Abstract:

When transcritical CO₂ expands at a high speed, the pressure and temperature drop sharply, and a non-equilibrium phase change occurs. The non-equilibrium condensation phase change of CO₂ would occur in natural gas supersonic separation equipment and supercritical CO₂ centrifugal compressor. Besides, the non-equilibrium flashing phase change of transcritical CO₂ occurs in the ejector primary nozzle in the ejector expansion refrigeration system. In order to solve the problem that the physical properties of transcritical CO₂ change sharply during the expansion process and it is difficult to simulate the non-equilibrium phase change, a new non-equilibrium phase change CFD model was constructed to study the transcritical CO₂ non-equilibrium condensation and flashing phase change process and expansion mechanisms in the supersonic converging-diverging nozzle. The model coupled the temperature-driven evaporation-condensation phase change mechanism and the pressure-driven cavitation-condensation phase change mechanism, and the accuracy of the model was verified by the experimental results in literature. The results showed that the pressure-driven condensation mass transfer had a major influence on condensation phase change. The pressure-driven condensation mass transfer mainly existed in the nozzle throat and internal flow zone, and the temperature driven condensation mass transfer mainly existed on the wall of the nozzle diverging section. The condensation mass transfer rate increased with the increase of inlet pressure and the decrease of inlet temperature, so that the non-equilibrium degree of condensation and the quality in the nozzle were reduced, and the position of speed of sound was delayed accordingly in the nozzle diverging section. In addition, the temperature-driven evaporation mass transfer dominated the flashing phase change, the evaporation phase change mainly occurred near the nozzle throat, the cavitation phase change mainly occurred in the nozzle diverging section, and the two-phase CO₂ reached speed of sound in the nozzle diverging section. With the increase of inlet pressure and the decrease of inlet temperature, the non-equilibrium degree of flashing increased and the quality in the nozzle decreased. This study was helpful to clarify the non-equilibrium flashing and condensation phase change mechanism in the rapid expansion of transcritical CO₂, and it provided a method for the analysis and optimization design of transcritical CO₂ expansion equipment.

Key words: carbon dioxide, flashing, condensation, supersonic nozzle, gas-liquid flow, CFD

摘要：

跨临界CO₂在高速膨胀时，压力和温度剧烈下降，会发生非平衡相变。其中在天然气超声速分离设备和超临界CO₂离心压缩机中CO₂会发生非平衡冷凝相变；在引射膨胀制冷系统中，跨临界CO₂在引射器主动喷嘴中发生非平衡闪蒸相变。为解决跨临界CO₂在膨胀过程中物性变化剧烈，非平衡相变模拟困难的问题，构建了新型非平衡相变CFD模型，以研究跨临界CO₂在超声速缩放喷嘴中的非平衡冷凝和非平衡闪蒸的相变过程和膨胀机理，模型耦合了温度驱动的蒸发-冷凝相变机制和压力驱动的空化-冷凝相变机制，并用文献中的试验结果验证了模型的准确性。研究结果表明，在冷凝相变过程中，由压力驱动的冷凝传质具有主要影响，压力驱动的冷凝传质主要存在于喷嘴喉部与内流区域，温度驱动的冷凝传质主要存在于喷嘴渐扩段壁面。冷凝传质速率随着进口压力的增加和进口温度的降低而增加，从而使冷凝的非平衡程度和喷嘴内的干度降低，喷嘴渐扩段内达到声速的位置也相应延后。在闪蒸相变过程中，由温度驱动的蒸发传质占据主导，蒸发相变主要发生在喷嘴喉部附近，空化相变主要发生在喷嘴渐扩段，两相CO₂在喷嘴的渐扩段达到声速。随着喷嘴进口压力的增加和进口温度的降低，闪蒸的非平衡程度增加，使喷嘴内的干度减小。本研究有助于厘清跨临界CO₂快速膨胀中的非平衡闪蒸和冷凝相变机理，并为跨临界CO₂膨胀设备的分析和优化设计提供参考。

关键词: 二氧化碳, 闪蒸, 冷凝, 超声速喷嘴, 气液两相流, 计算流体力学