Revisiting the CFD modeling of a virtual impactor with inverted split ratio: Highlighting some determinants of representativeness

Abstract

Dichotomous sampling of a mixture of gas and aerosol is looked for in many applications such as airborne SVOC (semivolatile organic compound) sampling. This article revisits the computational fluid dynamics simulation of the original SVOC aerosol dichotomous sampler (Kim & Raynor, 2009) to investigate which among some factors influence most the representativeness of such simulations. Considered modeling factors of influence are the 2D-axisymmetric reduction of space and the choice of turbulence model (realizable k-ε vs. k-ω BSL or k-ω SST). Considered physical factors of influence are the particles aerodynamic diameter (range 50 nm to 20 μm), the type of inlet condition for gas and particles (uniform or free-sampling condition), the particles turbulent dispersion and the particles initial radial position at inlet. Results are supported by an extensive numerical verification procedure and by available validation data. In this specific confined transitional two-phase flow, it is found that the axisymmetric simplification holds, that the turbulence model affects significantly the predicted flow pattern but less pressure drops and marginally the fate of aerosol particles. Turbulent dispersion of aerosols is found negligible, but the effect of inlet boundary condition appears decisive on aerosol motion despite a marginal influence on the flow pattern. In particular, for aerodynamic diameters above 2 μm, free-sampling conditions produce a strong focusing effect of particles which drastically helps in limiting wall deposition. These findings highlight particularly the importance of using a physically sound inlet condition for aerosol particles when simulating aerosol samplers, rejecting the usual simplistic uniform condition. Results also particularly emphasize the necessity of a thorough verification procedure for the computation of both phases when using CFD for similar samplers. Regarding the application to dichotomous sampling, it appears that any process that moves particles away from the wall upon entry improves the separation efficiency of the device while decreasing deposition.