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.
In this study the entropy generation minimization method is used to find the optimum channel dimensions in micro heat exchangers with a uniform heat flux. With this approach, pressure drop and heat transfer in the micro channels are considered simultaneously during the optimization analysis. A computational model is developed to find the optimum channel depth knowing other channel geometry dimensions and coolant inlet properties. The flow is assumed laminar and both hydrodynamically and thermally fully developed and incompressible. However, to take into account the effect of the developing length in the friction losses, the Hagenbach’s factor is introduced. The micro channels are assumed to have an isothermal or isoflux boundary condition, non-slip flow, and fluid properties have dependency on temperature accordingly. For these particular case studies, the pressure drop and heat transfer coefficient for the isoflux boundary condition is higher than the isothermal case. Higher heat transfer coefficient and pressure drop were found when the channel size decreased. The optimum channel geometry that minimizes the entropy generation rate tends to be a deep, narrow channel.
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