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13.2.2 Steps in Solving Heat Transfer Problems

The procedure for setting up a heat transfer problem is described below. (Note that this procedure includes only those steps necessary for the heat transfer model itself; you will need to set up other models, boundary conditions, etc. as usual.)

1.   To activate the calculation of heat transfer, enable the Energy Equation option in the Energy panel (Figure  13.2.1).

Define $\rightarrow$ Models $\rightarrow$ Energy...

Figure 13.2.1: The Energy Panel
figure

2.   (Optional, pressure-based solver only.) If you are modeling viscous flow and you want to include the viscous heating terms in the energy equation, enable the Viscous Heating option in the Viscous Model panel.

Define $\rightarrow$ Models $\rightarrow$ Viscous...

As noted in Section  13.2.1, the viscous heating terms in the energy equation are (by default) ignored by FLUENT when the pressure-based solver is used. (They are always included for the density-based solver.) Viscous dissipation should be enabled when the shear stress in the fluid is large (e.g., in lubrication problems) and/or in high-velocity, compressible flows (see Equation  13.2-9).

3.   Define thermal boundary conditions at flow inlets, flow outlets, and walls.

Define $\rightarrow$ Boundary Conditions...

At flow inlets and exits you will set the temperature; at walls you may use any of the following thermal conditions:

  • specified heat flux

  • specified temperature

  • convective heat transfer

  • external radiation

  • combined external radiation and external convective heat transfer

Section  7.13.1 provides details on the model inputs that govern these thermal boundary conditions. The default thermal boundary condition at inlets is a specified temperature of 300 K; at walls the default condition is zero heat flux (adiabatic). See Chapter  7 for details about boundary condition inputs.

figure   

If your heat transfer application involves two separated fluid regions, see the information provided below.

4.   Define material properties for heat transfer.

Define $\rightarrow$ Materials...

Heat capacity and thermal conductivity must be defined, and you can specify many properties as functions of temperature as described in Chapter  8.

figure   

If your heat transfer application involves two separated fluid regions, see the information provided below.



Limiting the Predicted Temperature Range: The Temperature Floor and Ceiling


For stability reasons, FLUENT includes a limit on the predicted temperature range. The purpose of the temperature ceiling and floor is to improve the stability of calculations in which the temperature should physically lie within known limits. Sometimes intermediate solutions of the equations give rise to temperatures beyond these limits for which property definitions, etc. are not well defined. The temperature limits keep the temperatures within the expected range for your problem. If the FLUENT calculation predicts a temperature above the maximum limit, the stored temperature values are "pegged'' at this maximum value. The default for the temperature ceiling is 5000 K. If the FLUENT calculation predicts a temperature below the minimum limit, the stored temperature values are "pegged'' at this minimum value. The default for the temperature minimum is 1 K.

If you expect the temperature in your domain to exceed 5000 K, you should use the Solution Limits panel to increase the Maximum Temperature.

Solve $\rightarrow$ Controls $\rightarrow$ Limits...



Modeling Heat Transfer in Two Separated Fluid Regions


If your heat transfer application involves two fluid regions separated by a solid zone or a wall, as illustrated in Figure  13.2.2, you will need to define the problem with some care. Specifically:

Figure 13.2.2: Typical Counterflow Heat Exchanger Involving Heat Transfer Between Two Separated Fluid Streams
figure


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