Numerical insights into ceiling-constrained fire evolution and thermal hazards of vented lithium-ion battery cells

Lithium-ion batteries are the leading energy storage technology for electric vehicles and portable electronics, but their increasing energy density has heightened safety concerns over thermal runaway. When the 18650 cell undergoes thermal runaway, the venting gases can ignite and develop into jet fires, posing serious hazards to battery modules. In this study, a three-dimensional computational fluid dynamics model is developed to investigate the fire behavior of a 3 × 3 18650 battery module triggered by single-cell thermal runaway. The model explicitly represented inter-cell spacing and vent hole geometry, employed the RANS turbulence framework with the EDC combustion model, and incorporated the P1 radiation model. Model validation against experimental data confirmed its accuracy in predicting flame extension length and peak heat release rate, establishing its reliability for parametric studies. Simulation results revealed that flame evolution beneath the ceiling proceeds through three stages: rapid horizontal spread, a quasi-stable extension phase, and eventual retraction toward the venting source. Ceiling height exerted the strongest influence on flame spread and gas distribution, with lower ceilings intensifying confinement, enhancing CO accumulation, and elevating ceiling-level temperatures. Battery surface temperature analysis indicated that cells adjacent to the failing cell heated more rapidly and to higher levels, making them more prone to secondary thermal runaway. Heat transfer analysis confirmed radiation as the dominant energy dissipation pathway, with convection providing a smaller but non-negligible contribution. Both mechanisms are highly sensitive to ceiling geometry, with a 10 mm ceiling significantly suppressing heat dissipation. Overall, the proposed model provides quantitative insights for optimizing enclosure design and improving the fire safety of lithium-ion battery systems. • 3D CFD model simulates fire from 18,650 cell thermal runaway. • Model validated against experiments for flame length and HRR. • Ceiling height strongly controls flame spread and gas buildup. • Adjacent cells heat faster, raising risk of secondary runaway. • Radiation dominates heat transfer; confinement suppresses cooling.