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7.25.4 Using the Heat Exchanger Group

To define a single heat exchanger that uses multiple fluid zones, or to connect the auxiliary fluid flow path among multiple heat exchangers, you can use heat exchanger groups. To use heat exchanger groups, perform the following steps:

1.   Enable the calculation of energy in the Energy panel.

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

2.   Specify the inputs to the heat exchanger group model, using the Heat Exchanger Group panel (Figure  7.25.9).

Define $\rightarrow$ User-Defined $\rightarrow$ Heat Exchanger Group...

Figure 7.25.9: The Heat Exchanger Group Panel
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(a)   Under Name, enter the name of the heat exchanger group.

(b)   Under Fluid Zones, select the fluid zones that you want to define in the heat exchanger group.

(c)   Click on the Model Data tab.

i.   Under Connectivity, select the Upstream heat exchanger group if such a connection exists.

ii.   Under Primary Fluid Flow Direction, specify the gas flow direction as either Width, Height, or Depth.

iii.   In the Heat Transfer Model drop-down list, choose either the ntu-model or the simple-effectiveness-model.

iv.   From the Core Porosity Model drop-down list, specify the core model that should be used to calculate the porous media parameters for the zones in the group.

v.   Provide the Heat Exchanger Performance Data according to the chosen model.

(d)   Click on the Geometry tab (Figure  7.25.10).

Figure 7.25.10: The Heat Exchanger Group Panel - Geometry Tab
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i.   Define the macro grid by specifying the Number of Passes, the Number of Rows/Pass, and the Number of Columns/Pass.

ii.   Specify the Auxiliary Fluid Inlet Direction and Pass-to-Pass Direction.

(e)   Click the Auxiliary Fluid tab (Figure  7.25.11) to specify the auxiliary fluid operating conditions.

Figure 7.25.11: The Heat Exchanger Group Panel - Auxiliary Fluid Tab
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i.   Specify the Specific Heat as either a constant-specific-heat or as a user-defined-enthalpy.

ii.    Auxiliary Fluid Flow Rate, Initial Temperature, and Inlet Pressure can be provided as a constant, polynomial or piecewise-linear profile that is a function of time.

(f)   If a supplementary auxiliary stream is to be modeled, click the Supplementary Auxiliary Fluid Stream tab.

Figure 7.25.12: The Heat Exchanger Group Panel - Supplementary Auxiliary Fluid Stream Tab
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i.   You can specify the Supplementary Mass Flow Rate as a constant, polynomial or piecewise-linear profile that is a function of time.

ii.   You can specify the Supplementary Flow Temperature as a constant, polynomial or piecewise-linear profile that is a function of time.

(g)   Click Create or Replace in the Heat Exchanger Group panel to save all the settings. Replace changes the parameters of the already existing group that is selected in the HX Groups list.

figure   

Creating or replacing any heat exchanger group initializes any previously calculated values for temperature and enthalpy for all macros.

3.   If a heat exchanger group is comprised of multiple fluid zones and you wish to override any of the inputs defined in the previous steps, click on the Set... button to open the Heat Exchanger Model panel (Figure  7.25.2). Select the particular fluid zone as usual. Notice that the individual heat exchanger inherits the properties of the group by default. You may override any of the following:

  • Number of Passes, Number of Rows/Pass, and Number of Columns/Pass

  • Auxiliary Fluid Inlet Direction and the Pass-to-Pass Direction

  • Core Porosity Model



Selecting the Fluid Zones for the Heat Exchanger Group


Select the fluid zones that you want to define in the heat exchanger group in the Fluid Zones drop-down list. The auxiliary fluid flow in all these zones will be in parallel. Note that one zone cannot be included in more than one heat exchanger group.



Selecting the Upstream Heat Exchanger Group


If you want to connect the current group in series with another group, choose the upstream heat exchanger group. Note that any group can have at most one upstream and one downstream group. Also, a group cannot be connected to itself. Select a heat exchanger group from the Upstream drop-down list under Connectivity in the Model Data tab of the Heat Exchanger Group panel.

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Connecting to an upstream heat exchanger group can be done only while creating a heat exchanger group. The connection will persist even if the connection is later changed and the Replace button is clicked. To change a connection to an upstream heat exchanger group, you need to delete the connecting group and create a new heat exchanger group with the proper connection.



Specifying the Auxiliary Fluid Inlet and Pass-to-Pass Directions


The Auxiliary Fluid Inlet Direction and Pass-to-Pass Direction, in the Geometry tab can be specified as directed in Section  7.25.3 in the Heat Exchanger Model panel. Note that the Update from Plane Tool will set the height, width, and depth as the average of the fluid zones selected in the Fluid Zones.



Specifying the Auxiliary Fluid Properties


The auxiliary fluid can be specified as having a constant-specific-heat, or a user-defined function can be written to calculate the enthalpy. The function should return a single value depending on the index:

The user-defined function should be of type

DEFINE_SOURCE(udf_name, cell_t c, Thread *t, real d[n], int index).

where n in the expression d[n] would be 0 for temperature, 1 for pressure, or 2 for quality. The variable index is 0 for enthalpy, 1 for temperature, or 2 for specific heat. This user-defined function should return

real value; /* (temperature or enthalpy or Cp depending on index). */



Specifying Supplementary Auxiliary Fluid Streams


The addition or removal of a supplementary auxiliary fluid is allowed in any of the heat exchanger groups. Note that auxiliary streams are not allowed for individual zones. You will input the mass flow rate, temperature, and quality of the supplementary auxiliary fluid. You will also need to specify the heat transfer for various flow rates of primary and auxiliary flows. The auxiliary stream has the following assumptions:



Initializing the Auxiliary Fluid Temperature


When the heat exchanger group is connected to an upstream heat exchanger group, FLUENT will automatically set the initial guess for the auxiliary fluid inlet temperature, $T_{\rm in}$, to be equal to the $T_{\rm in}$ of the upstream heat exchanger group. Thus the boundary condition $T_{\rm in}$ for the first heat exchanger group in a connected series will automatically propagate as an initial guess for every other heat exchanger group in the series. However, when it is necessary to further improve convergence properties, you will be allowed to override $T_{\rm in}$ for any connected heat exchanger group by providing a value in the Initial Temperature field. Whenever such an override is supplied, FLUENT will automatically propagate the new $T_{\rm in}$ to any heat exchanger groups further downstream in the series. Similarly, every time the $T_{\rm in}$ boundary condition for the first heat exchanger group is modified, FLUENT will correspondingly update every downstream heat exchanger group.

If you want to impose a non-uniform initialization on the auxiliary fluid temperature field, first connect the heat exchanger groups and then set $T_{\rm in}$ for each heat exchanger group in streamwise order.

All heat exchangers included in a group must use the fixed $T_{\rm in}$ option. All heat exchangers within a heat exchanger group must have the same $T_{\rm in}$. In other words, no local override of this setting is possible through the Heat Exchanger panel.


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