In designing a heat exchanger, it is generally assumed that the fluid is uniformly distributed through the heat exchanger core. In reality, the flow distribution is rarely uniform due to inlet and outlet header designs and flow velocity changes in the headers. The flow distribution through a plate-fin heat exchanger (straight Z-type flow) with parallel microchannels and minichannels is studied by using a Computational Fluid Dynamics (CFD) code FLUENT. It was found that the flow maldistribution is quite severe with constant cross-sectional area headers. A modified header design with tapered cross-section was employed and the flow and pressure distributions were investigated using the CFD model. Further, a mathematical model was used to study the effect of the tapered headers on the pressure difference available for each channel across its inlet and outlet ends. The pressure difference across each channel is responsible for the actual flow rate through the channel. Results from the CFD were compared with the model predictions.

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