Wettability-guided performance mapping of Ag, CuO, and MWCNT nanofluids in renewable copper heat pipe modules

Passive copper heat pipes for solar-assisted heat pump systems require working fluids that combine high heat transport, favourable wettability, and stable colloidal behaviour under variable heat input and orientation. This study compares deionized water with six 0.1 wt% nanofluids, Al₂O₃, CuO, TiO₂, ZnO, Ag, and MWCNT, in a straight copper heat pipe of 22 mm outer diameter, 450 mm length, and 60% filling ratio. Tests were performed at heat inputs of 30, 60, and 90 W and inclination angles of 0°, 30°, 60°, and 90°. Zeta potential, static contact angle on polished copper, and thermal conductivity from 25 to 100 °C were measured independently for each formulation. Response surface methodology was applied to model thermal resistance and condenser-side thermal efficiency as functions of heat input and inclination angle. Among the tested fluids, MWCNT nanofluid showed the largest enhancement, reducing thermal resistance by up to 36.7% compared with deionized water. At 90 W and 0° inclination, it decreased the evaporator temperature from approximately 90 °C to 71 °C and increased the thermal efficiency from 90–97%, corresponding to an absolute gain of 7% points and a relative improvement of 7.8%. The performance ranking was MWCNT > Ag > CuO > ZnO > Al₂O₃ > TiO₂. This trend correlated positively with the conductivity–aspect–ratio product and inversely with the contact angle. The optimum operating region was 90 W and 90° inclination, supporting wettability-guided nanofluid selection for passive solar heat pipe modules. These results provide a experimental basis for the design of compact thermal management systems for renewable energy applications.