Heat transfer characteristics of thermally developing flow in rectangular microchannels with constant wall temperature

Heat transfer at the thermal entrance region of rectangular microchannels is investigated for hydrodynamically fully developed but thermally developing flow with constant wall temperature. The effects of Reynolds number and aspect ratio on heat transfer properties have been calculated numerically. The local Nusselt number is found to be quite sensitive to the Reynolds number, especially for small Reynolds numbers. The slope of the Nusselt number curve is steeper with a lower Reynolds number when close to the inlet. It is found that the local Nusselt number is independent of the cross-sectional geometry at the channel inlet and the discrepancy of the local Nusselt number for different aspect ratios is gradually magnified along the streamwise direction. In consideration of the effects of Reynolds number and aspect ratio, new correlations are developed for actual Nusselt number and thermal entrance length of rectangular channels. A peak value of thermal entrance lengths can be discovered with the aspect ratio near 3. The present results may provide guidance for thermal design and optimization.