The heat exchanger model settings may be written into and read from the boundary conditions file (Section 4.6) using the text commands, file/write-bc and file/read-bc, respectively. Otherwise, the steps for setting up the heat exchanger models are as follows:
Define Models Energy...
Define User-Defined Heat Exchanger...
To use multiple fluid zones to define a single heat exchanger, or to connect the auxiliary fluid flow path among multiple heat exchangers, see Section 7.25.4.
Selecting the Zone for the Heat Exchanger
Choose the fluid zone for which you want to define a heat exchanger in the Fluid Zone drop-down list.
Specifying Heat Exchanger Performance Data
Based on the heat transfer model you choose in the Model Data tab, some performance data must be entered for the heat exchanger.
Specifying the Auxiliary Fluid Inlet and Pass-to-Pass Directions
To define the auxiliary fluid direction and flow path, you will specify direction vectors for the Auxiliary Fluid Inlet Direction and the Pass-to-Pass Direction in the Geometry tab. Figure 7.25.6 shows these directions relative to the macros.
For some problems in which the principal axes of the heat exchanger core are not aligned with the coordinate axes of the domain, you may not know the auxiliary fluid inlet and pass-to-pass direction vectors a priori. In such cases, you can use the plane tool as follows to help you to determine these direction vectors.
Defining the Macros
As discussed in Section 7.25.1, the fluid zone representing the heat exchanger core is split into macros. Macros are constructed based on the specified number of passes, the number of macro rows per pass, the number of macro columns per pass, and the corresponding auxiliary fluid inlet and pass-to-pass directions (see Figure 7.25.6). Macros are numbered from to ( ) in the direction of auxiliary fluid flow, where is the number of macros.
In the Heat Exchanger Model panel, in the Geometry tab, specify the Number of Passes, the Number of Rows/Pass, and the Number of Columns/Pass. The model will automatically extrude the macros to the depth of the heat exchanger core. For each pass, the Number of Rows/Pass are defined in the direction of the auxiliary flow inlet direction and the Number of Columns/Pass are defined in the direction of the pass-to-pass direction.
Number of Rows/Pass, as well as the
Number of Columns/Pass must be divisible by the number of cells in their respective directions.
Viewing the Macros
You can view the auxiliary fluid path by displaying the macros. To view the macros for your specified Number of Passes, Number of Rows/Pass, and Number of Columns/Pass, click the Apply button at the bottom of the panel. Then click View Passes to display it. The path of the auxiliary fluid is color-coded in the display: macro is red and macro is blue.
For some problems, especially complex geometries, you may want to include portions of the computational-domain grid in your macros plot as spatial reference points. For example, you may want to show the location of an inlet and an outlet along with the macros. This is accomplished by turning on the Draw Grid option. The Grid Display panel will appear automatically when you turn on the Draw Grid option, and you can set the grid display parameters there. When you click on View Passes in the Heat Exchanger Model panel, the grid display, as defined in the Grid Display panel, will be included in the macros plot (see Figure 7.25.7).
Selecting the Heat Exchanger Model
You can specify the model for your heat exchanger by selecting the ntu-model or the simple-effectiveness-model from the Heat Transfer Model drop-down list in the Model Data tab.
Specifying the Auxiliary Fluid Properties and Conditions
To define the auxiliary fluid properties and conditions, you will specify the Auxiliary Fluid Flow Rate ( ) in the Auxiliary Fluid tab. The properties of the auxiliary fluid can be specified using the Auxiliary Fluid Properties Method drop-down list. You can choose a Constant Specific Heat ( ) and set the value in the Auxiliary Fluid Specific Heat field below, or as a user-defined function for the enthalpy using the User Defined Enthalpy option and selecting the corresponding UDF from the Auxiliary Fluid Enthalpy UDF drop-down list.
Setting the Pressure-Drop Parameters and Effectiveness
The pressure drop parameters and effectiveness define the Core Porosity Model. If you would like 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:
If you do not choose a core porosity model, you will need to set the porosity parameters in the boundary conditions panel for the heat exchanger zone(s). To do this, follow the procedures described in Section 7.19.6.
The models you define will be saved in the case file.
Using the Default Core Porosity Model
FLUENT provides a default model for a typical heat exchanger core. To use these values, simply retain the selection of default-model in the Core Porosity Model drop-down list in the Heat Exchanger Model panel. (You can view the default parameters in the Heat Exchanger Model panel, as described below.)
The default-model core porosity model is a list of constant values from the Heat Exchanger Model panel. These constants are used for setting the porous media parameters.
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:
and the Core Friction Coefficient and Core Friction Exponent ( and , respectively, in Equation 7.25-3).
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 pressure drop parameters:
To read an external heat exchanger file, you will follow these steps:
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 panel). The values for that model from the database will be displayed in the Core Porosity Model panel.