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.

Visualized observations of trajectory and dynamics of unsteady tip cloud cavitating vortices in axial flow pump

“…Yoon et al (2006) demonstrated that tip vortex becomes strengthened and migrates upstream as the flow coefficient decreases, which was also validated by Tan et al (2014) found that the evolution of TLV becomes more abrupt, in terms of the initial location and breakdown process near the pre-stall condition. Furthermore, using high-speed imaging, Tan et al (2012) also discovered that the sheet cavitation is thin at BEP condition, while it becomes thickened as a result of an increase in the incidence angle of the flow entering the passage relative to the blade. Fig.6a shows hydraulic performance of the pump model with 3 blades (BEP is under rated condition).…”

Numerical study on the cavitation phenomenon for the head drop and unsteady bubble patterns with a difference in the incidence angle of a mixed-flow pump

“…Moreover, it becomes thicker due to the increase in incidence angle. 3 In contrast, the cloud cavity is eventually separated at the impeller shroud along the streamwise direction. [4][5][6][7] The cloud cavity is complex owing to the flow rates and blade geometries.…”

A Review of Tip Clearance in Propeller, Pump and Turbine

“…The three-dimensional structure and the evolution of TLV have been recently documented using high-speed imaging [20][21][22]. Figure 10a presents the major vortex structure from 33% chord to 67% chord, illustrating that the TLV gradually moves away from the blade along the blade chord.…”

“…This paper provides an indepth survey of the TLV structure and evolution in different perspectives. From a twodimensional perspective, Miorini, Wu, and Tan [16][17][18][19][20][21][22] systemically investigated the inner structure and turbulent evolution of TLV using PIV and high-speed imaging. For example, they conducted PIV measurements at several chord sections to observe the flow structure in an axial water-jet pump (see Figures 8 and 9).…”

“…Miorini, Wu, and Tan [16][17][18][19][20][21][22] Particle-Imaging Velocimetry High-Speed Imaging Two-dimensional and three-dimensional structure of TLV in an axial waterjet pump; five periods the evolution of TLV; vortex structures and corresponding flow characteristics along blade-chord direction.…”