Abstract: By using FDS numerical simulation software, the impact of the shaft aspect ratio on the smoke flow and the performance of shaft natural smoke exhaust under different ambient pressures is studied. The study considers the shaft aspect ratio and ambient pressures, and analyses parameters such as temperature and flow field distribution near the shaft, and the amount of each component in the smoke exhausted from the shaft. The study results showed that the increase of the shaft aspect ratio caused the smoke exhaust capacity to first decrease and then remain stable while keeping the shaft cross-sectional area constant. In the case of the shaft aspect ratio is too small, eddy will be generated at the shaft opening, which will reduce the smoke exhaust capacity. The increase in ambient pressure results in an increased stack effect and a slight increase in smoke exhaust capacity, while it can make plug-holing more likely to occur. Therefore, for the design of natural smoke exhaust systems in high-altitude tunnels, setting a proper shaft aspect ratio is convenient for exhausting more smoke and improves the smoke exhaust capacity.

Key words: tunnel fire, shaft, numerical simulations, smoke exhaust efficiency, natural ventilation, plug-holing