my sample fluent manual

8/7/2019 my sample fluent manual

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FLUENT MANUAL

INTRODUCTION: This report examines chemical species mixing and combustion of a gaseous fuel.

A cylindrical combustor burning liquid Hydrogen (H2) in liquid Oxygen (O2) is analyzed andstudied using the model in FLUENT.

In this report you will find how to:

Enable physical models, select material properties, and define

boundary conditions for a turbulent flow with chemical species

mixing and reaction

Initiate and solve the combustion simulation using the segregated solver

Compare the results computed with constant and variable specific heat

Examine the reacting flow results using graphics

PROBLEM DESCRIPTION: The cylindrical combustor considered in this report is shown below. The

flame considered is a turbulent diffusion flame. The fuel oxidizer mixture is prepared in the

combustion chamber through the feed nozzles of fuel and oxidizer at the entrance of thecombustion chamber.

BACKGROUND: In this report, generalized finite-rate chemistry model will be used to analyze the

hydrogen-oxygen combustion system. The combustion will be modeled using a global one-step

reaction mechanism, assuming complete conversion of the fuel to H2O. The reaction equation is

2H2 + O2 2H2O

This reaction will be defined in terms of stoichiometric coefficients, formation enthalpies, and

parameters that control the reaction rate. The reaction rate will be determined assuming that

turbulent mixing is the rate-limiting process; with the turbulence-chemistry interaction modeled

using the eddy-dissipation model.


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PREPARATION

1. Copy or locate the mesh file and open it in fluent.

2. Start the 2D version of FLUENT.

Step1: Grid

1. Read the grid from the *.msh (mesh file) following the command below.

FILE > READ >CASEAfter reading the grid file, FLUENT will report that quadrilateral fluid cells have been read, along with a number of boundary faceswith different zone identifiers.

2. Check the grid.

GRID > CHECKThe grid check lists the minimum and maximum x and y values from the grid, and reports on a number of other grid features that are

checked. Any errors in the grid would be reported at this time. For instance, the cell volumes must never be negative. Note that

the domain extents are reported in units of meters, the default unit of length in FLUENT. Since this grid was created in units of

millimeters, the Scale Grid panel will be used to scale the grid into meters.

3. Scale the grid.GRID > SCALEUnder Units Conversion, select desired units from the drop-down and scale up or down using the x and y values. Perform grid check

after scaling the model to ensure scaling without any errors to the model.Note: Because the default SI units will be used in this report, there is no need to change any units in this problem.

4. Display the grid.

DISPLAY > GRIDThis will display the grid after the scaling highlighting the basic model and the meshing of the medium of analysis.

Extra: You can use the right mouse button to check which zone number corresponds to each boundary. If you click the right mouse

button on one of the boundaries in the graphics window, its zone number, name, and type will be printed in the FLUENT console

window. This feature is especially useful when you have several zones of the same type and you want to distinguish between themquickly.


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Step 2: Models1. Define the domain as axi-symmetric, and keep the default (segregated) solver.

DEFINE > MODELS > SOLVER

2. Enable heat transfer by activating the energy equation.

DEFINE > MODELS > ENERGY


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3. Enable the k- turbulence model.

DEFINE > MODELS >VISCOUS

The panel will expand to provide further options. Click OK to accept the default Standard modeland parameters.


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4. Enable chemical species transport and reaction.

DEFINE > MODELS >SPECIES

(a) Select Species Transport under Model.

(b) Select Volumetric under Reactions.(c) Choose liquid H2 and O2 mixture in the Mixture Material drop-down list.The Mixture Material list contains the set of chemical mixtures that exist in the FLUENT database. By selecting one of the pre-defined

mixtures, you are accessing a complete description of the reacting system. The chemical species in the system and their physical and

thermodynamic properties are defined by your selection of the mixture material. You can alter the mixture material selection or

modify the mixture material properties using the Materials panel.

(d) Select Eddy-Dissipation under Turbulence-Chemistry Interaction.The eddy-dissipation model computes the rate of reaction under the assumption that chemical kinetics are fast compared to the rate at

which reactants are mixed by turbulent fluctuations (eddies).


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Step 3: Materials

DEFINE > MATERIALS

The Materials panel shows the mixture material, liquid H2 and O2 mixture, that is enabled in the

Species Model panel. The properties for this mixture material have been copied from the

FLUENT database and can be modified by user.

Here, user should modify the default setting for the mixture by enabling the gas law. By default,

the mixture material uses constant properties: user should retain this constant property

assumption for now, allowing only the mixture density to vary with temperature and composition.

1. Retain incompressible-ideal-gas in the Density drop-down list.

2. Click the Edit... button to the right of Mixture Species.


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This opens the Species panel.

You can add or remove species from the mixture material using this panel. Here, the species thatmake up the liquid H2 and O2 mixture are predefined and require no modification.

3. Click Cancel to close the panel after making the required changes.

4. In the Materials panel, click the Edit button to the right of the Reaction drop-down list.


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This will open the Reactions panel.

The eddy-dissipation reaction model ignores chemical kinetics and uses only the Mixing Rate

parameters in the Reactions panel. The Arrhenius Rate section of the panel is therefore inactive.

