Investigation on the heat transfer performance of synergistically modified h-BN nanofluids in synthetic ester coolant systems

h-BN is modified via synergistic KH550 and Span 80. BNKS nanosheets sustain stable dispersion in ester oil 1190 for 7 days. • BNKS nanofluids show less viscosity rise. BNKS-5 has 8.27% higher thermal conductivity (0.1693 W·m −1 ·K −1 , 20 ℃) vs ester oil. • BNKS-5 nanofluid lowers battery module maximum temperature difference to 4.80 °C at 4 L/min. Immersion cooling technology enables efficient heat dissipation through direct heat exchange between coolants and components. Ester oils are considered potential cooling media owing to their biodegradability and excellent thermal stability; however, their low thermal conductivity limits further improvement in cooling performance. Thus, this study proposed and implemented a synergistic mechanism combining chemical bonding and physical adsorption through the surface modification of h-BN with KH550 and Span 80. Nanofluids were prepared using ester oil 1190 as the base fluid, and their dispersion stability, rheological properties, and thermal conductivity were evaluated. Numerical simulations were further conducted to assess the cooling performance of the BNKS nanofluid in an immersion cooling system. The results demonstrated that the modified BNKS nanosheets exhibited improved short-term dispersion stability in ester oil 1190 compared with h-BN, maintaining stable dispersion after 7 days of static sedimentation. Moreover, surface modification reduced the viscosity increase of high-concentration nanofluids. At 20 °C, the BNKS-5 nanofluid achieved a thermal conductivity of 0.1693 W·m −1 ·K −1 , corresponding to an 8.27% enhancement compared with ester oil 1190. At a flow rate of 4 L/min, the BNKS-5 nanofluid decreased the maximum temperature difference of the battery module to 4.80 °C and maintained an average temperature of 26.78 °C, indicating enhanced temperature uniformity and cooling performance. This study proposed a synergistic surface modification strategy that integrated KH550-induced chemical bonding with the physical adsorption of Span 80 to improve the dispersion stability and thermal transport properties of ester oil-based h-BN nanofluids, highlighting their potential for immersion cooling applications.