Experimental study on the flame geometry and thermal flow characteristics of carriage fire in a tunnel under longitudinal ventilation

The increasing integration of energy systems into urban structures has led to a corresponding rise in fire risks. Metro tunnels, serving as critical underground transportation corridors, are characterized by their narrow, enclosed design and dense energy-related infrastructure. Meanwhile, rail transit systems are accelerating their transition toward clean energy, with energy-powered trains—driven by batteries, supercapacitors, or hydrogen fuel cells—being deployed on an expanding scale. This study experimentally and numerically investigates spilled fire behaviors in a narrow carriage under longitudinal ventilation. The evolution of flame overflow with ventilation velocity is divided into three stages within the intermittent spillage regime: Stage I: internal combustion without ventilation; Stage II: flame shifts toward the opening and is easily blown out under moderate ventilation; Stage III: flame is pressed back into the compartment and becomes unstable at higher velocities. The spilled flame height decreases with increasing ventilation velocity, and the decay rate gradually slows with larger opening width. The flame horizontal extension length increases gradually with increasing wind speed, and the growth is more rapid at low wind speeds. The flame depth increases first and then decreases with ventilation velocity, and is smaller than that in still air when the opening width is relatively small. Based on dimensional analysis, two impact factors—“Excessive Fuel Spillage” and “Absence of Facade”—are proposed to quantify flame geometry. Predictive models for flame height, horizontal extension length and depth, incorporating opening size, ventilation velocity, and heat release rate, are established to well describe flame geometric parameters.