Driving force for preventing smoke backlayering in downhill tunnel fires using forced longitudinal ventilation

The critical ventilation velocity plays an important role in smoke control in longitudinally ventilated tunnel fires. The magnitude of the critical velocity has been widely studied. In the present study, we carried out theoretical analyses and numerical simulations to investigate the driving force necessary for achieving the critical velocity in downhill tunnels. Theoretical models suggest that buoyant source location has a considerable effect on the condition for achieving the critical velocity. For a constant convective heat release rate of the fire, the driving force necessary for achieving the critical velocity increases with the height difference between the fire source and the outlet of the smoke. According to FDS simulation results, with the increase of the height difference between the fire source and the smoke outlet, the pressure loss induced by the stack effect increases significantly; however, the variation in the ventilation critical velocity is insignificant. Therefore, the variation in the driving force is mainly because of the stack effect rather than the variation in the critical velocity. In addition, the effects of the fire occurrence on the longitudinal ventilation velocity and pressure distribution in the tunnel are also numerically studied. If the pressure rise of the jet-fan keeps constant, the longitudinal velocity declines remarkably after the fire occurrence as a result of the pressure loss induced by the fire and the pressure loss induced by the stack effect. The phenomenon presented in the paper has implications for both tunnel ventilation and smoke control in inclined tunnels.