(The Rate Exponent and Arrhenius Rate entries are included in the database and are employed

when the alternate finite rate/eddy-dissipation model is used.)

5. Accept the default settings for the Mixing Rate constants by clicking the OK button.

6. Use the scroll bar to review the remaining properties. Click on the Change/Create button to

accept the material property settings and then Close the panel.


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The initial calculations will be performed assuming that all properties except density are constant.Using constant transport properties (viscosity, thermal conductivity, and mass diffusion

coefficients) is acceptable here because the flow is fully turbulent. The molecular transportproperties will play a minor role compared to turbulent transport. The assumption of constant

specific heat, in contrast, has a strong effect on the combustion solution.


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Step 4: Boundary Conditions

The boundary conditions in the model can be change or modified using the command panelshown below.

DEFINE >BOUNDARY CONDITIONS...


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Step 5: Initial Solution Using Constant Heat Capacity

1. Initialize the field variables.

SOLVE > INITIALIZE > INITIALIZE...

(a) Select all-zones in the Compute From drop-down list.

(b) Adjust the Initial Values for Temperature and Mass Fractions of liquid H2 and O2.

(c) Click Init to initialize the variables, and then close the panel.


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2. Set the under-relaxation factors.

The default under-relaxation parameters in FLUENT are set to high values. For a combustion

model it may be necessary to reduce the under-relaxation to stabilize the solution. Some

experimentation is typically necessary to establish the optimal under-relaxation.

SOLVE > CONTROLS > SOLUTION...

(a) Use the slider bar next to the Under-Relaxation Factors list to locate each species and set its

under-relaxation factor.


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3. Turn on residual plotting during the calculation.

SOLVE > MONITORS >RESIDUAL...

(a) Under Options, select Plot. This option plots the different changes in the properties taking

place during the combustion process.

(b) Click OK.


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4. Save the case file.

FILE > WRITE > CASE...

By doing this the plots and the analysis data will be stored in a file for future reference

(a) Keep the Write Binary Files button on to produce a smaller, unformatted binary file.(b) Enter the desired file name in the Case File text entry box.

(c) Click OK to proceed with the file writing.

5. Start the calculation by requesting desired no of iterations.

SOLVE > ITERATE...

.

6. Save the case and data files.

FILE > WRITE >CASE & DATA...

Note: FLUENT will ask you to confirm that the previous case file is to be overwritten.


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7. Review the current state of the solution by viewing contours of temperature.

DISPLAY >CONTOURS...

(a) Select Temperature and Static Temperature in the Contours Ofdrop-down list.

(b) Click Display.The temperature contours are shown in the contour plot below. The peak temperature, predicted

using a constant heat capacity of 1000 J/kg-K, is over 2900 K.


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TEMPERATURE CONTOURS: CONSTANTCP


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Step 6: Post processing

Review the solution by examining graphical displays of the results and performing surfaceintegrations at the combustor exit.

1. View contours of temperature.


(a) Select Temperature and Static Temperature in the Contours Ofdrop-down list.

(b) Click Display.

The temperature contours are shown in the contour plot. The peak temperature has dropped to

about 2200 K as a result of the temperature and composition-dependent specific heat (this can be

edited in the materials command box as shown in step 3).

TEMPERATURE CONTOURS: VARIABLECP


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2. Plot contours of specific heat

Contours of the mixture specific heat will show how it varies through the domain.


(a) Select Properties... and Specific Heat (Cp) in the Contours Ofdrop-down list.(b) Click Display.

The contours are shown in the contour plot below. The mixture specific heat is largest where the

H2is concentrated, near the fuel inlet, and where the temperature and combustion product

concentrations are large. The increase in heat capacity, relative to the constant value used before,

substantially lowers the peak flame temperature.

CONTOURSOF SPECIFIC HEAT


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3. Display velocity vectors

DISPLAY >VECTORS...

(a) Click the Vector Options button.

This opens the Vector Options panel.


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(b) Select the Fixed Length option and click Apply.

The fixed length option is useful when the vector magnitude varies dramatically. With fixed

length vectors, the velocity magnitude is described only by color instead of by both vector length

and color.(c) In the Vectors panel, reset the Scale to 0.1 and click Display.

The velocity vectors are shown in the plot below.

VELOCITY VECTORS: VARIABLECP


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4. Plot contours of stream function


(a) Select Velocity and Stream Function in the Contours Ofdropdown list.

(b) Click Display.


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The stream function contours are shown below.

STREAM FUNCTION CONTOURS: VARIABLECP


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5. Plot contours of mass fraction for each species.


(a) Select Species and Mass fraction of H2, O2 and H2O in the Contours Ofdrop-down list.(b) Turn on the Filled button under Options.

(c) Click Display.The mass fraction contours for O2, H2, and H2 O are shown below

CONTOUROF O2 MASS FRACTION


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CONTOUROF H2O MASS FRACTION

CONCLUSIONS: This report shows how to solve a 2D model in fluent. These steps can be followed

to solve a 3D model also but while starting the fluent 3D option instead of 2D option at the startof the fluent analysis.

my sample fluent manual

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