Performance evaluation of a virtual impactor with an additional pretreatment structure for particle separation

Abstract

In this paper, a proposed method based on a microfluidic chip that can arrange the collected particles in radial positions according to their sizes before virtual impactor separation was introduced. The main objective of this paper was to design a novel micro-separator structure composed of two major parts: a virtual impactor, and a pretreatment that consists of a sheath flow and a backward-facing step flow configuration. In the separation process, the particles firstly were aggregated on the middle channel by the sheath flow, then ordered in radial position of backward-facing step flow, and finally separated in virtual impactor. To determine the effects of the pretreatment, both simulations and experiments were conducted. A parameter optimization was performed on the micro-separator where the cut-off size of the virtual impactor was 2.5 µm. The results demonstrated that when the flow ratio between inlet flow rate and the total flow rate was set to 50% and the expansion ratio (Er) was 2. The cut-off size of the micro-separator was reduced by 20.77% from 2.436 µm to 1.93 µm, and the wall loss (WL) was acceptable compared with the result only using PM2.5 virtual impactor. The effect of pretreatment on PM1 and PM5 virtual impactor also saw an improvement of collection efficiency even when the additional pretreatment had varying Reynold numbers and nozzle sizes. Accordingly, the stk50 value was reduced by 44.39% from 0.669 to 0.372 by adding pretreatment before the virtual impactor.