Experimental investigation of liquid nitrogen cavitating flows in converging-diverging nozzle with special emphasis on thermal transition

The objective of this paper is to investigate the dynamic evolution of unsteady liquid nitrogen cavitating flows in a wide range of free-stream conditions and propose a thermal parameter to evaluate and predict the transition process of two typical cavitation dynamics in liquid nitrogen. The dynamic evolutions of liquid nitrogen cavitating flows in a converging–diverging (C-D) nozzle with a throat height of 2.0 mm under a wide range of free-stream conditions were experimentally investigated. Experiments were carried out in liquid nitrogen with the temperature range from 68 K to 86 K, the pressure in the tanks is within the range of 30–300 kPa. The results show that two typical cavitation dynamics, namely the quasi-isothermal mode and the thermo-sensitive mode were observed under similar cavitation number and Reynolds number with the increasing throat temperature. The cavitation dynamics transits from the quasi-isothermal mode to the thermo-sensitive mode with the increasing throat temperature, and the transition temperature (transition mode) is approximately at 77–78 K. In the quasi-isothermal mode, the shedding cavity with clear interface collapses immediately after shedding. The cavity area increases with the increasing temperature under similar cavitation number and Reynolds number. In the transition mode, the magnitude of cavity area, the time duration of the shedding process, as well as the cavitation aggressiveness reaches the maximum values. In the thermo-sensitive mode, the shedding cavity turns to be mushy and frothy, and the mushy interface collapses slowly after shedding. The cavity area decreases with the increasing temperature under similar cavitation number and Reynolds number. When the thermodynamic effects completely dominate the change of the cavitation dynamics in the thermo-sensitive mode, the cavitation process becomes more stable. The shedding cavity collapses more slowly, while moves more quickly. The thermal parameter C-factor could quantitatively evaluate and predict dynamics transition from the quasi-isothermal mode to the thermo-sensitive mode in liquid nitrogen cavitating flows. The transition mode (transition temperature) should be prevented from causing the maximum cavitation aggressiveness in liquid nitrogen apparatus or system.