Parabolic trough solar collector (PTC) is one of the most proven technologies in medium and high temperature solar thermal utilization field. The parabolic trough vacuum tube receiver, also called the heat collector element (HCE), plays a crucial role in the parabolic trough collector system. HCE performance directly affects the performance of solar energy heat utilization system. It is effective to improve the HCE performance by decreasing heat loss in annular space between the absorber and glass envelope. And right annulus gap size and pressure in annular space are important to decrease heat loss in annular space and improve the overall efficiency of the PTC. In order to optimize the annular space by using theoretical methods, a three-dimensional flow and heat transfer model is proposed. The cross-section profile of the solar energy flux on the outer surface of the absorber tube obtained by the SolTrace software is set as the thermal boundary condition in the model. To verify the validation of the model established in this paper, the thermal efficiency of PTC is calculated and compared with the experimental data obtained by literature. Satisfactory agreement is found and the average deviation between the simulation and the experiment is about 4.91%. On the basis of reliable results numerically simulated, the effects of the key parameters such as annulus gap size, pressure and filling gases on the heat-transfer characteristics of the receiver are numerically investigated under different conditions. Numerical simulation results indicate that temperature and radiation intensity distribution in annular space are not uniform because of the effects of non-uniform heat flux on the absorber tube wall. And thermal efficiency of PTC is significantly affected by annulus gap size. Average temperature of absorber tube's outer wall and glass envelope's inner and outer wall decreases with the increase of annular gap size. And heat loss in annular space decreases with the decrease of average temperature of glass envelope wall. Annular gas size below 20 mm can significantly affect the heat loss in annular space. Size above 20 mm, however, has slight influence on the heat loss in annular space. The HCE performance as a function of the pressure in the annular space is also studied. Average temperature of absorber tube's outer wall decreases and average temperature of glass envelope's inner and outer wall increases with the increase of annulus pressure. Annular pressures between 0.0001 and 0.01 Pa have slight influence on the HCE performance. But at annular pressure above 0.01 Pa, heat loss becomes a problem for HCE. Currently, the HCEs are manufactured with the annulus space under a vacuum of 0.01 Pa. The heat-transfer characteristics of the annular space are influenced by the type of gases in the annulus space. Average temperature of glass tube inner wall, as a function of annular pressure, is compared among air, hydrogen, helium, nitrogen and argon filled in the annulus space. The inert gas with a low thermal coefficient, i.e. argon, results in the lowest average temperature of glass envelope inner wall and the best HCE performance. However, air provides only a slight improvement on HCE performance. Hydrogen increases average temperature of glass tube inner wall significantly. The encouraging results in this paper will provide a fundamental reference for researching heat-transfer characteristics in annular space and optimization design of the PTC.%合适的间隙尺寸和真空度对减少接收器环形区域热损失、提高集热器系统效率非常重要.为了能有效利用理论方法对槽式太阳能真空管接收器环形区域进行优化研究,该文利用ANSYS软件建立了真空管接收器的三维流动、传热模型.该模型以SolTrace光学软件模拟得到的金属管周向热流密度为热边界条件,模拟计算了真空管接收器内三维流动及传热过程.为验证模拟结果准确性,与文献试验结果进行对比,吻合较好,平均相对误差为4.91%.在确定数值模拟可靠性前提下,对接收器环形区域传热性能进行了计算和分析.模拟结果显示,随着间隙尺寸增大,环形区域平均温度和热损失逐渐减小;间隙尺寸小于20 mm时变化趋势较快,大于20 mm时变化趋于平缓.随着环形区域压力增大,金属管外壁面平均温度逐渐降低,玻璃套管内、外壁面平均温度逐渐升高;压力为0.0001~0.01 Pa时壁面温度基本不变,压力大于0.01 Pa时壁面温度变化明显.环形区域的传热特性与渗入的气体种类也有关系,渗入气体导热系数越大,玻璃套管内壁面的平均温度越高.该研究对了解环形区域传热特性、优化环形区域结构、指导接收器设计具有一定的实用价值.
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