Numerical Research On Dynamic Characteristics Of Micro Droplet Impact On Surface And Liquid Film

With the development of a high degree of integration and high power density ofelectronic components, the thermal dissipation is more and more high. Conventionalcooling technologies are unsatisfactory to meet the requirement of the high heat fluxelectronic components. Spray cooling is an efficient heat transfer with phase changecooling technology. It has been more and more used in heat removing of high fluxdensity electronic components for high heat capacity and strict temperature controlability. Spray cooling is a complicated phase change heat transfer process. The heat istaken from the heat source surface mainly by the liquid film evaporation, heatconduction, convection and bubble formation and so on. There are more influencingfactors and the factors are mutual coupling, which bring certain difficulties for study.Therefore, dynamic behavior of a single droplet impinging wall and liquid film arestudied in this paper. The VOF (volume of fluid) method is taken to simulate the dropletimpact process. The characteristics of impinging drop spreading, retraction and rebounddynamic behaviors are explored. The liquid film shape change after drop impact andheat transfer characteristics after drop impact on hot wall are studied too. The airentrainment phenomenon during drop impact is also simulated in this work. The effectsof wall wettability, surface tension, viscosity and solid wall material on drop dynamicbehavior and heat transfer characteristics are obtained. The main conclusions of thispaper are summarized as follows:①When the drop impact on the surface at a room temperature, it will have thedynamic characteristics of spreading, retraction and rebound for different surfacecharacteristics and initial droplet kinetic energy. With the wall surface hydrophobic anddroplet surface tension enhanced and droplet viscosity reduced, the droplet maximumspread factor is reduced. The highly hydrophobic surface is beneficial for drop rebound.②Small drops pop up when the drop with a low impact velocity fuse in to theliquid film. While drop with a high impact velocity, the film kinetic energy is enough toovercome the surface tension to form splashing. Surface tension and viscosity effectboth inhibit drop splash. The film thickness is thicker, the radial velocity of flowseparation caused by impact on wall is smaller, which also inhibit the spray growth.③When droplet impacting on the hot wall, the cold drop spread on the hot walland heat exchange between the cold drop and hot wall. The wall heat flux at the wall center is fluctuant and the highest wall heat flux is near the three phase contact line. Thehot wall materials play an important role on the heat transfer characteristics. With theincrease of the thermal conductivity of the solid wall, its internal temperature tends tobe uniform, solid wall internal cooling area increased but the wall temperature is higher.④When the drop close to the wall, air between the liquid drop and wall iscompressed, leading to air pressure increased. The liquid droplet is extruded and adimple is created at the drop bottom, which becomes a bubble when the dropletimpinging on the wall. Air trapped also occurs in the retraction stage at the drop centercavity.⑤With the bottom curvature decreases and drop diameter increases, the amountof air entrainment increases. But the amount of air entrainment change irregularly withthe impact velocity increases. The amount of air entrainment little change with thecontact angle increases on the hydrophilic surface which contact angle less than90°.But on the hydrophobic surface, the amount of air entrainment increases rapidly withthe contact angle increases.