The design of gas-to-gas micro heat exchangers

In this paper the main results of an experimental campaign on the analysis of the thermal performances of gas-to-gas micro heat exchangers operating under different flow configurations (co-current, counter- current, cross flow) are presented. The results are compared with the predictions of conventional correlations developed for the design of the conventional-sized heat exchangers. The experimental analysis of this kind of devices has been coupled with a numerical investigation conducted by using a commercial CFD code (ANSYS Fluent). By means of the results of a series of numerical simulations, a double-layered microchannel heat exchanger based on a large number of parallel microchannels connected to two manifolds has been designed in order to be able to reproduce with the same device cocurrent, counter-current and cross flow arrangements. The core is housed in a shell made of polymer. On the contrary, the foil between the hot and cold gas flows is exchangeable. Several foils made from different materials (copper, peek, stainless steel, aluminum) and with different thickness have been investigated in order to put in evidence the effect due to the axial wall-fluid conjugate heat transfer on the thermal efficiency of the micro heat exchanger. It has been demonstrated that the accurate design of the inlet and outlet manifolds is a crucial point in order to limit the total pressure losses of this kind of device. The results show that the conjugate wall-gas heat transfer activated within the solid foil tends to reduce the effectiveness of counter-current micro heat exchangers and to increase the effectiveness of cross flow micro heat exchangers. On the contrary, in the case of a co-current arrangement the role of the wall axial conduction is always negligible