CFD Study on the Effects of the Large Particle Crossing Trajectory Phenomenon on Virtual Impactor Performance

Abstract

Two-dimensional CFD simulations were performed on a fullsection 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.

Comparison of Experimental and Numerical Studies of the Performance Characteristics of a Pumped Counterflow Virtual Impactor

“…This leads to decrease in the TE. We also observed a crossing trajectory phenomenon (Hu and McFarland 2008;Hari et al 2007) where the particles from one side of the nozzle cross the mid plane and impact on the opposite-side wall. Particle losses within the nozzle could also contribute to the decrease in the TE.…”

Optimization Study of Sampling Device for Semi-Volatile Oil Mist in the Industrial Workshop

“…According to previous studies, the configuration of the virtual impactor is inherently related to the cross-trajectory phenomenon; that is, the transmission efficiency drops sharply under large particle diameters, and changing the nozzle size, Reynolds number and other parameters will not prevent this phenomenon [38]. However, the use of acceleration nozzles with a certain slope and divergent collection nozzles can reduce the particlecrossing phenomenon by focusing particle trajectories, thereby improving the transmission efficiency of semi-volatile aerosols [39].…”

Effect of convergence angle on impactor performance

“…The length of nozzle throat was shortened to reduce velocity loss from friction along the nozzle wall, which also made the nozzle taper less aggressive to reduce the crossing trajectory phenomenon (Hari et al 2007). …”

“…The flat-plate orifice configuration, commonly used in cascade impactors, tends to focus particles closer to the centerline than the angled nozzle. Hari et al (2007) numerically studied the effect of the large particle crossing trajectory phenomenon on virtual impactor performance. The simulations reproduced 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.…”