Wetting Evolution And Heat Transfer Characteristics In Dropwise Condensation Of Steam-air Mixture On Superhydrophobic Surface

Interfacial effects of solid-liquid and gas-liquid are of great importance in enhancing condensation heat transfer, especially in the presence of noncondensable gas(NCG). Superhydrophobic surfaces have very low surface free energy and high contact angle. Droplet shows a nearly perfect spherical "pearl" and rolls easily on superhydrophobic surfaces. It seems to take great advantages of both the interfacial effects of solid-liquid and gas-liquid to enhance condensation heat transfer. In this thesis, condensation experiments of pure steam and steam-air mixture vapor were carried out to better understand mechanisms of heat transfer enhancement in terms of the wetting mode evolution of condensate droplet.Superhydrophobic surface (SHS) was prepared by chemical etching with K2S2O8aqueous solutions, followed by coating of n-octadecyl mercaptan self-assembled monolayer. Wenzel droplets were constructed by ethanol film pre-spreading and ultrasonic vibration inducing the wetting transition. Based on the observations of Wenzel and Cassie droplets, the condensate droplet wetting behaviors were proposed. During the pure steam condensation, the condensate droplets are in a new condensate wetting mode with condensate stagnating in the cavities of the micro-nanostructures. For the condensation in the presence of NCG, the wetting behavior of the condensate droplet shows a new condensate sinkage mode in addition to a Cassie-Baxter mode depending on the NCG concentration. During dropwise condensation (DWC) with high concentration of NCG, pulsating features are found during droplets coalescence movement. Springing-up of coalesced droplet was also observed induced by the strong effect of pulsating motion to overcome the pinning effect.The characteristics of droplet self-propelling induced by coalescence were investigated both experimentally and theoretically. The coalescence of two water droplets was promoted on roughness-induced superhydrophobic surface. The movement features of the coalescent droplet were analyzed from the capture images. The results have shown that the self-propelled droplet evolves a regular extension and retraction movement due to the flow liquid mixing and the effect of surface tension in the jumping movement. The smaller the initial droplet is, the higher the coalescent droplet jumps. Theoretical analysis is presented based on the energy conservation. The theoretical results have shown that droplet springing can only occur in a certain radius range of the coalescence droplets. A larger range of the coalescent droplet would spring in DWC in the presence of larger concentration of NCG. The droplet springing phenomena would occur only in the situation that the surface solid fraction is lower than a critical solid fraction for DWC with a given concentration of NCG. No springing phenomena would occur on superhydrophobic surface with solid fraction larger than the critical solid fraction. The larger the NCG is, the lower the critical solid fraction is.The effects of wetting mode and springing-up phenomena on condensate droplet size distribution were investigated. Condensation was conducted on a divided surface with condensate wetting mode (W region) and condensate sinkage mode (C region). Results have shown that bimodal size distribution forms for condensation in both W region and C region. For condensation with high concentration of NCG, transparent superhydrophobic surface (TSHS) and hydrophobic surface (THS) were applied for visualization of the dynamic behaviors of condensate droplets in dew point dropwise condensation process. Compared with THS surface, radii of most droplets on TSHS surface lie in10～15μm, and no bimodal size distribution forms.Repeated condensation experiments of pure steam and steam-air mixture vapor were carried out on Cu-1, Cu-2, HS, SHS-1and SHS-2condensing surfaces by the vertical tube and vertical plate equipment. Condensing surface between the condensate and the roughness-induced Cu-2, SHS-1or SHS-2surface is typically a composite solid-condensate-liquid interface during the pure steam condensation process, which results in a decrease in the liquid-solid surface free energy difference between the condensate and the condensing surface, and then decrease in the heat transfer performance of DWC on superhydrophobic surfaces. And the retardation of micro-nanostructure to the condensate also increases the thermal resistance. The contact angle hysteresis increases resulting in condensate dragging. Not similar to the situation of pure steam condensation, the heat transfer performance of DWC condensation in the presence of NCG on roughness-induced SHS-1or SHS-2surfaces is close to that on smooth HS surface at the experimental range. The wetting mode of condensate droplets in DWC with and without NCG can explain well the heat transfer performance of the SHS condensing surfaces.