CFD study on the effects of the large particle crossing trajectory phenomenon on virtual impactor performance
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Two-dimensional CFD simulations were performed on a full-section numerical model of an as-built slot virtual impactor prototype and its completely symmetric ideal counterpart. The simulations reproduce the trends of the experimentally observed performance including verification of a third region in the transmission efficiency curve, which is a drop-in transmission efficiency for large particle sizes. Visualization of simulated particle tracks show this decrease is attributed to a crossing trajectory phenomenon, whereby larger particles that acquire enough inertia in a chamfered acceleration nozzle, crossover the vertical mid-plane and impact on the opposite-side wall, particularly on the wall of the receiver section. Some experimental data presented in the literature for rectangular slot and round-nozzle virtual impactors with chamfered 45 half-angle acceleration nozzles (similar to the geometry tested herein), show a similar drop-in transmission efficiency that commences at a particle Stokes numbers of about 6. However, many studies do not demonstrate the drop in transmission efficiency because wall losses are not taken into account. The Reynolds number, based on the acceleration nozzle size and velocity, does not noticeably affect the onset of the phenomenon. The crossing trajectory phenomenon can severely restrict the size range over which a virtual impactor can be used as an efficient particle concentrator. Geometrical asymmetry from dimensional tolerance considerations in the construction of a virtual impactor exacerbates the impact of the crossing trajectory phenomenon. American Association for Aerosol Research.
