Influence of tunnel slope on smoke propagation characteristics and longitudinal ceiling temperature profile in metro carriage fires

• The double influence of the tunnel slope and the double narrow space is considered. • Theoretical analysis is conducted on asymmetric and nonlinear temperature fields. • Smoke propagation mechanism caused by overflow plume in carriage fires is clarified. • The ceiling temperature prediction models considering chimney effect are proposed. Fires within metro carriages stopped in inclined tunnels pose a unique and severe threat, creating a “double narrow space” where smoke dynamics are poorly understood. This study combines theoretical analysis, 1:15 reduced-scale experiments, and numerical simulations to investigate the influence of tunnel slope ( α ) on smoke propagation and longitudinal ceiling temperature profiles. The results reveal that a greater tunnel slope significantly enhances the chimney effect, suppressing smoke back-layering. Critically, when the slope exceeds 4 %, smoke spreads almost exclusively downstream. This leads to a complex phenomenon where the smoke mass flow from the downstream carriage doors can exceed that from the doors nearer the fire, creating multiple peaks in the ceiling temperature profile and reducing the downstream temperature decay rate. Consequently, while the maximum ceiling temperature decreases with increasing slope, existing fire models for single-line tunnels are inadequate. Based on our findings, we propose novel prediction models for both the maximum ceiling temperature and its longitudinal distribution, which are validated with predictive errors of less than ± 10 % and ± 16 %, respectively. These models provide a crucial scientific basis for designing more effective fire safety and evacuation strategies in inclined metro tunnels.