Fabrication Of Sandblasting-based Micro-nano Composite Structured Surface And Its Boiling Heat Transfer Performance

With the rapid development of modern industrial technology,devices are becoming more and more miniaturized,integrated and high-power,which leads to the total heat productivity per unit volume and the local temperature is too high.At this time,the equipment has a higher requirement for heat dissipation capacity.Boiling heat transfer,as a phase change convective heat transfer method,is one of the most effective heat transfer enhancement technologies at present.Enhanced boiling heat transfer can be divided into active techniques and passive techniques.As a passive technique,heat exchange surface modification technology has been widely used in industrial engineering because of its convenient manufacturing and no external conditions,which has attracted the attention of many scholars.In this thesis,taking copper as the research object,different structured surfaces were designed and prepared to enhance boiling heat transfer.On the basis of the sandblasting microstructures,the surface was etched by ultrasonic wet etching to prepare the sandblasting-based subtractive composite structured surface.At the same time,nanostructures were grown on the sandblasting microstructured surface by alkali-assisted oxidation technique to prepare sandblasting-based additive composite structured surface.Surfaces with different structures were characterized by morphology observation,roughness,surface area ratio and apparent contact angle.The boiling heat transfer performance and visualization phenomenon of different structured surfaces were studied,and the potential mechanism of enhanced heat transfer was analyzed.The main contents and conclusions of this thesis are as follows:（1）Taking the sandblasting microstructured surface as the substrate,the surface was etched by ultrasonic etching to prepare the sandblasting-based subtractive composite structured surface.The nanostructures were grown on the sandblasting microstructured surface by the alkali-assisted oxidation technique,and the sandblasting-based additive composite structured surface was prepared.The surfaces morphology were characterized,according to SEM image analysis,the micro-nano composite structured surfaces had more pits and higher cavity density than the smooth surface.The increase of the cavity leads to the increase of the vaporization core point,thereby enhancing the heat transfer performance.（2）Through quantitative analysis of the surface roughness and surface area ratio,it is found that the roughness and surface area ratio of the modified surfaces are larger than those of the smooth surface.In particular,the roughness and surface area ratio of the microstructured surface and the micro-nano composite structured surfaces are much larger than that of the smooth surface.Among them,the surface roughness and surface area ratio of the sandblasting-based additive composite structured surface are the largest,and its surface area ratio is 2.8 times higher than that of the smooth surface.The increase of roughness and surface area ratio can improve the critical heat flux（CHF）and heat transfer coefficient（HTC）,which is conducive to boiling heat transfer.（3）Using deionized water as the boiling medium,the pool boiling experiments were carried out on the prepared micro-nanostructured surfaces.The results of the boiling heat transfer characteristics showed that the heat transfer performance of all modified surfaces was improved to varying degrees.Among them,the CHF and maximum HTC of the sandblasting-based additive composite structured surface are the largest among all surfaces,which are increased by 55.6% and 218.0%,respectively,compared with the smooth surface.The second is the sandblasting microstructured surfaces and sandblasting-based subtractive composite structured surface,which also have substantial increases in CHF and maximum HTC.（4）The boiling processes of different surfaces were visualized from the perspective of bubble dynamics.Compared with the smooth surface,at low heat fluxes,the modified surfaces have more vaporization cores and smaller bubble detachment diameters,which is beneficial to the improvement of the boiling heat transfer performance.At medium heat fluxes,the modified surfaces have more isolated bubbles and less merging bubbles,while the smooth surface starts to form vapor column earlier due to the merging of vapor bubbles.When reaching the CHF,some isolated vapor bubbles can still be observed on the modified surfaces,especially the micro-nano composite structured surfaces,while smooth surface is completely enveloped by a mushroom-like vapor layer.Therefore,the micro-nano composite structures can effectively delay the appearance of the mushroom-like vapor layer,thereby delaying the triggering of CHF.