Experimental study of the moving flame behaviors and ceiling temperature characteristics under longitudinal ventilation in a tunnel

• Flame characteristics of moving fire source under forced ventilation are revealed. • The characteristics of ceiling heat transfer under the forced ventilation and vehicle movement are analyzed. • A dimensionless maximum temperature rise model of moving fire in a tunnel is proposed. The flame behaviors and heat transfer characteristics of moving fires under forced ventilation has not yet been revealed. The changes in heat release rate, flame tilt angle, and ceiling temperature of tunnel were studied. The results showed that the heat release rate showed a tendency of increasing and then decreasing because the flame tilt angle, which is a transient value, changed from deflecting upstream to deflecting downstream as the vehicle speed increased, resulting in a non-monotonous change in the fuel heat feedback. The coefficient of the moving flame tilt angle model decreased, due to the overall smaller heat release rate from a moving fire than a fixed fire. When the speed of the moving fire source is relatively low, the ceiling temperature reached the maximum value, due to the effect of heat accumulation time was greater than the effect of heat radiation. At relative velocities of less than zero, equal to zero, and greater than zero, the heat transfer was enhanced at the upstream, directly above, and downstream tunnel ceiling locations of the fire source, respectively, which in turn resulted in the maximum ceiling temperature occurring later than, equal to, and earlier than the time the fire reached this location. The temperature rise rate was faster in the rise stage, while the temperature drop rate was slower in the drop stage. Exponential laws for these two stages were obtained by selecting an appropriate temperature range for analysis. Finally, a new model was proposed to characterize the ceiling maximum temperature rise, which accounted for the important role of air entrainment and heat accumulation in the heat transfer process.