Single-phase immersion fluid selection for Li-ion battery modules: From the viewpoint of heat transfer

The immersion cooling exhibits potential for battery thermal management systems. However, the lack of evaluation indicators for diverse thermal management fluids hinders effective selection, and the impact of their properties on the battery's electro-thermal performance remains to be determined. The time-varying battery heat generation, the temperature-dependent properties of fluids, and the complicated heat exchange within the module also emphasize the importance of external heat transfer intensity. This paper aims to investigate the selection of immersion fluids through heat transfer. A precise platform for immersion-cooled battery modules was established. Under wide-temperature areas and various discharge C-rates, the effects of fluid types on module performance and natural convection heat transfer are analyzed. The results indicate that lower environmental temperatures (ETs) lead to a substantial increase in module temperature difference, exacerbated by higher C-rates. The electrical performance (η_e) of the module shows a stepwise reduction as ET decreases, experiencing a sharp decline of 53.8 % from 10 °C to −5 °C. Notably, different fluids exhibit significant disparities in η_e at lower ETs. The comprehensive indicator Mo/Mo₀ is firstly proposed to assess the heat transfer capacity of four fluids and ranks them as silicone oil-5cSt (#C2) > synthetic hydrocarbon (#C1) > silicone oil-20cSt (#C3) > fatty acid (#C4). The impact of viscosity on Mo/Mo₀ is pronounced, and ET has a more significant influence than C-rate. At variable C-rates, the stability of Mo/Mo₀ follows the order: #C2/#C3 > #C1 > #C4. Theoretical calculations reveal stable proportions of heat absorption by #C2 and #C3, maintaining approximately 43.0 % and 44.0 %, respectively. The correlations between C-rates and Nusselt numbers are also established under different ETs. This study provides a theoretical foundation and data support for selecting immersion fluids and offers valuable insights for predicting natural convection heat transfer intensity.