Effect of Ultra-Stokesian Drag and Particle Interception on Impaction Characteristics

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Abstract
The characteristics of impactors with round or rectangular nozzles have been determined by the numerical solution of the Navier-Stokes equations and of the equation of motion of the particles. The sensitivity of this solution to the system of grid lines was investigated by decreasing the spacing between them; these studies yielded sharper curves and higher √St50 for finer grids. The particle trajectory calculation program included an empirical ultra-Stokesian drag coefficient and a facility for handling particle interception. Studies comparing the results of runs made using the ultra-Stokesian drag law with those assuming Stokes law indicated that the latter underpredicts the √St20 by 5 to 10%. The influence of particle interception was found to be small except at very low Reynolds numbers, the characteristics for which demon strated low-efficiency tails. Dimensional analysis of the impactor system required the introduction of a second dimensionless quantity (in addition to the St) to char acterize the particle-fluid interaction for a given flow field. This dimensionless number, taken as (1/C)(ρ/ρ p ), was found to influence results at the high and low extremes of the Reynolds number only. For moderate values of the Reynolds number, a single curve relating collection efficiency to √St was found to characterize impactor systems adequately.

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