介绍了采用膨胀机的 CO2 跨临界循环的系统流程及典型工况 ,详细分析了膨胀中经历的亚稳态过程的成因和机理 ,指出气化的滞后对系统的危害 ,并对相变膨胀过程中的成核现象进行了热力学分析。在此基础上 ,结合 CO2 超临界膨胀过程的特点 ,阐明了气液相变介质中声速的确定方法 ,认为 CO2 超临界膨胀过程可近似按照经典热力学的准静态理论进行分析 The system flow and typical working conditions of transcritical CO2 cycle using expander are introduced. The causes and mechanism of metastable process experienced during the expansion are analyzed in detail. The hazard of gasification lag to the system is pointed out. The phase change and expansion process In the nucleation phenomenon of thermodynamic analysis. On this basis, combined with the characteristics of the supercritical CO2 expansion process, the method of determining the sound velocity in the gas-liquid phase change medium is expounded. The supercritical CO2 expansion process can be approximated by the classical thermodynamic quasi-static theory