In many engineering systems, such as compact heat exchangers and microcoolers used in electronics packaging, system performance depends on laminar flow development in microchannels. This study investigates effects of Reynolds number, hydraulic diameter, and channel aspect ratio on the entrance length in rectangular microchannels. Numerical investigations were performed for microchannels with hydraulic diameters between 100 and 500 μm, Reynolds numbers between 0.5 and 200, and channel aspect ratios between 1 and 5. The results show good agreement with available experimental data and are used to formulate new correlations for estimating the entrance length in microchannels. Compared to other correlations, these new correlations are shown to provide more accurate estimates of entrance length over a wider range of Reynolds numbers representative of practical flows in microchannels.

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