超临界二氧化碳自然循环并联通道多区间流动不稳定性特征研究
Study on the Characteristics of Multi-Region Flow Instability in Parallel Channels of Supercritical Carbon Dioxide Natural Circulation

超临界二氧化碳(S-CO₂)在拟临界区独特的热力学性能，被认为是自然循环的理想工质，现有研究表明，超临界二氧化碳在自然循环系统和并联通道结构中都可能发生流量失稳现象。为探究超临界二氧化碳在自然循环并联通道系统中的流动不稳定特征，本文在系统压力8.8~9.1 MPa，进口温度289~303 K的工况范围内开展了实验研究。通过改变加热功率，得到超临界二氧化碳自然循环流量变化曲线。实验结果表明，自然循环能力存在峰值，未到峰值前循环流量随功率增加快速提高，达到峰值后缓慢下降。当试验段出口温度达到拟临界温度时，流动不稳定性现象发生。在高、低加热功率区域都观察到了两种类型的流量振荡现象：高振幅、长周期的系统性振荡和低振幅、波动周期大的并联通道振荡。实验结果可为超临界二氧化碳非能动系统设计提供数据支撑。
Supercritical carbon dioxide (S-CO₂) is considered to be an ideal working fluid for natural circulation due to the unique thermodynamic changes in the pseudo-critical region. Existing studies have shown that supercritical carbon dioxide may have flow instability in both natural circulation systems and parallel channel structures. In order to explore the flow instability characteristics of supercritical carbon dioxide in the natural circulation parallel channel system, the experimental study was carried out in the range of system pressure of 8.8~9.1 MPa and inlet temperature of 289~303 K. By changing the heating power, the change curve of the natural circulation flow rate of supercritical carbon dioxide is obtained. The experimental results show that the natural circulation capacity has a peak value. Before reaching the peak value, the circulating flow rate increases rapidly with the increase of power, and then decreases slowly after reaching the peak value. When the outlet temperature of the test section reaches the pseudo-critical temperature, the flow instability phenomenon occurs. Two types of flow oscillations were observed in both high and low heating power regions: high-amplitude, long-period systematic oscillations, and low-amplitude, parallel-channel oscillations with large fluctuation periods. The experimental results can provide data support for the design of a supercritical carbon dioxide passive system.