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Research Papers

The Internal Structure of the Tip Leakage Vortex Within the Rotor of an Axial Waterjet Pump

[+] Author and Article Information
Rinaldo L. Miorini, Huixuan Wu, Joseph Katz

Department of Mechanical Engineering, The Johns Hopkins University, Baltimore, MD 21218

J. Turbomach 134(3), 031018 (Jul 15, 2011) (12 pages) doi:10.1115/1.4003065 History: Received July 05, 2010; Revised August 04, 2010; Published July 15, 2011; Online July 15, 2011

The complex flow field in the tip region of a turbomachine rotor, including the tip leakage flow and tip leakage vortex (TLV), has been studied for decades. Yet many associated phenomena are still not understood. This paper provides detailed data on the instantaneous and phase-averaged inner structures of the tip flow and evolution of the TLV. Observations are based on series of high resolution planar particle image velocimetry measurements performed in a transparent waterjet pump fitted into an optical refractive index-matched test facility. Velocity distributions and turbulence statistics are obtained in several meridional planes inside the rotor. We observe that the instantaneous TLV structure is composed of unsteady vortex filaments that propagate into the tip region of the blade passage. These filaments are first embedded into a vortex sheet, which is generated at the suction side of the blade tip, and then they wrap around each other and roll up into the TLV. We also find that the leakage vortex induces flow separation at the casing endwall and entrains the casing boundary layer with its counter-rotating vorticity. As it propagates in the rotor passage, the TLV migrates toward the pressure side of the neighboring blade. Unsteadiness associated with vortical structures is also investigated. We notice that, at early stages of the TLV evolution, turbulence is elevated in the vortex sheet, in the flow entrained from the endwall, and near the vortex core. Interestingly, the turbulence observed around the core is not consistent with the local distribution of turbulent kinetic energy production rate. This mismatch indicates that, given a TLV section, production likely occurs at preceding stages of the vortex evolution. Then, the turbulence is convected to the core of the TLV, and we suggest that this transport has substantial component along the vortex. We observe that the meandering of vortex filaments dominates the flow in the passage and we decompose the unsteadiness surrounding the TLV core to contributions from interlaced vortices and broadband turbulence. The two contributions are of the same order of magnitude. During late stages of its evolution, TLV breakdown occurs, causing rapid broadening of the phase-averaged core, with little change in overall circulation. Associated turbulence occupies almost half the width of the tip region of blade passage and turbulence production there is also broadly distributed. Proximity of the TLV to the pressure side of the neighboring blade also affects entrainment of flow into the incoming tip region.

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Copyright © 2012 by American Society of Mechanical Engineers
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Figures

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Figure 1

(a) Top view of the test loop. (b) Side view of the loop. (c) Perspective view of the transparent pump and the global frame of reference. A: top and side flat external surfaces, B: inlet section, C: stator, and D: rotor; the shaft spin is indicated by Ω. (d) Meridional section of the rotor and PIV laser sheet orientation. (e) Rotor blade profiles at different spanwise locations; note the origin at the tip profile LE; span <1 profiles are shifted to match the chordline middle points, chord length and stagger angle are indicated. (f) The tip profile is repeated to highlight the chosen convention for the chord fraction; horizontal lines indicate the trace of the laser sheet as it intersects the tip profile at different chord fractions.

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Figure 2

Efficiency (η) and head rise (H) curve of the waterjet pump. The dashed line indicates the working condition during the experiment.

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Figure 3

Tip leakage vortex visualization by means of cavitation. Blades are moving top to bottom and the mean flow is right to left. (a) Initial rollup at one-third of the chord length. (b) Meandering and breakup of the TLV core shown by clusters of bubbles.

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Figure 4

Chordwise distribution of the ensemble and spatially averaged axial velocity in the tip clearance normalized with the tip speed. The dashed line indicates overall mean value.

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Figure 5

Instantaneous circumferential vorticity distributions at (a) sc−1=0.44, (b) sc−1=0.56, (c) sc−1=0.67, (d) sc−1=0.83, and (e) sc−1=1.06. Note the differences in magnification and location, as indicated by differently scaled axes. A: vorticity peak at the PS edge; B: vortices shed by the tip; C: entrained vortices; D: end-wall vorticity; E: location of the blade wake; F: rollup of vorticity downstream of the trailing edge. Vectors in (c) and (d) are diluted horizontally (1:2) for clarity; vectors in (a), (b) and (e) are not diluted.

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Figure 6

Phase-averaged circumferential vorticity at (a) sc−1=0.33, (b) sc−1=0.50, (c) sc−1=0.72, (d) sc−1=1.00, and (e) sc−1=1.17. Note the substantial differences in scale and axial position. A: vorticity peak at the pressure side of the tip gap, B: initial TLV rollup, C: passage flow boundary layer separation, D: initial detachment of the end-wall boundary layer, E: detached vorticity entrained into the TLV perimeter, F: trace of the blade wake, and G: rollup of vorticity downstream of the trailing edge. Vectors in (a) are not diluted. Vectors in (b)–(d) are diluted horizontally (1:2). Vectors in (e) are diluted horizontally and vertically (1:2).

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Figure 7

Chordwise variation of axial and radial distances between the TLV center and the blade SS corner. The dashed line is drawn to highlight the linear trend of the axial displacement.

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Figure 8

Chordwise variation in circulation of major flow structures

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Figure 9

In-plane turbulent kinetic energy at sc−1=0.72; note the domain of higher magnification data set

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Figure 10

Distributions of (a) production rate of 2D TKE, (b) normal, axial contribution to the production rate, (c) normal, radial contribution to the production rate, and (d) shear production rate at sc−1=0.72. Vectors in (b) are diluted (1:2) both horizontally and vertically for clarity.

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Figure 11

(a) In-plane TKE at sc−1=1.06 and (b) associated planar TKE production rate

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Figure 12

A scatter plot of the position of (a) negative circulation vortex centers and (b) positive circulation vortex centers located upstream of the blade SS edge. Phase-averaged circumferential vorticity contours are shown in the background.

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Figure 13

(a) Total in-plane TKE surrounding the phase-averaged TLV core within the region marked with a black square contour in Fig. 9, (b) contributions of the interacting “large-scale” vortex filaments to the TKE statistics, and (c) remaining TKE after subtracting (b) from (a)

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