小尺度超临界二氧化碳快速多相节流膨胀过程物理机理研究

In the storage, transportation and use of supercritical CO2, the highly pressurized fluids would experience a rapid multiphase expansion process when an accidental structural failure occurred, and the high pressure jet flow as well as the extreme low temperature area appeared would bring a great safety threat to the storage devices and people nearby. This research mainly focused on the basic study of the supercritical CO2 rapid expansion process which is main concern of the supercritical CO2 safety and failure monitoring investigation. First of all, the supercritical CO2 rapid expansion in the open space is investigated. Combining development of the phase change of CO2 fluid and the basic experiment data in the fluid field and temperature field before and after the expansion, the complex gas dynamics structures such as choking flow and Mach discs are analyzed. Then the rapid expansion of supercritical CO2 in the confined space is studied. The physical mechanism of the dry ice bank formation, the two-phase flow with dry ice particles and extreme low-temperature zone are systematically studied and discussed through experimental investigation. The kinetic model of multiphase rapid expansion process of supercritical CO2 at different external space is established. Finally, the multiphase rapid expansion process of supercritical CO2 is simulated through two high precision numerical methods using a two-step L-W scheme and seventh-order WENO scheme. It aims to predict the development of pressure and temperature field of supercritical CO2 accurately and to reflect the variation of the complex kinetic flow structure including the Mach disk capture during the multiphase rapid expansion of CO2. Compared with the experimental data, the physical model of supercritical CO2 multiphase rapid expansion process is verified and modified. This research is helpful to the scientific understanding of the detection and evolution of the supercritical CO2 rapid expansion process. In addition, some important scientific support and application guidance could be provided for the development of the monitoring and identification of various supercritical CO2 devices.

超临界CO2在输运、存储发生结构失效泄漏过程中，形成的高压气体射流以及低温区将对存储设备以及周围人员带来极大的安全威胁。本项目针对超临界CO2安全监测中对于高压CO2节流膨胀物理过程研究中的薄弱环节，首先对于超临界CO2在开放空间内的节流膨胀中流体相态变化、流场和温度场的发展趋势进行研究，并分析了节流膨胀近场中出现的壅塞流、马赫盘等复杂气体动力学结构。随后对于超临界CO2在受限空间内节流膨胀过程进行研究，对其中干冰层现象、干冰颗粒、固液两相流和低温区形成及变化的物理机理进行系统性的探讨。最后，对超临界CO2多相节流膨胀过程进行高精度的数值模拟研究，较为真实的反映节流近场高压CO2自由射流高速膨胀过程中的流场的变化情况，准确捕捉激波，建立符合超临界CO2在不同外部空间内多相节流膨胀过程的动力学模型，为发展能够有效分析应对各种超临界CO2流体装置失效的综合监测识别方法提供有力的科学支撑。

减少CO2排放、降低大气中CO2的浓度，是亟待解决的世界性难题。碳捕捉与封存（CCS）技术是当前最具可行性的CO2减排的方法，其中CO2主要通过管道以超临界状态运输到封存地点。在管道运输过程中CO2如果发生泄漏，将对周围环境和人员生命安全产生直接威胁。.针对超临界态CO2管道泄漏的行为、特征和探测问题，本课题采用文献调研、实验、理论和数值模拟相结合的方法开展了系统研究，进行了如下工作：.（1）开展了超临界态CO2管道泄漏实验，对其中的质量流率、压力、速度、温度等气动参量演化规律研究。获得了超临界态CO2的初始压力与泄漏孔大小、初始管内压力及温度等多种因素的耦合关系；还原了超临界态CO2在泄漏口外的温度、压力、速度、形貌等物理量以及不同相态转换的真实发展过程；阐释了不同泄漏工况下管道内部的压力、温度场、射流速度场的变化规律。.（2）开展了泄漏射流流场的物理特性以及所泄漏CO2云团扩散蔓延行为研究。研究了不同初始压力下以及不同泄漏孔径的条件下CO2云团在泄漏口外的蔓延扩散行为；准确地记录下了CO2羽流在泄漏口外的发展以及变化情况、泄漏口附近干冰层的形成及变化，并对羽流的温度以及速度进行了准确的测量，获得了许多CO2羽流变化过程中重要的原始数据，为今后CO2泄漏危害的研究工作进行了前期的基础理论铺垫。.（3）开展了对泄漏探测特征参量如管道机械振动或噪声等的早期特征及发展规律的研究。结合实验结果检验了声发射检测方法用于超临界CO2泄漏检测的可行性，并设计了泄漏源振动以及声发射源测量的实验。.（4）发展了高压CO2管道泄漏算法，并以高阶数值格式对自由射流进行模拟研究。建立了高压运输管道泄漏时CO2在管内的特征参量预测算法以及在泄漏口外作高速膨胀的高精度CFD计算方法；该算法可以较为准确地预测管道内部和出口的压力、温度等参数变化。对泄漏口附近自由射流的模拟能反映出CO2高速膨胀过程中的复杂流场的变化情况，准确捕捉激波，并且和国际上成熟的数值模拟方法所得到的结论基本一致。