Study On Dynamic Behaviors Of Droplet Impacting On Heated Silicon-based Micro-pillar Array Surfaces And Boiling Characteristics

The phenomenon of droplet impacting on heated solid wall widely exist in the fields of fuel injection,metal quenching,spray cooling,etc.Among them,droplet impact velocity,solid surface morphology,and surface wettability contribute to the generation and evolution of droplet boiling behavior.Related research is of great significance for the in-depth understanding of spreading behavior and boiling heat transfer mechanism of droplet impacting on high-temperature solid surfaces,and to promote the development of related applications.In this thesis,silicon-based micropillar array surface with micropillar diameter close to 1 ?m and nano-sized grooves on the outer axis,having different wettabilities(superhydrophilic and superhydrophobic),were obtained by wet chemical etching method.The impact behavior of water droplet with diameter of 2 mm on heated micro-pillar array surface has been experimentally studied by high-speed visualization,revealing the characteristics of boiling behavior(including intermediate jet and atomization phenomenon),and obtaining the law of droplet contact time and spreading factor under boiling conditions.At the same time,a systematic study was carried out on the boiling Leidenfrost phenomenon in the case of droplet impact,and a prediction model of the Leidenfrost temperature changing with the Weber(We)number was proposed.The main contents are as follows:(1)The boiling behavior of droplets on the superhydrophilic(?=4.2°)micropillar array surface includes intermediate jet,transition boiling,atomizing boiling and film boiling.The transformation of boiling behavior is affected by We and surface temperature.The unique formation of the intermediate jet is closely related to the uneven pressure distribution caused by the bubble cavitation at the bottom of the droplet.The generation of the jet is mainly determined by the surface temperature,while We affects the jet speed.The contact time of the droplets decreases with the increase of We.The boiling behavior of the droplets affects the contact time which is also changing with the surface temperature.When the jet phenomenon occurs,the contact time is extremely short,and the contact time of the droplets increases with increasing surface temperature.Contact time in atomization boiling is basically independent of the surface temperature.(2)Using FAS coating to modify micropillar array surface to make it superhydrophobic(?=162°),and study its effect on the boiling behavior of droplets.The droplet boiling behavior on the surface of the superhydrophobic micropillar array mainly includes transition boiling,atomizing boiling and film boiling.No intermediate jet phenomenon was observed on the superhydrophobic surface,which is caused by the difference in the way of bubbles nucleation on different wettability surfaces,indicating that the occurrence of intermediate jet phenomenon is closely related to the surface wettability.In the transitional boiling stage,the droplet can bounce off the surface completely,and its contact time is always about 10 ms,regardless of the surface temperature and We.After the dimensionless treatment of the contact time,the correlation formula ?r=1.05We^0.47 related to the We number is obtained.However,the droplet contact time will be greatly reduced during the atomization boiling stage,which is related to the decrease of the liquid film thickness and the expansion of the thermal boundary layer.It is found that the separation time is proportional to Re^-0.78.In addition,in the film boiling and transition boiling states,the maximum spreading factor of the droplet is only related to the We,and has nothing to do with the surface temperature and surface conditions.The maximum spreading factor and the We satisfy the relationship of ?max?We^3/10.(3)Analysis of the transition temperature of droplet boiling behavior(including Leidenfrost temperature)indicated that when We<10 on a superhydrophilic surface,the Leidenfrost temperature has nothing to do with the We,and the surface superheat for the Leidenfrost phenomenon satisfies:T_w-T_sat?(s??_vh_fg)/(k?_vR).In the middle We region,the Leidenfrost temperature increases with We.The transition temperature and the initial impact velocity satisfy:T_i-T_sat?V₀^1/2We^1/5,the transition temperature in the region predicted by the model is basically in line with the experimental structure.On the superhydrophobic surface,the Leidenfrost phenomenon can only be observed at low We(We<10);when We>10,the film boiling behavior will no longer appear,replaced by atomization boiling behavior.The transition temperature from transitional boiling to atomized boiling increases with the increase of We.The statistical analysis of the number of droplets produced by atomizing boiling on the superhydrophobic surface indicated that the number of atomized droplets increased with the increasing We,but decreased with the increasing surface temperature,increased first and then decreased over time.