Flow Visualization Using Cavitation Within Blade Passage of an Axial Waterjet Pump Rotor

Abstract

Cavitation phenomena within an axial waterjet pump, AxWJ-2 [1,2] operating at and below the best efficiency point (BEP) are investigated using high-speed imaging. The purpose of these preliminary observations is to provide an overview of the physical appearance of several forms of cavitation under varying flow and pressure conditions. These observations provide a motivation for upcoming detailed velocity and turbulence measurements. The experiment is conducted using a transparent pump installed in an optically index-matched facility, which facilitates unobstructed visual access to the pressure and suction sides of the rotor and stator blade passages. By varying the cavitation index within the facility, the observations follow the gradual development of cavitation from inception level to conditions under which the cavitation covers the entire blade. Cavitation appears first in the tip gap, as the fluid is forced from the pressure side (PS) to the suction side (SS) of the rotor blade. Bubbly streaks start at the SS corner, and penetrate into the passage, and are subsequently entrained into the tip leakage vortex (TLV) propagating in the passage. Sheet cavitation also develops along the SS of the rotor leading edge and covers increasing fractions of the blade surface with decreasing cavitation number. At BEP conditions, the sheet is thin. Below BEP, the blade loading increases as a result of an increase in the incidence angle of the flow entering the passage relative to the blade. Consequently, the backward leakage flow also increases, further increasing the incidence angle in the tip region, and thickening the sheet cavitation there. Consistent with previous observations on swept hydrofoils, a re-entrant jet that flows radially outward develops at the trailing edge of the sheet cavitation. Only near the tip corner the trailing edge of the sheet cavitation is opened as the radial re-entrant flow is entrained into the TLV, forming an unstable and noisy spiraling pattern. Within a certain range of cavitation indices, when the sheet cavitation length at the blade tip extends to about 50–60% of the blade spacing, the sheet cavitation on every other blade begins to expand and contract rapidly, generating loud low-frequency noise. With further decrease in pressure, persistent alternating cavitation occurs, namely, the cavitating region on one blade becomes much larger than that in the neighboring one. The mechanisms involved and associated instabilities are discussed based on previous analyses performed for inducers. As the cavitation number is lowered even further, the sheet cavitation on the “heavily-cavitating” blade grows, and eventually passes the trailing edge of the rotor blade. At this condition, cavitation begins again to expand and contract rapidly on the “less-cavitating” blade, covering a significant portion of SS surface. At a lower pressure, all the blades cavitate, with the sheet cavitation covering the entire SS surface of the rotor blade. The large cavities on alternate rotor blade surfaces re-direct flow into the neighboring passages with the smaller cavities. As a result, there is a lower flow rate in the passage with the larger cavitation and higher flow rate in the neighboring passage. As the flow with the cavitating passage arrives to the leading edge of the stator flow rate, it increases the incidence angle at the entrance to the stator, causing intermittent sheet and cloud cavitation on the stator blade.