Study On Hydrodynamics And Heat Transfer Of Droplets Impacting On Wall

The droplet impact phenomenon is widely present in industrial and agricultural applications,such as inkjet printing,spray cooling,and pesticide spraying.The complex application environment makes the droplet impact phenomenon show various changes.Therefore,studying the process of droplets impacting the wall can not only improve the gas-liquid two-phase flow characteristics and heat transfer theory system of the free interface,but also provide an important theoretical basis for the research of practical engineering applications.Firstly,High-speed camera is used to investigation the impact of droplets on the wall,and the impact velocity,wall contact angle,glycerin solution and surfactant concentration on the impact process are discussed.The experimental results show that the droplet impacts the wall through four stages: before impact,spread,rebound and oscillation stabilization;the droplet spreading width increases with the rise of the impact velocity as well as the surfactant concentration,and decreases with the rise of glycerin solution concentration and contact angle.The droplet rebound height increases with the rise of the wall contact angle,and decreases with the rise of the concentration of the glycerin solution and the surfactant.In particular,it needs to be pointed out that the rebound height decreases with the increase of velocity only in the first cycle.Secondly,the coupled level set and volume of fluid method(CLSVOF)was employed to simulate and study the droplet collision process,and the impact of the above experimental factors and the droplet diameter on the impact process is deeply investigated.In the simulation conclusion,the process of droplets impacting the wall also goes through the above four stages,but in the stable stage of oscillation,the spreading width amplitude is more obvious.The spreading width of the droplet and its oscillation period increase with the rise of diameter and surfactant concentration,and decrease with the rise of glycerol concentration and contact angle,and varies with velocity only in the spreading stage with obvious regularity.Moreover,the effect of the increase in contact angle and glycerin concentration on the rebound height is consistent with the experimental results,and the droplet diameter increases and the rebound height increases significantly,while increasing the surfactant concentration only increases the rebound oscillation period.Finally,a prediction equation of the maximum spreading coefficient is proposed,and the predicted value coincides with the simulation results.Finally,the effects of droplet initial velocity,diameter,viscosity,surface tension,wall temperature and contact angle on the liquid-wall heat transfer characteristics were further explored in the simulation of droplet impact on the hot wall using the CLSVOF method.As a result,the varies of the local heat flux and the average heat flux of the wall are mainly affected by the droplet spreading range and the liquid-wall temperature difference.On the one hand,rising the temperature of the wall will increase the liquidwall thermal differential,and increase the local and average heat flux on the wall.On the other hand,increasing the droplet velocity,diameter,and surfactant concentration can improve the heat exchange area,resulting in increased local and average heat flux to the wall;but rising the droplet viscosity and contact angle will reduce the heat exchange area,thus suppress the change of wall heat flux.