Compressibility and rarefaction effect on heat transfer in rough microchannels

High pressure drop and high length to hydraulic diameter ratios yield significant compressibility effects in microchannel flows, which compete with rarefaction phenomena at the smaller scale. In such regimes, flow field and temperature field are no longer decoupled. In presence of significant heat transfer, and combined with the effect of viscous dissipation, this yields to a quite complex thermo-fluid dynamic problem. A finite volume compressible solver, including generalized Maxwell slip flow and temperature jump boundary conditions suitable for arbitrary geometries, is adopted. Roughness geometry is modeled as a series of triangular shaped obstructions, and relative roughness from 0% to 2.65% were considered. The chosen geometry allows for direct comparison with pressure drop computations carried out, in a previous paper, under adiabatic conditions. A wide range of Mach number is considered, from nearly incompressible to chocked flow conditions. Flow conditions with Reynolds number up to around 300 were computed. The outlet Knudsen number corresponding to the chosen range of Mach and Reynolds number ranges from very low value to around 0.05, and the competing effects of rarefaction, compressibility and roughness are investigated in detail. Compressibility is found to be the most dominant effect at high Mach number, yielding even inversion of heat flux, while roughness has a strong effect in the case of rarefied flow. Furthermore, the mutual interaction between heat transfer and pressure drop is highlighted, comparing Poiseuille number values for both cooled and heated flows with previous adiabatic computations.