Research Papers: Multiphase Flows

Experimental Investigation of the Role of Large Scale Cavitating Vortical Structures in Performance Breakdown of an Axial Waterjet Pump

[+] Author and Article Information
David Tan

Department of Mechanical Engineering,
Johns Hopkins University,
223 Latrobe Hall,
3400 N. Charles Street,
Baltimore, MD 21218
e-mail: dtan4@jhu.edu

Yuanchao Li

Department of Mechanical Engineering,
Johns Hopkins University,
223 Latrobe Hall,
3400 N. Charles Street,
Baltimore, MD 21218
e-mail: yli131@jhu.edu

Ian Wilkes

Department of Mechanical Engineering,
Johns Hopkins University,
223 Latrobe Hall,
3400 N. Charles Street,
Baltimore, MD 21218
e-mail: iwilkes1@jhu.edu

Elena Vagnoni

Facoltà di Ingegneria Industriale,
Politecnico di Milano,
via La Masa 34,
Milano 20156, Italy
e-mail: elena.vagnoni@gmail.com

Rinaldo L. Miorini

Department of Mechanical Engineering,
Johns Hopkins University,
223 Latrobe Hall,
3400 N. Charles Street,
Baltimore, MD 21218
e-mail: rmiorin1@jhu.com

Joseph Katz

Department of Mechanical Engineering,
Johns Hopkins University,
122 Latrobe Hall,
3400 N. Charles Street,
Baltimore, MD 21218
e-mail: katz@jhu.edu

1Corresponding author.

Contributed by the Fluids Engineering Division of ASME for publication in the JOURNAL OF FLUIDS ENGINEERING. Manuscript received January 5, 2015; final manuscript received May 5, 2015; published online June 16, 2015. Assoc. Editor: Olivier Coutier-Delgosha.

J. Fluids Eng 137(11), 111301 (Nov 01, 2015) (14 pages) Paper No: FE-15-1005; doi: 10.1115/1.4030614 History: Received January 05, 2015; Revised May 05, 2015; Online June 16, 2015

Flow phenomena and mechanisms involved in cavitation breakdown, namely, a severe degradation of pump performance caused by cavitation, have been a longstanding puzzle. In this paper, results of high-speed imaging as well as pressure and performance measurements are used to elucidate the specific mechanism involved with cavitation breakdown within an axial waterjet pump. The experiments have been performed using geometrically identical aluminum and transparent acrylic rotors, the latter allowing uninhibited visual access to the cavitation phenomena within the blade passage. The observations demonstrate that interaction between the tip leakage vortex (TLV) and trailing edge of the attached cavitation near the rotor blade tip that covers the suction side (SS) of the blade plays a key role in processes leading to breakdown. In particular, the vortical cloud cavitation developing at the trailing edge of the sheet cavity near the blade tip is entrained and re-oriented by the TLV in a direction that is nearly perpendicular to the blade SS surface, and then convected downstream through the blade passage. Well above breakdown cavitation indices, these “perpendicular cavitating vortices” or PCVs occur in the region where blades do not overlap, and they only affect the local flow complexity with minimal impact on the global pump performance. With decreasing pressure and growing sheet cavitation coverage of the blade surface, this interaction occurs in the region where two adjacent rotor blades overlap, and the PCV extends from the SS surface of the originating blade to the pressure side (PS) of the neighboring blade. Cavitation breakdown begins when the PCV extends between blades, effectively blocking the tip region of the rotor passage. With further decrease in pressure, the PCVs grow in size and strength, and extend deeper into the passage, causing rapid degradation in performance. Accordingly, the casing pressure measurements confirm that attachment of the PCV to the PS of the blade causes rapid decrease in the pressure difference across this blade, i.e., a rapid decrease in blade loading near the tip. Similar large perpendicular vortical structures have been observed in the heavily loaded cavitating rocket inducers (Acosta, 1958, “An Experimental Study of Cavitating Inducers,” Proceedings of the Second Symposium on Naval Hydrodynamics, ONR/ACR-38, pp. 537–557 and Tsujimoto, 2007, “Tip Leakage and Backflow Vortex Cavitation,” Fluid Dynamics of Cavitation and Cavitating Turbopumps, L. d'Agostino and M. Salvetti, eds., Springer, Vienna, Austria, pp. 231–251), where they extend far upstream of the rotor and cause global flow instabilities.

