Experimental and numerical investigation into effect of vortex fields on flame plume behavior and smoke temperature distribution in tunnel spill fires

Abstract

The study of vortex structures is instrumental in elucidating the flame behavior and smoke dispersion mechanisms during tunnel spill fires. This work combines numerical simulations with experimental analysis to investigate the flame plume behavior and ceiling temperature distribution throughout both the diffusion and stable stages of spill fires, with a particular emphasis on the temperature-flow field, velocity-flow field and vorticity field. The findings indicate that the interaction of cold air and hot smoke generates unstable vortex structures, which subsequently affect the flame morphology. During the diffusion stage, the vortex structures tear the flame, resulting in bifurcation behavior. In contrast, in the stable stage, the diminishing vortex structures allow the flame to stabilize and merge. From the diffusion to the stable stage, the flame oscillation frequency gradually decreases and approaches a constant value, primarily due to the deflection of the flow field at the flame root and the heat exchange between the hot smoke and cold air, a predictive model for the flame oscillation frequency throughout the entire process has been developed. Ceiling temperature variations are influenced by the combustion process, leading to the establishment of models for maximum temperature rise and longitudinal temperature attenuation over the entire process. During the diffusion stage, increased turbulence from vortex structures gradually elevates ceiling temperatures. Conversely, in the stable stage, the disappearance of vortex structures near the ceiling results in no significant changes in ceiling temperature. In the high vorticity zone, there is a pronounced rise in ceiling temperature, whereas in the low vorticity zones, the mixing of hot smoke and cold air tends to reach equilibrium, resulting in a gentler temperature increase.