The effect of interception on particle collection by spheres and cylinders

Abstract

Collection or separation of particles of greater than 1 μm in diameter from a fluid as it flows around a spherical or cylindrical target is predominantly due to the inertia of the particle carrying it across the fluid streamlines. If the particle is also of a significant size compared to the collector the efficiency of collection can be increased by the effect of interception. The usual method of accounting for interception uses the trajectory of a particle with zero inertia which follows the fluid streamlines and is assumed to make contact at the collector equator. This method has been shown by particle trajectory calculations to be inappropriate for the majority of practical situations, particularly at high collector Reynolds numbers. The increase in collection efficiency due to interception has been calculated over a wide range of conditions and the results presented in graphical form. In addition, two methods of accounting for the effect of interception are presented based on two dimensionless groups which characterize the system and the inertial collection efficiency that is widely available in the literature. The first uses a correction factor to the diameter of the area far upstream of the collector, within which particles are collected by inertial collection. The second assumes the increase in collection efficiency due to interception is linear with the particle to collector diameter ratio for any particular set of conditions, and fits equations to the calculated values.