Experimental study on flame behaviors and ceiling smoke temperature of two weak plumes in a tunnel under natural ventilation

Vehicle collisions in tunnel can easily cause accidental fires, which could result in fire spread and finally lead to multiple fire sources burning simultaneously. Therefore, the quick fire detection and efficient operation of controlling systems is extremely important at the initial stage of a tunnel fire. By now, few researches have been conducted on tunnel fire with two weak plumes. This paper focused on tunnel fire considering two weak plumes and aimed to investigate flame behaviors and ceiling smoke temperature. A series of reduced-scale experiments were done in a model tunnel, considering two gas burners with six heat release rates (HRRs) and six varying fire separation distances. A set of thermocouples were positioned near the tunnel ceiling to monitor the temperature of smoke flow, while a camera was utilized to record the flame behaviors. The experimental findings indicated that with the enlarging fire spacing, the flames exhibite three stages: complete merging, intermittent merging with small merging probability and separate flames. The flame merging probability exhibites a sudden decrease with increasing fire spacing, while the peak ceiling temperature transforms from one point to two points. An expression was developed to forecast the maximum temperature rise of two weak plumes. Furthermore, a correlation with the effect of fire spacing considered was presented to forecast the longitudinal temperature decay below ceiling for two weak plumes. This work could offer basic knowledge for the recognition of flame behaviors and ceiling smoke temperature induced by two weak plumes in a tunnel.