Thermal runaway jet flames of lithium iron phosphate batteries under inclined ceilings: Flame dynamics, ceiling thermal exposure, and safety evaluation

Abstract

Battery thermal runaway fires in ceiling-confined spaces can generate jet flames, ceiling impingement, attached flame propagation, and enhanced thermal feedback. The effects of state of charge (SOC) and ceiling inclination angle ( θ ) on the thermal runaway (TR) behavior and jet flame hazards of 32 Ah lithium iron phosphate (LFP) batteries were experimentally investigated. Three SOC levels (0%, 50%, and 100%) and θ (15°, 30°, and 45°) were considered. Battery voltage, surface temperature, expansion force, venting pressure, ceiling temperature, and flame images were recorded. Results show that SOC dominates TR severity and jet combustion intensity. As SOC increases from 0% to 100%, both the peak venting pressure and estimated jet velocity increase significantly. Additionally, higher SOC produces larger flame areas, stronger intermittent eruptive jet fires, and larger flame extension lengths ( L e ). Ceiling inclination mainly regulates post-venting flame attachment and thermal feedback. Lower θ promotes sustained attached flame propagation and stronger ceiling thermal exposure, whereas θ of 45° weakens continuous flame extension. An analytic hierarchy process (AHP)-based comprehensive evaluation method was further developed. The results indicate that increasing SOC accelerates failure development and intensifies TR hazards, whereas θ mainly modifies the relative TR hazards within each SOC group. This study provides experimental evidence and evaluation guidance for fire safety design in energy storage cabinets, battery compartments, and other ceiling-confined battery systems.