Effects of hydrophilic surface on heat transfer performance and oscillating motion for an oscillating heat pipe

In this paper, a series of experiments were performed to investigate the effects of superhydrophilic and hydrophilic surfaces on the heat transfer performance and slug motion for oscillating heat pipes (OHPs). Deionized water was used as the working fluid. The surfaces of the OHPs were copper, superhydrophilic, hydrophilic and hydrophobic surfaces with contact angles of 73.4°, 0°, 12.9° and 141.5°, respectively. The heat transfer performance in six-turn OHPs was higher than in four-turn OHPs. Experimental results showed that the surface wettability remarkably influenced the slug motion and thermal performance of OHPs. The liquid slug movements became stronger both in superhydrophilic and hydrophilic OHPs as opposed to the copper OHP, while the global heat transfer performance of the superhydrophilic and hydrophilic OHPs increased in the six-turn OHP. For six-turn OHPs, the maximum displacement of the liquid slug in the hydrophilic OHPs and superhydrophilic OHPs increased by 5-60% and 25-60%, respectively, in comparison with that of copper OHPs. The heat transfer performance of superhydrophilic OHPs and hydrophilic OHPs increased by 5-15% and 15-25%, respectively, in comparison with that of copper OHPs. The hydrophilic surface improved the heat transport capability of OHPs, and the maximum displacement and velocity of the liquid slug increased. The startup temperature of the four-turn and six-turn OHPs varied from 45 to 55 °C and 35 to 45 °C, respectively. The startup temperature of the four-turn and six-turn superhydrophilic OHPs varied from 40 to 55 °C and from 35 to 50 °C, respectively. The startup temperature of the four-turn and six-turn hydrophilic OHPs varied from 40 to 50 °C and from 30 to 40 °C, respectively. The hydrophilic OHP also showed a better startup performance than the copper OHP. At heat input of 150 W, the thermal resistance for the four-turn hydrophobic OHPs was about two times greater than that of copper OHPs. The global heat transfer performance of the superhydrophilic OHP for the four-turn OHP was lower than that of the four-turn OHPs with hydrophilic and pure copper OHPs.