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Fig. 1

(a) Top and (b) side views of the test facility

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Fig. 2

Trends of the vapor pressure with temperature for a 63%-by-weight solution of sodium iodide in water, obtained from Refs. [19,20-19,20]

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Fig. 3

A sketch of the AxWJ-2 pump showing the location of pressure measurements

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Fig. 4

Performance curves of the waterjet pump and conditions for the cavitation tests. Results are scaled with the BEP conditions (ψBEP=2.46 and ϕBEP=0.76). NSWCCD data and dashed lines representing the uncertainty bounds are obtained from Chesnakas et al. [24]. Hollow squares show the JHU tests with an acrylic rotor. Solid symbols represent conditions during cavitation tests described in Table 2, with the symbol indicating values for the specified σ. The arrows represent the range of conditions occurring during the cavitation breakdown tests, as the cavitation index is reduced below a critical level.

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Fig. 5

Trends of the head (ψ/ψBEP) and flow (φ/φBEP) coefficients as the cavitation number (σ) is reduced during tests B, C, and D (Table 2)

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Fig. 6

(a) A sample side view image of the Al rotor recorded at ϕ/ϕBEP = 0.915 and σ = 0.56 (test C), in which observed cavitation phenomena are highlighted. Rotor blade edges outlined in white; (b) top view of the waterjet pump rotor showing the high-speed camera location; (c) high magnification view of cavitation in tip gap; and (d) rotor blade passage cavitation phenomena.

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Fig. 7

Progression of cavitation with decreasing cavitation number: (a) σ = 0.59, (b) 0.45, (c) 0.41, (d) 0.38, and (e) 0.30, and (f) head breakdown curve, for test C, which starts with φ/φBEP = 0.915. SS surface without cavitation encircled with dashed lines.

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Fig. 8

Sequence of images at σ = 0.45 showing the trailing edge of sheet cavitation being entrained near the tip. Angular displacement of the rotor between each frame is 6 deg (i.e., time between frames is 1.11 ms).

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Fig. 9

(a) A sample image demonstrating the interaction between the sheet cavity trailing edge and the TLV for test C, at σ = 0.45. Both tip leakage cavitation sheet cavitation are outlined, and the perceived directions of rotation are indicated by arrows. (b) The TLV being entrained underneath the PCV (test C: σ = 0.43).

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Fig. 10

Sample images demonstrating the effect of cavitation number on apex angle, β

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Fig. 11

PCV being re-oriented and formed at sheet cavitation trailing edge (test A: φ/φBEP = 0.987 and σ = 0.62)

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Fig. 12

Sequence of images showing PCV formation within the acrylic rotor blade passage, looking through the blade PS (test A: φ/φBEP = 0.987 and σ = 0.32), Δt = 0.85 ms: (a) T = t0, (b) T = t0 + 3Δt, (c) T = t0 + 6Δt, and (d) T = t0 + 11Δt

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Fig. 13

Backflow cavitating vortices (Reproduced with permission from Tsujimoto, Ref. [51])

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Fig. 14

Cp1 circumferential distribution for all cavitation numbers

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Fig. 15

Cp2 circumferential distribution for all cavitation numbers

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Fig. 16

Variations of head rise, flow rate, and pressure coefficient difference across the blade at P1 and P2 with cavitation number (test D)

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Fig. 17

(a) A sample sequence of cavitation images with the specific blade phase indicated and (b) the corresponding averaged pressure coefficients of transducer P1, at σ = 0.56 (test D)

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Fig. 18

A comparison between sample cavitation images and phase-averaged pressure signals of transducer P1 in test D. The horizontal lines indicate phase, and curved vertical lines indicate the location of the transducers relative to the blade.

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Fig. 19

A comparison between sample cavitation images and phase-averaged pressure signals of transducer P2 in test D. The horizontal lines indicate phase, and curved vertical lines indicate the location of the transducers relative to the blade.



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