High-velocity inertial impactors

Abstract

Cascade impactors with large pressure drops across individual stages are useful for classifying submicron aerosols even though the flow becomes compressible when the ratio of the downstream to upstream stagnation pressure falls below r = 0.95 and sonic when r < 0.53 (for air). The variation of collection efficiency with pressure ratio is examined experimentally. The cutoff Stokes number does not exhibit significant dependence on the pressure ratio in the range from r = 0.97 (incompressible flow) to r = 0.19 (choked flow). The impactor stage pressure ratio may be estimated by modeling the stage as a nonideal, compressible flow nozzle with a discharge coefficient which is well correlated with the jet Reynolds number divided by the nozzle throat aspect ratio. Predictions of the variation of the cutoff diameter with flow rate, temperature, and inlet pressure are presented for impactors with high Mach number jets.