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

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.