Effect of vehicular blockage on critical ventilation velocity and tunnel fire behavior in longitudinally ventilated tunnels

Abstract Most studies have investigated tunnel fires in tunnels without vehicles, even though vehicles can obstruct ventilation flows and alter tunnel fire behavior. This study conducted small-scale experiments and numerical simulations to investigate the effects of vehicular blockage on tunnel fire behavior and critical ventilation velocity, ucr, in longitudinally ventilated tunnels. The 7 m-long tunnels had cross sections of 0.6 m(H)×0.6 m(W) or 0.4 m(H)×0.6 m(W). Three vehicles types in two or three arrays, occupying 5–31% of the tunnel cross section, were positioned upstream of fires. Fires were located on the tunnel center line or downstream of vehicles on the vehicle center line. Fuel was gasoline in square pans with dimensions of 6.3×6.3 cm2 or 9.0×9.0 cm2. Experimental data indicate that ucr decreased due to vehicular obstruction when ventilation flow reached the fires; the reduction ratio approximately equals the vehicle blockage ratio by the continuity equation. Non-uniform distribution of flow velocity caused by the relative positions of fires and vehicles when ventilation flow passed fires and vehicles played a secondary role. However, ucr increased when the vehicle obstructions existed and ventilation flow did not reach the fires since the heat release rate increased due to heat feedback from vehicle obstructions. The mechanism of vehicle blockage is discussed.