14.3 Using the Ungrouped Macro Heat Exchanger Model

The heat exchanger model settings may be written into and read from the boundary conditions file (Section  4.7) using the text commands, file/write-settings and file/read-settings, respectively. Otherwise, the steps for setting up the ungrouped macro heat exchanger model is as follows:

1.   Enable the calculation of energy in the Energy dialog box.

Models Energy Edit...

2.   Enable the Ungrouped Macro Model option and click the Define... button in the Heat Exchanger Model dialog box (Figure  14.3.1) to access the Ungrouped Macro Heat Exchanger dialog box.

Models Heat Exchanger Edit...

Figure 14.3.1: The Heat Exchanger Model Dialog Box

3.   Specify the heat exchanger inputs in the Ungrouped Macro Heat Exchanger dialog box.

Figure 14.3.2: The Ungrouped Macro Heat Exchanger Dialog Box Displaying the Model Data Tab

(a)   In the Fluid Zone drop-down list, select the fluid zone representing the heat exchanger core.

(b)   Under the Model Data tab, choose Fixed Heat Rejection or Fixed Inlet Temperature, as required (Figure  14.3.2).

(c)   Specify the heat exchanger model as either the default ntu-model or the simple-effectiveness-model.

(d)   Specify the Core Porosity Model if you want ANSYS FLUENT to use the pressure loss coefficient function to automatically compute (and update) the porous media coefficients in the cell zones condition dialog box, as described in this section in the separate Theory Guide. More information is available in Section  14.3.6.

(e)   If the ntu-model is chosen, a Heat Transfer Data... button will appear under Heat Exchanger Performance Data. Clicking the Heat Transfer Data... button will open up the Heat Transfer Data Table dialog box with information on the fluid flow rates and heat transfer data (Figure  14.3.3). More information is available in Section  14.3.2.

Figure 14.3.3: The Heat Transfer Data Table Dialog Box for the NTU Model

(f)   Enter the Auxiliary Fluid Temperature and the Primary Fluid Temperature for the ntu-model. These are the fixed inlet temperatures at which the test was performed to obtain the heat transfer data.

(g)   If the simple-effectiveness-model is chosen, then clicking the Velocity Effectiveness Curve... button, under the Heat Exchanger Performance Data, allows you to set the velocity and corresponding effectiveness for each point. More information is available in Section  14.3.2.

(h)   In the Geometry tab, define the macro mesh using the Number of Passes, the Number of Rows/Pass, and the Number of Columns/Pass fields. The Number of Rows/Pass is along the auxiliary flow direction (height) and the Number of Columns/Pass is defined in the pass-to-pass (width) direction. Also, enter the Auxiliary Fluid Inlet Direction and Pass-to-Pass Direction. You may want to snap the plane tool to either the inlet or outlet of the heat exchanger using the Update from Plane Tool. Note that the plane tool must be attached exactly and oriented so that its green arrow points in the auxiliary flow direction, and its blue arrow points in the pass-to-pass direction.

To attach the plane tool exactly, exact coordinates (printed in the console by probing) of the three corner nodes must be entered in the plane tool (x0, x1, x2) in a specific order. Also, note that x0 to x1 is the auxiliary flow direction and x1 to x2 is the pass-to-pass direction.

More information is available in Sections  14.3.3 and 14.3.4.

Figure 14.3.4: The Ungrouped Macro Heat Exchanger Dialog Box Displaying the Geometry Tab

Figure 14.3.5: The Ungrouped Macro Heat Exchanger Dialog Box Displaying the Auxiliary Fluid Tab

(i)   In the Auxiliary Fluid tab, specify the Auxiliary Fluid Properties Method, either as a constant-specific-heat or as a user-defined-enthalpy.

(j)    Auxiliary Fluid Flow Rate, Heat Rejection, Inlet Temperature, and Inlet Pressure can be provided as a constant, polynomial or piecewise-linear profile that is a function of time. If user-defined-enthalpy is selected as the Auxiliary Fluid Properties Method, you will need to specify the Inlet Quality and the Pressure Drop. More information is available in Section  14.3.5.

(k)   Click Apply in the Ungrouped Macro Heat Exchanger dialog box to save all the settings. Once you click the Apply button, the NTU matrix will be computed from the raw data. Therefore, make sure you click Apply at the very end of your setup.

When you click Apply, look for any error or warning message in the ANSYS FLUENT console. Some of the common errors you may see displayed are due to the NTU computations not converging. In such cases, check that you have entered the data correctly the values of the data are reasonable the operating condition for the auxiliary fluid flow rate is not too far from the range of the heat transfer data Other error messages you may encounter may be due to macros not getting any cells assigned to it. In such cases, make sure that the heat exchanger core is rectangular the directions are correct you are using uniformly spaced cells in both directions the mesh is either a hexahedra or wedge and is structured

(l)   Repeat steps (a)-(k) for any other heat exchanger fluid zones.

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  14.4.