超临界二氧化碳矩形回路自然循环稳态数值研究

Steady State Numerical Study on the Natural Cyclic of Supercritical Carbon Dioxide in a Rectangle Loop

超临界二氧化碳在超临界区具有良好的热力学性能,作为布雷顿循环的工质,相比于传统的朗肯循环有着更高的效率,自然循环是提升系统非能动安全性的技术基础,超临界二氧化碳循环系统的稳态特性是设计的关键。为研究超临界二氧化碳自然循环稳态特性,采用计算流体力学(CFD)方法,对超临界二氧化碳在小型矩形回路内自然循环换热特性进行三维稳态数值模拟,冷热段高差1.0 m,压力范围7~11 MPa,加热壁面温度320~370 K。模拟结果显示,当流体温度达到拟临界温度附近时,质量流量、换热功率和表面换热系数显著升高。由于在拟临界点附近剧烈的物性变化,受浮升力和重力影响,水平管内局部温度呈现底部向顶部逐渐升高。基于模拟结果拟合了稳态雷诺数Re和修正格拉晓夫数Grm的关系式。为超临界二氧化碳非能动安全系统的设计和分析提供数据基础。

Due to the admirable thermal characteristics in the supercritical region, carbon dioxide is considered for use as working fluid of Brayton cycle, which could achieve higher efficiency compared with steam Rankine cycle. Natural circulation is the technical basis for improving passive safety of systems, and the steady-state characteristics are essential for the design of supercritical carbon dioxide power conversion system. In order to study the steady-state characteristics of the natural circulation of supercritical carbon dioxide, a three-dimensional steady-state simulation of the heat transfer characteristics of the natural circulation of supercritical carbon dioxide in a small rectangular loop was carried out by using the CFD method. The simulation parameters are as follows: the height difference of cold and hot section is 1.0 m, the pressure range is 7 - 11 MPa, and the heating wall temperature range is 320 - 370 K. The simulation results show that the local temperature in the horizontal tube increases gradually from the bottom to the top due to the drastic thermal properties changes near the quasi-critical point and the influence of buoyancy and gravity. When the fluid temperature reaches near the quasi-critical temperature, the mass flow rate, heat transfer power and surface heat transfer coefficient increase significantly. Based on the simulation results, the relationship between steady-state Reynolds number Re and modified Grashof number Grm is fitted. The simulation results can provide reference for the design and analysis of passive safety system of supercritical carbon dioxide.