Synergistic effects of nanoparticles on heat pipe performance: A study on orientation, concentration, and fluid type

Heat pipes are widely used in electronics cooling, renewable energy, and aerospace systems due to their high thermal conductivity and passive operation; however, their performance can be limited by the properties of the working fluid and device orientation. Nanofluids have emerged as a promising solution to enhance thermal transport, but a lack of comparative data across different particle types, concentrations, and inclination angles limits practical optimization. This study addresses the gap by systematically evaluating the thermal performance of a copper heat pipe with a stainless-steel mesh wick charged with water and five nanofluids, including single-component and hybrid formulations. Experiments were conducted at nanoparticle concentrations of 0.1 %, 0.5 %, and 1.0 % and inclination angles of 0°, 30°, 45°, and 60°, under a constant 60 W heat load, with thermal resistance and effective conductivity calculated from steady-state temperatures. The TiO2–MWCNT hybrid nanofluid delivered the highest performance, reducing thermal resistance by up to 34.7 % and increasing effective thermal conductivity by 54 % compared to water, while the Al2O3–CuO hybrid achieved a 30 % reduction and 43 % increase, respectively. Single nanofluids also improved performance, with CuO–Water showing the best results among them. Increasing nanoparticle concentration consistently enhanced heat transfer, though stability considerations suggest optimal ranges. These findings indicate that hybrid nanofluids, particularly those incorporating carbon nanostructures, significantly boost heat pipe efficiency, especially at non-vertical orientations. The results provide design guidance for high-performance thermal management, and further studies on long-term stability, material compatibility, and extreme operating conditions are recommended to advance the integration of nanofluid heat pipes.