The effect of an orifice on the collection efficiency and wall loss of a slit virtual impactor was investigated both numerically and experimentally. The ratios of the collection nozzle width (W c ), distance between acceleration nozzle and collection nozzle (S), length of acceleration nozzle (T), inlet width (D), and nozzle span (l) to the acceleration nozzle width (W a ) were fixed at W c /W a = 1.4, S/W a = 1.5, T/W a = 1.1, D/W a = 6, and l/W a = 10, respectively. The minor-to-total flow ratio was set to 0.1 in laminar flow regime. The collection efficiency and wall loss of the slit virtual impactor were found to be characterized by the square root of the Stokes number. For the slit virtual impactor without an orifice, the square root of the Stokes number corresponding to the cut-off diameter was determined to be (Stk 50 ) 1/2 = 0.77 and the maximum wall loss at the collection nozzle reached 30% or 40%. When an orifice having the same width as the acceleration nozzle was placed upstream of the acceleration nozzle at a distance of 20W a , the value of (Stk 50 ) 1/2 decreased to 0.68 and the wall loss at the collection nozzle decreased below 5%.

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A horizontal inlet was employed to improve the collection efficiency of a rectangular-slit-nozzle impactor. A numerical and experimental study of the collection efficiency of rectangular-slitnozzle impactors, with either typical inlets or horizontal inlets, was conducted. In the comparison of typical inlet impactors and horizontal-inlet impactors, parameters such as the nozzle width, impaction plate width, nozzle-to-plate distance, and aerosol flow rate were held constant, and only the inlet shape was changed. A parametric study was conducted to examine the effects of the horizontal inlet dimensions on the collection efficiency of rectangularslit-nozzle impactors. It was found that a horizontal inlet could reduce the square root of the Stokes number corresponding to the cutoff size from 0.77 to 0.60, compared with a typical inlet.

Airflow around an eight-passenger-car subway train running in the underground tunnel at a cruise speed of 70 km/h was numerically simulated, and the trajectories of the particles that were assumed to be re-suspended from the ground or generated at the contact points between the wheels and rails were predicted. In addition, field experiments were conducted to measure airflow velocity and PM 10 mass concentration under a T-car (trailer car without a driving cab) during the running of a subway train in straight sections of the underground tunnel of the Seoul Subway Line 5. The numerically predicted airflow velocities agreed well with the experimental data with the error of less than 30%, and the predicted particle distribution showed a similar tendency to the experimental results. The airflow under the T-car was predicted to be relatively uniform compared to the airflow under other passenger cars. Both numerical results and experimental data signified that a lot of particles could drift under the T-car by showing a higher particle concentration in the central area of the space under the T-car than in the edge area. As a result, the space underneath the T-car is anticipated to be a good place for installing a dust-removal system.

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