Experimental study on flame merging behaviors and smoke backlayering length of two fires in a longitudinally ventilated tunnel

A set of experimental tests were conducted to investigate the flame characteristics of two tunnel fires and the backlayering length of smoke flow induced by two tunnel fires. The experiments were performed in a reduced scale model tunnel with longitudinal ventilation by considering various heat release rates, burner separation distances and ventilation velocities. The image processing method was used to investigate the flame merging behaviors of two fires. Results show that with the increasing burner separation distance, the flames of two fires undergo three stage: fully merging flame, intermittent merging flame and non-merging flame. A piecewise function considering the new dimensionless heat release rate, the dimensionless ventilation velocity and the dimensionless burner separation distance was developed to predict the flame merging probability of two fires. The backlayering length of smoke flow induced by two fires was determined by measuring the temperature of smoke flow beneath the tunnel ceiling. It was found that the backlayering length decreases with the increase in burner separation distance. There is a transition in backlayering length at the burner separation distance of 0.9 m. At burner separation less than or larger than 0.9 m, the impact of burner separation distance on backlayering length is negligible. A piecewise function considering the heat release rate and the ventilation velocity was proposed to predict the backlayering length. It was found that the proposed expression can well correlate the experimental data in this study.