Visualization Experiments of Cavitation at a Rocket Tubopump Inducer in Liquid Nitrogen and Water

― For Clarifying Causes of the Thermodynamic Effect ―

Abstract

The world’s first test facility that allows the visualization of cavitation on a rotating inducer in both cryogen and water was used for comparing the cavitation features in liquid nitrogen and water. 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 inducer was made of Inconel superalloy, and the transparent casing was made of quartz glass. In addition, a mechanism to preserve the alignment of the casing with the inducer axis during thermal expansion or compression was used, so the tip clearance remained almost uniform and constant. The appearance of the cavitation was “foggy” in nitrogen and “foamy” in water. This difference can be explained by the maximum stable size of cavitation bubbles derived from the critical Weber number theory. Based on this theory, each cavitation bubble in nitrogen is four times smaller than that in water. The difference of each cavitation bubble size seems to be a cause of the thermodynamic effect.