Cavitation flow instability of subcooled liquid nitrogen in converging-diverging nozzles

The cavitation flow instability of subcooled liquid nitrogen in two types of converging-diverging (C-D) circular nozzles with throat diameters of 1.5 and 2.0 mm was experimentally investigated. Flow observations were also performed to clarify the instability phenomenon and the differences in cavitation behavior between the two nozzles. The cavitation mode changed from continuous mode to intermittent mode as the temperature of the subcooled liquid nitrogen decreased. This change occurred in both C-D nozzles when the temperature of the liquid reached approximately 76 K. Occurrence of the intermittent mode accompanying very large pressure-oscillations was considered to be caused by a drastic reduction of the speed of sound in the single-component, vapor-liquid flow because the speed of sound restricted the throat velocity in the C-D nozzle during cavitation. Oscillation pressure values in intermittent mode were much larger than those in continuous mode, peaking between 74 and 76 K. The magnitude of the oscillation pressure in intermittent mode could be evaluated from the difference between the throat static-pressure immediately prior to the occurrence of cavitation and that during cavitation.