Study On Manufacturing And Boiling Heat Transfer Performance Of Nanoporous Copper Surface

Boiling heat transfer technology is widely used in traditional industrial areas, likethermal power, nuclear engineering, solar energy, chemical, food engineering and cryogenicengineering, as well as space technology and microelectronics cooling. The development ofboiling enhancement technology can improve heat transfer efficiency and provide a solutionfor the heat dispersing problem in small space with high heat flux. The nanoporous coppersurface with high specific surface area, excellent thermal conductivity, good wettability aswell as a high density of potential bubble nucleate sites, is a promising heating wall forenhancing boiling heat transfer. In this thesis, a simple method which combined hot-dipgalvanized and dealloying was proposed for the fabrication of nanoporous copper surface.The key physical and chemical characteristics related to boiling heat transfer applications ofthe porous surface were analyzed. Finally, the enhancement of boiling heat transferperformance of the nanoporous copper surface was studied. The main contents of this thesisinclude:1. Hot-dip galvanizing and component control of the coatingHot-dip galvanizing was selected for the surface alloying on copper substrate. With theobjective of getting a Cu/Zn layer which meets the component requirement for dealloying, theoperation parameters of the hot-dipping process were optimized based on the thermodynamicequilibrium phase diagram and reaction-diffusion theory. The microstructure of the coatinglayer and the growth pattern of Cu-Zn intermetallic compounds were analyzed by means ofmetallography and SEM. Two obvious phase interfaces were found in the coating layer. Thegrowth rates of the phase layers are different, and are all controlled by diffusion. The XRDanalysis indicates γ-CuZn and β′-CuZn are the main components of the coating layer.2. Fabrication of nanoporous copper surface by dealloyingThe dealloying process of the multiphase Cu-Zn alloy layer formatted by hot-dipgalvanizing was systematically studied. The effects of electrolytes, the concentration ofelectrolyte solution, corrosion time and the anodic potential employed in dealloying on theformation of nanoporous structure and cracks were discussed. Homogeneous nanoporous copper structure can be obtained by free corrosion in an alkaline environment. It was foundthat nanoporous copper would be destroyed in a hydrochloric acid environment beyond10wt.%in concentration. A higher dealloying rate can be obtained by using electrochemical corrosioncompared to that of chemical corrosion, however, more cracks on the dealloyed surface.According to the linear sweep voltammetry analysis, the γ-CuZn and β’-CuZn phases whichmainly compose the coating layer can be selectively dissolved during the dealloying process.3. Stability of NPC in hydrothermal environment and scale effect of porosityCu0.4Mn0.6was prepared by a process involved melting and solution treatment. NPCmaterials with uniform porosity were obtained by electrochemical and free dealloying,respectively. The stability and evolution of the nanoporous-copper structure in saturatedboiling water were experimentally investigated by means of SEM. Meanwhile, the changes inchemical composition of the nanoporous materials before and after boiled were analyzed byEDS. The NPC structure shows excellent chemical stability, however, keeps coarsening in thehydrothermal environment. The coarsening process shows high dependence on porosity scale,and is controlled by the surface diffusion mechanism. In addition, the contact anglemeasurement and infrared imaging technology were employed to reveal the changes insurface wettability and capillarity resulted from the scale effect of the porous structures.4. Boiling heat transfer performance on the nanoporous copper surfaceSaturated pool boiling experiments were conducted on a visualization platform. Thenucleate boiling heat transfer performance of nanoporous copper surface prepared by HDGDprocess was investigated compared to that of unstructured surface. The mechanism of theenhancement of boiling heat transfer was discussed based on the physical and chemicalproperties of the nanoporous surface and the observations of bubble dynamics phenomena.The improvement of wettability, higher nucleation sites density, and more entrapped gasvolume of the nanoporous surface contribute to the significant enhancement of boiling heattransfer. Furthermore, the bubble kinetic characteristics on the nanoporous copper surface andthe smooth surface show significant differences in the saturated pool-boiling tests. Bubbleswith smaller departure diameter and higher departure frequency were observed on nanoporous surface. The experimental data was compared with the reported results, and the effect ofthermal properties of the heating wall on the boiling heat transfer performance was discussed.