Experimental study on temperature characteristics in a subway train carriage with lateral openings in a longitudinally ventilated tunnel

• Scenario of double long-narrow confined space was considered. • Effect of lateral train openings on the ceiling gas temperature was addressed. • Upstream temperature decay in the train was systematically analyzed.. • Fire risk in the train was assessed based on the smoke layer height.. Under the rapid development of subway systems, a fundamental understanding of subway fire safety is critically needed to benefit its designs and assessment. Previous studies on train carriage fires are limited to single-carriage fires under natural ventilation. However, it is still unclear how longitudinal ventilation affects the smoke movement in the train carriage with lateral openings. Therefore, a series of 1:3 reduced-scale experimental tests were conducted to investigate the smoke temperature characteristics in a train carriage located in a longitudinally ventilated tunnel. Under various fire sizes and ventilation rates, the ceiling gas temperature and smoke layer height were measured. Compared to the traditional fire scenario with end openings, the maximum ceiling gas temperature and the downstream temperature distribution decay factor are relatively smaller because of the lateral flame tilt and less heat loss during smoke propagation. The effects of ventilation velocity on the maximum ceiling gas temperature are significantly weakened under the blockage effect of the train carriage. The downstream temperature distribution followed a single exponential attenuation function, independent of the fire size and ventilation rate. However, the upstream temperature distribution is much more complex under the restriction of the end wall. A four-segment model was proposed to predict the upstream temperature distribution based on the relationship between the back-layering length and half the carriage length. The fire size and ventilation rate also show limited influence on the smoke stratification, with a layer height of 0.316 m in the stable region. The research outcomes offer adeeper understanding of the smoke propagation of the subway train carriage fire and a theoretical guide on the designs of future tunnels.