Abstract: There is no numerical simulation and theoretical data support for diffusion patterns of pollutants in high-altitude railway tunnels during operation period. Thus, the diesel-hauled train pollutants ventilated by nature wind using piston wind in high-altitude tunnels is numerically simulated. A three-dimensional model of a high-altitude single-tube single-track railway tunnel is established using CFD software. The CO emissions from diesel locomotives is taken as the characteristic pollutant, and based on simulation results under different natural wind speeds and conditions, the impact of external natural wind speed and direction on pollutant diffusion, the attenuation characteristics of piston wind, and the spatiotemporal variation of CO concentration are quantitatively analyzed. These provide a theoretical support for the design of operational ventilation in high-altitude tunnels, ensuring the safety, effectiveness, and scientific rigor of the ventilation scheme. The results indicate that: (1) The diffusion of pollutants from a diesel-hauled train follows an exponential decay pattern. (2) Under positive wind speed, the CO attenuation distance decreases with increasing wind speed; whereas it increases with increasing wind speed under reverse wind speed. (3) Under a wind speed of over 3 m/s, the concentration of pollutants in the tunnel typically remains below 30×10^-6, regardless of wind direction; under a wind speed of 2 m/s, a 4 km long high-altitude tunnel can achieve air replacement within 20 min using piston wind, ensuring that the air quality meets safety standards.

Key words: high altitude, natural ventilation, operational ventilation, pollutant dispersion, railway tunnel, diesel-hauled trains, airflow characteristics