Experimental study on the coupling effect of filling ratio and inclination angle of dropwise condensation enhanced heat pipes with graphene coatings

The introduction of dropwise condensation (DWC) in the heat pipe condenser can significantly improve the condensation heat transfer and the overall effective thermal conductivity. However, the intricate effects of the filling ratio and inclination angle caused by unique discrete condensing droplets bring new challenges to the condensation enhanced heat pipe technique. In this work, sustainable DWC on the nickel-graphene nanocomposite coatings (Ni-Gr) is introduced into conventional thermosyphon to explore the new effect of filling ratio and inclination angle on its thermal performance. The optimal filling ratio combined with inclination angles for DWC enhanced Ni-Gr heat pipe is identified. Results indicate that the optimal filling ratio was 61.8 % at inclination angle of 15°. When the inclination angles were 25°, 55°, and 90°, the optimal filling ratio was 44.7 %. The maximum effective thermal conductivities of the Ni-Gr heat pipe were improved by 85 %, 69.2 %, 75.4 %, and 70.3 % compared to the pure Ni heat pipe at 15°, 25°, 55° and 90°, respectively. At low filling ratios (17.1 % – 25.2 %), the effective thermal conductivity of the Ni-Gr heat pipe was significantly affected by the inclination angles due to the condensing droplet retention, degrading the evaporation efficiency at the evaporator by the lack of working fluid. During the filling ratios of 44.7 % – 74.8 %, the retention effect of condensing droplets on the evaporator region was mitigated and the average liquid film thickness was reduced compared to the pure Ni heat pipe, enhancing both condensation heat transfer and evaporation heat transfer. Evaporative flooding with low heating powers was observed to be mitigated by DWC at various inclination angles when the filling ratio was greater than 36.6 %. Furthermore, an empirical fitting model for the Ni-Gr heat pipes was developed using the response surface methodology, which could predict the effective thermal conductivity under different working conditions. This work provides new insights into the coupling effect of the filling ratio and inclination angles of heat pipes modified by Ni-Gr coatings, providing feasible design guidelines for DWC enhanced heat pipes.