Reduced Pressure Effect on Smoke Layer Plug-Holing Behavior of Tunnel Fires with a Naturally Ventilated Vertical Shaft

The fire smoke exhaust behavior of tunnels with naturally ventilated vertical shafts at high altitude area is different from that at standard atmospheric pressure due to the unconventional fire smoke transportation at reduced pressure. Smoke layer plug-holing is an unfavorable phenomenon for the smoke exhaust efficiency of tunnels with vertical shafts. Therefore, theoretical analysis and numerical simulation of the smoke layer plug-holing behavior of the naturally ventilated vertical shafts at ambient pressure of 60 kPa, 70 kPa, 80 kPa, 90 kPa and 101 kPa were conducted. Since the flame height for tunnel fires with the same heat release rate increases at reduced pressure, the smoke temperature and flow velocity beneath the ceiling are enlarged, which increases the horizontal inertia force of smoke beneath the bottom of the shaft to cause high smoke temperatures and violent smoke diffusion within the shaft, and then the degree of smoke layer plug-holing is weakened. The critical Richardson number (Ri′) for predicting the occurrence of plug-holing falls to 1.08 at 60 kPa and has a non-linear relationship to the pressure coefficient. Similarly, the plug-holing height decreases with the decrease in ambient pressure, and a global correlation for the plug-holing height that is positive to the 1.77 power of the Richardson number is proposed based on the theoretical analysis and simulated results.