A CFD study on the effects of air cleaner and mechanical ventilation on the particulate matter concentration at a semi-outdoor space

One of the approaches to mitigate the outdoor particulate matter (PM) concentration is the use of air cleaners. In the present study, the effects of air cleaners as well as mechanical ventilation units on the PM reduction of a semi-outdoor bus terminal are examined by using computational fluid dynamics (CFD) simulations. The governing equations for the fluid flow and the PM concentration are the steady incompressible three-dimensional Reynolds-averaged Navier-Stokes equations (RANS) with the realizable k-ε model and a passive scalar transport equation with the standard gradient-diffusion hypothesis, respectively. As per the numerical results, the air cleaners alone significantly reduce the PM concentration of the bus terminal. The effect of mechanical ventilation units on the PM reduction is positive without air cleaners but negative with air cleaners. To understand the role of ventilation on the PM reduction, a simple theoretical model is derived based on the mass conservation of the PM. This model considers the effects of traffic-related PM emission, air cleaners, and ventilation. The PM concentrations predicted by the proposed model show a good correlation with those from the CFD simulations. From the model, we suggest that the ratio of the mass emission rate of PM to the clean air delivery rate of air cleaners is an important value in determining the effect of ventilation.