Optimization of TiO2 nanofluid heat pipes for thermal energy storage and management systems: Independent and coupled effects of filling ratio and inclination angle

To overcome the thermal bottlenecks in high-density Energy Storage Systems (ESS), such as thermal runaway risks in batteries and slow charging/discharging rates in thermal energy storage, this study investigates the independent and coupled effects of the filling ratio and inclination angle on the thermal performance of a heat pipe using TiO 2 nanofluids as the working fluid. Efficient heat removal is critical for prolonging the cycle life of energy storage devices. Computational Fluid Dynamics (CFD) simulations were employed to visualize complex phase change processes, analyzing boiling and natural convection heat transfer from startup to the steady state. The impacts of these parameters were evaluated based on wall temperature distribution, thermal resistance, and the surface heat transfer coefficient of the evaporator. The results indicate that the filling ratio determines the baseline performance, while the inclination angle exerts a regulatory effect characterized by distinct flow regime differentiation across different filling ratio ranges. At low filling ratios (60%), although the surface heat transfer coefficient at an 80% filling ratio increased by 95.1% (reaching 4608 W/(m 2 ·K)) relative to other cases, the overall thermal resistance decreased by only 2.6% due to hydrodynamic resistance limitations. This study provides a theoretical basis for the optimal integration of nanofluid heat pipes into energy storage modules, offering design guidelines to ensure thermal safety and enhance the dynamic response of next-generation storage systems.