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Research Papers: Multiphase Flows

Shear Cavitation in an Annular Jet Pump Under Recirculation Conditions

[+] Author and Article Information
Longzhou Xiao, Junqiang Zhang

School of Power and Mechanical Engineering,
Wuhan University;
Key Lab of Jet Theory and
New Technology of Hubei Province,
Wuhan 430072, Hubei, China

Xinping Long

School of Power and Mechanical Engineering,
Wuhan University;
Key Lab of Jet Theory and
New Technology of Hubei Province,
Wuhan 430072, Hubei, China
e-mail: xplong@whu.edu.cn

1Corresponding author.

Contributed by the Fluids Engineering Division of ASME for publication in the JOURNAL OF FLUIDS ENGINEERING. Manuscript received October 28, 2014; final manuscript received December 22, 2015; published online March 15, 2016. Assoc. Editor: Satoshi Watanabe.

J. Fluids Eng 138(6), 061303 (Mar 15, 2016) (14 pages) Paper No: FE-14-1617; doi: 10.1115/1.4032487 History: Received October 28, 2014; Revised December 22, 2015

Recirculation accompanied by shear cavitation is a key flow feature in annular jet pumps (AJPs). In this study, a high-speed camera was used to capture the recirculation region and various types of cavity clouds. By monitoring the trajectories of the small bubbles, the main recirculation regions under each flow rate ratio were obtained. As the flow rate ratio decreases, the recirculation region continued expanding with the separation point moving upstream, while the reattachment point remained nearly stationary regardless of the decreasing flow rate ratio. Hill's spherical vortex theory was adopted to evaluate the variations of the recirculation regions. Moreover, the minimum local static wall pressure in the recirculation region decreases as well, which can promote the inception and development of shear cavitation. There are numerous vortices simultaneously induced by the large velocity gradient in the shear layer, the core of which becomes a potential site for cavitation. Consequently, with the growth of the recirculation region, three types of cavity clouds, viz., the ribbonlike, annular, and merged cavity clouds, appear in turn. The movement characteristics of these cavity clouds, including their inception, distortion, and collapse, are illustrated based on the high-speed imaging results. The ribbonlike and annular cavity clouds are both induced by the small vortices in the shear layer because of the low local pressure in the vortex core. However, the merged cavity clouds are caused by a combination of several ribbonlike and annular cavity clouds, which provides a larger scale and a longer life span. Hence, the collapse of the merged cavity clouds can cause a large pressure pulsation near the reattachment point of the recirculation region. The corresponding frequency spectra were also demonstrated based on the fast Fourier transform (FFT) method.

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Figures

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Fig. 1

Schematic drawing of AJP (unit: mm)

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Fig. 2

Configuration of the nozzle (unit: mm)

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Fig. 3

Sketch of the experimental rig: 1, 8, 11, and 13—values; 2 and 12—magnetic flow meter; 3—air release valve; 4 and 7—pressure gauge; 5—AJP; 6—pressure transducer; 9—water tank; and 10—baffle plate

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Fig. 4

Layout of the high-speed video system and lamps: (a) top view of the test section and (b) side view of the test section

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Fig. 5

Pump efficiency under different flow rate ratios

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Fig. 6

Sketch of inner flow in an AJP

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Fig. 7

A typical process for a shed bubble collapsing near the nozzle lip

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Fig. 8

A typical process of a suctioned bubble being entrained into the shear layer

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Fig. 9

A typical process of small bubbles being entrained into the recirculation region

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Fig. 10

Recirculation region under different flow rate ratios

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Fig. 11

Cp distributions of four tested working conditions

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Fig. 12

A typical ribbonlike cavity cloud forming in the shear layer (q = 0.43)

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Fig. 13

Ribbonlike cavity clouds under different flow rate ratios: (a) q = 0.43, (b) q = 0.31, (c) q = 0.19, and (d) q = 0.09

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Fig. 14

Formation and disappearance of a typical annular cavity cloud

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Fig. 15

Different annular cavity clouds under q = 0.19

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Fig. 16

Time evolution of a typical cavity cloud merging (q = 0.09)

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Fig. 17

A comparison of the intensity of cavity clouds under different flow rate ratios

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Fig. 18

Time evolution of wall pressure at (a) x/Dt = −1.618 and (b) x/Dt = −0.500 (corresponding to the first and third pressure tap in the suction chamber; sampling frequency: 1000Hz)

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Fig. 19

Typical frequency spectra of pressure pulsation (p − patm) at (a) x/Dt = −1.618 and (b) x/Dt = −0.500

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