Pool Boiling Enhancement For Metal Surface Of Mixed Wettability

Microstructure metal surfaces had displayed excellent prospects for improving the wettability of interface surface,enhancing flow drag reduction,enhancing boiling heat transfer rate and generating interface slip.In recent decades,the special hydrophobic or super-hydrophobic of microstructures have attracted the attention of scientists in and flow resistance reduction and convective heat transfer,and this performance have been successfully applied in chemical industry,aerospace,medicine,precision instruments and other fields.With the development of Micro/Nano electro-mechanical system,the study of mechanism heat transfer enhancement on micro-scale metal surface was expected to analyze surface effect which was brought by micro-scale to solve the problem of heat dissipation of ultra-high heat flux in micro-space.In this paper,molecular dynamics simulation method was used to study the mechanism of the influence of micro groove and mixed wettability on boiling for copper surface from a molecular view.The essence of boiling heat transfer was disclosed and the design theory of interface structure for enhanced boiling was put forward.According to the theoretical research to design surface structures,the five-axis CNC machining center was used to manufacture the micro-groove structure on copper plates.The boiling heat transfer experiments were carried out on the boiling experimental platform which was built.Depending on the collection and analysis of the data such as the temperature of copper and water,bubble nucleation and bubble escape movement,the influential factors of boiling heat transfer were verified based on the scale effect.The specific research contents were as follows:(1)Numerical simulation of metal surface wettability.LAMMPS,as an open source software,was used to build copper atomic layer and water molecular droplets based on molecular dynamic computational simulation method.Cu-O potential well depth parameters were adjusted to simulate the different wettability of copper atoms and water molecules.The mapping relationship between Cu-O potential well depth and surface wettability was established.(2)Numerical simulation of the influence of mixed wettability for smooth metal surface on boiling heat transfer.Four uniform wettability and six mixed wettability smooth surfaces were established and the influence of micro-nano structure on heat transfer was excluded from the study by molecular dynamic method.All the data of surface water molecule movement velocity,density visualization analysis,copper water temperature and water molecule evaporation rate were compared and analyzed.It was revealed that there were obvious microfluidics between the solid hydrophilic region and the contact region of water molecules,which can ensure the full contact of solid and liquid.By analyzing the density and mechanical energy of water molecules in hydrophilic and hydrophobic regions,it was concluded that the density of water molecules in hydrophobic regions in contact with water molecules was significantly lower than that in hydrophilic regions,and the potential energy of water molecules showed a repulsive force,which can be considered as nucleation points.It was proved that hydrophilic and hydrophobic mixed wettability surface 9 had the best heat transfer performance than uniform wettability and other mixed wettability surfaces.(3)Numerical simulation of the influence of mixed wettability for groove structures on boiling heat transfer.Passing on the smooth surface and three different heights micro groove structure,including uniform wettability and mixed wettability,eight kinds of surfaces were analyzed.It was found the nano groove structure above can form hydrophobic properties which help to bubble nucleation;Hydrophilic nano rib can form micro fluid layer adsorbed water molecules,constantly into a groove into core zone to supplement;The increase of the height of the nano convex edges leaded to a stable supply and the copper temperature fluctuates less.The hydrophilic and hydrophobic mix ed properties of nano groove structures were proposed from the perspective of molecular energy.(4)Put forward the surface design theory of enhanced heat transfer.The essential reason of the influence of water molecular mechanical energy and water molecular flow on boiling heat transfer was summarized through simulation analysis.Enhanced boiling heat transfer was developed based on surface design theory.The groove of six kinds of optimization was constructed depending on design theory of vertical surfaces to meet enhancing boiling heat transfer,and the optimized surfaces improved the critical heat flux density and boiling heat transfer coefficient to verify the design theory.(5)The micro groove structures of copper with different wettability were designed and manufactured,and the boiling heat transfer experiments of that surfaces were carried out.On the basis of simulation theory,the surface and smooth surface of six groups of different micro-grooves were designed,and five-axis CNC machining center was used to manufacture the surfaces of copper plates.The static contact angles of all surfaces were measured by contact Angle measuring instrument,and the mapping relationship between microgroove structure and wettability was obtained.Through the comparison of boiling experiments on the self-built boiling experimental platform,it was revealed that the heat transfer performance of all groove surfaces was significantly higher than that of smooth surfaces.The influence of groove depth on heat transfer was analyzed,and the influence of solid-liquid contact area on heat transfer performance was verified.