An analysis of tunnel fire characteristics under the effects of vehicular blockage and tunnel inclination

It is known that the blockage and inclination conditions of tunnels are among the important factors affecting fire safety considerations, as these factors could change the characteristics of possible fire incidents as well as smoke movement in tunnels. In the present work, we analyze the variations of the tunnel fire burning rate, heat release rate and smoke backlayering as being functions of these two factors. Ethanol pools were used as fire sources in a reduced scale tunnel model with longitudinal ventilation ranging between 0 and 1.5 m/s. The blockage ratio of the tunnel, which was defined as the ratio of the cross-sectional area of the blockage to that of the tunnel, was tested under three cases: 0% (i.e., no blockage), 14% and 56% blockage. The latter two blockage ratios respectively correspond to that of a typical small vehicle and a railroad carrier. The tunnel inclination grade was varied between −6% and +3% to represent uphill and downhill slopes. Numerical simulations were also performed using Fire Dynamics Simulator (FDS) to rationalize some of the experimental results. Measurements and predictions indicated that the blockage affects the burning rate of tunnel a fire due to changes in the air entrainment at wake flow, local ventilation over the fire and flame dragging. Increasing the fire-blockage separation distance had an adverse effect on the burning rates. The temperature results emphasized the effect of blockages on tunnel ceiling temperatures, which increased as high as 300% compared to that of the no blockage case. The results indicated the major effects of the tunnel sloping grade on the fire heat load as well as the tunnel ceiling temperature. The critical ventilation velocity was achieved in the range of 0.75-1.25 m/s for the limiting cases of −6% and +3% tunnel inclination, for which a fit was proposed as a function of inclination grade. Finally, a statistical model based on an analysis of variance approach was applied on the obtained results, which demonstrated that among the factors contributing to the fire heat release rate variations in this study, the ventilation velocity accounted for 45% of the variation, followed by tunnel blockage at 25%, and inclination at 19%.