Experimental Study On The Dynamic Wetting Characteristics Of The Droplet During Impacting The Wall

When the nuclear reactor under accident scenario such as in a Loss of Coolant Accident(LOCA),whether in the spray cooling of the containment or in the dispersed flow film boiling(DFFB)during the core reflood,a complex heat transfer,such as convection,radiation and contact heat transfer,wi ll occur among dispersed droplets,superheated walls and steam.Among them,the direct contact heat transfer between the droplets and the superheated wall surface also plays a very important role in taking away the core’s residual heat or stabilizing the temperature and pressure of the containment vessel.However,the dynamic wetting of the droplet and the heat transfer process between the droplet and the wall during the droplet impacts the wall are very complicated,and there is no model that can describe this well.The complexity is that after the droplet impacts the wall,there will be a variety of different dynamic behaviors and heat transfer mechanisms,and under different working conditions,these mechanisms will change between each other;in addition,the mechanism of wetting and heat transfer near the three-phase contact line of the droplet is not clear.In the current reactor system program,the direct contact heat transfer between the droplet and the wall surface mainly depends on some empirical relationships based on macroscopic parameters such as the volume fraction of the two phases,the average diameter of the droplet,etc.,without considering the complex transient process of droplets impacting the wall.Therefore,it is necessary to carry out a fundamental research on the dynamic wetting behavior of droplets impacting the wall.The research in this paper is mainly divided into two parts.The first is to carry out dynamic wetting experiments of droplets under adiabatic conditions.Through image processing,quantitative extraction of the velocity of the triple-phase contact line and dynamic contact angle of the droplet.In the case of droplet deposition,the effects of different initial impact velocities,different surface microstructures,different surface wettability and wall inclination on the triple-phase contact line behavior of droplets were analyzed.Then,a model was established to describe the transient force of the droplet under adiabatic and inclined conditions.In addition,for the heating part,the dynamic behavior of the droplet is classified by the dimensionless Weber number and the wall temperature.Under deposition conditions,the effect of the dynamic contact angle to the boiling heat transfer during the droplet spreading and receding will be discussed,separately.Also,an analysis of the droplet breaking mechanism when the wall temperature is above the Leidenfrost temperature is made.The basic method of this study is that due to the complexity of the droplet process and the experimental conditions,it is difficult to directly measure its micro-physical process.Therefore,a high-speed cameras are used to capture the transient images of the dynamic behavior during droplet impact on the wall surface under different conditions.The effects of thermal factors,wall factors,gravity fields and inertial forces on the wetting and heat transfer of the droplets were analyzed.Finally,based on its macroscopic dynamic behavior,some mechanisms affecting the dynamic wetting behavior of the droplets and the heat transfer from the wall to the droplets can be deduced,and provide more physical evidence for building more accurate predictions and a wider range of models and correlations.