Effect Of High-frequency Vibrating Wall Surface On Heat And Mass Transfer Characteristics Of Nano Droplets

Droplet impact on high temperature surface widely exists in energy,metallurgy,spraying,cooling and other fields.When the temperature of the wall is too high,a stable vapor film will be generated on the wall after the droplet hits the high temperature wall with rapid evaporation,and the film boiling state will be achieved.The effect of film boiling under different conditions has both advantages and disadvantages.Therefore,it is necessary to accurately regulate the dynamic behavior of the droplet.As a simple and effective active regulation method,the application of ultra-high frequency vibration(GHz)to the wall surface can realize the precise regulation of droplets,and has the advantages of high frequency and small amplitude,which will not affect the safe operation of the equipment.In this paper,the dynamic behavior of micro and nano droplets after impact on ultrahigh frequency vibrating high temperature wall is studied by molecular dynamics method.The research content is as follows:In this paper,the impact of droplet on the high-temperature smooth surface was studied first.Different wall temperatures,different droplet impact velocities,and different hydrophilic and hydrophobic wall surfaces were set respectively to study the influence of the above parameters on the heat and mass transfer process of droplet.Within the range of parameters set in this paper,two evaporative and boiling modes,namely contact boiling and film boiling,will be produced after droplets collide with high temperature hydrophilic surface,and with the increase of wall temperature and droplet impact velocity,film boiling is more likely to occur,while only contact boiling occurs on hydrophobic surface within the range of parameters set in this paper.Then,ultra-high frequency vibration is applied to the wall surface.The results show that with the increase of vibration frequency and amplitude,the hydrophilic and hydrophobic surface can promote the droplets to reach film boiling,and the time from droplet impact to film boiling is shorter than that on the static wall surface.The higher the vibration frequency,the larger the amplitude,and the more obvious this law is.However,when the frequency and amplitude increase further,the droplet will reach the state of film state-popping and film state broken-popping after hitting the wall.Under the same vibration condition,the hydrophobic surface is more prone to film state-popping or film state broken-popping.Secondly,wall surfaces with different morphologies,roughness factors and phase area fractions were constructed to compare the dynamics of droplet under different roughness walls.The results showed that,when the wall surface was stationary,the droplet impaction on the high temperature rough wall surface with different morphologies(900 K),film boiling would all occur,and it took the shortest time for film boiling to occur on the triangular wall.It takes a long time for film boiling to occur on matrix wall,but film boiling does not occur on smooth wall under the same initial conditions,indicating that rough wall can promote film boiling of droplets.After the droplet impinges on the palisade wall with different roughness factors,film boiling occurs,and the time required for film boiling to occur after the droplet impinges on the wall fluctuates.After the droplet impinges on the palisade wall with different phase area fractions,film boiling occurs,and the time required for film boiling is basically the same.Then,ultrahigh frequency vibration is applied to the wall surfaces with different morphologies.In a certain range,the time required for film boiling after the droplet impinges on the wall is shorter than that under the static wall,and the time required for film boiling on the matrix wall is reduced more.When the vibration condition is further increased,the droplet impinges on the rough wall,and then the film state-popping or film state breaking-popping will occur.