Differences in thermal characteristics of longitudinally ventilated tunnel fires induced by fire location

Tunnel fires are high-risk incidents in confined spaces. However, the impact of fire location randomness on fire development and smoke control has not been fully explored. This study experimentally analyzed how fire location influences tunnel fire characteristics under longitudinal ventilation. The findings revealed distinct differences when the fire was near the entrance (S/d ≤ 2, where S was the distance from fire source to tunnel entrance and d was the side length of pool fires) vs inside (S/d > 2). When the fire source was located inside the tunnel, the fully developed and thick smoke layer generates stronger airflow by its compression effect, which significantly affected flame tilt. This led to a steeper flame tilt angle, a reduced flame height, and an intensified radiative heat flux to the tunnel floor. However, when the fire was near the entrance, the maximum ceiling gas temperature might not be fully captured, and smoke was more easily exhausted, leading to a reduction in smoke backlayering length and critical velocity. Incorporating fire location effects, characterization models were developed for key parameters of flame behavior, including flame geometries and radiation heat flux, and smoke control, including smoke backlayering length and critical velocity. This work enhances insights into the stochastic nature of tunnel fires, expands fire dynamics research frameworks, and supports improved thermal safety in tunnels.