Research On The Dynamic And Thermodynamic Characteristics Of Droplet Impacting On Wall Under The Phase Interface Action

Droplet impingement on the wall is a widespread phenomenon in the industry,which has different requirements for the droplet dynamic characteristics in different engineering applications.Droplet impingement on the wall is a complex process of multi-field coupling,involving two-phase flow,heat transfer and interaction between the droplet and the wall,with numerous influencing factors.Therefore,studying the dynamic and thermodynamic characteristics of droplet impact on the wall and its internal evolution mechanism is significant.In this paper,based on theoretical analysis,a combination of experimental observations and numerical simulations is used to systematically study the dynamic characteristics of droplet impact on the wall for the differences between non-metal and metal droplets.The results and conclusions are briefly given as follows:(1)Achieved the visualized experimental study of droplet impact on wall.The whole impact process is divided into four stages according to the dynamic characteristics of non-metal droplets on the wall,namely:motion stage,spreading stage,relaxation retraction stage,and equilibrium stage.Differing from non-metal droplets,metal droplets have a relatively simple dynamic characteristics due to their own solidification characteristics,which can be divided into three stages of the wall impact process:motion stage,spreading stage,and equilibrium stage.The initial diameter and impact velocity are important parameters affecting the droplet impact characteristics.Experiments show that the spreading of droplets increases with the increase of initial diameter and impact velocity.Furthermore,by comparing the experimental values of the maximum spreading rate of droplets in the range 0<We<100 with those obtained by Tushar Srivastava et al.,the results were found to be very similar,which also verifies the accuracy of the experimental results in this paper.(2)Based on the principle of energy conservation,the process of droplet impact on the wall is analyzed,and the kinetic energy,surface energy,and gravitational potential energy of the droplet before impact and the deformation energy,adhesion energy,viscous dissipation and gravitational potential energy of the droplet at the maximum spreading extent are analyzed,and the functional equations of spreading ratio and Reynolds number,Weber number and other dimensionless parameters are established to predict the maximum spreading diameter of the droplet impact on the wall,i.e.,the theoretical model of maximum spreading ratio prediction.Comparing the predicted values obtained under the same conditions with the experimental values,as well as the predicted values of the Chandra model,the predicted values of the model proposed in this paper are consistent with the experimental values and have good accuracy.In addition,we can derive two limiting solutions for the surface tension and viscosity regions,which are ξmax ∝ We 0.5 and ξmax ∝ Re0.2.(3)By defining the metal droplet density as a correlation function of temperature,the effect of thermal shrinkage on the spreading morphology,heat transfer characteristics,and energy evolution of metal droplets is explored,and it is shown that thermal shrinkage has an important effect on the spreading solidification of metal droplets,mainly in the following ways:considering that metal droplets under thermal shrinkage conditions will have voids after impacting the wall,which hinders the spreading flow,heat transfer efficiency,and leads to various defects,such as voids,porosity,and discontinuities.Therefore,thermal shrinkage is a non-negligible influencing factor in the numerical research on the process of droplet impingement on the wall,especially for metal droplets.(4)The spreading characteristics of droplets on the wall can be measured by the spreading ratio and spreading time.By studying the initial diameter,impact velocity,and wettability on the droplet impact characteristics,it is found that the maximum spreading ratio of droplets increases with the increase of initial diameter,impact velocity,and the decrease of contact angle.The spreading time of droplets decreases with the decrease of initial diameter,the increase of impact velocity and contact angle.The voids inside the droplet gradually decrease with the increase of impact velocity and contact angle,which provides a new approach to reducing the effect of thermal shrinkage on the formation of voids inside the droplet.(5)Combined with the numerical simulation analysis of non-metal droplet and metal droplet impact process,the regression analysis of the maximum spreading ratio of droplets with Weber number and Reynolds number corresponding to the variation law.Empirical equations for Weber number and Reynolds number for non-metal and metal droplets were obtained to predict the spreading of droplets on the wall.