Thermally Driven Flows for Species Separation in a Microcavity

This study investigates the separation of binary gas mixtures in a small thermally driven square cavity, focusing on slip and transitional flow regimes. We simulated flow in rarefied regimes at micro-scales using the Direct Simulation Monte Carlo (DSMC) method. The cavity, with a 600 K temperature difference between its upper and lower walls, has a lower wall divided between specular and diffusive surfaces. Key findings include analyzing gas flow behaviors and improving separation efficiency using non-mechanical setups with varying surface characteristics and temperature differences. Helium accumulates on the heated surface and xenon on the cooled surface, resulting in a 17.5% improvement in separation efficiency at a Knudsen number of 0.05.

Two main flow mechanisms were identified: thermal edge flow along the lower wall and thermal creep from the upper to the lower wall. As Knudsen numbers increase, temperature jumps significantly affect temperature distributions and streamline orientations. Helium moves away from cooler areas faster than xenon, leading to distinct distribution patterns due to thermal gradients and molecular properties.

Separation efficiency peaks at 17.59% at a Knudsen number of 0.05 but decreases with higher Knudsen numbers due to increased molecular collisions and reduced temperature gradients. The cavity's specular-diffusive surface alters concentration gradients, impacting efficiency. Increasing the normalized temperature difference enhances the temperature gradient, intensifying thermal edge flow and resulting in greater species separation.