Evaporation Characteristics and Morphological Evolutions of Fuel Droplets After Hitting Different Wettability Surfaces

To solve the wall-wetting problem in internal combustion engines,the physical and chemical etching methods are used to prepare different wettability surfaces with various microstructures.The evaporation characteristics and morphological evolution processes of diesel and n-butanol droplets after hitting the various surfaces are investigated.The results show that the surface microstructures increase the surface roughness(Ra),enhancing the oleophilic property of the oleophilic surfaces.Compared with n-butanol droplets,the same surface shows stronger oleophobicity to diesel droplets.When a droplet hits an oleophilic property surface with a lower temperature,the stronger the oleophilicity,the shorter the evaporation time.For oleophilic surfaces,larger Ra leads to a higher Leidenfrost temperature(TLeid).The low TLeid caused by enhanced oleophobicity,dense microstructures and increased convex dome height facilitates droplet rebound and promotes the evaporation of the wall-impinging droplets into the cylinder.The evaporation rate of the droplets is not only related to the characteristics of the solid surfaces and the fuel droplets but also affected by the heat transfer rate to the droplets in different boiling regimes.The spreading diameter of a droplet on an oleophobic surface varies significantly less with time than that on an oleophilic surface under the same surface temperature.

Morphological Development of Fuel Droplets after Impacting Biomimetic Structured Surfaces with Different Temperatures

To improve the controllability for the evaporation process of fuel spray impinging on the cylinder wall,an experimental study on the development of morphological process of different fuel droplets on aluminium alloy surfaces is carried out.The metal surfaces with different wettability are prepared by laser etching and chemical etching for the experiments.In total,three different fuels are tested and compared under different surface temperatures,including diesel,n-butanol and dimethyl carbonate(DMC).The results show that under a lower wall temperature,the surface wettability,viscosity and surface tension of the fuels have significant effects on spreading and rebounding behaviour of the droplets.As the wall temperature rises over the boiling points of the fuel but below its Leidenfrost temperature,the contact angles between the fuels and surfaces are varying according to the surface wettability,boiling point and Leidenfrost temperature of the fuels.When the temperature of the surface exceeds the Leidenfrost temperature of all the fuels,after impacting the surfaces,different fuel droplets tend to have the same development pattern,regardless of the surface wettability.The rebound level is mainly affected by the amount of fuel vapour generated during the wall-hitting process.Viscosity,surface tension and other properties of the fuel have little effect on post-impacting behaviour of the droplet when the wall temperature is higher than the Leidenfrost temperature of the fuel.