Sang-Hee Woo

Generation of Nanoparticles from Friction between Railway Brake Disks and Pads

In this study, we measured the size distribution of particles ranging in size from 5.6 to 560 nm that were emitted between brake disks and pads under various braking conditions to observe and analyze changes to the resulting particle size distribution over braking time. A peak of 178-275 nm (200 nm peak) was observed in all braking conditions. However, the generation of spherical particles of a 10 nm range was observed only when the disk speed and brake force were above certain levels and intensified only when speed and brake force further increased. The total number concentration of ultrafine particles (no larger than 0.1 μm; PM0.1) generated was found to correlate with disk speed and brake force. Thus, the generation of nanoparticles resulting from disk speed and brake force was attributable primarily to increases in the contact surface temperature. The critical temperature for the generation of nanoparticles of a 10 nm range was found to be about 70 °C, which is the average temperature between the surface and the inside of the disk. If the speed or brake force was higher, that is, the temperature of the contact surface reached a certain level, evaporation and condensation took place. Vapor then left the friction surface, met with the air, and quickly cooled to form nanoparticles through nucleation. When the newly generated particles became highly concentrated, they grew through coagulation to form agglomerates or the vapor condensed directly onto the surface of existing particles of about 200 nm (formed by mechanical friction).

Investigation of airflow and particle behavior around a subway train running in the underground tunnel

ABSTRACT 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 PM10 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.

Local Behavior of Deposition Velocity onto a Flat Plate in Horizontal Airflow under the Influence of Thermophoresis

In this study, the Gaussian Diffusion Sphere Model (GDSM) and the Statistical Lagrangian Particle Tracking (SLPT) approach were employed and adjusted to calculate the local deposition velocity onto a flat plate in horizontal airflow. The GDSM and the SLPT approach were validated by comparing the predicted local deposition velocities with those determined by solving the equation of convective diffusion. Both the GDSM and the SLPT approach were found to be accurate in calculating the local deposition velocity onto a flat plate in horizontal airflow. In addition, the GDSM was much more efficient than the SLPT approach in terms of the calculation time. Finally, a parametric study on the local deposition velocity onto a flat plate exposed to horizontal airflow was performed using the GDSM with the consideration of the effects of the gravity, convection, diffusion, and thermophoresis. Copyright 2015 American Association for Aerosol Research