Effect of Width Ratio of Collection Nozzle to Acceleration Nozzle on the Classification Performance of Rectangular Jet Virtual-Impactor

Abstract

The effect of width ratio of collection nozzle to acceleration nozzle on the classification performance has been studied by using a rectangular jet virtual-impactor. Numerical calculations showed that smaller cut-size would be obtained if the impactor was set with larger nozzle width ratio. The sharpness of the classification was also expected to be higher for larger nozzle width ratio. The experimental results showed, however, that if the nozzle width ratio was larger than about 1.7, cut-size data disagreed with the theoretical values. The data for the sharpness showed similar tendency.
 Flow visualization experiments have been carried out in order to find out the causes of the disagreement. It was found that the flow in the impactor of large nozzle width ratio was separated from the wall of the collection nozzle and large eddies were generated there. Some of the particles separated into the collection nozzle would be caught by the eddies and exhausted to the separation zone, resulting in larger cut-size and less sharp separation than expected by the numerical calculations.
 The ratio of undeflected flow rate to total flow rate was increased, and clean air flows were introduced so as to eliminate the eddies. Then smaller cut-size and sharper classification were obtained.