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 (E_r) 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 PM_2.5 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.