Development and Optimization of Passive Control Methods to Alleviate Cavitation Breakdown in Axial Waterjet Pumps
Abstract
Development and Optimization of Passive Control Methods to Alleviate Cavitation Breakdown in Axial Waterjet Pumps. Project summary The objective of this proposal is to develop methods to alleviate the adverse effects of cavitation breakdown on the performance of axial turbo pumps in general, and Naval waterjet pumps in particular. Cavitation breakdown refers a severe and rapid degradation of pump performance when the extent of cavitation exceeds a certain threshold level. The mechanisms causing this phenomenon have been a longstanding puzzle and subject for many studies. Recent measurements performed in our laboratory have shown that processes leading to the breakdown involve interactions between the tip leakage vortex (TLV) and the railing edge of the attached cavitation on the suction side (SS) of the blade. The vortical cloud cavitation at the trailing edge of the attached cavitation is entrained and re-oriented by the TLV in a direction that is perpendicular to the blade surface. When the resulting perpendicular cavitating vortices (PCVs) develop in the region where two adjacent rotor blades overlap, they extend from the SS surface of the originating blade to the pressure side (PS) of the neighboring blade, effectively blocking the tip region. With decreasing pressure, the PCVs grow in size and extend deeper into the passage, causing rapid degradation in performance. Based on the recent observations, it appears that altering the orientation, structure and strength of the TLV in order to prevent the formation of PCVs is a reasonable approach to alleviating or delaying the performance breakdown by cavitation. A related phenomenon is the axial backflow cavitating vortices, which develop upstream of heavily loaded rocket inducers, and play prominent roles in destructive flow instabilities. Previous successful efforts to ‘harness’ these vortices using circumferential casing grooves support the notion that casing treatment might be an effective mean of suppressing cavitation breakdown in waterjet pumps as well. Furthermore, casing grooves of different orientations and sizes have been used extensively to suppress/delay the onset of rotating stall in aviation compressors. Hence, we propose develop and test the application of circumferential (initially), axial and inclined casing grooves aimed at realigning the TLV, altering its structure, and preventing it from entraining the cloud cavitation in the regions where blades begin to overlap. The location, shape, orientation, number and size of grove(s) required to alleviate the cavitation breakdown will be determined, along with the effects of these grooves on the pump performance, as well as the flow and cavitation structures in the tip region. The cavitation visualizations, as well as detailed performance and velocity measurements will be performed in the unique optically index-matched facility at JHU using both transparent and metallic rotors of a waterjet pump (AxWJ2). With a transparent index-matched casing, this facility enables us to perform unobstructed observations on the cavitation, and using the acrylic rotor, perform stereo PIV measurements at any point within the machine, including the tip gap, and even the complex flow passages within the treated casing. Consequently, we will be able to obtain direct evidence about the effect of the grooves the TLV structure, distribution of leakage flow, associated changes to the extent of cavitation, and resulting changes to pump performance. Furthermore, in spite of numerous prior studies, there is little direct experimental evidence of how casing grooves affect the flow structure and stability tip leakage flows in axial turbomachines. Consequently, the proposed velocity measurements will have far reaching significance to a broad community that extend well beyond the primary objective of suppressing cavitation breakdown.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Aug 12, 2016
- Source ID
- N000141512561
Entities
People
- Joseph Katz
Organizations
- Johns Hopkins University
- Office of Naval Research
- United States Navy