Pressure-driven Nitrogen Flow in Divergent Microchannels with Isothermal Walls

TL;DR

This study uses DSMC simulations to analyze nitrogen flow and heat transfer in divergent microchannels, revealing rarefaction effects, heat flow reversal, and thermal separation phenomena influenced by geometry and boundary conditions.

Contribution

It provides detailed insights into thermal and flow behaviors in microchannels under rarefied conditions, highlighting effects not captured by classical models.

Findings

Heat flow can oppose mass flow direction due to rarefaction effects.

Thermal separation and anti-Fourier heat transfer occur in divergent microchannels.

Flow and heat transfer are strongly influenced by geometry and boundary conditions.

Abstract

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.