Modeling flat heat exchangers in KULI

In the constant effort for increasing the efficiency of commercial vehicles, new concepts for improving the cooling system are developed. One idea is to use surfaces of the vehicle for cooling purposes. With the help of sufficiently large flat heat exchangers coolant can be cooled by the air flowing around the vehicle. The aerodynamics would not be influenced, perhaps even improved. In particular the underbody region can be used for such a concept.

Figure 1: Position and shape of flat heat exchangers


Within the European research project “COnVENienT” (COmplete Vehicle ENergy-saving Technologies), in which three major OEMs of commercial vehicles and several suppliers and research institutes are involved, the development of such FHX is included. CFD simulation was performed by ECS to determine suitable FHX locations. Afterwards, one task was to include a suitable model of the FHX into the full cooling system KULI model. Such an FHX consists of a single coolant layer, where the upper side (interface to engine room) is made of composite material and the lower side (interface to the ambient air under the vehicle) is made of aluminum, ensuring good heat transfer. Since none of the existing KULI components could be used out of the box to model such an FHX, ECS used the signal path and existing components to set up an FHX model.

Figure 2: Model of the FHX in KULI


Essentially, the FHX is modeled as a tube (for modeling the pressure drop) and a heat source (for modeling the heat that is being removed from the coolant). The value of the heat source is then calculated with the signal path. The amount of heat transferred on the composite and aluminum side is calculated independently and then summed up. Since for very low coolant flow rates the performance of the FHX is limited by the coolant heat capacity flow, a separate calculation controller was included to take care of this limitation.

The model was calibrated based on CFD results of the component itself that were provided by the FHX supplier. KULI optimization was used to automatically determine the effective area of the FHX with which the model was calibrated.

The complete FHX model was then saved as a subsystem and was included into the full cooling system model at different positions. The model turned out to be robust  and quick.