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Gas flow and heat transfer in confined geometries at micro and nano scalesdiffer considerably from those at macro-scales, mainly due to nonequilibriumeffects such as velocity slip and temperature jump. The nonequilibrium effectsenhance with a decrease in the characteristic length-scale of the fluid flow orthe gas density, leading to the failure of the standard Navier-Stokes-Fourier(NSF) equations in predicting thermal and fluid flow fields. The directsimulation Monte-Carlo (DSMC) method is employed in the present work toinvestigate pressure-driven nitrogen flow in divergent microchannels withvarious divergence angles and isothermal walls. The thermal fields obtainedfrom numerical simulations are analysed for different inlet-to-outlet pressureratios (1.5 $\leq \Pi \leq$ 2.5), tangential momentum accommodationcoefficients and Knudsen numbers (0.05 $\leq \mathrm{Kn} \leq$ 12.5), coveringslip to free-molecular rarefaction regimes. The thermal field in themicrochannel is predicted, heat-lines are visualised, and the physics of heattransfer in the microchannel is discussed. Due to the rarefaction effects, thedirection of heat flow is largely opposite to that of the mass flow. However,the interplay between thermal and pressure gradients, which are affected bygeometrical configurations of the microchannel and applied boundary conditions,determines the net heat flow direction. Additionally, the occurrence of thermalseparation and cold-to-hot heat transfer (also known as anti-Fourier heattransfer) in divergent microchannels is explained.

Pressure-driven Nitrogen Flow in Divergent Microchannels with Isothermal Walls