Effects of Hydrophilic and Hydrophobic Surfaces on Start-Up Performance of an Oscillating Heat Pipe

Slug oscillations and heat transfer performance in the start-up stage of oscillating heat pipes (OHPs) with different surface wetting characteristics were investigated experimentally. The inner surfaces of the OHPs were superhydrophilic surface, hydrophilic surface, copper, hydrophobic surface, and superhydrophobic surface, respectively. There was a thin liquid film between the vapor bubble and the surface in the hydrophilic OHP which was different from hydrophobic OHP. Results showed that start-up performance was improved in hydrophilic OHP due to the low flow resistance and deteriorated in hydrophobic OHP as opposed to the copper OHP. Heat transfer results showed that wall temperature fluctuations were observed at the start-up stage. Compared with the copper OHP, start-up time and start-up temperature were reduced by 100 s and 3.32–4.41 °C in the hydrophilic OHP at the start-up stage. Slug oscillation frequency and temperature oscillation amplitude increased with heat input; however, slug oscillation amplitude increased first and then decreased with heat input. Compared with the copper OHP, with the increasing of 0–57% in slug oscillation amplitude and 0–100% in slug oscillation frequency, the thermal performance was enhanced by 0–67% in the hydrophilic OHP. Although the slug oscillation frequency in the superhydrophobic OHP was higher than that in the copper OHP, with the decreasing of 0–70% in the slug oscillation amplitude, the thermal resistance in superhydrophobic OHP was significantly increased and was 1.5–5 times higher than that in the copper OHP.