The dual cell heat exchanger consists of two porous fluid zones, namely a primary zone and an auxiliary zone. The two zones are solved simultaneously and are coupled only through heat transfer. The common region in each zone, where heat transfer takes place, represents the heat exchanger core. The cores for both primary and auxiliary zones occupy the same physical space, as shown in Figure 6.3: Core with Matching Quad Meshes for Primary and Auxiliary Zones in a Crossflow Pattern. The cells in the two cores should overlap completely in the physical space to ensure conservative heat transfer. Heat transfer occurs between cells in close proximity based on the cell centroid. That is, a primary zone cell exchanges heat with one, and only one, auxiliary zone cell and vice versa. Therefore, if one of the core (say primary) mesh is too coarse or fine relative to the other core (say auxiliary), conservation of heat transfer is not ensured. Heat transfer calculations in the dual cell model are based on the NTU method, regardless of whether you provide the raw heat rejection data or NTU data.

Figure 6.3: Core with Matching Quad Meshes for Primary and Auxiliary Zones in a Crossflow Pattern

 

6.2.3.1. NTU Relations

When you provide raw heat rejection data, the NTU values are calculated using Equation 6–9 and the Effectiveness-NTU Relation specified in the Performance Data tab of the Set Dual Cell Heat Exchanger dialog box. In a crossflow pattern, the NTU values are calculated as in Equation 6–10. The equation is solved iteratively using the Newton-Raphson. For parallel flow, the NTU value is calculated as follows:

(6–22)

and for counter flow, the following equation is used:

(6–23)

Otherwise,

(6–24)

where is the heat capacity ratio and is the effectiveness.

 

6.2.3.2. Heat Rejection

Heat rejection is computed for each cell in the two cores (primary and auxiliary) and added as a source term to the energy equation for the respective flows. This is illustrated in Figure 6.4: Core with Primary and Auxiliary Zones with Overlap of Cells.

Figure 6.4: Core with Primary and Auxiliary Zones with Overlap of Cells

(6–25)

(6–26)

(6–27)

(6–28)

(6–29)

(6–30)

where
	= auxiliary cell temperature
	= primary cell temperature
	= the product of the overall heat transfer coefficient () and the total heat transfer area ()
	= scaled minimum heat capacity rate
	= volume of cell
	= volume of the heat exchanger core

You can supply the NTU values, or it can be calculated using the supplied raw data and the effectiveness-NTU relation that you specify.

To learn how to use the dual cell heat exchanges model, refer to Using the Dual Cell Heat Exchanger Model in the User's Guide.