Multi-parameter coupling mechanisms of smoke back-layering in asymmetric V-shaped tunnel fires driven by stack-buoyancy effects

Underwater tunnels are often designed with asymmetric V-shaped longitudinal profiles to accommodate complex geological and hydraulic conditions. During tunnel fires, high-temperature smoke exhibits flow behavior distinct from that in horizontal tunnels because of the combined effects of asymmetric stack pressure and thermal buoyancy. Slope-induced thermal differences can substantially alter the upstream smoke back-layering length, which is a critical parameter for tunnel ventilation and smoke-extraction design. Therefore, smoke back-layering characteristics in asymmetric V-shaped sloped tunnel fires are investigated through 120 simulation cases in which the heat release rate (HRR), tunnel slope, and downstream gradient length range from 5–50 MW, 3%–8%, and 600–1200 m, respectively. The results indicate that higher HRRs and steeper slopes significantly reduce the smoke back-layering length, whereas increasing the downstream gradient length produces only a weak suppressive effect. Asymmetric longitudinal slopes increase the bilateral difference in stack pressure and promote buoyancy-driven directional smoke exhaust, thereby more effectively suppressing smoke back-layering. Under matched boundary conditions, slope asymmetry reduced the smoke back-layering length by approximately 22% on average compared with the corresponding symmetric configurations, although the reduction magnitude varied with HRR and slope combination. Furthermore, the dimensionless back-layering length is found to follow a negative power-law relationship with the primary influencing factors. A multi-factor coupled prediction model is developed to provide a reliable tool for tunnel ventilation design and fire safety optimization.