The F correction factor charts, or the ε-NTU relations all assume that the overall heat transfer coefficient is constant across the heat exchanger. The short lengths and the comparatively thick walls through which heat is conducted in microchannel heat exchangers preclude the existence of thermally fully developed flow over a large portion of the heat exchanger. In this study, a parallel flow heat exchanger is simulated numerically to determine the impact of different parameters on the performance and the accuracy of constant heat transfer coefficient assumption. It is shown that there is significant change in the overall heat transfer coefficient in the developing region and that the three-dimensional heat transfer in the heat exchanger wall must be included in the analysis.

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