CFD–DPM Analysis of Coal-Dust Transport and Near-Portal Dispersion from an Open-Top Coal Train in a Railway Tunnel

Coal dust carried by open-top freight trains can undergo complex transport and redistribution in confined railway tunnels, where train-induced airflow links in-tunnel particle motion to near-portal dispersion. However, how particle size and source position jointly influence transport across the train–tunnel–portal system remains insufficiently understood. A three-dimensional transient CFD–DPM model was developed for an open-top coal train traveling at 80 km/h through a 200 m local tunnel section and adjoining portal air domains. Four controlled cases combined two prescribed particle sources—a coal-surface source and a near-ground source—with representative diameters of 10 and 350 μm. In the simulated cases, the maximum air speed over the exposed coal surface increased from approximately 24 to 39 m/s during tunnel entry. The 350 μm particles exhibited stronger inertial settling and preferential migration toward the lower tunnel, whereas the 10 μm particles were more strongly coupled to the airflow and transported toward the portal by the train wake. Under the same prescribed source strength, the near-ground-source cases produced higher source-normalized concentration responses than the coal-surface-source cases, indicating a stronger suspended-transport response for particles introduced near the tunnel floor. In the 10 μm near-ground-source case, fine particles passed through the outlet portal and formed a transient elevated plume that spread downstream and laterally. Within the prescribed-input cases examined here, the simulations illustrate the joint influence of particle size and source position on cross-region coal-dust transport and organize the transport pathways into four particle-transport regions: the coal-surface, lower-tunnel, train-wake, and near-portal regions.