Study On Preparation Method Of Non-lipophilic Metal Surface And Development Process Of Fuel Droplets After Wall Collision

Due to internal combustion engine works and the limitation of its geometric structure,wall wetting phenomenon is inevitable,high-temperature cracking of the attached oil film,harmful pollutants such as incompletely burned HC compounds and CO are produced,so it is necessary to conduct an in-depth study on detailed fluid-solid coupling development process between the fuel droplets and the surface of the combustion chamber after wall wetting,to explore the possibility for conducting active control to the fuel spreading and evaporation process.This article relies on the National Natural Science Foundation of China?5167060406?,combined with the latest research progress in bionics,taking the wetting of metal materials of engine combustion chambers as the breakthrough point,different metal preparation methods are used to reconstruct the micromorphology of the of the experimental substrate surface,and the dynamic morphological development of fuel droplets on different surfaces is studied with the boundary conditions.The metal material preparation platform was built,and the micromorphology and surface free energy of brass,45 steel and aluminum alloy are reconstructed by chemical dissolution,electrochemical etching and laser etching.With the help of scanning electron microscope,the micromorphology of the prepared metal wall surface is observed and analyzed,The effect of different preparation methods on the contact angle of gasoline and diesel droplets at room temperature is studied by means of contact angle measuring instrument.The experimental results show that:On the surface of FeCl₃ oxidized 45#,multi-level rock-like structure appeared,after modification by low-surface energy materials,the maximum contact Angle of diesel droplets is 108.6°.FeCl3 oxidizes the surface of Cu,after being modified by low surface energy substances,the contact angle of gasoline and diesel droplets increases with the increase of etching time,but did not reach the oleophobic state.After two-step etching of the Al surface,a labyrinth structure of rectangular pit boss appears on the surface of the substrate,the pits and bosses are covered with nano-scale needle-shaped boehmite,after modification by low-surface energy materials,the contact Angle of diesel droplets reaches 148.8°.The surface of the electrochemical Cu forms a micro-nano-potato micromorphology,the contact Angle was up to 117.7°after being modified by a low-surface energy materials.The surface of electrochemical etching Al shows a micro-nano boss pit step structure,the etching time has little effect on the micromorphology of the metal surface and the contact angle of diesel droplets,after modification by low-surface energy materials,the maximum contact angle of diesel droplets is 150°.The surface of laser etching Al,a periodic and orderly convex hull groove matrix and nanocluster structure appeared,after modification by low-surface energy materials,the super lipophilic characteristics of gasoline cannot be changed,and the contact Angle of diesel droplets is 161.8°at most.A high-temperature test platform for droplets of gasoline and diesel was built.In this paper,the substrate surface with the maximum contact Angle of diesel droplets under different preparation methods is selected as the test surface,and the impact test of droplets of gasoline and diesel on the wall is carried out under different boundary conditions.The MATLAB image processing platform is built to study the changing trend of fuel spreading/retraction coefficient over time.The results show that:with the increase of the wall temperature,fuel droplets experienced four heat transfer forms:convection,nuclear boiling,transition boiling and film boiling.When the fuel droplets are in convective heat transfer and nuclear boiling state,the spreading/retraction coefficient is large,while in the convective heat transfer state,the spreading/retraction coefficient changes gently,and the nuclear,transition and film boiling states boil changes greatly.When the diesel droplets are in the convective heat transfer state,the better the oleophobicity of the wall surface,the more air layers intercepted by the microstructure of the substrate surface,the heat transfer coefficient between the droplet and the wall surface decreases,and the spread/retraction coefficient decreases.Laser etching Al surface has the best oleophobicity,from 323K to423K,the diesel droplets will bounce,and the number of bounces and the time of staying in the air both show a decreasing trend as the temperature increases.When the gasoline droplets are in the convection heat exchange state,due to the large temperature difference,the corresponding surface tension gradient is generated within the droplets.The droplets begin to retract under the drive of Marangoni effect,and the rebound height increases;The geometrical scale of the micromorphology of the laser etching Al surface is relatively large,and the thermal conductivity of the material of the oxidized 45#surface is poor,The Marangoni effect only delays the spreading rate of gasoline droplets.When the gasoline droplets impact the surface of aluminum alloy,it reaches the film boiling state at 423K.The oleophobicity of the wall reduces the starting point of the temperature of the film boiling of the gasoline droplet.As the wall temperature increases,the time of gasoline droplets in the air increases,the maximum spreading/retraction coefficient is basically stable at 1.5;From423K to 473K,the electrochemical Cu surface has the best oleophobicity,with a small heat transfer coefficient,the gasoline droplets adhere to the substrate surface and continue to evaporate without rebound.