Fire source elevation effects on smoke transport in inclined tunnels: scaled experiments, synergistic mechanisms, and predictive model enhancements

Fire-induced smoke dynamics form a critical foundation for tunnel safety design. Existing studies on inclined tunnel fires predominantly adopt simplified ground-level fire source assumptions, neglecting the impact of elevation variations caused by vehicle type differences on smoke transport in urban scenarios. This study systematically investigates the influence mechanisms of plume state, fire source elevation, tunnel slope, and heat release rate (HRR) on induced air inflow velocity and smoke back-layering length through scaled experiments. Results reveal that while fire source elevation, slope, and HRR collectively alter plume-ceiling impingement states (e.g., transitions between weak/strong plumes), such alterations do not significantly alter the overall trends of induced air inflow velocity and smoke back-layering length. Induced air inflow velocity and smoke back-layering length show significant negative correlations with fire source elevation, though their sensitivity nonlinearly diminishes with increasing slope. Induced air inflow velocity exhibits synergistic enhancement with concurrent increases in HRR and slope, driven respectively by amplified tunnel temperature differentials and elevation differences. Conversely, smoke back-layering length demonstrates pronounced slope-dependent responses to HRR variations: higher HRRs amplify smoke back-layering length in low-slope configurations due to dominant thermal buoyancy effects, but this effect diminishes in steep slopes. Building upon these mechanisms, the revised predictive model of induced air inflow velocity and smoke back-layering length incorporating fire source elevation is developed and validated against experimental data, showing improved accuracy across varied fire elevation scenarios. These findings provide critical insights for optimizing smoke control strategies in urban inclined tunnels.