Spread dynamics and heat transfer mechanism of tunnel spill fire under longitudinal ventilation

Abstract

This study systematically investigated the effects of longitudinal ventilation on tunnel spill fires within a velocity range of 0 to 2.0 m/s, using scaled model tunnel experiments at a ratio of 1:8. The research described the dynamic spreading process of spill fire in detail, revealed the stress mechanism of the fuel layer, and highlighted that heat feedback was the critical factor controlling the spread area of the spill fire, despite an increase in ventilation velocity enhancing wind force. The spread process of tunnel spill fires was divided into initial spread, shrinking, quasi-steady burning, and extinction stages. The research also uncovered the coexistence of natural and forced convection within tunnels, highlighting the substantial impact of natural convection on forced convection. On the upwind side, low wind velocities increased the combustion rate, whereas high velocities had the opposite effect. Conversely, on the downwind side, the interplay between natural and forced convection led to an initial decrease in combustion rate at low wind velocities, followed by an increase as wind velocity increased. This study overcame the constraints of traditional unidirectional spread experiments and established a new method for the study of simultaneous bidirectional spread of spill fires in tunnel environments. These findings yielded novel insights into the behavioral characteristics of tunnel spill fire behavior under longitudinal ventilation.