

3.25e+01

2.96e+01

2.67e+01

2.38e+01

2.09e+01

1.80e+01

1.51e+01

1.22e+01

9.34e+00

6.45e+00

3.55e+00

6.59e-01

Figure 6: Velocity Vectors

Contours of Stream Function (kg/s)FLUENT 6.2 (axi, swirl, segregated, ske)

Oct 26, 2005

2.69e-02

2.55e-02

2.42e-02

2.29e-022.15e-02

2.02e-02

1.88e-02

1.75e-02

1.61e-02

1.48e-02

1.34e-02

1.21e-02

1.08e-02

9.41e-03

8.07e-03

6.72e-03

5.38e-03

4.03e-03

2.69e-03

1.34e-03

0.00e+00

Figure 7: Contours of Stream Function



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Contours of Progress Variable Oct 26, 2005FLUENT 6.2 (axi, swirl, segregated, ske)

1.00e+00

0.00e+00

5.00e-02

1.00e-01

1.50e-01

2.00e-01

2.50e-01

3.00e-01

3.50e-01

4.00e-01

4.50e-01

5.00e-01

5.50e-01

6.00e-01

6.50e-01

7.00e-01

7.50e-01

8.00e-01

8.50e-01

9.00e-01

9.50e-01

Figure 8: Contours of Progress Variable

Contours of Static Temperature (k) Oct 26, 2005FLUENT 6.2 (axi, swirl, segregated, ske)

1.91e+03

3.00e+02

3.81e+02

4.61e+02

5.42e+02

6.22e+02

7.03e+02

7.83e+02

8.64e+02

9.44e+02

1.02e+03

1.11e+03

1.19e+03

1.27e+03

1.35e+03

1.43e+03

1.51e+03

1.59e+031.67e+03

1.75e+03

1.83e+03

Figure 9: Contours of Static Temperature



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5. Display contours of species mass fractions.

(a) Mass fraction ofCH4 (Figure 10).

Select Species... and Mass Fractions of ch4 under Contours Of.

(b) Mass fraction ofH2

O (Figure 11).(c) Mass fraction ofCO2 (Figure 12).

(d) Mass fraction ofCO (Figure 13).

(e) Mass fraction ofN2 (Figure 14).

Contours of Mass fraction of ch4 Oct 26, 2005FLUENT 6.2 (axi, swirl, segregated, ske)

4.53e-02

0.00e+00

2.27e-03

4.53e-03

6.79e-03

9.06e-03

1.13e-02

1.36e-02

1.59e-02

1.81e-02

2.04e-02

2.26e-02

2.49e-02

2.72e-02

2.94e-02

3.17e-02

3.40e-02

3.62e-02

3.85e-02

4.08e-02

4.30e-02

Figure 10: Mass Fraction ofCH4



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Contours of Mass fraction of h2o Oct 26, 2005FLUENT 6.2 (axi, swirl, segregated, ske)

9.09e-02

0.00e+00

4.55e-03

9.09e-03

1.36e-02

1.82e-02

2.27e-02

2.73e-02

3.18e-02

3.64e-02

4.09e-02

4.55e-02

5.00e-02

5.46e-02

5.91e-02

6.37e-02

6.82e-02

7.28e-02

7.73e-028.18e-02

8.64e-02

Figure 11: Mass Fraction ofH2O

Contours of Mass fraction of co2 Oct 26, 2005FLUENT 6.2 (axi, swirl, segregated, ske)

1.11e-01

0.00e+00

5.56e-03

1.11e-02

1.67e-02

2.23e-02

2.78e-02

3.34e-02

3.90e-02

4.45e-02

5.01e-02

5.56e-02

6.12e-02

6.68e-02

7.23e-02

7.79e-02

8.35e-02

8.90e-02

9.46e-02

1.00e-01

1.06e-01

Figure 12: Mass Fraction ofCO2



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Contours of Mass fraction of co Oct 26, 2005FLUENT 6.2 (axi, swirl, segregated, ske)

2.06e-04

0.00e+00

1.03e-05

2.06e-05

3.10e-05

4.13e-05

5.16e-05

6.19e-05

7.23e-05

8.26e-05

9.29e-05

1.03e-04

1.14e-04

1.24e-04

1.34e-04

1.45e-04

1.55e-04

1.65e-04

1.76e-041.86e-04

1.96e-04

Figure 13: Mass Fraction ofCO

Contours of Mass fraction of n2 Oct 26, 2005FLUENT 6.2 (axi, swirl, segregated, ske)

7.67e-01

7.28e-01

7.30e-01

7.32e-01

7.34e-01

7.36e-01

7.38e-01

7.40e-01

7.42e-01

7.44e-01

7.46e-01

7.48e-01

7.50e-01

7.52e-01

7.53e-01

7.55e-01

7.57e-01

7.59e-01

7.61e-01

7.63e-01

7.65e-01

Figure 14: Mass Fraction ofN2



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Results

The partially premixed model in FLUENT can be used to simulate a combustion system,where the combustion process is neither purely premixed nor purely non-premixed. Bothpremixed and non-premixed properties can be investigated using the postprocessing results

as shown above.

Summary

Application of the partially premixed model, based on both non-premixed (mixture-fractionbased) and premixed (mean progress variable based) models has been demonstrated.


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