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