Abstract
The rapid electrification of the energy and transport sectors has increased the demand for efficient thermal management of Lithium-Ion Batteries (LIBs). LIBs operate optimally within a narrow temperature range (15-40 °C), effective Battery Thermal Management Systems (BTMS) are essential to prevent accelerated aging and performance degradation. Immersion cooling using dielectric fluids has emerged as a promising solution; however, a systematic comparison of their thermal and hydraulic performance remains limited. This study evaluates six dielectric fluids-Silicone Oil, Mineral Oil, Sunflower Oil, AmpCoolAC-100, E5-TM410, and Deionized Water-using Computational Fluid Dynamics (CFD) simulations of a lithium-ion battery pack composed of 32 fully immersed 18650 cells. The analysis focuses on key thermophysical properties, particularly the Prandtl number and thermal diffusivity, as criteria for coolant selection. The results demonstrate a strong correlation between cooling performance and the Prandtl number. Fluids with lower Prandtl numbers provide superior heat dissipation and lower hydraulic losses. Deionized water exhibited the best overall performance, maintaining maximum cell temperatures at 31.04 °C under a 2 LPM flow rate and achieving up to 31.33% better cooling than silicone oil. In contrast, high-viscosity oils showed poor thermal performance and significantly higher pressure drops. This work establishes the Prandtl number as a critical and reliable parameter for selecting dielectric fluids in immersion-based BTMS, highlighting deionized water as the most effective coolant among those evaluated.