Size effect on single-phase channel flow and heat transfer at microscale

The size effects on microscale single-phase fluid flow and heat transfer are reviewed and discussed. The physical mechanisms for the size effects on the microchannel flow and heat transfer were divided into two classifications: (a) The gas rarefaction effect occurs when the continuum assumption breaks down as the characteristic length of the flow becomes comparable to the mean free path of the molecules; (b) Variations of the predominant factors influence the relative importance of various phenomena on the flow and heat transfer as the characteristic length decreases, even if the continuum assumption is still valid. Due to the larger surface to volume ratio for microchannels, factors related to surface area have more impact to the microscale flow and heat transfer. Among them are: The surface friction induced flow compressibility in microchannels makes the fluid velocity profiles flatter and leads to higher friction factors and Nusselt numbers; The surface roughness of the microchannel is likely responsible for the early transition from laminar to turbulent flow and the increased friction factor and Nusselt number; The importance of viscous force in natural convection modifies the correlation between Nu and Ra for natural convection in a microenclosure and, other effects, such as the axial heat conduction in the channel wall, the channel surface geometry, and measurement errors as well, could lead to different flow and heat transfer behaviors from that at conventional scales.