On the vortical characteristics and cold-to-hot transfer of rarefied gas flow in a lid driven isosceles orthogonal triangular cavity with isothermal walls

Rarefied gas flows in nano-scale isosceles triangular cavities with a motion away from the square corner are considered over a wide range of flow rarefactions regimes, i.e., 0.01 ≤ Kn ≤ 10, using the direct simulation Monte Carlo (DSMC) method. We put emphasis to the rarefaction effects on the vortical behaviors and heat transfer patterns of rarefied monatomic and diatomic gases. We show that the primary vortices appearing in the triangular cavity are Moffat-type vortices, which disappear as velocity slip increases over the walls. However, an additional vortex is observed at Kn > 1 on the inclined wall attributable to the balance of the thermally driven flows from the cold to the hot region and flow induced by the primary vortex. We also report variations in the effective length scale of the fluid circulation with the Knudsen number. Our investigations specify that the competition between the Fourier term and shear stress gradient component of the heat flux constitutive relation results in the cold-to-hot transfer in triangular cavities. Also, heat flux patterns predicted by the asymptotic theory of the Boltzmann equation for the weakly non-linear flow and by the linearized form of the Regularized 13 moments (R13) equations are compared with the DSMC solution suitably at low Knudsen number regimes.