Experimental and theoretical study of the smoke back-layering length in a tunnel with cross-passage: effects of longitudinal fire source locations

The diversion effect caused by the linked structure in a metro tunnel with cross-passage complicates the impact of longitudinal fire source location on the smoke backflow layering behavior that has not been clarified, despite the fact that the scenario exists in practice. A series of laboratory-scale experiments were conducted in this study to investigate the smoke back-layering length in a model tunnel with cross-passage. The heat release rate, the velocity of longitudinal air flow, and the location of the fire source were all varied. It was found that the behavior of smoke backflow for the fire source located at the upstream of bifurcation point resembles a single-hole tunnel fire. As the fire source's position shifts downstream from the bifurcation point, the length of smoke back-layering progressively increases. A competitive interaction exists between airflow diversion and smoke diversion during smoke backflow, significantly affecting the smoke back-layering length in the main tunnel. The dimensionless smoke back-layering length model was formulated in a tunnel featuring a cross-passage, taking into account the positions of longitudinal fire sources. The dimensionless smoke back-layering length exhibits a positive correlation with the 17/18 power of total heat release rate Q and a negative correlation with the 5/2 power of longitudinal ventilation velocity V.