Numerical and Experimental Study on Jet Flame Behavior and Smoke Pattern Characteristics of 50 Ah NCM Lithium-Ion Battery Thermal Runaway

This paper investigates the flame behavior and smoke pattern characteristics of lithium-ion battery (LIB) fires using an integrated experimental and numerical simulation approach. Based on fire dynamics theory, a jet flame model for LIB thermal runaway (TR) is developed to analyze the flame height and dynamic characteristics. The results reveal two distinct regimes in LIB jet flames: momentum-controlled dominance in the early TR stage (lasting approximately 3 s) and buoyancy-controlled dominance in subsequent combustion. The jet flame shifts from a momentum-dominated regime (Fr > 5) to a buoyancy-dominated plume (Fr < 5) as the vent velocity decays below 12 m/s. The simulated flame heights align with experimental measurements and the Delichatsios model, validating the numerical approach. Furthermore, the distribution of flame components (e.g., H2, CO, CO2, CH4, C2H4) is analyzed, highlighting the influence of multi-component gases on combustion heterogeneity. Smoke pattern analysis demonstrates that soot deposition varies significantly between momentum- and buoyancy-controlled stages, with the former producing darker, concentrated deposits and the latter yielding wider, lighter patterns. These findings provide a theoretical basis for forensic fire investigation (accident reconstruction) and targeted suppression strategies for different combustion stages.