Influence of taper ratio on maximum temperature and smoke backlayering in the tapered cross-section tunnel

With the increasing complexity of urban transportation systems, tapered cross-section tunnels are widely applied in merging and diverging sections, while studies on fire smoke movement in such tunnels remain limited. In this study, a full-scale numerical model of the tapered cross-section tunnel is developed, incorporating 2 typical geometric configurations and 4 sets of parameters, with a taper ratio ranging from -0.05∼0.05. The effects of taper ratio and fire source location on the temperature field and smoke backlayering are systematically investigated by varying the fire source position, heat release rate, and longitudinal ventilation velocity. The results indicate that the maximum smoke temperature rise and smoke backlayering length are governed by the coupled effects of taper ratio and fire source location. Both parameters are greater in expanding tunnels than in reducing tunnels, and the maximum smoke temperature rise ranges from 0.46 to 1.39 times that in horizontal tunnels with uniform cross-sections. Based on the local hydraulic diameter at the fire source location and a dimensionless coupling factor, predictive models for the maximum smoke temperature rise and smoke backlayering length are proposed. The models achieve a goodness of fit of R 2 ≥0.95, with prediction errors within 20%. These findings provide a theoretical basis for fire safety design and smoke control in tapered cross-section tunnels.