COMMON FEATURES IN THE STRUCTURE OF TIP LEAKAGE FLOWS

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Date
2015-03-20
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Johns Hopkins University
Abstract
There are many common features in the flow structure within the tip region of turbomachines. However, neither detailed measurements nor accurate simulations of tip leakage flows are currently available to enable a meaningful comparison between significantly different turbomachines. Using a combination of time-resolved two-dimensional Particle Image Velocimetry (PIV), and high-resolution Stereoscopic PIV (SPIV), as well as cavitation visualization within a naval waterjet pump and an aviation compressor, the tip region flow structures are elucidated and several commonalities are found. Time-resolved PIV and SPIV measurements characterize the behavior of the tip leakage vortex (TLV), as it rolls up, grows, migrates across the blade passage, and eventually bursts. The high-temporal resolution measurements within the compressor show that the TLV consists of multiple interlacing vortices that never roll up into a single structure. SPIV measurements, recorded at several meridional planes, provide phase-averaged distributions of all three velocity components, and in-plane vorticity in high spatial resolution, resolving the flow within the tip gap. Turbulence statistics identify localized regions of elevated turbulence, and demonstrate high inhomogeneity and anisotropy. Analyses of individual components of the turbulence production rate pinpoint the flow mechanisms responsible for turbulence production. As the TLV migrates to higher chord fractions, it exhibits characteristics that indicate vortex bursting: a sharp fall in peak vorticity and turbulent kinetic energy (TKE), and the spread of elevated vorticity and TKE across a substantial fraction of the blade passage. Blockage effects, associated with regions of low momentum fluid growing with increasing chord fraction, are also found in both machines. Extensive cavitation testing using high-speed imaging and casing pressure measurements identifies the mechanism responsible for cavitation breakdown in the waterjet pump, namely, the interaction between the TLV and trailing edge of the attached cavitation near the rotor blade tip that covers the blade suction side. This interaction results in the formation of vortical cavitating structures aligned perpendicularly to the blade suction side. When these “Perpendicular Cavitating Vortices” form in the blade overlap region, and extend across the blade passage to the pressure side of the neighboring blade, rapid performance degradation occurs.
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Keywords
Turbomachinery, Compressor, Waterjet Pump, Tip Leakage Vortex, Tip, Leakage Flows, SPIV, PIV, Cavitation, Cavitation Breakdown
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