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Multiphase Flows

Influence of Flow Coefficient and Flow Structure on Rotational Cavitation in Inducer

[+] Author and Article Information
Naoki Tani1

Japan Aerospace Exploration Agency, JAXA’s Engineering Digital Innovation Center, 2-1-1 Sengen, Tsukuba, Ibaraki 305-8505, Japantani.naoki@jaxa.jp

Nobuhiro Yamanishi

Japan Aerospace Exploration Agency, JAXA’s Engineering Digital Innovation Center, 2-1-1 Sengen, Tsukuba, Ibaraki 305-8505, Japanyamanishi.nobuhiro@jaxa.jp

Yoshinobu Tsujimoto

 Osaka University, Graduate School of Engineering Science, 1-3, Machikaneyama, Toyonaka, Osaka 560-8531, Japantujimoto@me.es.osaka-u.ac.jp

1

Corresponding author.

J. Fluids Eng 134(2), 021302 (Mar 06, 2012) (13 pages) doi:10.1115/1.4005903 History: Received September 07, 2011; Accepted January 23, 2012; Revised January 23, 2012; Published March 06, 2012; Online March 06, 2012

Cavitation instability is a major vibration source in turbopump inducers, and its prevention is a critical design problem in rocket-engine development. As reported by Kang , (2009, “Cause of Cavitation Instabilities in Three Dimensional Inducer,” Int. J. Fluid Mach. Syst., 2 (3), pp. 206–214), the flow coefficient plays an important role in the onset of cavitation instabilities such as rotating and asymmetric cavitation. At high flow rates, various cavitation instabilities occur; on the other hand, as the flow coefficient is reduced, these cavitation instabilities either become absent or may change in character. The purpose of the present study is to investigate the relationship between rotating cavitation and flow coefficient through numerical simulations using the Combustion Research Unstructured Navier-stokes solver with CHemistry (CRUNCH) computational fluid dynamics (CFD) code (Ahuja , 2001, “Simulations of Cavitating Flows Using Hybrid Unstructured Meshes,” J. Fluids Eng.Trans ASME, 123 (2), pp. 331–340), and to investigate the internal flow. As a first step, the interaction between the tip vortex and inducer blade was investigated through steady-state simulations. The tip vortex was identified by a vortex detection variable, i.e., the Q-function, a second invariant of the velocity tensor, and the distance between the blade and Q-function peak was measured. For a better understanding of cavitation instabilities, unsteady simulations were also performed for two different flow coefficients. The internal flow was carefully investigated, and the relation between cavity collapse/growth and the change in angle of attack was evaluated. The tip-vortex interaction is not a primary cause of unsteady cavitation, but the negative flow divergence caused by cavity collapse has a great influence on the flow angle. Moreover, changes in flow angle also introduce backflow from the tip clearance; these two factors are primary causes of cavitation instability. When the flow coefficient is large, the backflow is weak, and the interaction with the cavity collapse is strong. In contrast, as the flow coefficient decreases, stronger backflow occurs, and the interaction between backflow, cavity collapse, and flow angle weakens.

Copyright © 2012 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Suction performance and a map showing the onset regions of AC, RC, and CS [6]

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Figure 2

Test inducer geometry

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Figure 3

Computational region and mesh

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Figure 4

Static pressure performance

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Figure 5

Axial position of upstream edge of backflow

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Figure 6

Comparison of suction performance obtained from steady-state CFD simulations (a) and comparison of cavity shape (b). Superimposed AC, RC, and CS ranges are equivalent to Fig. 1

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Figure 7

Schematic diagram of tip-vortex identification and definition of tip vortex and blade leading distance Dt . Isosurface is for Q = 0.6 s−1

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Figure 8

Distance between blade tip and tip vortex. Upstream direction is a positive value. Superimposed AC, RC, and CS ranges are equivalent to Figs.  16

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Figure 9

Comparison of suction performance obtained from unsteady CFD simulations. Superimposed AC, RC, and CS ranges are equivalent to Fig. 1

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Figure 10

Spectral analysis of shaft-force fluctuations at φ/φd  = 100% based on the absolute frame

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Figure 11

Shaft-force orbit on the relative frame at φ/φd  = 100%

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Figure 12

Force direction and cavity shape (φ/φd  = 100%, top: σ = 0.036, bottom: σ = 0.044. Isosurface is 10% void fraction)

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Figure 13

Void fraction and radial velocity distribution at y = 0 plane (φ/φd  = 100%, σ = 0.044, T/Trev  = 1.8)

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Figure 14

Time histories of void fraction, velocity divergence, and angle of attack upstream of the leading edge (φ/φd  = 100%, σ = 0.044, A: T/Trev  = 0.98, B: T/Trev  = 1.62, C: T/Trev  = 1.30)

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Figure 15

Void fraction at 95% radius. The white circles are the measurement points of Fig. 1 (φ/φd  = 100%, σ = 0.044; the above figure shows two rounds)

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Figure 16

Velocity divergence at 95% radius. The white circles are the measurement points of Fig. 1 (φ/φd  = 100%, σ = 0.044; the above figure shows two rounds)

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Figure 17

Flow angles based on inlet blade angle at 95% radius. The white circles are the measurement points of Fig. 1 (φ/φd  = 100%, σ = 0.044; the above figure shows two rounds)

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Figure 18

Axial velocity at 95% radius. The black color denotes the backflow region. The white circles are the measurement points of Fig. 1 (φ/φd  = 100%, σ = 0.044; the above figure shows two rounds)

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Figure 19

Schematic drawing of rotating cavitation

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Figure 20

Cavity length along 95% radius. Blade 1 is the focusing blade in Fig. 1

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Figure 21

Void fraction, flow angle, velocity divergence, and axial velocity at 95% radius. Black color denotes backflow region. (φ/φd  = 50%, σ = 0.040; the above figure shows two rounds)

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Figure 22

Schematic diagram of the influence of the flow-angle change on the cavity

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