Performance Evaluation of Boiling-Based Two-Phase Immersion Cooling for Lithium-Ion Battery Modules Using Different Coolants and Discharge Rates

In this numerical study, a boiling-based two-phase immersion cooling system is designed for lithium-ion batteries. However, systematic cross-coolant comparisons linking boiling onset, vapor generation, and module temperature uniformity under a common hydrodynamic condition remain limited. To address this gap, the Volume of Fluid (VOF) method, together with the transient solution method and the Lee model, is used to simulate boiling, evaporation, and two-phase flow of coolants after heating in the lithium-ion battery cooling module. FS49, SF33, Novec 7000, and R141b are considered as the coolants. All coolant cases were evaluated at a fixed Reynolds number, Re = 500, defined using the liquid-phase inlet properties and the inlet diameter, to provide a common hydrodynamic basis for comparison. The module’s temperature distribution, average temperature, maximum temperature difference, temperature standard deviation, and gas-phase volume fraction results are examined at different discharge rates. At 9C, the low-boiling R141b triggered nucleate boiling earlier, reducing the maximum temperature by 14.81 °C (25.4%) compared to single-phase FS49. The coolant boiling point is identified as a critical parameter; phase-change media reduced internal temperature differences by 10.77–14.8 °C relative to single-phase flow by effectively flattening temperature rise curves via latent heat. R141b, with a gas-phase generation of approximately 0.7, has the highest gas-phase volume fraction among the four coolants, exhibiting superior heat-dissipation capabilities, with an average temperature of only 37.3 °C, a 27.1% reduction compared to FS49. The design of a two-phase immersion cooling system must balance the benefits of latent heat of vaporization against the adverse effects of bubble accumulation on the flow field. Ultimately, this work quantifies the roles of latent heat and convection, offering a vital numerical reference for optimizing high-power battery cooling media and flow configurations.