Effect Of Surface Wettability And Gravity On Dropwise Condensation With Non-Condensable Gases

Dropwise condensation with a higher heat transfer efficiency, which has extensive applications in many fields, such as refrigeration industry, petrochemical industry, aviation navigation and other fields. Droplet motion is an important factor in the process of dropwise condensation. Heat transfer performance of dropwise condensation is influenced by droplets characteristics, such as shape, size, and the dynamic behavior. The droplets characteristics droplets influenced by the condensation surface wettability, the inclined angle and non-condensable gas content. Furthermore, when condensation surface slope, the contact angle of the droplet in the three-phase contact line changes, thus this dissertation focus on the influence of surface wettability, the inclined angle and non-condensable gas content on droplets characteristics, droplet motion and thermal performance during dropwise condensation.In this paper, completely hydrophobic surface tube and completely superhydrophobic surface tube was prepared to examine the influence of different surface wettability of mixture vapor condensation process in different non-condensable gas content, vertical annular tube and other different angles. Hydrophobic-superhydrophobic hybrid surface tube was designed to study the effect of surface wettability and gravity on dropwise condensation with non-condensable gases.The visualization results showed the condensing droplet contact angle hysteresis on hydrophobic surface tube increased with the increasing of non-condensable gas content. The contact angle hysteresis on superhydrophobic surface increased with a decreasing of non-condensable gas content. Contact angle hysteresis on inclined tubes was obviously greater than vertical tubes. Contact angle hysteresis at the top of the tubes was smaller than the droplet at the bottom. Critical droplet radius on hydrophobic surface increased as the non-condensable gas content increased. Critical droplet radius on superhydrophobic surface increased as the non-condensable gas content decreased. The inclined angle also impacted the critical radius, which increased with an increasing of inclined angle. For hybrid surface, critical droplet radius on hydrophobic region was greater than superhydrophobic region. Non-condensable gas content and the inclined angle showed little effect of critical droplet radius on hydrophobic region, the law of critical droplet radius superhydrophobic region was same as completely superhydrophobic surface. For hydrophobic-superhydrophobic hybrid surface tube, as non-condensable gas content of 30% and 45%, the droplet moved from hydrophobic region to superhydrophobic region, non-condensable gas content of 60%, transfer direction was opposite. This phenomenon confirmed the non-condensable gas content affect the surface wetting form.Droplet dynamic characteristics are also remarkably affected by the parameters above mentioned. The droplet on the completely hydrophobic and superhydrophobic surface was flushed along the vertical downward direction. When the inclined angle was 60 o, hybrid surface changes condensation droplets off tracks, the droplets were deviate from the direction of gravity flushing along hydrophobic ring. Inclined angles also have a certain influence on the tracks of the droplets jump. Horizontal velocity component of droplet jump on inclined tube was less than the value when tube was vertical.Effect of non-condensable gas content on the heat transfer performance was greater than inclined angle and surface wettability. When the non-condensable gas content was 30%, the heat transfer flux on hydrophobic surface at the same inclined angle was better than the heat transfer flux on superhydrophobic surfaces. Heat transfer performance of vertical hydrophobic tube displayed better than inclined tube, superhydrophobic surfaces as well. For hydrophobic-superhydrophobic hybrid surface tube, when the non-condensable gas content was 30%, heat flux on the vertical tube was the largest, and the second largest was 60 o. When inclined angle was 60 o, droplet dynamic characteristics significantly affected by the interfacial interaction, and heat transfer performance was enhanced. Hybrid surface can provide new ideas for designing an inclined mixed vapor condensing heat exchanger.