Dynamic And Heat Transfer Characteristics For Droplet Impacting On Hydrophilic-Hydrophobic Hybrid Surfaces

With the advance of electronic technology,electronic power consumption keep increasing.Traditional cooling methods cannot meet the demand for the cooling of the electronic components nowadays.Some of the new cooling techniques based on phase change have attracted a lot of attention.The spray cooling technology,with its advantages of strong cooling heat transfer,lower surface temperature,and less demand of cooling medium,is considered to be preferred to solve the heat dissipation of high power electronic devices.The droplet dynamic and evaporation process on the heated surface is the basic phenomena in spray cooling.The hydrophilic-hydrophobic hybrid surface consists of hydrophilic regions and hydrophobic regions.The hydrophilic-hydrophobic hybrid surface can promote the droplet splitting to improve the heat transfer efficiency in film evaporation.Besides,the hydrophilic-hydrophobic hybrid surface can provide a large number of gasification core to promote the droplet nucleate boiling.Therefore,the hydrophilic and hydrophobic hybrid surface is a functional surface that promotes the evaporation heat transfer of the droplet.This work focused on the hydrophilic-hydrophobic hybrid surface.The droplet dynamic and evaporation process on the hydrophilic-hydrophobic hybrid surface were studied systematically,and the heat transfer performance of the spray cooling on the hydrophilic-hydrophobic hybrid surface was further studied.The main contents and conclusions are as follows:Firstly,the two-phase flow characteristics of droplets after impinging on the hydrophilic surface,hydrophobic surface,and hydrophilic-hydrophobic hybrid surface were studied by the numerical simulation method.It can be found that the hydrophilic-hydrophobic hybrid surface can promote the droplet to split.The influence of various parameters on the droplet movement and splitting was investigated.The results revealed that for the hydrophilic-hydrophobic hybrid surface,the droplet was subjected to different forces in hydrophilic and hydrophobic areas,which resulted in the edge of the droplet appearing wavy during the spreading process and further led to the droplet splitting during the retraction process.The number of satellite droplets increases with increasing the maximum spread factor of the droplet.As the hydrophilic static contact angle decreases,or the hydrophobic static contact angle increases,the number of satellite droplets is increasing.The energy of the droplet at the impact moment and the maximum spread moment was analyzed,the maximum spread factor of the droplet on the hydrophilic-hydrophobic hybrid surface was predicted.The droplet splitting on different hydrophilic-hydrophobic hybrid surfaces was studied,and the method to strengthen the splitting effect was proposed.The results revealed that when the number of stripes spanned by the droplet at maximum spread moment was less than 2,it could not split.When the number of stripes spanned by the droplet at maximum spread moment was greater than 3,the triple-phase contact line length of the droplet was the largest,and the splitting effect was the best.For hydrophilic-hydrophobic hybrid surfaces with different hydrophobic patterns,the dimensionless boundary length of the hydrophilic region and hydrophobic region spanned by the droplet at the maximum spread moment determined the splitting effect.The film evaporation process of the droplet on the hydrophilic surface,superhydrophobic surface,and hydrophilic-hydrophobic hybrid surface were studied experimentally.The influence mechanism of the hydrophilic-hydrophobic hybrid surface on droplet film evaporation was investigated.The experimental results revealed that the droplet splitting on the hydrophilic-hydrophobic hybrid surface could increase the triple-phase contact line length of the droplet,thus accelerating the droplet evaporation and enhancing the heat transfer between the droplet and the heating surface.Compared with the hydrophilic surface,the endothermic power of the droplet on the hydrophilic-hydrophobic hybrid surface can be increased by 50.94%.Due to smaller size,when the satellite droplets enter the CCA evaporation mode,the main droplet is still in CCR evaporation mode,resulting in a CCR-CCA mixed evaporation mode.When the satellite droplet is larger,the CCR-CCA mixed evaporation mode can be delayed.After maintaining a larger triple-phase line length for a long time,the heat transfer between the droplet and the heating surface can be enhanced.The droplet nucleate boiling on the hydrophilic surface,and the hydrophilic-hydrophobic hybrid surface was studied experimentally.The influence mechanism of the hydrophilic-hydrophobic hybrid surface on droplet nucleate boiling was investigated.The experimental results revealed that both reducing surface energy and adding microstructure could decrease the superheat needed for the droplet to enter the nucleate boiling state.Combined with lower surface energy and microstructure,the superhydrophobic structure could further reduce the superheat required for the droplet to enter the nucleate boiling state.When the superheat is 20K,the endothermic power of the droplet on the hydrophilic-hydrophobic hybrid surface is 542.66%higher than that on the hydrophilic surface.In addition,when the superhydrophobic pattern on the hydrophilic-hydrophobic hybrid surface is larger,the promoting effect of the superhydrophobic structure on the droplet nucleate boiling is stronger,leading the heat transfer between the droplet and the superheated surface is stronger.The spray cooling process on the hydrophilic surface,super-hydrophobic surface,and hydrophilic-hydrophobic hybrid surface were studied by the experimental method.The influence mechanism of the hydrophilic-hydrophobic hybrid surface on spray cooling was analyzed combined with the droplet nucleate boiling.A new method to enhance the heat transfer performance of spray cooling was proposed.The experimental results revealed that when the mass flow was 6.3kg/h,the temperature at which the liquid film on the hydrophilic-hydrophobic hybrid surface entering the nucleate boiling was lower than that on the hydrophilic surface.Thus,the heat transfer performance of the spray cooling on the hydrophilic-hydrophobic hybrid is higher.When the surface temperature is 130?,the heat flux and heat transfer coefficient of the spray cooling on the hydrophilic-hydrophobic hybrid surface reaches 200.53W/cm~2 and 19262.96W/m~2K,which are 50%higher than that on the hydrophilic surface.As the mass flow increases,the fast-moving liquid film impedes the growth and separation of bubbles on the hydrophilic-hydrophobic hybrid surface.It leads that the spray cooling performance on the hydrophilic-hydrophobic hybrid surface is close to that on the hydrophilic surface.