Experimental study on confinement velocity in tunnel fires with longitudinal ventilation

A series of experiments were carried out in a model-scale tunnel to investigate the confinement velocity in the tunnel fire with longitudinal ventilation. The significance of longitudinal ventilation to the smoke control in tunnel fire was comprehensively elaborated by both the variation of smoke back-layering length in the upstream and the characteristics of smoke layer in the downstream which was visualized by the laser-sheet and determined objectively by the measured vertical temperature profile. Experimental results show that the smoke back-layering length exhibits a common trend with the increasing longitudinal ventilation velocity, first decreases sharply and then becomes not very sensitive after reaching a turning point. By analyzing the experiment results, it was found that the ratio of confinement velocity, i.e., the velocity corresponding to the turning point to critical velocity was basically identical which could be determined as 0.86. In addition, a quantitative model for directly predicting the confinement velocity was proposed and compared with previous models. The study is expected to provide some references for the application of confinement velocity in actual tunnel fire-fighting.