| In recent years,solid-liquid interface drag reduction has become a hot research topic.This technology has broad application prospects in underwater navigation,fluid transportation and microfluidic devices.Fluoride is usually used to modify the surface of solid-liquid interface to decrease the surface free energy,but its price is high,so there's a high cost.And also,it is easy to hydrolyze and fail in the preparation process,and it's difficult to degrade,which will damage the ecological environment.Drawing inspiration from bionics,the efficient drag reduction performance of the solid-liquid interface can be realized,and the effect of reducing the friction force of the interface between the two phases can be realized by constructing a special surface morphology through physical and chemical preparation methods.Recently,the research on drag reduction by using hydrophobic property has gradually attracted the attention of many scientific researchers.Drag reduction by superhydrophobic surface is a method of reducing the drag using the contact angle of water on the solid surface greater than150°.Its drag reduction effect is good,but its theoretical research is not perfect.Metal is usually used as the basic material of liquid transportation pipelines in the engineering field,which makes the study of surface friction resistance characteristics of various metals and their alloys especially important.In this dissertation,the superhydrophobic drag reduction at solid-liquid interface on metal surface was studied theoretically and experimentally,including building simulation model based on triangular microstructure to analyze the drag reduction law of solid-liquid interface;The wetting and drag reduction characteristics of different superhydrophobic metal substrate surfaces obtained by a series of preparation methods were studied experimentally.The specific research contents are as follows:(1)The solid-liquid interface drag reduction model of the triangular microstructure is designed,and the effects of the degree,depth and spacing of the vertex angle on the drag reduction are analyzed.The results show that the vertex angle of the triangle microstructure affects the velocity gradient near the wall,the slip length and the solid-liquid interface friction resistance,and the smaller the vertex angle of the triangle is,the better the drag reduction characteristic is.Superhydrophobic surface with triangular microstructure is prepared experimentally to test the drag reduction characteristics.It is used to verify the theoretical results of drag reduction with the triangular microstructure,and measure the frictional resistance between superhydrophobic surface and water at different triangle vertex angles.The experimental results show that it is consistent with the influence law of triangle microstructure vertex angle and solid-liquid friction resistance in the simulation,and the error between the measured average frictional resistance and the simulation results is less than 10%.(2)Superhydrophobic surface is prepared on the surface of the copper substrate by a one-step electrodeposition method and characterized the micro-morphology,through which we obtain that the static contact angle of the surface is 158.8±0.2°,and the rolling angle is 1°.It can be seen that the super-hydrophobic surface microstructure is formed on the surface of the copper substrate.The drag reduction tests at the solid-liquid interface show that the surface has a drag reduction of approximately 22.5%at a velocity of 1.78 m/s.As the metallic copper has higher surface free energy,the surface prepared by this method is obviously different from the super-hydrophobic surface prepared by two steps in the conventional method.It simplifies the traditional preparation process,and improves the preparation efficiency and the corrosion resistance.(3)Fluorine-free super-hydrophobic surface of the aluminum alloy substrate is made by a chemical corrosion method,and the whole process is simple,green,and non-toxic.Three kinds of natural hydrophobic substances,namely,rosin,paraffin and carbon black,are used to replace the traditional fluoride to prepare the superhydrophobic surface.It was found through the solid-liquid interface drag reduction test that the superhydrophobic surface of aluminum alloy modified by carbon black coating has the best hydrophobicity,the rolling angle is as low as1.38°and the contact angle is 155°.When the inflow velocity is 1.78 m/s,its drag reduction rate reaches 22.2%.The corrosion voltage is-0.379 V and the current density is 6.96×10-6 A·cm-2,when it's tested in simulated seawater,which indicates that its corrosion resistance is good.(4)A method for preparing aluminum-base superhydrophobic surface is proposed by the method of an organic-inorganic composite coating combined with an anodic oxidation.Wherein low-cost raw materials are adopted to reduce the dosage of a fluorinating agent,oxidation time and electrolyte concentration parameters in the anodic oxidation method are regulated,the composite coating is used for enhancing the bonding force with a micro-structured metal substrate,thus enhancing the mechanical stability of the surface.Optimal preparation parameters are obtained by using the matrix analysis method,and a superhydrophobic surface with a static contact angle of 160°and a dynamic rolling angle of about 1°is obtained.The results of superhydrophobic drag reduction characteristic show that the superhydrophobic surface has more excellent super-hydrophobic and drag reduction characteristic,compared with the superhydrophobic surface of copper substrate by one-step electrodeposition method and that of aluminum alloy substrate by chemical corrosion. |
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