LAUSR

Abstract Virtual impactors can be used as aerosol concentrators as well as particle size classifiers. The flow field and particle trajectory inside a linear slit-type virtual impactor were investigated using a commercial computational fluid dynamics software package. Effects of flow and slit geometry, including the inlet flow ( Q in ), the ratio of minor to total flow ( r ), and the configuration of the taper-lip slit nozzle and the collection slit nozzle, were studied. A new modified Stokes number ( Stk c ) for the virtual impactor was proposed to predict the d 50 at different r values. Stk c,50 0.5 was found to be retained at approximately 0.9 under different Q in or r values, and it can be considered the characteristic performance parameter for the slit virtual impactor. The numerical simulation results show that the particle loss starts increasing when Stk c 0.5 is larger than 2.0 due to a particle-crossing phenomenon. The range of the maximum concentration factor ( CF max ) is influenced by the particle-crossing phenomenon. An expression was also developed to predict the size range of particles with the maximum CF under different inlet flow rates. Regarding the geometrical configuration, the arc-lip nozzle could postpone particle-crossing and lessen the internal losses of coarse particles, whereas it increased particle losses near d 50 . However, the divergent collection slit nozzle could effectively decrease particle losses on the wall of the nozzle without this side effect. The designing guideline of a linear-slit virtual impactor was briefly summarized based on these findings.