A control-volume numerical approach has been used to study rarefaction effects in simultaneously hydrodynamically and thermally developing 3D micro flows in rectangular channels for Kn ≤ 0.1. The effects of velocity slip, thermal creep and temperature jump on the key flow parameters are examined in detail. Low Reynolds number flows (Re ≤ 1) for different channel aspect ratios (0 ≤ α* ≤ 1) are considered. The effects of rarefaction on the global features of the flow and thermal development in the entrance region are studied. Dramatic reductions in the friction coefficient are observed in the entrance region due to rarefaction effects, which are enhanced by thermal creep. For the fluid heating case considered here, thermal creep increases slip at the wall and thereby further reduces the friction coefficient and enhances heat transfer. For an identical heat flux applied to the microchannel walls, thermal creep effects become more pronounced at lower Reynolds numbers since it results in higher axial temperature gradients. Present results for Kn = 0.1 indicate that the flow and thermal fields are greatly influenced by thermal creep at Re = 0.1.

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