A nanoparticle virtual impactor was constructed and its performance under different operating conditions was investigated. Experimental evaluations showed that the nanoparticle virtual impactor has a 50% cutoff size ranging from 15 to 60 nm. Further, the cutoff size of 60 nm can be achieved at an impactor chamber pressure of 220 torr when the nozzle upstream pressure is 760 torr. This pressure level is much higher than that of thin-plate orifice nozzle impactors, which require 12 torr to achieve the cutoff size of 66 nm. Thus, the proposed virtual impactor can be operated with a small vacuum pump, which is more preferable for practical applications. In this study, the effects of design parameters on the impactor performance have also been experimentally investigated. The parameters include the separation distance between the collection probe and the acceleration nozzle, the pressure ratio of the upstream and downstream chambers, the diameter ratio of the collection probe and the nozzle, the flow ratio of the minor and total flows, total mass flow rates and the upstream pressure. The experimental data obtained were then scaled with the Stokes number defined by previous researchers. The performance of the proposed nanoparticle virtual impactors can therefore be estimated when the operating variables are given or measured. An important finding in this parametric study is that the optimal diameter ratio of collection probe to nozzle is around 1.8. It is different from the value of 1.4 recommended in previous studies with virtual impactors for submicron particle applications.
- Virtual impactors