The current work presents a study of qualitative relationships between the flow unsteadiness, induced by nozzle geometry variation, and non-equilibrium condensation behavior at high temporal scales for supercritical carbon dioxide (sCO${}_2$) flow. A closed sCO${}_2$ loop facility is used to drive a supercritical carbon dioxide flow through a rectangular converging–diverging channel. As the flow traverses through the channel, it experiences a local pressure reduction in a stalled region, eventually leading to non-equilibrium condensation of the working fluid. The flow is visualized using high-speed shadowgraphy and schlieren techniques to characterize the unsteady flow dynamics at the diverging section of the nozzle. Spectral information of the condensation behavior is retrieved from the optical diagnostics using power spectral density calculations, revealing multiple dominant frequencies in the diverging section of the nozzle. Three different nozzles are studied to understand how these dominant frequencies of condensation phenomenon change with respect to the nozzle geometry. Each nozzle design is distinguished with a unique dominant frequency, which is significantly impacted by the effect of flow throat velocity and the degree of adverse pressure gradient. Nozzles with divergence of 15° shows a dominant frequency in the range of 5 – 6.25 kHz in condensing behavior while nozzles with 6° divergence exhibited half the frequency of the phase change. These effects resulted from the extent of adverse pressure gradient imposed by the divergence angle of the nozzle. Additionally, these dominant frequencies are also compared to the wall-pressure signals acquired from high-frequency pressure transducers. Spectral analysis of wall-pressure signals, which are directly linked to nozzle designs, reveals a strong coupling between the condensation behavior and the wall-pressures.

当前的工作提出了由喷嘴几何形状变化引起的流动不稳定性与超临界二氧化碳（sCO）在高时间尺度上的非平衡冷凝行为之间的定性关系的研究${}_{\mathrm{2个}}$） 流。封闭的上合组织${}_{\mathrm{2个}}$回路设备用于驱动超临界二氧化碳流经矩形的会聚-扩散通道。当流体横穿通道时，它会在失速区域经历局部压力降低，最终导致工作流体的不平衡冷凝。使用高速影印术和schlieren技术将流动可视化，以表征喷嘴发散部分处的不稳定流动动力学。使用功率谱密度计算从光学诊断中检索冷凝行为的光谱信息，从而揭示喷嘴发散部分中的多个主频。对三种不同的喷嘴进行了研究，以了解冷凝现象的这些主要频率如何相对于喷嘴几何形状发生变化。每种喷嘴设计均具有独特的主导频率，这主要受到流喉速度和不利压力梯度程度的影响。发散角为15°的喷嘴在冷凝行为中显示的主频率在5 – 6.25 kHz范围内，而发散角为6°的喷嘴显示的是相变频率的一半。这些效果是由喷嘴的发散角所施加的不利压力梯度的程度所导致的。此外，还将这些主频率与从高频压力传感器获取的壁压力信号进行比较。与喷嘴设计直接相关的壁压信号的频谱分析显示出冷凝行为和壁压之间的强耦合。喉咙流速和逆压力梯度程度的影响会极大地影响这一点。发散角为15°的喷嘴在冷凝行为中显示的主频率在5 – 6.25 kHz范围内，而发散角为6°的喷嘴显示的是相变频率的一半。这些效果是由喷嘴的发散角所施加的不利压力梯度的程度所导致的。此外，还将这些主频率与从高频压力传感器获取的壁压力信号进行比较。与喷嘴设计直接相关的壁压信号的频谱分析显示出冷凝行为和壁压之间的强耦合。喉咙流速和逆压力梯度程度的影响会极大地影响这一点。发散角为15°的喷嘴在冷凝行为中显示的主频率在5 – 6.25 kHz范围内，而发散角为6°的喷嘴显示的是相变频率的一半。这些效果是由喷嘴的发散角所施加的不利压力梯度的程度所导致的。此外，还将这些主频率与从高频压力传感器获取的壁压力信号进行比较。与喷嘴设计直接相关的壁压信号的频谱分析显示出冷凝行为和壁压之间的强耦合。发散角为15°的喷嘴在冷凝行为中显示的主频率在5 – 6.25 kHz范围内，而发散角为6°的喷嘴显示的是相变频率的一半。这些效果是由喷嘴的发散角所施加的不利压力梯度的程度所导致的。此外，还将这些主频率与从高频压力传感器获取的壁压力信号进行比较。与喷嘴设计直接相关的壁压信号的频谱分析显示出冷凝行为和壁压之间的强耦合。发散角为15°的喷嘴在冷凝行为中显示的主频率在5 – 6.25 kHz范围内，而发散角为6°的喷嘴显示的是相变频率的一半。这些效果是由喷嘴的发散角所施加的不利压力梯度的程度所导致的。此外，还将这些主频率与从高频压力传感器获取的壁压力信号进行比较。与喷嘴设计直接相关的壁压信号的频谱分析显示出冷凝行为和壁压之间的强耦合。这些影响是由喷嘴的发散角所施加的不利压力梯度的程度引起的。此外，还将这些主频率与从高频压力传感器获取的壁压力信号进行比较。与喷嘴设计直接相关的壁压信号的频谱分析显示出冷凝行为和壁压之间的强耦合。这些效果是由喷嘴的发散角所施加的不利压力梯度的程度所导致的。此外，还将这些主频率与从高频压力传感器获取的壁压力信号进行比较。与喷嘴设计直接相关的壁压信号的频谱分析显示出冷凝行为和壁压之间的强耦合。