14.3.6 Setting the Pressure-Drop Parameters and Effectiveness

The pressure drop parameters and effectiveness define the Core Porosity Model. If you would like ANSYS FLUENT to set the porosity of this a heat exchanger zone using a particular core model, you can select the appropriate model. This will automatically set the porous media inputs. There are three ways to specify the Core Porosity Model parameters:

• Use the values in ANSYS FLUENT's default model.

• Define a new core porosity model with your own values.

• Read a core porosity model from an external file.

If you do not choose a core porosity model, you will need to set the porosity parameters in the cell zone conditions dialog box for the heat exchanger zone(s). To do this, follow the procedures described in Section  7.2.3.

The models you define will be saved in the case file.

Using the Default Core Porosity Model

ANSYS FLUENT provides a default model for a typical heat exchanger core. The default-model core porosity model is a list of constant values from the Ungrouped Macro Heat Exchanger dialog box. These constants are used for setting the porous media parameters. To use these values, simply retain the selection of default-model in the Core Porosity Model drop-down list in the Ungrouped Macro Heat Exchanger dialog box. (You can view the default parameters as described below.)

Defining a New Core Porosity Model

If you want to define pressure-drop and effectiveness parameters that are different from those in the default core porosity model, you can create a new model. The steps for creating a new model are as follows:

1.   Click the Edit... button to the right of the Core Porosity Model drop-down list, for which default-model should have been selected. This will open the Core Porosity Model dialog box (see Section  14.3.6) (Figure  14.3.8).

Figure 14.3.8: The Core Porosity Model Dialog Box

2.   Enter the name of your new model in the Name box at the top of the dialog box.

3.   Under Gas-Side Pressure Drop, specify the following parameters used in this equation in the separate Theory Guide :

Minimum Flow to Face Area Ratio   ( $\sigma$)

Entrance Loss Coefficient   ( $K_c$)

Exit Loss Coefficient   ( $K_e$)

Gas-Side Surface Area   ( $A$)

Minimum Cross Section Flow Area   ( $A_c$)

and the Core Friction Coefficient and Core Friction Exponent ( $a$ and $b$, respectively, in this equation in the separate Theory Guide).

4.   Click the Change/Create button. This will add your new model to the database.

Reading Heat Exchanger Parameters from an External File

You can read parameters for your Core Porosity Model from an external file. A sample file is shown below:

("modelname"
  (0.73 0.43 0.053 5.2 0.33 9.1 0.66))

The first entry in the file is the name of the model (e.g., modelname). The second set of numbers contains the gas-side (primary-side) pressure drop parameters:

( $\sigma$ $K_c$ $K_e$ $A$ $A_c$ $a$ $b$)

To read an external heat exchanger file, you will follow these steps:

1.   In the Core Porosity Model dialog, click the Read... button.

2.   In the resulting Select File dialog box, specify the HXC Parameters File name and click OK. ANSYS FLUENT will read the core porosity model parameters, and add the new model to the database.

Viewing the Parameters for an Existing Core Model

To view the parameters associated with a core porosity model that you have already defined, select the model name in the Database drop-down list (in the Core Porosity Model dialog box). The values for that model from the database will be displayed in the Core Porosity Model dialog box.