Numerical Research On The Freezing Process Of Supercooled Saline Water Droplets Impacting On The Cold Wall

Ships and offshore structures often face the icing problem during navigation or operation in the polar low temperature environment.The large amount of ice not only affects the stability of the ship hull and the reliability of deck equipment,but also poses a great threat to the safety of personnel operations,directly hindering the rational development and utilization of the polar region.For the icing of naval architecture and ocean structures,scholars at home and abroad have carried out a lot of research about such macro scale aspects as the sea spray and its movement process formed by the collision between waves and ships,and the ice accretion prediction,while less research has been carried out on the influence of the movement and freezing process of the supercooled saltwater droplet on the macroscopic icing.In view of this,the icing problem on the surface of typical constructional element of ships and offshore structures superstructures is taken as the engineering background in this paper.The tracking of gas-liquid and solid-liquid interfaces is regarded as the core,and the research is mainly carried out by numerical simulation supplemented by the experimental method.On the basis of verifying the accuracy of the model,around the dynamic and thermodynamic behaviors in the movement and freezing process of the supercooled salt water droplets,the ice crystal growth and main freezing process of the supercooled water droplet,the movement and freezing process of the water droplets impacting the horizontal and vertical cold surfaces,the freezing process of multiple water droplets impacting the vertical isolated cold plate and the seawater solidification process in the two-dimensional cavity are simulated,and the icing mechanism and influencing factors during each process are analyzed.In this paper,firstly,around the freezing process of supercooled saltwater droplets impacting the cold wall,literature researches are carried out on the numerical simulation methods of the ice crystal growth inside the supercooled water droplet,the supercooled water droplet main freezing,the single water droplet impacting and freezing on the cold wall and saline water film icing.Then,the research status of the above numerical researches at home and abroad is introduced.The contents that still need to be improved in the current research are summarized.The review of the research progress can lay a foundation for this study.To solve the problem of ice crystal growth in the nucleation and regeneration stage of supercooled water droplet freezing,the simulation of the pure water ice crystal growth process is realized based on the Kobayashi and Wheeler phase field models.The simulation results are basically consistent with the existing laboratory observation results.On this basis,the Wheeler model is used to combine the phase field parameters with the real physical field,and the solute field equation is added to the model to realize the growth of sea ice crystals.The influences of factors such as the latent heat constant and concentration on the simulation results are analyzed and discussed.In view of the phenomenon of solid-liquid interface movement during the main freezing process of the supercooled water droplet,the freezing model of the standing water droplet in the air is built based on the VOSET gas-liquid interface tracking method and the EnthalpyPorosity method,the freezing process simulation of the standing water droplet on the cold wall is realized,and the accuracy of the model is verified through the calculation example of metal gallium melting process.In order to reflect gas-liquid-solid three-phase simultaneously,the volume fraction of the VOSET method and the liquid fraction of the Enthalpy-Porosity model are unified by introducing mixing fraction.The temperature evolutions inside the water droplet during freezing process are discussed,and the influences of the contact angle,supercooling,air convection and other factors on the freezing process are analyzed.At the same time,the freezing experiment of the standing water droplet is carried out,and the observed experimental phenomena are compared with the numerical results.Aiming at the dynamic and thermodynamic behaviors of the water droplet impacting the cold surface,such as spreading,retracting,sticking,freezing and springing off,a twodimensional axisymmetric motion-freezing model is built based on the static water droplet freezing model,and the simulations of the water droplet motion-freezing process are realized.Firstly,the process of the water droplet at room temperature impacting the horizontal wall is simulated,and the hysteresis phenomenon during the movement process of the water droplet is considered.The real-time calculation of three-phase contact line velocity is realized,which provides input parameter for the application of the dynamic contact angle boundary condition.Secondly,the freezing processes of the pure water droplet and the saltwater droplet impacting the horizontal cold wall are simulated,and the effects of the contact angle,impact velocity,salinity and other factors on the maximum spreading factor and stable spreading factor of the water droplets are analyzed.At the same time,the process simulation of the single water droplet impacting the vertical cold wall is carried out to lay the foundation for the subsequent multiple water droplets impacting the vertical isolated cold plate.For the phenomenon that many water droplets form a liquid film after impacting the cold surface accompanied by ice accretion,a freezing model of multiple water droplets impacting an isolated cold plate is established on the basis of the previous fluid-solid single-phase coupling model.First,the process of a single water droplet impacting a horizontal isolated cold plate is simulated,and the accuracy is verified by comparing with the relevant experimental results.Then,the process of a single water droplet impacting a vertical isolated cold plate is simulated,and the phenomenon of air entrainment is reproduced.Four forms of the water droplet impacting the wall are sorted out,and some typical shape distribution maps are given.At the same time,the process of forming liquid film and freezing when multiple water droplets impacting the rectangular cold plate is simulated,and the effects of the contact angle,the initial velocity and the release interval are analyzed.In view of the brine channel phenomenon formed in the sea water freezing process,the problem is simplified to a binary solution solidification scene in a two-dimensional square cavity.Based on the previous binary alloy solidification model and the pure matter freezing model in this paper,the binary solution lateral solidification model is built.The concentration control equation is introduced into the model,and the latent heat update mode in the original pure substance freezing model is modified to reflect the micro factors such as solute backdiffusion.The model is used to simulate the solidification process of three groups of ammonium chloride aqueous solutions with different solute concentrations,and the simulation results are compared with the previous numerical and experimental results.On the basis of verifying the accuracy of the model,the solidification process of sodium chloride aqueous solution is simulated,and the causes and influencing factors of the brine channel are analyzed,which provided a reference for the prediction of sea water icing.