Mechanisms affecting ring-shaped particle deposition patterns in an inertial impactor

Experiments and simulations are presented showing the particle deposition patterns obtained from inertial impactors, particle sizing devices that are commonly used in field and laboratory applications.  This work focuses on low S/W impactors (S/W ~ O(0.01)), where W is the nozzle diameter and S is the distance of the nozzle to the impactor plate.  This low S/W condition results in ring-shaped particle deposition patterns where the ring diameter D is inversely proportional to the particle diameter d.  This behavior may enable development of single-stage impactors capable of obtaining particle size distributions over a range of diameters, something not possible in current multi-stage impactor implementations where S/W~1 and the deposition pattern is a disk and whose geometry is not sensitive to d.  The goal of the research presented here was to determine how S/W affects the relationship between D and d.  A subsequent goal was to then use simulations to determine the physical mechanisms that control the deposition patterns that are experimentally observed, thereby facilitating designs that maximize the sensitivity of D to d.  The experimental results show that while ring patterns require a small S/W, the sensitivity of D to d increases with S/W indicating that there is an optimal value of S/W.  Simulations reveal that the formation of rings and their diameter is determined by deviations of particles from their streamlines in a region very close to the impactor surface.  A discussion is presented of possible practical implementations of this small S/W approach for obtaining particle size distributions.