# Experimental Study on the Condensation Heat Transfer on a Wettability-Interval Grooved Surface
## 3 Results and Discussion
### Heat Transfer Performance  
From the analysis of condensate dynamic behaviors on different functional surfaces, it can be seen that the dropwise condensation induced by the hydrophobic modification, especially the spontaneous suction effect and directional drainage behaviors induced by the wettability-interval microgrooves, are beneficial to the enhancement of the condensation heat transfer performance in the humid air. To quantitatively analyze the condensation heat transfer performance of different functional surfaces (including plain hydrophilic surface, plain hydrophobic surface, hydrophilic grooved surface, hydrophobic grooved surface, and wettability-interval grooved surface) in humid air, the corresponding condensation heat fluxes over time at subcooling of 12 K were compared in Figure 9.  
As expected, surface wettability and microgrooves have a significant impact on the condensation heat flux in the humid air. As shown in Figure 9, when the condensation reached a steady state after a period of time, the heat flux of the hydrophobic surface was higher than that of the hydrophilic surface. The heat flux of the plain hydrophilic surface was the lowest, only 700 W/m\({}^{2}\), which was due to the large liquid film pinned on the plain hydrophilic surface. The increasing thermal resistance brought by large liquid film deteriorated condensation heat transfer performance. However, on the plain hydrophobic surface, the droplets could slide off rapidly from the surface owing to the low adhesion, resulting in lower thermal resistance. The heat flux of the plain hydrophobic surface  
Figure 8: A typical shedding process on the wettability-interval grooved surface, including the progress of suck, film flow, and sweeping.  
was 1030 W/m\({}^{2}\), much higher than that of the plain hydrophilic surface. In addition, for the surfaces with microgrooves, the heat flux of the hydrophobic grooved surface was 1110 W/m\({}^{2}\), which was higher than that of the hydrophilic grooved surface (768 W/m\({}^{2}\)).  
Even under the same surface wettability modification, the heat flux of the surface with microgrooves was higher than that of the plain surface in the humid air condensation. This was mainly since the microgroove structure increased the heat transfer area compared with the plain surface, and the microgroove structure could enhance the drag of condensate and accelerate the removal of the condensate on the surface. In addition, the heat flux of the plain hydrophobic surface was still higher than that of the hydrophilic grooved surface, so the surface wettability was the key factor affecting the condensation heat transfer performance. In some cases, surface wettability modification was more efficient than surface structure modification in enhancing condensation heat transfer performance.  
Compared with the surfaces with uniform wettability, the wettability-interval grooved surface had the largest condensation heat flux. This was because large droplets, liquid film or liquid blocks pinned on the surface were easy to form on the surface with uniform wettability, which increased the heat transfer resistance of the condensation surface. However, the phenomenon of spontaneous suction and directional drainage through hydrophilic grooves could not only remove condensate quickly but also suppress the formation of the flooded liquid film. Therefore, the condensation heat flux on the wettability-interval grooved surface was the highest, which was 1280 W/m\({}^{2}\), about 1.8 times that of the plain hydrophilic surface (700 W/m\({}^{2}\)), and 15% higher than that of the hydrophobic grooved surface (1110 W/m\({}^{2}\)). This study demonstrated that the combined spontaneous suction and directional drainage phenomena were more efficient than the single gravity-driven sweeping mode in humid air with a large amount of non-condensable gas.