The physical mechanisms for the size effects on the flow and heat transfer have been divided into two classifications: (a) variations of the predominant factors influence the relative importance of various phenomena in the flow and heat transfer as the characteristic length decrease, even if the continuum assumption is still valid; (b) the continuum assumption breaks down as the characteristic length of the flow becomes comparable to the mean free path of the molecules. The departure of most flow and thermal phenomena in the MEMS from conventional ones is due to the variation of predominant factors in the flow and heat transfer problems, rather than that Navier-Stokes equation and Fourier heat conduction equation etc are no longer valid. Due to the larger surface to volume ratio for microchannels and microdevices, factors related to surface effects have more impact to microscale flow and heat transfer. For example, surface friction induced flow compressibility makes the fluid velocity profiles flatter and leads to higher friction factors and Nusselt numbers; surface roughness is likely responsible for the increased friction factor, the early transition from laminar to turbulent flow and Nusselt number; and other effects, such as the axial heat conduction in the channel wall, the channel surface geometry, surface electrostatic charges, and measurement errors, could lead to different flow and heat transfer behaviors from that at conventional scales. The condensation/evaporation across the liquid-vapor interface and the liquid-vapor nucleation are processes at nanometer scale. In these cases the macroscopic approach is hard to reveal the details at nanometer scales, while the molecular dynamic simulation is a powerful tool to describe such microscopic processes, and has been applied to investigate the density and temperature profiles across the liquid-vapor interface. The condensation coefficient on the liquid-vapor interface under thermodynamic equilibrium condition has been well predicted based on the characteristic time method, which can distinguish the condensed particles from the reflected particles. Molecular dynamics simulations show that liquid-vapor nucleation undergoes three stages, namely, cavity growth, cavity coalescence and bubble formation.

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