Experimental study on the burning characteristics and smoke temperature distribution of continuous spill fires in sloped tunnels

•A series of spill fire experiments were conducted in a sloped tunnel.•The diffusion and burning characteristics of tunnel spill fires were discussed.•A model to characterize the liquid fuel diffusion area was developed.•The effect of the tunnel slope on the maximum temperature was revealed.•The smoke temperature decay model of tunnel spill fires was proposed. A series of tunnel spill fire tests were carried out, considering different tunnel slopes and discharge rates. Characteristic parameters, including diffusion distance, diffusion area, mass burning rate per unit area, and smoke temperature, were measured, and the influence of the tunnel slope on the characteristic of these parameters was analyzed. The results show that the combustion process of the tunnel spill fire could be separated into four distinct stages: the combustion diffusion stage, the combustion shrinking stage, the quasi-steady combustion stage, and the extinction stage. The occurrence of the shrinking stage could be attributed to the increase in the fuel’s mass burning rate per unit area. Moreover, the transition between combustion stages primarily depends on the relative magnitude of the fuel’s mass burning rate and discharge rate. A correlation model was developed to characterize the diffusion area and diffusion distance of the liquid fuel. During the quasi-steady combustion stage, the average diffusion width significantly decreases with increasing tunnel slope. Therefore, the maximum temperature decreases as the slope increases. Furthermore, the mass burning rate per unit area increases as discharge rate increases. This is primarily due to the flame hitting the ceiling, increasing the radiant heat flux to the liquid fuel. Additionally, the maximum temperature rise decreases as the slope increases. However, when beyond a specific slope threshold, additional increases in slope have a minor effect on the maximum temperature rise. This could be attributed to the variation of fuel's diffusion width with the tunnel slope. Finally, a dimensionless model was developed to characterize the decay of smoke temperature below the tunnel ceiling. This study could serve as a critical reference for assessing disaster risks and executing emergency rescue operations in the case of tunnel spill fire incidents.