| How to remove the heat flux of electronic device is a great challenge for its development.Spray cooling has become a popular cooling method in the electronic field due to its advantages of high heat transfer coefficient,small mass flow,good temperature uniformity and no hysteresis of boiling.The modification of the hot surface with micro/nano-structure is an effective way to enhance the heat transfer of spray cooling.But the mechanism of interaction between micro/nano-structured surface and spray droplets as well as the effect of micro/nano-structure on spray cooling are still unclear.On this basis,a visual experimental system for droplets impacting on the surface and an open spray cooling experimental system were built in this paper.The dynamic characteristics of droplets impacting on microstructured surfaces,the evaporation characteristics and the influence of the microstructured surface on spray cooling were analyzed respectively,which would lay the experimental foundation for the optimization of spray cooling.In this paper,2mm nickel plate was selected as the substrate,which was used as the reference surface after grinded and polished and its contact angle was 73°.The anisotropic regular micro structure with large and small groove arrays arranged on the substrate was fabricated by UV-LIGA(deep lithography and electroplating)process.The contact angles in parallel and vertical directions of the microgroove were 135° and 114°,respectively.In addition,the rough and disordered irregular microstructure was fabricated by electrochemical corrosion process on the substrate and its contact angle was 126°.A high-speed camera was used to record the dynamic spreading process of droplets impacting on the surfaces,and the influence of structural characteristics,surface temperature and impact velocity on the dimensionless spreading diameter and height of the droplets were discussed quantitatively.The results showed that the shapes of droplets during spreading and retracting were affected by the microstructure on the surface.The parallel microgroove and the corroded structure reduced the maximum dimensionless spreading diameter and its appearance time of the droplet.Basically,the surface temperature had no effect on the maximum dimensionless spreading diameter.The retraction of droplet on polished surface showed a clear surface temperature dependence,and the microstructure on the surface would weaken the effect.The maximum dimensionless spreading diameter of the droplet and its corresponding dimensionless time increased with the impact velocity increasing.Due to the pinning effect in the parallel direction and the free motion in the vertical direction of the microgroove structure,a beat phenomenon was observed on the dimensionless height curve of droplet within a certain range of We number.By comparing the total evaporation time of droplet on the polished surface and the microstructure surface,it was found that both microstructures had a slight deterioration effect on the droplet evaporation process at non-boiling temperature.However,when the surface temperature exceeded the boiling temperature of the liquid,the two microstructures enhanced the heat transfer of the droplet.An open spray cooling heat transfer experiment system was used to study heat transfer of spray cooling on the microstructured surfaces,and the influence of structural characteristics,spray height,surface orientation and surfactant on the spray cooling were analyzed.The results showed that the microgroove surface enhanced heat transfer in the boiling stage.The microgroove structure played an important role in increasing the active nucleation,homogenizing liquid film distribution and reducing surface temperature fluctuation,but the direction of the microgroove did not have a significant effect on heat transfer.Corroded surface deteriorated heat transfer during the whole cooling process.When the nozzle was close to the hot surface,the heat transfer coefficient of spray cooling was higher.And the optimal spray height corresponding to the maximum heat transfer coefficient of the microgroove surface was slightly larger than that of the polished surface.The heat flux of the polished surface and microgroove surface oriented at 0=90° was higher than that at ?=180°(? is the angle between the normal of the heat transfer surface and the opposite direction of gravity),but the difference would get smaller and smaller during the boiling stage.Small concentration of surfactant enhanced the foaming ability of the cooling medium,so that the surface superheat required for initial boiling was reduced. |
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