# Experimental Study on the Condensation Heat Transfer on a Wettability-Interval Grooved Surface
## 1 Introduction
[21] carried out experimental research to investigate the heat transfer performances of tubes with hybrid hydrophilic-hydrophobic surfaces and functional fins. It is proven that the outer hydrophilic surface can decrease the resistance of droplet nucleation, while the inner hydrophobic surface can enhance the removal of droplets. Compared with the above plane surfaces, the hydrophilic groove structure can help reduce the thickness of the liquid film on the top of the groove and transport the condensate at the bottom of the groove in time by surface tension, which reduces the thermal resistance of the liquid film in the groove. This contributes to the enhancement of filmwise condensation heat transfer performance [22; 23; 24; 25]. Inspired by this, [PERSON] et al. [26] enhanced condensation heat transfer by arranging the network of super-hydrophilic grooves on the hydrophobic stainless steel surface. In the work, super-hydrophilic grooves were used to replace the plain hydrophilic stripes on the traditional hybrid surface, and the transport performance of liquid film on the hydrophilic area was enhanced. Their results showed that the condensate on the hydrophobic region could be sucked away in time, reducing the droplet diameter and accelerating the departure of the condensate. The heat transfer coefficient was up to 3.4 times that of the plain hydrophobic surface. Differing from the super-hydrophilic-hydrophobic network hybrid surface proposed by [PERSON] et al., [PERSON] et al. [27] prepared a 3D hybrid surface with superhydrophobic ridge and hydrophilic microchannels. The droplets on the superhydrophobic ridge could move spontaneously into the hydrophilic groove, and the liquid bridges formed on the surface could also be removed through the groove, which increased the droplet detachment frequency on the hybrid surface at high subcooling. In their pure steam experiment, the heat transfer coefficient of the 3D hybrid surface was higher than that of the plane hydrophobic surface, and the maximum heat flux was about 3 times higher at subcooling 18 K.  
Though most experiments and theoretical studies are based on pure steam, the condensation heat transfer performance and dynamics behaviors of droplets on the hybrid surface in humid air are also worth exploring. Because non-condensable gas is an important factor affecting the condensation heat transfer performance, the condensation surface can show different condensation characteristics in pure steam and humid air. Despite the superior condensation heat transfer performance of pure steam on hybrid surfaces [27], it is still unclear whether the hybrid surface improves the thermal performance of humid air condensation. In addition, the dynamic behaviors of the droplet on wettability-interval grooved surfaces during humid air condensation are not fully understood. To provide further insight into humid air condensation on hybrid surfaces, an experiment is conducted to investigate the condensation phase change process on wettability-interval grooved surfaces via a visualization system with high-speed CCD, to explore the droplet dynamic behaviors. In addition, the droplet dynamic behaviors and heat transfer performance of condensation on a wettability-interval grooved surface are compared with several corresponding surfaces, including the plain hydrophilic surface, plain hydrophobic surface, hydrophilic grooved surface, and hydrophobic grooved surface. The current visualization experiment identifies the thermal improvement of humid air condensation by using a wettability-interval micro-grooved surface.