With the rapid adoption of lithium-ion batteries in electric vehicles and energy storage systems, thermal runaway propagation raises growing safety concerns, risking fires, explosions and module failures. Conventional boiling models are computationally heavy for large-scale use, demanding efficient simulation methods. This study proposes a simplified boiling model to simulate a 5×5 battery module immersed in HFE7000, assessing how insulation thicknesses (0 mm, 0.1 mm, 0.2 mm, 0.3 mm) affect thermal runaway starting from a corner cell. The model offers high efficiency and scalability for rapid safety design simulations, with results meeting practical needs. Notably, immersion cooling shows excellent thermal management: effective even without insulation, and enhanced by thicker layers. Data shows: at 0 mm, 1 cell fails at 50 s, 12 at 200 s, full module runaway at 338 s; 0.1 mm delays to 1 at 50 s, 6 at 200 s (50% reduction), full runaway at 488 s (44% extension); 0.2 mm limits to 1 at 50 s, 3 at 200 s (75% reduction), 5 cells affected by 306 s; 0.3 mm confines to the initial cell (1 at 50 s and 200 s). Propagation direction shifts with thickness—lateral for thinner layers, to thickness direction at 0.2 mm due to smaller side areas, larger front areas and higher lateral thermal resistance.
 

Heat propagation; Immersion cooling; Lithium-ion battery; Pool boiling; Thermal runaway.