Analysis Of Heat Transfer Characteristics Of Water-based Nanofluids In Large-volume Boiling

Energy is an indispensable resource for human survival,economic development and social progress.In order to realize the Chinese dream of great rejuvenation of the Chinese nation,complete the process of urbanization and industrialization,and realize modernization in an all-round way,China's energy consumption will inevitably increase substantially.Although the total amount of energy in China is very large,the per capita energy share is still relatively small,and the future energy prospects are still not optimistic.Improving the energy utilization rate and reducing the loss of thermal power plants can alleviate the severe energy waste to a certain extent.At present,the cycle efficiency of water-based Rankine cycle is relatively low and the loss is serious.The analysis shows that the loss mainly occurs in condensers and boilers.In order to improve the circulating efficiency of water-based Rankine cycle system,the nanofluids with concentration of 0.001 wt%,0.01 wt%,0.1 wt%,1 wt%,2 wt% of water-based alumina and water-based copper oxide were prepared by dispersion method,and their stability was analyzed.Pool boiling heat transfer experiments of deionized water,water-based alumina and water-based copper oxide nanofluids at 0.1 MPa system pressure were carried out.The boiling heat transfer effects of pure water with different heat flux density and nanofluids with different concentration of water-based alumina and water-based copper oxide were analyzed from the aspects of superheat and boiling heat transfer coefficient,and the mechanism was analyzed,which provided experimental basis for the application of water-based nanofluids in Rankine circulation system.(1)The preparation methods and stability influencing factors of nanofluids were systematically described.Water-based nanofluids with different concentrations were prepared by "two-step method".Aluminum oxide and copper oxide nanoparticles with particle size of 20 nm were selected.Sodium dodecylbenzenesulfonate,which accounts for 5 wt% of the nanoparticles,was selected as dispersant.The sedimentation experiments of the prepared nanofluids were carried out and the nanofluids were analyzed by visual method.The stability of the fluids shows that the prepared nanofluids can fully meet the experimental requirements.(2)The boiling heat transfer experiments of nanofluids in a pool were carried out on a large-volume boiling heat transfer experimental platform.The effects of boiling heat transfer of nanofluids on particle concentration,particle type,dispersant and pipe fittings were studied under different heat flux densities.Viscosimeter and surface tensiometer were used to measure the viscosity and surface tension of nanofluids under different working conditions.Leong's improved formula was used to calculate the thermal conductivity of nanofluids,and Rohsenow's relationship was used to verify the experimental results.(3)The variation mechanism of boiling heat transfer coefficient of nanofluids was analyzed.When the concentration of water-based alumina and copper oxide nanofluids is 0 wt%-1 wt%,the thermal conductivity of nanofluids increases with the increase of the critical diameter of bubbles,the liquid film layer of nanoparticles and the aggregate of nanoparticles,and the heat transfer coefficient of nanofluids increases with the increase of the particle concentration.When the concentration of nanofluids exceeds 1 wt%,the heat transfer coefficient of nanofluids decreases with the increase of concentration because the deposition of nanoparticles reduces the distribution density of vaporization core on the wall.