超临界二氧化碳双通道流动不稳定性三维CFD数值研究

Three-Dimensional CFD Numerical Study of Supercritical Carbon Dioxide Two-Channel Flow Instability

超临界二氧化碳在拟临界点附近具有良好的导热性能和易压缩性,采用超临界二氧化碳作为换热工质的布雷顿循环系统拥有可观的系统热效率,但物性的剧烈变化可能会导致流动不稳定性问题。采用三维数值模拟方法对超临界二氧化碳双通道流动不稳定性问题进行了研究,与实验进行了对比,分析了不稳定起始至结束整个过程中双通道间的状态变化。已有的数值结果表明超临界二氧化碳流动不稳定性分为三个阶段,随着加热功率的线性增加,流动状态经历波动起始、持续波动和波动结束三个过程。质量流量、流体密度和主流温度均会呈现正余弦波动状态,三者存在一定的相位延迟。壁面温度和表面换热系数会随着流体的波动而出现相应的变化,壁面出现最大温度点和最大表面系数换热点。

Supercritical carbon dioxide has outstanding thermal conductivity and compressibility near the pseudo-critical point. The Brayton cycle system which uses supercritical carbon dioxide as heat exchanger has considerable thermal efficiency. However, the drastic change of physical properties may lead to the problem of flow instability. Three-dimensional numerical simulation is used to study the supercritical carbon dioxide two-channel flow. The calculated results are compared with the experimental results and the state changes between the two channels are analyzed during the whole process. The numerical results show that flow instability of the supercritical carbon dioxide is divided into three stages. With the linear increase of heating power, the flow state goes through three processes: the beginning of fluctuation, the continuous and the end of fluctuation. The mass flow rate, fluid density and flux temperature show sine and cosine fluctuation state, and there is a certain phase delay. The wall temperature and surface heat transfer coefficient will change with the fluctuation of the fluid, and the maximum temperature and the maximum surface heat transfer coefficient will occur on the wall.