Study On Surface Boiling Heat Transfer Characteristics Of Microstructures

With the development of the times,in order to maintain the performance of the equipment,a large amount of heat dissipation is needed to keep the temperature of the material below the specified limit.Boiling heat transfer is one of the most effective heat transfer methods due to the latent heat of phase change.Therefore,how to improve the boiling heat transfer coefficient（HTC）and the critical heat flux（CHF）of boiling heat transfer is the focus of boiling heat transfer research.Because the bubble growth in boiling heat transfer process is greatly affected by the surface wettability,the boiling heat transfer phenomenon of modified surface has been widely concerned by researchers.In this paper,the influence of surface wettability on heat transfer of nuclear boiling was studied by building a visualization experimental platform of pool boiling.Using chemical etching of the prepared hydrophilic copper surface,coating prepared copper surface hydrophobicity,mechanical grinding process,the smooth surface of the copper and passing on the hydrophilic surface blotted out different sizes of six different hydrophobic teflon coating preparation area ratio of the mixed copper surface wettability,under ordinary conditions of deionized water as working substance,pool boiling experiment was carried out on the surface.Through the analysis of experimental data,the mechanism of the influence of wettability on boiling heat transfer was obtained.The main contents of this paper are as follows:（1）According to the obtained boiling curve and heat transfer coefficient curve,the boiling heat transfer characteristics of three kinds of copper surfaces with single wettability are analyzed.The results show that the boiling heat transfer is mainly determined by the bubble covering on the surface at low heat flux,and the boiling heat transfer efficiency on the hydrophobic surface is better than that on the hydrophilic surface.With the increase of heat flux,the water vapor channel connecting the large bubble to the wall plays an important role in enhancing heat transfer.Bubbles on the hydrophobic surface polymerize to form bubble film,which destroys the vapor channel and makes HTC and critical heat flux significantly smaller than those on the hydrophilic surface.The HTC of hydrophilic surface can reach54.85k W·m^-2K^-1,which is 2.3 times that of hydrophobic surface.（2）By comparing the boiling curve and the heat transfer coefficient curve of each surface,it is found that when boiling heat transfer approaches the CHF,the HTC of boiling surface still increases.In order to evaluate boiling heat transfer efficiency better,the concept of heat flux growth rate per unit superheat is introduced.The dynamic behavior of single bubble and multi-bubble on each wettability surface was recorded and the boiling heat transfer mechanism was studied.The results show that the bubble behavior is in good agreement with the rate of increase of heat flux per unit superheat.（3）Based on the boiling heat transfer characteristics of single wettable copper surface,the boiling heat transfer characteristics of mixed wettable copper surface were further investigated by pool boiling experiment.The boiling heat transfer performance of six kinds of boiling surfaces was compared by drawing boiling curve and heat transfer coefficient curve.The results show that the optimum hydrophobic area ratio of the mixed wettability copper surface is 39.6%,which approaches the critical heat flux state at 93.97W/cm²,and the maximum heat transfer coefficient is 50.23k W·m^-2·K^-1,which is 27.8%and 47.56%higher than that of the hydrophilic surface,respectively.When the hydrophobic area ratio is less than 39.6%,the boiling heat transfer performance becomes stronger with the increase of the hydrophobic area ratio.When the hydrophobic area ratio is greater than 39.6%,the opposite is true.The larger the hydrophobic area ratio is,the worse the boiling heat transfer performance is.The mechanism of boiling heat transfer on the mixed wettability copper surface was analyzed by the dynamic behavior of bubbles recorded by a high-speed camera.The visualization results show that the hydrophobic region on the mixed wettability copper surface can reduce the superheat of ONB,thus promoting the boiling,and improving the boiling heat transfer performance at low heat flux state.However,at medium and high heat fluxes,the bubbles with high separation frequency are generated in the hydrophilic region and merge with the bubbles in the hydrophobic region to produce large bubbles,thus taking away more heat and improving boiling heat transfer performance.