| The impacting and freezing processes of droplets on cold surfaces are widespread in industrial production and daily life.Such as the raindrops impact on cold surfaces of aircraft,wind turbines,power transmission lines in low temperature environments.These impacts can cause ice to cover cold metal surfaces,which can affect the dynamics and functional properties of aircraft,turbines,power transmission lines and other equipment.Therefore,understanding the dynamics of droplets impact and freezing characteristics on cold surfaces is very important in the field of industrial production.At present,the impact-freezing process of a single droplet impacting a cold surface has been studied in depth by many researchers.However,the impactfreezing process of a droplet impacting a cold surface is often accompanied by the generation of multiple droplets.Therefore,studying the impact-freezing process between double-droplet helps us to understand the impact-freezing properties of multiple droplets on a cold surface.These findings may deepen our understanding of the mechanism of impact freezing on a cold surface,it provides reference for the associated applications and technologies in icing/antiicing.Thus,the main work of this paper is shown below:Firstly,a two-dimensional axisymmetric swirl model as well as a three-dimensional rectangular model were developed.After verification of grid-independence,Use ANSYS Fluent to investigate the impact-freezing process of a supercooled droplet impacting a cold superhydrophobic surface.The result were compared with the experiments of Zhang et al.to confirm the correctness and reliability of the model.Secondly,numerical simulations were carried out to study the impact-freezing process of double-droplet continuously and simultaneously impacting on cold surfaces with different wettability,and to obtain the morphology evolution,internal phase change,as well as the wetting spread factor and area ratio wetting factor in the two modes,and the result shows:For the double-droplet continuously impact on cold surfaces with different wettability,the droplet spacing has less influence on the hydrophilic surface at low impact velocities,as the contact angle increases,the influence of droplet spacing on the double-droplet gradually increases,the double droplet contact mode can be divided into spreading phase contact polymerization and retraction phase contact polymerization.At the same time,compared to the impact of a single droplet,the maximum spreading factor of continuously impacting double-droplet on a hydrophilic surface increases by 26-38% and 15-30% for spreading and contraction contacts,respectively.While,upon the superhydrophobic surface,the maximal spreading coefficients exhibit negligible disparity between single and double droplet impact,with variances below 2%.Additionally,three typical impact results were observed for continuous impacts of double droplet on a superhydrophobic cold surface under different velocity conditions: full rebound,partial rebound,and full adhesion.For the double-droplet simultaneously impacting cold surfaces with different wettability,in the early spreading stage,the double-droplet impact behaves in the same way as the singledroplet.At the contraction stage,the internal fluid flow of the droplets is changed as the droplets then polymerize under the influence of surface tension and inertial forces,and the polymerized large droplets begin to oscillate and contract horizontally.Temperature conditions have a significant influence on the retraction stage of the double-droplet impact.The lower the temperatures are,the stronger the adhesion of the wall,and the larger the corresponding wetting area.At the same time,compared to a single droplet,the maximum wetting area factor of double-droplet on a hydrophilic surface is reduced by 27%,and this difference reaches its maximum on a supercooled droplet impacting a supercooled hydrophilic surface,with a deviation of-38%.When it comes to the superhydrophobic surface,the maximum wetting area factor is then reduced to about 2%.Additionally,three typical impact results were observed for simultaneously impacts of double-droplet on a superhydrophobic cold surface under different velocity conditions: full rebound,adhesive avulsion,and full adhesion.The above results reflect the competing effects between fluid flow inside the large droplet and heat transfer from the cold superhydrophobic surface after droplet aggregation during double-droplet continuously impacting and simultaneously impacting.These findings can deepen our understanding of the impact-freezing mechanism after droplet impact on cold surfaces,it provides reference for the impact-freezing mechanism after multiple droplet impacts on cold surfaces and related applications and techniques for icing/anti-icing. |
