Rarefaction, Compressibility, and Viscous Heating in Gas Microfilters

Gas flows through microfilters are simulated in the continuum and slip flow regimes as a function of the Knudsen, Reynolds, and Mach numbers. The numerical simulations are based on the spectral element formulation of compressible Navier-Stokes equations, which utilize previously developed high-order velocity slip and temperature jump boundary conditions. Both slip and no-slip simulations are used to identify the rarefaction effects. The simulation results show skin friction and form-drag reduction with increased Knudsen number. Pressure drops across the filters are compared against several empirical scaling laws, available in the literature. Compressibility becomes important for high-speed flows, creating large density fluctuations across the microfilter elements. For high-Mach-number flows interactions between thermal and kinetic energies of the fluid are observed. It is also shown that viscous heating plays a significant role for high-speed gas flows, which impact heat-transfer characteristics of microfilters.