A numerical investigation is conducted to study the heat transfer characteristics of laminar ?ow in three-dimensional rectangular microchannels with aspect ratios of 0.1–1 and Reynolds numbers of 5–400. In this paper, the effects of aspect ratio and Reynolds number on the local and mean Nusselt numbers are presented in detail. Good agreement on Nusselt numbers is observed be tween the present results and the data reported in the literature. The results show that the Reynolds number has a great impact on the local Nusselt number in the thermally developing region, especially at lower Reynolds number, while the Reynolds number is independent of the developed Nusselt number. The local Nusselt number is found to increase with decreasing aspect ratio at the same Reynolds number. Additionally, the effects of these parameters on the dimen sionless thermal entrance length are also investigated. The dimensionless thermal entrance length for rectangular microchannels monotonously increases by increasing aspect ratio and decreasing Reynolds number, whereas the effect of Reynolds number on the dimensionless thermal entrance length can be basically ignored when Reynolds number is more than 50. Generalized correlations are developed for the local Nusselt number and the dimensionless thermal entrance length.
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