Research On Condensation Heat Transfer Enhancement Of Metal-based Nanoarray Surface

Condensation,as an efficient phase change heat transfer way,has been widely used in the fields sush as power generation,energy,water desalination,thermal management,and environmental control,and improving the efficiency of condensation heat transfer has been attracted much attention.Compared with filmwise condensation,dropwise condensation can form discrete droplets on the condensation surface,thereby reducing the heat transfer resistance and improving the heat transfer efficiency,and some studies shows that the superhydrophobic surface can promote the droplet jumping and further enhance heat transfer.However,the transition of the wetting state of the condensed droplets inhibits jumping phenomenon and limits the improvement of heat transfer performance.The key issue is the insufficient understanding of the mechanism of condensed droplets wetting state transition.Therefore,based on the controllable preparation of metal-based nanoarrays,this paper systematically explored the effect of surface structure and surface subcooling on the growth behavior and heat transfer performance of condensed droplets through experiments and theoretical analysis methods,and revealed the mechanism of condensed droplets wetting state transition,and optimized the surface structure to promote efficient jumping condensation The main research contents and results are as follows:(1)The liquid-phase growth method was used to prepare the ZnO nanoarray structure on stainless steel surface.The results indicated that increasing the number of pulling can increase nanoarray density;increasing the heat treatment temperature can increase nanoarray diameter and destroy the uniformity;increasing the concentration of the seed solution can improve the uniformity and verticality of nanoarray;nanoarray can formed only when the concentration of the growth solution is moderate;increasing the growth temperature can reduce the diameter and spacing of the nanoarray;increasing the growth time can not only increase the height of the nanoarray,but also cause the agglomeration phenomenon at the top of the nanoarray;within the appropriate DAP concentration range,increasing the concentration of DAP can further reduce the top diameter of nanoarray and makes it closer to tapered.(2)The hydrophobic properties of nanoarrays were tested under different preparation parameters,and the growth behavior of condensed droplets on different nanostructures surfaces were observed through air condensation experiment.The results indicated that hydrophobic angle of all nanostructures with a certain spacing and aspect ratio were above 150°.The macroscopic wetting performance cannot accurately predict the wetting state of the droplets on the nanostructure surfaces.Whether it can jumping and the strength of jumping phenomenon mainly depended on the surface nanostructure characteristics,and the nanostructure with high aspect ratio and small top diameter can promote the formation of Cassie state droplets to achieve stable jumping condensation.(3)The effect of surface subcooling on the growth behavior and heat transfer performance of condensed droplets was explored through steam condensation experiment.The results indicated that compared with smooth hydrophobic surface,nanostructure surface was improve the heat transfer performance by more than 50% in the range of 0?4 K.With the increase of subcooling,the nucleation size of the droplets droped sharply,causing the nucleation sites to extend from nanostructure top into nanostructure gap,which leaded to the transition of wetting state to inhibit heat transfer.Increasing the height of the nanostructure can further inhibit the transition of wetting state under high subcooling.(4)The dropwise condensation heat transfer model was modified in two parts: the thermal resistance of the nanostructure layer and the detachment size.The predicted results of the heat transfer model showed a good agreement with the experimental under low subcooling.The effect of surface nanostructure parameters on heat transfer performance was analyzed through theoretical analysis method.The results indicated that under Cassie wetting state,the top diameter and spacing of nanostructure mainly affected the heat transfer performance by changing the solid-liquid contact area,and its effect on the heat transfer performance was small.The increase of the height nanostructure can increase the thermal resistance of nanostructure layer and weaken the heat transfer.As the height of nanostructure increased,the influence amplitude gradually decreased.The increase of thermal conductivity of nanostructure can strengthen heat transfer,and the influence amplitude gradually decreased with the increase of thermal conductivity.The idea of constructing appropriate height of nanostructure for different subcooling requirements to achieve jumping condensation heat transfer was proposed.