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Experimental investigations on transient dynamics of cryogenic cavitating flows under different free-stream conditions

机译：不同自由流条件下低温空心流动瞬态动力学的实验研究

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The objective of this study is to investigate the transient dynamics of liquid nitrogen cavitating flows under a wide range of free-stream conditions. The temporal and spatial resolution of cavity images was improved to analyze in detail the cavity structures and unsteady behaviors. The Proper orthogonal decomposition (POD) method based on the clear experimental images was applied to investigate the spatial structures. The unsteady evolution of discrete bubbles at different temperatures was compared to analyze the effects of heat transfer on the collapse process. The results show that: (1) as the cavitation number increases, four typical cavitation patterns through the convergent-divergent (C-D) nozzle, namely choke cavity, large-scale cloud cavity, sheet cavity and incipient cavity are observed. (2) As the temperature increases, the significant re-entrant jet becomes invisible, and the large-scale cloud cavity is absent. The contributions of the lowest POD modes decrease with the increasing temperature. The dominant POD structure changes from a single large area to several counter-rotating structures with increasing temperature. This reflects the thermal effects significantly change the shedding behaviors of the detached cloud cavities. (3) The significant thermal effects decrease the condensation rates of the discrete cavity bubbles. The collapse mechanisms of the discrete bubbles in the inertial cavitation and thermal cavitation are different. For the inertial cavitation, the condensation rates of cloud cavities and the discrete bubbles are close. For the thermal cavitation, the condensation rates of cloud cavities are much larger than that of a single bubble.
机译：本研究的目的是在广泛的自由流条件下研究液氮空化流量的瞬态动态。腔图像的时间和空间分辨率得到改善以详细分析腔结构和不稳定的行为。基于清除实验图像的适当正交分解（POD）方法应用于研究空间结构。比较不同温度下离散气泡的不稳定演变，以分析传热对塌陷过程的影响。结果表明：（1）由于空化数增加，观察到通过会聚 - 发散（C-D）喷嘴的四个典型的空化图案，即扼流腔，大规模云腔，板腔和初始腔。 （2）随着温度升高，显着的再参赛射流变得不可见，不存在大规模的云腔。最低豆荚模式的贡献随着温度的增加而降低。主导荚结构从单个大面积变为几个反向旋转结构，随着温度的增加。这反映了热效应显着改变了分离的云腔的脱落行为。 （3）显着的热效应降低了离散腔气泡的冷凝率。惯性空化和热空化中的离散气泡的折叠机构是不同的。对于惯性空化，云腔和离散气泡的冷凝率接近。对于热空化，云腔的冷凝速率远大于单个泡沫的冷凝率。

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