Study On Thermal Protection And Drag Reduction Of Hydrophobic Surface

Superhydrophobic surface is the surface with a contact angle larger than150degrees. It received a growing interest for its excellent properties. With the development of the manufacturing process, researchers have proposed many new methods for fabricating superhydrophobic surfaces, which provide many possible applications of superhydrophobic surfaces.The drag force due to the relative motion between the solid and the fluid is composed of two parts, frictional drag force and shape drag force. For a long time, fluid dynamics studies have relied upon the assumption that the liquid molecules are stationary relative to its adjacent solid, and no-slip boundary condition is used to calculate the drag force between Newtonian liquids and solid surface. But when water flows over a superhydrophobic surface, the slippage between water and solid surface will reduce the drag force effectively. At the same time, due to the low thermal conductivity of air, heat transfer efficiency of the whole structure can be reduced by the air layer covering the rough superhydrophobic surface. In this way the superhydrophobic surface can decrease the thermal stress and improve the reliability of the structure that suffers thermal shock. It is obvious that the key for these excellent characteristics of superhydrophobic material is the air layer existing within the surface rough micro-structure. As a result, it is necessary to study how to keep the air film stable on the solid surface.There are four chapters in this dissertation. The research work mainly covers the following aspects:In chapter1, theory and models of superhydrophobic surface are introduced, and boundary slip in Newtonian liquids and heat transfer are introduced briefly as well.In chapter2, the author designs and fabricates a new copper compound structure, which is hydrophobia The thermal shock resistance of this compound structure is discussed based on experiment when it is flushed by hot water. The temperature increase of the new compound structure is slower than that of the copper structure with smooth surface. Thus, this compound structure is effective to decrease thermal stress and avoid fatigue failure.In chapter3, the storage conditions of air in the dead-end tubular structure will be discussed. With different water pressures, the airtightness of the capillary is compared in both static and mechanical vibration conditions in order to find the critical diameter of the capillary to allow water intrusion. This is taken as the reference for the micro-structure design. In chapter4, based on the excellent surface force-support capability of superhydrophobic material, the author designed a boat with superhydrophobic grille structure. It enables the boat to float on the water surface, which reduces the direct contact area between the solid surface and water. Therefore, the friction drag force between the water and boat bottom can be decreased. Moreover, the grille structure can also reduce the shape drag force. Compared with flat boat made of hydrophilic material, the drag force of miniature boat with superhydrophobic grille is decreased significantly.