Thermal rarefied gas flow investigations through micro-/nano-backward-facing step: comparison of DSMC and CFD subject to hybrid slip and jump boundary conditions

This paper evaluates the suitability of a newly developed hybrid “Langmuir–Maxwell” and “Langmuir–Smoluchowski” slip/jump boundary conditions in the Navier–Stokes–Fourier equations for nano-/micro-backward-facing step geometry flow which experiences separation and reattachment. Additionally, this paper investigates the effect of different parameters such as step pressure ratio, inflow temperature, and wall temperature on the separation zone in the nano-/micro-step geometry. We chose nitrogen as the working gas and use two direct simulation Monte Carlo (DSMC) solvers to assess the accuracy of the computational fluid dynamics (CFD) solutions. DSMC results showed that the increase of the inlet temperatures extends the length of the separation zone and raises the mass flow rate. The change of pressure ratio does not affect the separation length while the increase of the step wall temperature decreases the length of this zone for both CFD and DSMC results. Compared to the DSMC results, the hybrid slip/jump boundary conditions predict better surface pressure, surface gas temperature, and slip velocity in the separation zone than the standard Maxwell/Smoluchowski boundary conditions