Experimental Study On Heat Transfer Enhancement Characteristics Of Supercritical CO₂ Fluid In Heat Absorber For Tower Solar Thermal Power Generation

With the adjustment of China's energy structure and the strategic planning of energy sustainable development,solar thermal power generation,as an important part of the clean energy field,will play a key role in the low-carbon revolution and the development of green economy.In recent years,in solar thermal technology,supercritical CO2 Brayton cycle technology has gradually become a research focus.Compared with conventional Rankine cycle,supercritical CO2 Brayton cycle has higher energy conversion efficiency and higher economy,which is expected to promote the large-scale and low-cost development of solar thermal power generation plants.In this new solar thermal power station,supercritical CO2 fluid is the working medium of solar-thermal energy conversion,and its flow heat transfer characteristics directly restrict the efficiency and safety stability of energy transfer process.However,there are many influencing factors for the heat transfer characteristics of supercritical fluid.Especially under the condition of high heat flux,the heat transfer characteristics of supercritical CO2 is very complicated due to the effects of variability and buoyancy.Therefore,this paper carried out the experimental research on the heat transfer characteristics of supercritical CO2 under various working conditions,and explored the technical measures to enhance the heat transfer of supercritical CO2.The main work and achievements are as follows:(1)Design and build the supercritical CO2 flow heat transfer experiment platform,electrical heating method are adopted to simulate the working condition of high heat flux,use of constant flow pump,heater,condenser,the back pressure valve,high pressure storage tank and other equipment to build the stable high pressure closed loop,distributed real-time data acquisition and processing system is developed,thus formed the supercritical CO2 flow heat transfer test device and method.(2)The convective heat transfer test of supercritical CO2 in smooth circular tube was carried out,and the influences of thermal parameters,buoyancy effect and other factors on the heat transfer of supercritical CO2 were systematically studied.At lower heat flux,the heat transfer coefficient has a peak value in the quasi-critical region.However,under the condition of high heat flux and low flux,significant heat transfer deterioration occurred,and the inducing factor was sharply increased buoyancy lift effect(Gr/Re2).By comparing the experimental data with the empirical correlations of heat transfer in literature,the results show that the existing correlations cannot well predict the heat transfer under the working conditions in this paper.Therefore,the corresponding correlations of heat transfer are put forward by using the least square method,and the prediction error is±15%.(3)The convective heat transfer test of supercritical CO2 was carried out in the inner convex tube.The results show that the inner tube can effectively enhance the heat transfer performance of supercritical CO2 and the PEC factor is 1.24?1.52.According to the test data,the heat transfer correlation formula suitable for convex tube was fitted,and the prediction error was ±10%.The research results provide theoretical basis and technical support for the design and optimization of supercritical CO2 energy conversion device in solar thermal power station.