Experimental study on the effect of different sealing ratios on the behavior of continuous spill fires in sloped tunnels

Abstract

Several spill fire experiments were carried out in a reduced-scale sealed tunnel, considering different discharge rates, sealing ratios, and initial sealing time. The burning characteristics of tunnel spill fires were analyzed. The results show that in the scenario of continuous fuel leakage, tunnel spill fires will eventually fall into three different burning behaviors as the sealing ratio increases: quasi-steady combustion, unsteady combustion diffusion, and self-extinction. The fundamental reason that the sealing ratio influences the burning characteristics of tunnel spill fires is that it alters the ventilation factor at the tunnel opening, thereby affecting the maximum burning rate of the liquid fuel. This leads to variations in the relative magnitudes of the discharge rate and evaporation rate, resulting in different diffusion regimes of the tunnel spill fire. Moreover, when the tunnel exits are sealed at the quasi-steady combustion stage, the three regimes of tunnel spill fires observed are similar to those observed when the exits are sealed at the initial moment. The transition mechanisms of various diffusion regimes were analyzed, and a correlation model between the critical sealing ratio and the discharge rate was established. Additionally, the temperature distribution law of the smoke below the tunnel ceiling under various sealing ratios was studied. The maximum smoke temperature rise exhibits two distinct regions of variation as the sealing ratio increases. When the sealing ratio is small, the maximum smoke temperature rise remains relatively steady with the increase of the sealing ratio. However, as the sealing ratio increases, the tunnel spill fire transitions to the ventilation-controlled, resulting in a reduced burning rate and a rapid decrease in the maximum smoke temperature. Empirical models characterizing the maximum temperature rise were developed for different sealing situations. This study could provide an essential reference for conducting emergency rescue operations during tunnel spill fires.