Smoke control using natural ventilation shafts in a tunnel during construction under the effect of geothermal condition

An increasing number of tunnels experiencing geothermal hazards are being constructed in the western regions of China. During construction, tunnels have a confined structure, and shafts are typically installed to facilitate transportation and enhance ventilation. Once a fire occurs in these tunnels, their management faces the added challenges posed by geothermal hazards. To investigate the performance of natural smoke exhaust from the construction shaft under the influence of geothermal conditions, a series of fire tests in a 1/20 scale model tunnel have been conducted. Results indicate that the hot air and initial temperature field generated by geothermal conditions reduce heat loss during smoke movement. The smoke temperature and smoke layer thickness behind the shaft decrease significantly with increasing shaft height, while both increase with an increase in geothermal temperature. The smoke movement patterns within the shaft are affected by the geothermal temperature. Specifically, the boundary layer separation is intensified, while the plug-holing is mitigated as the geothermal temperature increases. The mass flow rate exhausted from the shaft increases with both increasing shaft height and geothermal temperature, and a dimensionless model is proposed to calculate the mass flow rate of mixed smoke. The heat exhaust efficiency of the shaft initially increases and then decreases with shaft height due to the plug-holing phenomenon, while it reduces with increasing geothermal temperature for a given shaft height.