14.1 Overview and Restrictions of the Macro Heat Exchanger Models

To use the heat exchanger models, you must define one or more fluid zone(s) to represent the heat exchanger core. Typically, the fluid zone is sized to the dimension of the core itself. As part of the setup procedure, you will define the auxiliary fluid path, the number of macros, and the physical properties and operating conditions of the core (pressure drop parameters, heat exchanger effectiveness, auxiliary fluid flow rate, etc.).

It is highly recommended that the free-form Tet mesh is not used in the macro heat exchanger model. Instead, evenly distributed Hex/Wedge cells should be used for improved accuracy and a more robust solution process.

Additional information about macro heat exchangers can be found in this section in the separate Theory Guide. For the theory behind these models, refer to this section in the separate Theory Guide.

ANSYS FLUENT provides two heat transfer models: the default ntu-model and the simple-effectiveness-model. The simple-effectiveness-model interpolates the effectiveness from the velocity vs effectiveness curve that you provide. For the ntu-model, ANSYS FLUENT calculates the effectiveness, $\epsilon$, from the NTU value that is calculated by ANSYS FLUENT from the heat transfer data provided by the user in tabular format. ANSYS FLUENT will automatically convert this heat transfer data to a primary fluid mass flow rate vs NTU curve (this curve will be piecewise linear). This curve will be used by ANSYS FLUENT to calculate the NTU for macros based on their size and primary fluid flow rate.

The ntu-model provides the following features:

• The model can be used to check heat capacity for both the primary and the auxiliary fluid and takes the lesser of the two for the calculation of heat transfer.

• The model can be used to model heat transfer to the primary fluid from the auxiliary fluid and vice versa.

• The model can be used to model primary fluid-side reverse flow.

• The model can be used with variable density of the primary fluid.

• The model can be used in either the serial or parallel ANSYS FLUENT solvers.

• Transient profiles can be used for the coolant inlet temperature and for total heat rejection.

• Transient profiles can be used for auxiliary mass flow rates.

The simple-effectiveness-model provides the following features:

• The model can be used to model heat transfer from the auxiliary fluid to the fluid.

• The auxiliary fluid properties can be a function of pressure and temperature, thus allowing phase change of the auxiliary fluid.

• The model can be used by serial as well as parallel solvers.

• The model can be used to make a network of heat exchangers using a heat exchanger group (Section  14.4).

• Transient profiles can be used for the coolant inlet temperature and for total heat rejection.

• Transient profiles can be used for auxiliary mass flow rates.