CFD Study on Compound Impaction in a Jet-in-Well Impactor

In a classical jet impactor, the jet is assumed to impact on an infinite plate; however, the typical construction of an impactor necessitates that a wall surrounds the jet. If the wall is close to the jet, secondary impaction can take place on the wall. Also, for some impactor designs, e.g., large particle scalping devices in air sampling inlets, the jet is purposefully placed in a well and the overall performance of the impactor is the compound effect of the jet impacting on a collection plate and on the wall of the well. In this study we primarily use CFD to examine the performance of compound round jet impactors, where the ratio of well diameter to jet diameter is from 3 to 15 and the particle size is in the range of 5 to 15 μm AD. The onset of the secondary impaction occurs at a ratio of well diameter to jet diameter of about eight with the collection on the side wall increasing for smaller ratios. Compound impaction can significantly enhance the overall particle collection and can result in a much smaller value of Stk 0.5 . At a ratio of well diameter to jet diameter of 3, the value of Stk 0.5 is 0.07, as compared with a classical circular jet impactor where Stk 0.5 is about 0.24 for a typical set of operational and configuration parameters. Numerical predictions for the jet-in-well impactors are compared with experimental results using liquid particles and good agreement is obtained. Photos taken over a range of experimental conditions verify the strong side wall impaction for certain conditions.