超临界二氧化碳（S-CO2）动力循环可利用工质无相变、密度大、压比低等特点，在核能、高温余热回收等应用领域中提供一种大幅提升能源利用率的变革性方案。S-CO2在拟临界区发生物性畸变，这既是降低压缩功耗、提高系统效率的关键所在，又可能诱发传热退化使得加热壁面过热失效。但当前对于S-CO2物性畸变和传热异化机理的研究并不完善，制约了S-CO2动力循环的工程研发。论文结合微观尺度的分子动力学模拟以及介观尺度的平均场理论和标度理论建立了S-CO2物性畸变的机理模型，并从传质和传热两个角度研究了物性畸变对于宏观尺度工程设计的影响。论文首先采用分子动力学模拟在微观尺度上研究了S-CO2体系的涨落行为、物性畸变特性和结构转变，建立了物性畸变的涨落理论。通过分析对分布函数、配位数等结构参数证明了超临界流体中存在类气-类液两种结构，定义了两种结构的划分方法。基于该方法提出了超临界流体团簇分析方法，揭示了超临界流体类气-类液态共存与亚临界气液相变的不同内在机制。论文从介观尺度的平均场理论和标度理论出发，建立了一种考虑外场的气液相变模型。模型实现了物性畸变的定量预测，预测结果与文献对比验证良好。通过分析温度压力主导的外场和体系内涨落对于热力学势的影响，最终将亚临界相变、临界现象和超临界流体拟临界行为纳入统一普适的理论描述。针对超临界流体的传质问题，宏观尺度上的氮气/S-CO2体系的分界面演化实验表明，随着温度的升高，超临界流体微观结构的转变导致S-CO2与氮气之间的分界面逐渐消失，最终完全互溶。结合分子动力学模拟和相平衡理论分析，论文指出了在S-CO2系统中采用无分隔的氮气稳压器设计的缺陷，提出了采用蓄能器的解决方案，并探索了S-CO2混合工质物性调控技术的应用方向。在传热问题方面，论文开展了亚临界和超临界工况下的CO2池式传热可视化实验，总结出近临界沸腾、超临界流体拟膜态沸腾和沸腾模式自蔓延的特殊传热现象。结合数值模拟和理论分析，揭示了基于热力学稳定性、水力学不稳定性和固体导热的池式传热机理。论文工作为S-CO2动力循环的研发提供技术支撑与理论基础，同时可拓展应用至与超临界流体应用相关的化学工程、航空航天领域。
Supercritical carbon dioxide （S-CO2） power cycle offers a transformative solution to significantly enhance energy utilization and thermal efficiency in nuclear energy and waste heat recovery fields, which has advantages of high density, and low compression ratio and without phase change. In the pseudo-critical region, the S-CO2 property undergoes a anomaly, which is not only crucial for reducing compression power consumption and improving system efficiency but also may induce heat transfer degradation leading to overheating failure of the heating surfaces. However, the current understandings of S-CO2 property distortion and heat transfer anomaly are not comprehensive, which hampers the engineering development of S-CO2 power cycles. This thesis combined molecular dynamics simulations at the microscopic scale with mean-field theory and scaling theory at the mesoscopic scale to establish a mechanistic model of property distortion. Then this thesis investigated the impact of these anomalies on macroscopic engineering design from both mass transfer and heat transfer perspectives.The thesis utilized molecular dynamics simulations to investigate the fluctuation behaviors, thermodynamic quantities, and structural transitions of S-CO2 at the microscopic scale, establishing a fluctuation theory of property distortion. By analyzing structural parameters such as distribution functions and coordination numbers, it was demonstrated that there existed two types of structures, gas-like and liquid-like, in supercritical fluids, and that a method for dividing these two structures was defined. Based on this method, a cluster analysis method for supercritical fluids was proposed, revealing the distinct underlying mechanisms of coexistence of gas-like and liquid-like states in supercritical fluids compared to subcritical gas-liquid phase transitions.Based on the mean field theory and scale theory of mesoscale scale, a gas-liquid phase transition model considering the external field was established. The model realized the quantitative prediction of property distortion, and the prediction results were well verified by comparison with the literature. By analyzing the influence of temperature and pressure dominated external field and internal fluctuation on the thermodynamic potential, the subcritical phase transition, critical phenomenon and pseudo-critical behavior of supercritical fluids were finally included in the unified and universal theoretical description.For mass transfer of supercritical fluids, at the macroscopic scale, experiments on the evolution of interfaces in nitrogen/S-CO2 systems indicated that with increasing temperature, the transformation of the microstructure of the supercritical fluid led to the gradual disappearance of the interface between S-CO2 and nitrogen, ultimately resulting in complete miscibility. Combining molecular dynamics simulations and phase equilibrium theory analysis, the thesis identified the drawbacks of using a non-segregated nitrogen pressurizer design in S-CO2 systems and proposed a solution using a bag accumulator. Additionally, the thesis explored the application directions of property control techniques for mixed working fluids of S-CO2.For heat transfer of supercritical fluids, the thesis conducted visual experiments on pool boiling heat transfer under subcritical and supercritical conditions of CO2, summarizing the special heat transfer phenomena including near-critical boiling, pseudo-film boiling of supercritical fluids, and self-propagating boiling mode. Combining numerical simulations and theoretical analysis, it revealed the pool boiling heat transfer mechanisms based on thermodynamic stability, hydraulic instability, and solid conduction.The works provided technical support and theoretical foundation for the development of S-CO2 power cycles, while also offering potential applications in chemical engineering and aerospace fields related to supercritical fluid applications.
