Particle collection efficiency of different impactor designs

In this study, three different impactor designs were used to investigate the effect of solid particle bounce on the collection efficiency. Design no. 1 is a common impactor with a flat impaction surface surrounded by a retainer ring. Design no. 2 is similar to a particle trap impactor but has a shallower cylindrical cavity, whereas design no. 3 is a regular particle trap impactor. In some cases, a minor flow at 10% of the total flow rate was drawn under the filter substrate to improve the collection efficiency. The experimental data of solid particles show that for design no. 1 without minor flow, the particle collection efficiency increases with Stokes number (Stk) and peaks at 75% at root Stk of 0.50 (when jet-to-plate distance S/W = 1) or 70% at root Stk of 0.60 (S/W = 4). The collection efficiency drops thereafter with increasing Stokes number because of particle bounce. The minor flow increases the collection efficiency by 10-20%. The solid particle collection efficiency of design no. 2 is lower than that of design no. 1 except when root Stk is close to 1.0. The collection efficiency of design no. 2 increases from 30 to 50% when root Stk is increased from 0.5 to 1.1. The effect of the minor flow on the increase of the collection efficiency, which is 20-30%. is more pronounced than for design no. 1. For design no. 3 without minor flow, the solid particle collection efficiency is found to increase with increasing Stokes number, and the shape of the collection efficiency curve is quite different from the other two designs. The collection efficiency increases monotonically from 10 to 70% when root Stk is increased from 0.4 to 1.7.