Simulation of the Capture of Airborne Particles by Sprays

Suspended particulate matter in ambient air is today recognised as an important process and environmental issue. Besides adverse health effects, the performance of many machines and industrial processes are adversely affected by suspended particles. A number of devices, wet and dry, are in use for capture of particles. One such wet scrubbing device, gravitational scrubbers are simple in construction, have low operating costs and are used widely in industries. The prediction of the efficiency of these scrubbers and some basic issues on particle capture on drops have been the focus of this study. Capture at intermediate drop Reynolds numbers (1 to 1000) relevant to gravitational towers, were studied as also the boundary layer effect. Particle drag was computed from local particle Reynolds numbers instead of Stokesian drag as in most reported studies and it shows that inertial impaction efficiencies are overestimated by 2-3% by Stokesian drag formulation. On including boundary layer effect, the local drag formulation showed a large decrease, up to 40%, in inertial impaction efficiency for drop Reynolds numbers below 100; this difference decreases with ReD and at about 1500 it is about 2 to 3%. A particle size dependent shadow zone, devoid of particles, exists behind a drop. A sharp increase in particle concentration, i.e. bunching-up, is observed at the edge of the shadow zone that exponentially smoothes out to the upstream value radially outwards. In in-line arrangement of two drops, there is no additional capture by the trailing drop at drop Reynolds number up to 32 (limit for potential flow model), however, in viscous flow (ReD< 1) additional capture occurs on the trailing drop also. In offset arrangement of two drops, additional capture occurs in viscous and intermediate drop Reynolds number flow regimes. Potential flow-based models of counterand cross-flow gravity towers significantly overpredicts (about 40%) the tower efficiency for small particles (less than 5 iv pm) in comparison to the model with intermediate drop Reynolds numbers for drops at their terminal velocity. Accelerating or decelerating drop motion affects tower efficiency, it increases with increasing injection velocity, vertical air velocity (in counter-flow arrangement) and liquid-to-gas flow rate ratio. For similar operating conditions, a counter-flow gravity tower is more efficient than a cross-flow gravity tower when drops start from rest, but at higher injection velocities, their efficiencies are comparable. In a cross-flow gravity tower, capture efficiency for particles up to 51.IM is maximum for 400 p.m drops, and for larger particles, tower efficiency decreases with increasing drop size. Increasing the tower height does not significantly improve the efficiency of cross-flow towers with 200 p.m drops for small particles (< 3 pm). The injection angle does not affect tower efficiency for small drops but a marginal increase is observed for 500 pm drops. Significant decrease (about 50%) in the efficiency of a counter-flow gravity tower occurs when a two-drop unit with inhomogeneous distribution in between as against the traditional homogeneous particle distribution model.