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Research Papers: Multiphase Flows

Impact Load Measurements in an Erosive Cavitating Flow

[+] Author and Article Information
Jean-Pierre Franc

 LEGI, Grenoble University, BP 53, 38041 Grenoble Cedex 9, FranceJean-Pierre.Franc@legi.grenoble-inp.fr

Michel Riondet

 LEGI, Grenoble University, BP 53, 38041 Grenoble Cedex 9, France

Ayat Karimi

 Swiss Federal Institute of Technology (EPFL), 1015 Lausanne, Switzerlandayat.karimi@epfl.ch

Georges L. Chahine

 DYNAFLOW , Inc., Jessup, Maryland 20794glchahine@dynaflow-inc.com

J. Fluids Eng 133(12), 121301 (Dec 19, 2011) (8 pages) doi:10.1115/1.4005342 History: Received May 11, 2011; Revised October 14, 2011; Published December 19, 2011; Online December 19, 2011

Impact load measurements were carried out in a high-speed cavitation loop by means of a conventional pressure sensor flush-mounted in the region of closure of the cavity where maximum damage was observed. The sensor was dynamically calibrated by the ball drop test technique. Pressure pulse amplitudes were measured at different velocities and constant cavitation number and cavity length. It was found that pressure pulse height spectra follow a simple exponential law, which depends upon two parameters interpreted as a reference peak rate and a reference load. By exploring the dependence of both parameters on flow velocity, it was possible to show that the various histograms measured at different velocities can be reduced to a unique non-dimensional one and derive scaling laws, which enable to transpose results from one velocity to another. The measured values of impact loads are compared to similar data in the literature, and the impact load spectra are discussed with respect to pitting test results available from a previous investigation. It is concluded that an uncertainty remains on the measured values of impact loads and that a special effort should be made to compare quantitatively pitting test results and impact load measurements. To evaluate the coherence of both sets of data with each other, it is suggested to introduce two-dimensional histograms of impact loads by considering the size of the impacted area in addition to the measured impact load amplitude. It is conjectured that the combination of impact load measurements and pitting test measurements should allow the determination of such two-dimensional histograms, which are an essential input for analyzing the material response and computing the progression of erosion with exposure time.

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Copyright © 2011 by American Society of Mechanical Engineers
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Figures

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Figure 1

View of the cavitating test section. The white region attached to the nozzle exit is a ring type cavity.

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Figure 2

View of an eroded sample (left) and of a sample instrumented with a pressure sensor (right). The sample is 100 mm in diameter. Erosion takes the form of a ring centered on the cavity closure. The sensor is mounted at the point of maximum erosion.

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Figure 3

View of the transducer at the end of the test campaign. The metallic membrane is deformed by cavitation erosion. The outer diameter of the transducer is 6.2 mm.

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Figure 4

Comparison of the transducer response at the beginning (virgin transducer) and at the end of the campaign (pitted transducer shown in Fig. 3). The upstream pressure was 10 bars.

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Figure 5

Signal delivered by the transducer during calibration by the ball drop test technique. Eight runs are superposed and show a good reproducibility. The initial drop height was h1=0.5 m.

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Figure 6

Typical visualizations of the signal for different operating conditions. The circles indicate all pulses detected above threshold 0.8 V. (a) Upstream pressure: 10 bars – Velocity on cavity: 45 m/s. (b) Upstream pressure: 20 bars – Velocity on cavity: 63 m/s. (c) Upstream pressure: 40 bars – Velocity on cavity: 89 m/s.

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Figure 7

Impact load spectra at different velocities and upstream pressures. The vertical axis is the peak rate per unit time and unit surface area. The horizontal axis is the peak amplitude measured either in Volts (bottom) or in Newtons (top). These spectra are cumulative histograms.

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Figure 8

Influence of flow velocity on peak rate (a) and load (b). (a) Reference peak rate N·0 versus flow velocity. (b) Reference load F0 versus flow velocity.

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Figure 9

Non-dimensional representation of the impact load spectra presented in Fig. 7. Non-dimensional load is defined by (F/F0)/(V/V0)0.64 and non-dimensional peak rate by (N·/N·0)/(V/V0)2.9 where V0=44.6 m/s is the minimum velocity corresponding to an operating pressure of 10 bars. Moreover, F0=26 N and N·0=9630 peaks/cm2 /s are the two parameters of the impact load spectrum at velocity V0 as defined by Eq. 2.

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Figure 10

Typical pressure pulse (upstream pressure is 20 bars). (a) Time evolution. (b) Frequency spectrum.

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Figure 11

Comparison of (i) peak rate (peaks/cm2 /s) for different thresholds in load and (ii) pitting rate (pits/cm2 /s) for different thresholds in diameter on (a) aluminum 7075 and (b) nickel aluminum bronze (NAB). The exponents of the power law for peak rate are respectively 4.5, 5.8, 7.1, 8.4, 8.2, and 9.6 for the load thresholds 112 N, 165 N, 218 N, 276 N, 329 N, and 387 N considered here. (a) Case of Al7075. The exponents of the power law for pitting rate are respectively 5.6, 5.8, and 6.8 for the diameter thresholds 10 μm, 50 μm, and 100 μm. (b) Case of NAB. The exponents of the power law for pitting rate are respectively 5.1, 5.6, and 8.4 for the diameter thresholds 10 μm, 50 μm, and 100 μm.

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