Lecture 3: Review of Single-Phase Forced Micro-Convection for Consolidation of Rarefaction and Non-Rarefaction Scaling Effects

Micro-convection is a strategic area in transport phenomena, since it is the basis for a wide range of miniaturised high-performance applications. Microchannel heat-sinks are appealing solutions to the increasing heat dissipation demands, especially in electronic devices. Literature reveals that in the single-phase continuum regime, the behaviour of micro-convection significantly differs from that in the conventionally-sized channels. These differences have been attributed to scaling effects, which surface at the microscale: due to axial conduction in wall and fluid, variation of fluid thermophysical properties, and surface roughness. In gas micro-flows, the analysis is further complicated by rarefaction and compressibility effects. Most gas micro-flow applications are in the slip or transition flow regimes. Hence, microscale slip-flows are comprehensively reviewed to identify the salient research findings. Based on the review analysis, the critical focus now is to objectively demark the continuum and freemolecular flow regimes, so that the computationally inexpensive continuum models can be extended to higher Knudsen numbers. This review suggests that early transition regime behaviour is characterised by conjugate effects due to variable fluid thermophysical properties, axial conduction, compressibility, and surface roughness; rather than rarefaction alone.