Application of effectiveness-NTU relationship to parallel flow microchannel heat exchangers subjected to external heat transfer

In this paper the thermal performance of parallel flow microchannel heat exchangers subjected to constant external heat transfer has been theoretically analyzed. Equations for predicting the axial temperatures as well as the effectiveness of the fluids of a microchannel heat exchanger operating under laminar flow conditions have been developed. In addition, an equation for determining the heat transfer between the fluids has also been formulated. Irrespective of the heat capacity ratio, for a specific NTU, external heating always decreases and increases the effectiveness of the hot and cold fluid, respectively. The opposite trend in the effectiveness of the fluids is observed when they are subjected to external cooling. Moreover, under unbalanced flow conditions (heat capacities of two fluids are not equal) the effectiveness of the fluids depended on the fluid with the lowest heat capacity. Among the two unbalanced flow conditions (heat capacities of the two fluids are not equal) the effectiveness of the fluids is greatest when the hot fluid has the lowest heat capacity. At a given NTU, reduction in heat capacity ratio improved the effectiveness of the fluids. Under certain operating conditions temperature cross over was observed in the heat exchanger.