Enhanced Boiling Heat Transfer With Heterogeneous Wetting Micro/nano Structures In Microchannel

Heat exchangers are widely used in various fields of energy conversion systems.The life expectancy and heat transfer performance of heat exchangers have a tremendous impact on the overall performance of energy utilization systems.With the improvement of traditional industry and the development of high-tech industry,there is an increasing demand for the anticorrosive heat exchangers.The development of new high-efficiency anticorrosive heat exchangers is of great significance to improve the utilization rate of traditional energy and develop new energy sources.In order to meet the requirement of high-efficiency anticorrosive heat exchangers,a copper-based open microchannel heat exchanger with graphene oxide/nickel micro-nano composite coatings was proposed and fabricated by electro brush plating.The preparation technology was developed,the experimental open-microchannels were prepared and the surface of micro-nano structure characterization was quantitative carried out.Flow boiling heat transfer characteristics and flow pattern visualization in microchannels with different surfaces were carried out,and the influence mechanism of different surface structures and wetting characteristics on boiling heat transfer was revealed.The morphology characteristics of the graphite oxide/nickel composite surfaces were characterized by scanning electron microscope.It was found that in the process of brush plating,graphene oxide could change the crystallization process of nickel,interrupted the normal growth of the nickel grain and resulted in the production of new crystal nucleus,thus changing the crystallization process of nickel and producing smaller crystal particles.Compared with pure nickel surface,graphene oxide/nickel composite micro/nano composite surfaces have many porous cavity structures.The experimental surfaces of flow boiling in microchannels can be divided into three categories:metal surfaces(Cu,Ni and Ni-F),uniform wetting micro/nano composite surfaces(Ni/GO,Ni/GO-F)and heterogeneous wetting micro/nano composite surfaces(H-Ni/GO,H-Ni/GO-F).Among them,the heat transfer coefficient of H-Ni/GO-F surface are maximum.The maximum heat transfer coefficient is 21.55 kW/m2K,which is 50.28%higher than that of Cu surface.The minimum heat transfer coefficient with Ni surface is 12.37 kW/m2K.which is 13.73%lower than that of copper surface.The microchannels with H-Ni/GO-F surface have three mechanisms for heat transfer enhancement:(1)By fabricating graphene oxide/nickel micro/nano composite structures,the size distribution and number of nucleation cavity are increased,the nucleation density is increased,and thereby the nucleation boiling heat transfer is enhanced;(2)The fluorination treated surface exhibits hydrophobicity,and its surface energy is low,bubbles are easy to nucleate.Thus,the number of bubbles increases;(3)Because of the existence of inter lines among different wettability area,there are many active nucleation cavities on the inter lines and this can promote the formation of large amount of small bubbles,leading to a higher heat transfer coefficient.