Compressible flow inertial impactors

Abstract

The scaling criteria used to predict the size cuts of inertial impactors are strictly valid only so long as the jet Mach number is small, i.e., M² 1. In order to improve size resolution of submicron particles, a number of impactors have been operated with Jet Mach numbers as large as one. Unless the jet Mach number is small, fluid properties, e.g., viscosity, μ, mean free-path, λ and density, p, may vary in the jet impingement region, making the aerodynamic drag coefficient of a particle vary with position. A theoretical investigation of inertial impaction from sonic jets (M = 1) was undertaken in order to determine the extent to which fluid compressibility influences impactor size cuts. A finite difference solution to the inviscid, compressible flow is used to explore the conditions for particle impaction in round sonic jet impactors. The flow from an underexpanded sonic jet (pressure ratio less than 0.53) expands to supersonic velocities before decelerating through a shock wave between the jet outlet and the impaction plate. The pressure near the impaction plate approaches that of the stagnation conditions upstream of the jet inlet. The impaction parameters characteristic of an impactor stage should, therefore, be defined in terms of the gas viscosity and mean free-path at the stagnation conditions above the jet entrance. The impaction parameter, Ψ₅₀ defined in this manner does not appear to vary significantly with the jet pressure ratio, Mach number, or the particle Knudsen number.