Study on the variation rule of smoke back‐layering length of tunnel ceiling jet induced by strong fire plume

Tunnel fire is a major research topic in tunnel safety. Because of the tunnel's narrow and enclosed structure, smoke movement plays an important role in the investigation of the tunnel fire. In this paper, the Froude similarity principle is used to study the variation rule of the smoke back‐layering length of tunnel ceiling jet induced by strong fire plume with different heat release rates, longitudinal ventilation velocities, and effective heights of fire source analyzed. It is found that the smoke back‐layering length of tunnel ceiling jet induced by strong fire plume increases with the increase of effective height of fire source and decreases with the increase of longitudinal ventilation velocity. When the heat release rate is relatively small (), the smoke back‐layering length of the tunnel ceiling jet induced by strong fire plume increases as the heat release rate increases. However, for the larger heat release rate (), the smoke back‐layering length of the tunnel ceiling jet induced by strong fire plume is mainly related to the longitudinal ventilation velocity. Based on Thomas's model and Li's model, by introducing the correction coefficient, correction formulas of the smoke back‐layering length of tunnel ceiling jet induced by strong fire plume changing with the effective height of fire source are proposed.