Research On Morphological Development Processes And Evaporation Characteristics Of Wall-impinging Fuels Based On Wall Wettability

With the increase of vehicle ownership,the problems of energy shortage and environmental pollution are becoming more and more serious.Fuel consumption regulations and emission regulations for internal combustion engines are becoming more and more stringent.Higher fuel economy and lower emission levels are the motivations for the development of vehicle engines.New combustion modes such as homogeneous charge compression ignition,premixed charge compression ignition,reaction controlled compression ignition and low temperature combustion can effectively improve the performance of internal combustion engines and reduce emissions,and have good development prospects.However,in the new combustion modes for internal combustion engines,due to the limitation of the working modes of internal combustion engines,the wall-wetting phenomenon of the fuel sprays is inevitable.Wall wetting has great effects on the combustion boundary controllability and combustion stability of homogeneous charge compression ignition engines and premixed charge compression ignition engines.The formation and vaporization of the wall-attached fuel films after spray-wall impingement are the main factors that affects the emissions of unburn hydrocarbon（HC）and CO in internal combustion engines.Therefore,it is very important to actively control the spreading,rebound,vaporization and other behaviors of the fuel droplets after hitting the walls according to the fluid-solid coupling processes between the fuel droplets and the combustion chamber walls.Aiming at the wall-wetting problem of the fuels in internal combustion engines,in this study,the surface wettability was regarded as a controllable boundary.The surface construction platforms were built based on the methods of laser etching,chemical etching and low surface free energy modification.The surfaces with different microstructures and wettability were prepared.A platform for analyzing the microscopic morphology of the surfaces was built,which can record the microscopic morphologies of the surfaces obtained by different preparation methods.A platform for testing the wettability of the surfaces was built to realize the determination of the wetting characteristics of different fuel droplets on various surfaces.The fluid-solid coupling processes between the wall-impinging fuel droplets and the walls were explored with the high-speed photography test platform,the laser-induced fluorescence（LIF）test platform,the schlieren test platform and the molecular dynamics（MD）simulation platform.The effect laws of wall property on the processes of spreading,rebound and evaporation of the wall-impinging fuels under different boundary conditions were revealed,and the active control strategies for the wall-wetting problem of the fuels in internal combustion engines were proposed.The main research contents and conclusions of this paper are as follows:（1）Based on a platform for analyzing the microscopic morphology of the surfaces,the microscopic morphologies of different surfaces were recorded.It is found that the micron-scale convex domes and grooves and the nano-scale protrusions and cluster-like structures are formed upon the surfaces after laser etching.The micron-scale bosses and pits and the nano-scale needle-like structures are formed upon the surfaces after chemical etching.The effects of surface property and droplet characteristic on the wetting abilities of the fuel droplets were explored based on a platform for testing the wettability of the surfaces.It is found that increasing the roughness of the oleophilic surfaces can enhance the oleophilicity of the surfaces.The contact angles of diesel and n-butanol droplets after contacting the oleophilic surfaces prepared by the methods of laser etching and chemical etching rapidly reduce to 2°.The surface free energy of the walls can be reduced through low surface free energy modification,so the surface oleophobicity is significantly enhanced.Among all the experimental surfaces,the contact angles of diesel and n-butanol droplets are maximized on the laser-etched surface LF1 modified with low surface free energy,which are 161.8°and 127.3°,respectively.The contact angle of a diesel droplet is higher than that of a n-butanol droplet on the same experimental surface.（2）The effects of surface property and droplet characteristic on the Leidenfrost temperature of the droplets were studied experimentally.It is found that for the oleophilic surfaces with sparser microstructures,the Leidenfrost temperature of the droplets rises with increasing surface roughness.Compared with the oleophilic surfaces,the oleophobic surfaces facilitate the formation of vapor layer,resulting in a lower Leidenfrost temperature of the droplets.The surfaces with denser microstructures can reduce the Leidenfrost temperature of the droplets.The Leidenfrost temperature of a diesel droplet is higher than that of a n-butanol droplet on the same experimental surface.It is pointed out that when the thermal atmosphere in the engine cylinder is poor,enhancing the oleophobicity of the surfaces where the fuels hit is beneficial to the rebound of the fuel droplets and can reduce the Leidenfrost temperature of the droplets,which is conducive to the evaporation of the droplets in the gas-phase space of the cylinder.When the thermal atmosphere in the engine cylinder is good,enhancing the oleophilicity of the surfaces where the fuels hit is conducive to the spreading and evaporation of the droplets,which improves the atomization quality.The influence of surface property on the evaporation processes of the wall-attached fuel films was investigated based on the LIF test platform.It is found that enhancing the surface oleophilicity is conducive to increasing the spreading rates of the fuel films,which can promote the evaporation of the fuel films.