Study on the heat transfer characteristics of a flat plate heat pipe with a wettability-gradient grooved Wick structure

The limited capillary pumping capability of conventional grooved wicks can restrict condensate return and lead to insufficient liquid replenishment and local dryout, particularly under high heat-flux conditions. To address this limitation, a cactus-spine-inspired wettability-gradient grooved wick was developed for flat heat pipes. Unlike conventional constant-depth grooves, the proposed wick employs depth-gradient microgrooves to generate continuous variations in surface roughness and apparent contact angle, thereby introducing an additional directional driving force while retaining the high permeability and low flow resistance of grooved structures. This wettability gradient promotes condensate transport from the condenser to the evaporator and improves working-fluid redistribution. The thermal and two-phase flow characteristics of the proposed heat pipe were investigated using validated numerical simulations, and the effects of filling ratio, heat flux, heat-pipe height, and length were systematically evaluated. Results showed that, compared with a conventional heat pipe reported in the literature, the proposed biomimetic heat pipe reduced the thermal resistance from 3.28, 2.04, and 1.55 K/W to 1.78, 1.48, and 1.39 K/W at heating powers of 3, 5, and 7 W, corresponding to reductions of 45.7%, 27.5%, and 10.3%, respectively.A filling ratio of 15% produced the highest average heat-transfer coefficients of 11,872 W/(m 2 ·K) in the evaporator and 1362 W/(m 2 ·K) in the condenser among the investigated filling ratios. Increasing the heat flux from 80,000 to 320,000 W/m 2 increased the corresponding heat-transfer coefficients by 24.3% and 32.7% and shortened the transition time to stable working-fluid circulation from 2.72 to 1.05 s. Increasing the heat-pipe height from 3 to 5 mm increased the evaporator and condenser heat-transfer coefficients by approximately 125% and 126%, respectively, whereas increasing the heat-pipe length weakened the wettability-gradient driving force and hindered condensate return. These results demonstrate that the proposed biomimetic wettability-gradient wick enhances directional liquid transport, alleviates insufficient evaporator replenishment, and effectively reduces the thermal resistance of flat heat pipes, providing a promising passive thermal-management strategy for high-heat-flux electronic devices.