Research On Freezing And Impact Processes Of Supercooled Water Droplet And Their Coupling Characteristics

Impact freezing of supercooled water droplet widely exists in many engineering fields such as aerospace,power,communication,meteorology,and cryogenic refrigeration.Since the supercooled water is metastable,its freezing process is significantly different from the phase change process at a steady and equilibrium state.The macroscopic icing/frosting in real production and life is actually a process that a single supercooled water droplet begins to freeze after interacting with a cold wall and gradually forms an ice layer.In order to reduce the damage caused by icing,studies on the freezing delay and rate as well as ice accretion are done in this research.They mainly focused on the freezing of sessile water droplet,impact dynamic of water droplet,impact freezing of supercooled water droplet and the macroscopic icing model considering variable-properties rime ice.The purposes of this work are to clarify the coupling mechanism of freezing and impact of supercooled water droplet and to help better understand the heat transfer and flow problems in the impact freezing process of supercooled water droplet,providing theoretical basis and technical reference for improving the icing/anti-icing/de-icing applications in engineering.Freezing and nucleation experiments are conducted on sessile supercooled water droplets.The temperature transitions,volume and profile evolutions of water droplets in the freezing and melting process are obtained.The statistical characteristics of nucleation temperature and rate of supercooled water droplets are analyzed and the volume and time effects on nucleation temperature and rate are quantitatively studied.The results show that the nucleation temperature approximately satisfies a normal distribution,the nucleation temperature goes down with increasing droplet volume,the nucleation rate increases as the volume increases and time goes by.Besides,an improved theoretical model considering the supercooling effect is developed,it can more accurately predict the freezing time,the evolution of droplet profile as well as the final freezing tip.A simulation model is also established to calculate the evolution of the freezing front and the temperature distribution in the droplet.The influences of surface temperature,contact angle and droplet volume on the final freezing time are investigated and a correlation for calculating the freezing time is proposed,it has a deviation of ±25%.The effects of Weber number and contact angle on the spreading and height factors are investigated experimentally.Using the Kistler dynamic contact angle,a simulation model is established to calculate the impact process of an ellipsoidal water droplet.The effect of initial shape on the impact dynamics of an ellipsoidal water droplet is studied and the results show that a larger aspect ratio yields a larger maximum spreading factor.The aspect ratio is used to modify the viscous dissipation term,and an improved correlation for the maximum spreading factor are obatianed,with a deviaton of-5% ? 25%.The impact freezing experiment on a supercooled droplet is conducted and the supercooled effect on the impact freezing process is analyzed.A simlution model is devoleped to calculate the impact freezing process of a supercooled water droplet.The droplet behaviors in the impact freezing process are divided into three patterns,including complete rebound,partial rebound and complete adhesion,which actually reflects the coupling mechanism of freezing and impact.A pattern map is proposed to describe the effects of Weber number,conctanct angle and supercooling degree on the impact freezing process,and the condition for complete rebound is obtained.After an analysis of the relationship between the impact freezing process of a single microscopic supercooled water droplet and the macroscopic ice accretion process,a macroscopic icing model considering the variable-property rime ice is developed.The effects of the airflow parameters on the ice accretion characteristics yielded by different models are compared and some suggestions are given for the model selection for different airflow parameters.