||Experimental Observationof Backflow-vortex Cavitationon a Rotating Inducer
Experimental Observationof Backflow-vortex Cavitationon a Rotating Inducer
伊藤, 優 ,
ITO, Yu ,
長崎, 孝夫NAGASAKI, TAKAO
This study was for comparing the cavitation features in liquid nitrogen at 77.9 K and water at 292.5 K and 333.5 K on a rotating inducer. The test inducer was a triple-threaded helical one with a diameter of 65.3 mm and a rotational speed range of 3500–6000 rpm. The size of tip-vortex cavitation bubbles, and characteristics of backflow-vortex cavitation were quantitatively measured in the considered fluids. From the results, it was inferred that the backflow-vortex cavitation orbital rate, its orbital diameter around the inducer axis, and the number of backflow-vortex cavitation columns depended on the head coefficient but were almost independent of the cavitation number and kind of fluid. Especially, the number of backflow-vortex cavitation columns decreased as the head coefficient increased. This tendency was explained by a theoretical analysis in which backflow vortices were regarded as an array of vortex filaments orbiting along the casing around the inducer center. On the other hand, the size of each tip-vortex cavitation bubble and the diameter of each backflow-vortex cavitation column depended on the thermodynamic properties of the considered fluids. The size of each tip-vortex cavitation bubble was explained by the critical Weber number theory. Based on this theory, each cavitation bubble at the tip-vortex regions in nitrogen was four times smaller than that in water. This affected the appearance of the cavitation: the cavitation was “foggy” in nitrogen and “foamy” in water. The diameter of each backflow-vortex cavitation column depended on the head coefficient, cavitation number, and kind of fluid. At the same head coefficient and cavitation number, the diameter in liquid nitrogen was 2.4 times smaller than that in water because of the thermodynamic effects.