（3）Aiming at the morphological development processes of the wall-impinging fuel droplets,the liquid phase development processes of diesel droplets after hitting different heated walls were studied based on the high-speed photography test platform.It is found that as the surface oleophobicity increases,the spreading factor of a droplet decreases,and the rebound factor increases.When the wall temperature is 423 K,the maximum rebound factors of diesel droplets after hitting the surfaces SN,LF1 and CF are 0.56,1.18 and 0.82,respectively.Based on the schlieren test platform,the vapor phase diffusion processes of diesel droplets hitting different heated walls were further studied.It is found that when the surface temperature is low,the gas phase diffusion areas of the wall-impinging droplets are related to the spreading areas of the droplets.For oleophilic surfaces,increasing the surface roughness can increase the evaporation rates of the droplets.Compared with the oleophilic surfaces,the heat exchange capacity between the oleophobic surfaces modified with low surface free energy and the droplets is weakened,resulting in a decrease in the evaporation rate of the droplets.For the droplets in different regimes,when the droplets are in the transition boiling regime,the heat transfer rate between the heated walls and the droplets is higher,which makes the diffusion width,diffusion height and diffusion area of the vapor phase of the fuel droplets hitting the walls increase significantly.（4）The effects of solid-liquid interaction coefficient and surface microstructure on the static wetting behaviors of n-hexadecane droplets were studied from the microscopic scale based on the molecular dynamics simulation platform.Four solid-liquid interaction coefficients ofε₁,ε₂,ε₃ andε₄ are selected,and the intrinsic contact angles of n-hexadecane droplets on the smooth surfaces under the above four solid-liquid interaction coefficient conditions are 0°,60.02°,93.17°and 120.10°,respectively,which meant that the smooth surfaces are superoleophilic,oleophilic,neutral and superoleophobic,respectively.It is found that the structures of the rough surfaces have an inhibitory effect on the spreading of the droplets compared with the smooth surfaces.As a result,the apparent contact angles of the droplets on the surfaces with different microstructures are larger than their intrinsic contact angles on the smooth surfaces.As the solid-liquid interaction coefficient decreases,the wetting state can change from the Wenzel state to the Cassie state.Also,the difficulty of the droplets reaching the Cassie state is affected by the proportion of the solid-liquid contact surface in the composite contact surface.When the solid-liquid interaction coefficient isε₂,the apparent contact angle of a n-hexadecane droplet on the secondary boss-shaped surface is 51.73°higher than the intrinsic contact angle on the smooth surface,that is,the secondary boss-shaped structures can significantly enhance the surface oleophobicity.By applying an external force to a droplet in an equilibrium state,the effects of solid-liquid interaction coefficient and surface microstructure on the dynamic wetting behaviors of the droplets were further studied.Under the action of an external force,the droplet moves on the surface in a rolling manner rather than in a translational manner.At the solid-liquid interaction coefficients ofε₂,ε₃ andε₄,the values of the contact angle hysteresis of n-hexadecane droplets on the secondary boss-shaped surfaces reduce by 7.79°,17.27°and 11.73°,respectively,compared with those on the smooth surfaces.With the decrease of the solid-liquid interaction coefficient,the pinning effect of the wall on the droplet is weakened,so the contact angle hysteresis decreases.（5）The effects of surface temperature and rough structure on the behavior developments of n-hexadecane droplets under different solid-liquid interaction coefficients were studied based on the molecular dynamics simulation platform.It is found that the shape changes of the droplets are complicated during the evaporation processes.On the surface with the same temperature,as the solid-liquid interaction coefficient increases,the thermal resistance of the solid-liquid interface decreases,which is conducive to the transfer of heat from the heated surface to the droplet.As a result,the number of the droplet molecules that change from the liquid phase to the vapor phase increases,and the evaporation rate of the droplet rises.When the solid-liquid interaction coefficients areε₁ andε₂,as the surface temperature increases,the total energy and temperature of the droplet molecules on the heated surfaces increase,the interaction between the droplet molecules and wall atoms is weakened,and the mean square displacement of the droplet molecules increases.As the solid-liquid interaction coefficient decreases fromε₁ toε₂,ε₃ andε₄,the effects of surface temperature on the energy between the droplet molecules and the wall atoms and the mean square deviation of the droplet molecules are weakened.In addition,under the conditions of larger solid-liquid interaction coefficients ofε₁ andε₂,the total energy and temperature of the droplet molecules on the walls with different microstructures and the interaction between the droplet molecules and the wall atoms all increase first and then tend to be stable with the increase of time.When the solid-liquid interaction coefficient decreases fromε₂ toε₃ andε₄,the order of total energy and temperature of the droplet molecules on the walls with different microstructures is smooth surface>pit-shaped surface>grid-shaped surface>boss-shaped surface>secondary boss-shaped surface.