Effect of inclination angle on thermal performance of microchannel-nanoporous membrane composite ultra-thin heat pipes

The integration of micro-nano hybrid architectures serves as a critical strategy for augmenting the thermal efficacy of ultra-thin heat pipes. In this work, a novel composite heat pipe is fabricated by synergistically combining silicon-based microchannel structures with anodic aluminum oxide (AAO) membranes. The liquid-vapor interface dynamics and thermal performance were systematically investigated under horizontal, positive gravity, and reverse gravity directions. Results show that gravitational assistance enhances capillary-driven flow with increasing inclination angles, accelerating liquid reflux and intensifying interfacial fluctuations in the evaporator. At 90° inclination angle, gravity significantly improves capillary liquid supply capacity, resulting in excellent heat transfer performance. Although HFE-7100 enables rapid startup, anhydrous ethanol demonstrates superior maximum heat load capacity. A higher filling ratio generally improves effective thermal conductivity and temperature uniformity by sustaining a stable liquid film. As the filling ratio rises, the liquid content within the ultra-thin heat pipe increases, accelerating evaporation and condensation rates and significantly lowering the evaporator section temperature. At a filling ratio of 1.7 and a 90° inclination, the maximum effective thermal conductivity reached 1193 W/(m∙K), highlighting the synergistic effect of optimal liquid charging and gravitational assistance on heat transfer enhancement.