Experimental study on flame behaviour characteristics induced by unequal double fires in a tunnel under different ventilation modes

Unequal double fires may cause more serious disasters due to their complex interaction mechanisms, especially in the case of longitudinal ventilation. However, previous studies have paid little attention to this spot. Therefore, this study presents an experimental investigation into flame behaviour characteristics induced by unequal double fires in a tunnel under different ventilation modes. Using a reduced‐scale tunnel model based on Froude scaling, the effects of varying heat release rates (HRR), fire source separation distances (S), and longitudinal ventilation velocities (v) on flame merging, tilt angles, and mean flame lengths are systematically examined. These findings reveal that unequal HRR configurations yield lower merging probabilities compared to equal HRR scenarios under natural ventilation. Moreover, the flame merging probability exhibits a non‐monotonic relationship with ventilation velocity, and reaches a peak value at v = 0.2 m/s. Furthermore, the flame tilt behaviour and the role of its differences in flame merging are elucidated. Notably, the mean flame length of one fire is relatively insensitive to variations in the HRR of the adjacent fire, and the normalised mean flame length is independent of the merging state. Based on the detailed analysis, novel prediction models for flame tilt angles, incorporating the influence of pressure differential forces, and for normalised mean flame length are proposed. Although the experiments are limited to low ventilation velocities (v ≤ 0.5 m/s), the results provide practical insights for tunnel fire safety design. These results represent the first quantification of flame tilt angles and mean flame lengths under differential pressure forces in longitudinal ventilation, thereby bridging a critical gap in tunnel fire dynamics research.