Numerical and experimental study of virtual impactor design and aerosol separation

• This study developed an innovative active virtual impactor to separate particles.
• The two sheath flows included can squeeze gas flow to avoid particle wall loss.
• CFD simulation accurately predicted aerosol separation within the virtual impactor.
• The novel virtual impactor has good ability to separate and purify particles.
• The virtual impactor can be further developed to separate polydisperse aerosols.

PM10 and PM2.5 are the most harmful particles affecting the human respiratory system in the environment or at the workplace. In this study, an innovative active virtual impactor (AVI) was developed to separate particles. The operation parameters of different flow rates regarding sample flow velocity, side flow velocity, and sheath flow velocity were established based on computational fluid dynamics (CFD) simulation results. Simulation results were also examined experimentally for validation purposes. The optimal numbers of structured grids for numerical simulations were between 10,000 and 150,000. The flow ratios of sheath velocity versus sample flow velocity were in the range from 0 to 20. The simulated particle size and side flow velocities ranged from 1.5 to 20 µm and 0 to 3 m/s, respectively. In the experiment, single-sized (1.5 μm) particles were generated and measured using a fluidized bed aerosol generator and an aerosol spectrometer, respectively. The ratio of the sheath flow velocity to the sample flow velocity can only range from 0 to 4. Simulation results showed that particles were increasingly separated when the side flow velocity increased. When the ratio of the sheath flow velocity to the sample flow velocity increased, the required side flow velocity to separate the specific particle size also increased. The experimental results agreed with simulation results. The sheath flow design could maintain the particle flow in the middle of the flow channel, and without loss on the walls of virtual impactor. The CFD simulation tool can be successfully applied to predict the particle separation efficiency of the impactor and related operational parameters. The designed AVI can thus improve the traditional virtual impactors with respect to the ease of flow control and separation efficiency.