| dc.description.abstract | The virtual impactor was well known as it could avoid particle bounce and overloading typically encountered with inertial impactors. In this study, the flow field and particle trajectory in a slit type virtual impactor was simulated numerically by a commercial Computational Fluid Dynamics (CFD) software (COMSOL Multiphysics v.4.3b). Effects of flow and slit geometry, including the total flow (Qin), the ratio of minor to total flow (r), the taper slip nozzle (45⁰ chamfer lip and arc lip) and the collection probe configuration (straight and divergent), were investigated. The performance parameters for evaluating the performance were the collection efficiency (CE), the particle loss, the cutoff size (d50), and the concentrating factor (CF). The results show that the d50 is strongly depending on either Qin or r. For instance, the d50 could be reduced form 1.4 μm to 0.2 μm as Qin increased from 13 LPM to 400 LPM at r = 0.1. When Qin was fixed at 80 LPM, the d50 will decrease from 0.63 μm to 0.21 μm as minor flow ratio increasing from 0.05 to 0.4. In addition, a new modified Stokes number for virtual impactor to include the effect of r was proposed. It was further found the square root of this modified Stokes number was retained at about 0.9 under different Qin or r, which can be considered as the characteristic performance parameter for this slit type virtual impactor. On the other hand, the numerical simulation results show particle loss would increase when Stk^0.5 lager than 1.9 due to the particle crossing phenomenon. The particle crossing in acceleration nozzle caused particle deposit on collection probe and CF would decreased. In geometry analysis, the gradual curving lip could postpone particle crossing and decrease the internal loss of larger particles, but the particle loss near cutoff size was increased. Moreover, the divergent probe could decrease the particle loss near the d50 and collection probe. At last, the preliminary experimental tests were conducted to validate the numerical simulation results. | en_US |