EXPERIMENTAL STUDY ON THE EVOLUTION OF LIQUID NITROGEN CAVITATING FLOWS THROUGH CONVERGING-DIVERGING NOZZLE

The objective of this paper is to investigate the unsteady characteristics of liquid nitrogen cavitating flow in a converging-diverging (C-D) nozzle via a cryogenic experimental facility. A high-speed camera with high resolution was employed to study the evolution of cavity with varying cavitation numbers σ under T_throat ≈ 77 K. In order to quantitatively analyze the unsteady characteristics and temporal-spatial evolution, processed data such as the length and area of cavitation based on experimental images were obtained. The results show that: (1) As the cavitation number decreases and under similar free-stream velocity and temperature, the liquid nitrogen cavitation shows four typical flow patterns, with the cavitation length within 2.5 h for incipient cavitation, between 2.5 h and 7.5 h for sheet cavitation, between 7.5 h and 15 h for large-scale cloud cavitation, and over 15 h for double-cloud cavitation, Additionally, a significant phenomenon of re-entrant jet is captured in the large-scale cloud cavitation and double-cloud cavitation; (2) as the liquid nitrogen cavitating flow evolves from incipient cavitation to double-cloud cavitation, the scale of shedding cavity increases gradually, meanwhile, the amplitude and quasi-period of cavity area pulsation is getting longer. In addition, it is observed that the blockage effect on the cavitation flow at the throat is significantly enhanced in the large-scale cloud cavitation and double-cloud cavitation; (3) compared with incipient cavitation, the travel distance of shedding cavities increases by 0.97, 2.65 and 2.68 times in sheet cavitation, large-scale cloud cavitation and double-cloud cavitation, and the collapse time increases by 1.18, 3.59 and 4.47 times, respectively. For the double-cloud cavitation, there are two significantly different evolutions of shedding cavity.