Aerosol Capture in Particle Laden Granular Beds in the Impaction Dominated Regime

The single grain capture efficiency η differs from the value η 0 for a clean bed when the number N of captured aerosol particles per grain becomes sufficiently large. The dependence of η on N was found by numerically solving population balance equations for N k where N k is the number of clusters per grain containing k primary aerosol particles. The model requires two functions of k: β k , the fraction of the total flow past one grain that flows past a single cluster of size k; and η k , the efficiency of capture from that flow. For this work we used β k = β 1 (1 + )/2 and (η k − η 0 ) = (η 1 − η 0 ) . The resulting dependence of η on N is given with reasonable accuracy by η = η 0 + (η 1 − η 0 )β 1 N/[1 + g(β 1 N)] with g(β 1 N) = 0.13β 1 N for β 1 N < 6 and g(β 1 N) = 0.33 for β 1 N > 6. The penetration of monodisperse aerosols through initially clean granular beds was measured for sufficiently long challenge times that significant reductions in penetration occurred. Aerosol diameters D p ranged from 1.5 to 3.5 μm; superficial velocity U, from 0.2 to 0.6 m/s; bed depth, from 25 to 100 mm; and grain diameter, D G , was 1.9 mm. For these conditions, capture in clean beds is dominated by impaction. Values of β 1 (η 1 − η 0 ) were determined by comparison of the measured penetration history with calculations from the model. The precision of the measurements is not sufficient to allow separate values of β 1 and (η 1 − η 0 ) to be deduced. The value of β 1 can be estimated from geometrical considerations to be of the order of 2D p /πD G . Using this estimate we found (η 1 − η 0 )/η 0 to average 0.14 with a standard deviation of 0.06. The model is based on unsubstantiated expressions for β k and η k . Nevertheless, with the expressions adopted, the model does reproduce the trends observed with a suitable value for (η 1 − η 0 )/η 0 .