Evaluating the smoke back-layering length and critical velocity in branched tunnel fire with sloped mainline: An experimental study

The branched tunnel with inclined mainline is increasingly common to link ground and underground transportation network. The smoke movement characteristics and critical velocity in branched tunnel with uphill mainline are still unclear. This work investigated the smoke back-layering length and critical velocity in branched tunnel with inclined upstream and horizontal downstream via model-scale (1/20) experiments. The mainline slope varied from 1% to 7%, and three bifurcation angles of 10 degrees, 20 degrees and 30 degrees were considered. Results show that the increasing of the uphill mainline slope promotes the shorter backflow distance in inclined mainline that requires smaller wind speed to control smoke return. The smoke back-layering length monotonically ascends with the heat release rate, but the effect of bifurcation angle is fluctuant. The critical velocity is higher for greater heat release rate. The logarithmic function can well express the relationship between Q*/V*3 and L/Hc that the correction factor of bifurcation angle is used to modify the expression. A new model of critical velocity is characterized by considering the mainline slope and bifurcation angle. The proposed model for critical velocity shows good agreement with experimental results, which can apply to horizontal branched tunnel and branched tunnel with uphill mainline.