| Droplet dynamic is a determinant of dropwise condensation,and removing the droplets timely from the condensing surface is the transformation from the film condensation to the dropwise condensation,which is the basis of achievement of efficient dropwise condensation heat transfer.Gravity is commonly used for droplet removal,however,this efficiency of this method is very low.Due to the slight solid-liquid adhesion,the microscale droplets on superhydrophobic surfaces may spontaneously jump away from the surface after coalescence,regardless of gravity.Utilizing this droplet-jumping dynamic characteristic can improve the efficiency of droplet removal,without any external force or field.A deep study on coalescenceinduced self-propelled jumping of droplet is not only conducive to enhancement of dropwise condensation heat transfer,but also provides a crucial and fundamental theory for more engineering applications.In this dissertation,the volume-of-fluid(VOF)method is applied to numerically simulate coalescence-induced self-propelled jumping behaviors,and the law of energy conservation is utilized to calculate and analyze the energy transition characteristics,aiming at revealing the kinetic mechanism of droplet jumping,and mastering its dynamic characteristic and law of energy transition.Firstly,the effcts of the liquid properties,the radius ratio(the ratio of the large droplet radius to the small droplet radius)and the contact angle on droplet jumping are studied.The dimensionless number Ohnesorge(Oh)is a characteristic parameter that reflects the dropletjumping characteristics: if Oh is constant,the varied liquid properties have little effect on the four characteristic stages of droplet jumping(the liquid bridge expansion,the impact between the liquid bridge and surface,jumping away from surface,and deceleration and oscillation in the air),however,they will change the values of all energy terms during the coalescence and jumping.With the increase of Oh number,the effect of viscosity is significant,leading to the rise of the dimensionless solid-liquid contact time and the decrease of the dimensionless jumping velocity at droplet detachment time.According to the different grey relational values,the relation between the viscosity and the real solid-liquid contact time is greater;meanwhile,there is a greater relation between surface tension and the real jumping velocity.When the radii of two droplets are closed to each other,the change in jumping velocity possesses a unique peak value,namely the maximum velocity.With the decrease of radius ratio,the jumping velocity experiences several peak values,meanwhile,the type of jumping changes from the vertical motion to the upward rotation.Both the jumping velocity and the energy terms decline with the decrease of radius ratio,and the droplet cannot jump after coalescence when the radius ratio is less than a critical value.However,this threshold is not constant,which descends with the increase of contact angle.The coalescence-induced self-propelled jumping of two droplets has a very low energy conversion efficiency(less than 3%),and this efficiency will further decrease when the radius ratio decreases.Subsequently,the effects of different arrangements on multi-droplet jumping are given,taking an example of three droplets.For concentrated configuration,two different forming mechanisms of liquid bridge caused by different droplet distribution angles exhibit different dynamic behaviors.The critical distribution angle resulting in the overall non-monotonic changes in the jumping velocity and the energy conversion efficiency ranges between 110° and120°(0.02 ≤ Oh ≤0.16).Compared with the symmetry,the geometric characteristic of droplet arrangement(the concentricity between the center of circumscribed circle of two lateral droplets with widest location and the center of mass of three droplets)is the criterion for multi-droplet jumping performance.The spaced configuration of droplet arrangement prolongs the coalescence time,intensifies the deviation of concentricity between the center of mass and the center of circumscribed circle of geometric distribution,and destroys the symmetrical distribution of mass,which is detrimental to droplet jumping.In the two typical unequal-sized multi-droplet arrangements(concave and convex arrangements),although the convex arrangement is beneficial to more excess surface energy,it also causes more energy loss.There is little difference in the jumping velocities between the concave and the convex arrangements when the droplet jumps away from the surface,therefore,the concave arrangement has a larger energy conversion efficiency.Finally,two kinds of structures with different scales on the surfaces are utilized to change liquid bridge dynamics,so as to enhance the droplet jumping.For a microscale cuboid protuberance on the surface,the repelling effect from the contribution of surface tension and protuberance superhydrophobicity is the key to the enhancement of droplet jumping and the improvement of the energy conversion efficiency.The geometric factors of the cuboid protuberance influencing the jumping velocity are listed in the descending order: height(parallel to the normal direction of the surface),length(parallel to the connecting line of two sphere centers),width(perpendicular to the plane formed by height and length),height-width interaction,and height-length interaction.To further explore the structural effect of the surface on the droplet jumping dynamics,the size of the structure on the surface is extended from the microscale to the macroscale: when the wettability of the horizontal plane and the vertical plane for double-plane structure is the same,the jumping angle is 45°;otherwise,the jumping direction tends to the side of the plane with worse wettability.Both two macroscale planes from double-plane structure can change the liquid bridge dynamics to significantly increase the jumping velocity and the jumping kinetic energy,and the corresponding energy conversion efficiency is larger than that under the effect of microscale protuberant structure. |
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