A study on the effects of the slope on the critical velocity for longitudinal ventilation in tilted tunnels

Motivated by controversy of the grade correction factor in the critical velocity model recommended in most tunnel design codes, the effect of slope on the critical velocity in a tilted tunnel was studied by simulation and experimental methods. A 1:8 reduced-scale tilted tunnel model was built for the experimental studies. When a tunnel is tilted, smoke movement is asymmetrical about the fire source due to buoyancy along the longitudinal tunnel axis. In this paper only the worse scenario of downhill longitudinal ventilation to suppress smoke back-layering uphill is considered. Both the simulation and experimental results in the present study show that the grade correction factor, referred to the critical velocity for horizontal tunnel in Danziger and Kennedy's model, depends on the slope in a linear manner. The values of the critical velocity in the present study agree well with those predicted by the model of Wu and Bakar (2000), but are somewhat higher than the values predicted by the model of Danziger and Kennedy (1982).