Flame characteristics and downstream thermal radiation from elevated fires in tunnel energy systems: Influence of fire source geometry and ventilation

Elevated fires in longitudinally ventilated tunnels pose significant thermal hazards, yet existing radiation models do not account for the flame downwash phenomenon that occurs when wind momentum overcomes buoyancy behind an elevated carriage. This study experimentally investigates the flame geometry and downstream radiative heat flux of elevated fires with varying burner aspect ratios (n = 1-9), carriage heights, and ventilation velocities. Based on image processing of 336 reduced-scale tests, the flame horizontal length Lf,t and downwash length Hf,d are quantified. Dimensionless correlations are established by considering the competition among wind momentum, buoyancy, and air entrainment. The horizontal length scales with a modified dimensionless heat release rate and Froude numbers, while the downwash length follows a combination of momentum–buoyancy, hydrostatic pressure, and source size length scales, incorporating the carriage obstacle effect. A tilted triangular prism solid flame model, originally developed for ground fires, is modified by including the downwash flame volume to calculate downstream radiation. The modified model predicts the radiative heat flux within 20% error across a wide range of conditions, compared to approximately 50% error without the downwash correction. The model is applicable when the downstream target lies outside the flame envelope. Consequently, these findings provide a practical framework for assessing thermal radiation hazards from elevated tunnel fires.