Experimental study on thermal performance enhancement of heat pipes based on grooved condensation surface with hybrid wettability

Heat pipes are commonly utilized in various energy systems owing to their excellent heat transfer performance. The introduction of dropwise condensation (DWC) within the heat pipe is a promising way to further enhance the thermal performance significantly. However, the departure of unique discrete condensing droplets highly depends on the orientation and the dynamic wettability of condensing surfaces, which brings new challenges to the DWC enhanced heat pipe technologies. In this work, a condensation surface with periodic superhydrophilic grooves and hydrophobic ridges was proposed to address the orientation dependency of the DWC inside the heat pipe. A visualization experimental system for flat heat pipes was developed to investigate the phase change dynamics and the thermal performance. Results indicate that the periodic superhydrophilic grooves optimize the sizes of condensing droplets. Under the Laplace pressure induced by the wettability difference, the condensing droplets are rapidly removed by the superhydrophilic grooves, achieving a 62% reduction in condensation thermal resistance compared to conventional heat pipes at 0°. Compared to the uniform hydrophobic surface, the droplet departure radius and growth cycle time on the hybrid wettability grooved surface were reduced by 51.9% and 10.6%, respectively, at 90°. The corresponding condensation thermal resistance and evaporation thermal resistance of the hybrid wettability grooved surface were reduced by 45% and 17%, respectively, compared to those of the conventional grooved surface. Furthermore, the effective thermal conductivity of the proposed heat pipe was enhanced by 117.5% and 54% at inclination angles of 0° and 90°, respectively, compared to the conventional grooved heat pipe at 60 W. The optimal filling ratio for the proposed heat pipe is revealed to be 18%-25% at an inclination angle of 90° and 42% at an inclination angle of 0°. This work demonstrates promising solutions in enhancing the current thermal performance of heat pipes with and without the assistance of gravity, providing new guidance for advancing DWC enhancement within the closed two-phase devices.