Stagnation point displacement: Control of losses on a conically shaped aerosol distributor

Abstract

A cone was placed just upstream of the second stage of a Two-Stage Circumferential Slot Virtual Impactor in an attempt to uniformly distribute 10 μm aerodynamic diameter (AD) aerosol particles so a circumferential inlet of the second stage, which was located below the cone, could uniformly aspirate aerosol. Large losses were observed near the apex of the cone; however, the losses can be significantly reduced by injecting a small counter-flow jet of sheath flow through the apex of the cone. The jet displaces the stagnation point of the main flow and thereby alters the particle trajectories. Computational Fluid Dynamics (CFD) was the primarily tool used to investigate the concept through simulating the effects of different operational conditions such as the ratio of counterflow jet velocity to main flow velocity and the Stokes number. The velocity of the counterflow jet should be approximately 1–2 times the main airflow velocity to achieve low particle losses. The jet becomes asymmetric when the ratio of its velocity to the main flow velocity exceeds a critical value, which varies from 2.6 to 1.34 for sheath flow Reynolds numbers over a range of values from 170 to 420. Experimental results were compared with CFD predictions and good agreement was achieved. For a special case of the two-stage circumferential slot virtual impactor, where there was a narrow beam of 10 μm AD aerosol particles exhausted from the 1st stage of a virtual impactor, 1.8% sheath air (relative to flow rate directed towards the cone) increased the transmission of the beam through the two-stage system from 4% to 90%. © 2009 Taylor & Francis Group, LLC.