Experimental investigation on the influence of longitudinal fire location on critical velocity in a T-shaped tunnel fire

A series of fire experiments were performed in a reduced-scale T-shaped bifurcated tunnel to explore the influence mechanism of longitudinal fire location on critical velocity. The smoke temperature and critical velocity characteristics were recorded and identified. The results show that in the current experimental range, the change in critical velocity with the parameter L denoting the longitudinal location of the fire can be separated into two regions: the increasing region and the stable region. When -0.2 m <= L<0.7 m, that is, the fire source is located at the T-junction or near upstream of the T-junction, the critical velocity increases with L. When 0.7 m <= L <= 1.3 m, that is, the fire source is located at downstream of the T-junction, the critical velocity almost remains constant. Based on the application of Bernoulli's equation, a proportionality coefficient kl/kc is proposed to normalize the critical velocity at different fire source locations, and an empirical correlation of the critical velocity of the Tshaped bifurcated tunnel is proposed. By comparing current experimental data and previous scholars' test results, the effectiveness of the proposed empirical correlation for critical velocity in bifurcated tunnel fire scenario is well verified. The current study can serve as a reference for the ventilation and smoke control of fire scenarios in complex-branch tunnel structures.