Experimental study on temperature profile and critical velocity in bifurcated tunnel fire with inclined transverse cross-passage

A series of model-scaled bifurcated tunnel fire tests were conducted to explore the effect mechanism of branch slope. Different fuel pool sizes and a range of branch slopes between -6 degrees similar to 9 degrees were considered. The temperature profiles, velocity data and critical velocity were measured and analyzed. Results show that within the current test range of -6 degrees <= slope <= 9 degrees and 6.63 kW <= HRR <= 35.53 kW, as the branch slope changes from negative to positive, the thermal smoke entering branch tunnel and the corresponding high-temperature region obviously increase. And the amount of thermal smoke diverted by branch tunnel also affects the critical velocity of main tunnel, which is mainly attributed to the stack effect induced by the inclined branch tunnel. For the fire scenario with positive branch slope, stack effect is conducive to the discharge of thermal smoke, and the critical velocity of main tunnel shows a decrease tendency with the increase of slope. For the fire scenario with negative branch slope, the pressure difference caused by stack effect prevents the spread of thermal smoke inside branch tunnel to some extent, and the critical velocity of main tunnel increases with the increase of branch slope. Theoretically, there may be a critical slope for the fire scenario with negative slope, at which the thermal smoke cannot diffuse out from branch tunnel. Finally, an empirical correlation for the critical velocity considering the effect mechanism of branch slope was proposed for current experiment. This work is expected to provide some reference for the fire safety issue in bifurcated tunnel structures.