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Research Papers: Multiphase Flows

Estimation of Cavitation Limits From Local Head Loss Coefficient

[+] Author and Article Information
Raúl Sánchez

Departamento de Ingeniería Rural, Technical University of Madrid (UPM), ETSI Agrónomos, Ciudad Universitaria s/n, Madrid 28040, Spainraul.sanchez@upm.es

Luis Juana

Departamento de Ingeniería Rural, Technical University of Madrid (UPM), ETSI Agrónomos, Ciudad Universitaria s/n, Madrid 28040, Spainluis.juana@upm.es

Francisco V. Laguna

Departamento de Ingeniería Civil: Hidráulica y Energética, Technical University of Madrid (UPM), ETSI Caminos, Canales y Puertos, C/Profesor Aranguren s/n, Madrid 28040, Spainhe15@caminos.upm.es

Leonor Rodríguez-Sinobas

Departamento de Ingeniería Rural, Technical University of Madrid (UPM), ETSI Agrónomos, Ciudad Universitaria s/n, Madrid 28040, Spainleonor.rodriguez.sinobas@upm.es

J. Fluids Eng 130(10), 101302 (Sep 04, 2008) (9 pages) doi:10.1115/1.2969453 History: Received October 11, 2007; Revised May 19, 2008; Published September 04, 2008

Cavitation effects in valves and other sudden transitions in water distribution systems are studied as their better understanding and quantification is needed for design and analysis purposes and for predicting and controlling their operation. Two dimensionless coefficients are used to characterize and verify local effects under cavitating flow conditions: the coefficient of local head losses and the minimum value of the cavitation number. In principle, both coefficients must be determined experimentally, but a semianalytical relationship between them is here proposed so that if one of them is known, its value can be used to estimate the corresponding value of the other one. This relationship is experimentally contrasted by measuring head losses and flow rates. It is also shown that cavitation number values, called cavitation limits, such as the critical cavitation limit, can be related in a simple but practical way with the mentioned minimum cavitation number and with a given pressure fluctuation level. Head losses under conditions of cavitation in sharp-edged orifices and valves are predicted for changes in upstream and downstream boundary conditions. An experimental determination of the coefficient of local head losses and the minimum value of the cavitation number is not dependent on the boundary conditions even if vapor cavity extends far enough to reach a downstream pressure tap. Also, the effects of cavitation and displacement of moving parts of valves on head losses can be split. A relatively simple formulation for local head losses including cavitation influence is presented. It can be incorporated to water distribution analysis models to improve their results when cavitation occurs. Likewise, it can also be used to elaborate information about validity limits of head losses in valves and other sudden transitions and to interpret the results of head loss tests.

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Copyright © 2008 by American Society of Mechanical Engineers
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Figures

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Figure 1

Cavitating region downstream a sharp-edged orifice and piezometric levels indicated by a differential air manometer

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Figure 2

(a) Valve between two tanks. (b) Head losses as a function of flow under different system-imposed boundary conditions.

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Figure 3

View of cavitating flow downstream of one of the tested sharp-edged orifices

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Figure 4

Hydraulic schematic diagram of the test rig

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Figure 5

Experimental and predicted results of head loss tests on sharp-edged orifices of diameter d in a pipe with an internal diameter D=16mm

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Figure 6

Results of the proposed analysis applied to data from Fig. 5

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Figure 7

Experimental results of head loss tests and predicted curves on the sharp-edged orifice of diameter d=8.93mm in a pipe with an internal diameter D=16.2mm

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Figure 8

Results of the proposed analysis applied to data from Fig. 7

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Figure 9

Experimental and prediction results of head loss tests on a butterfly valve with a nominal diameter of 75mm and a closing disk at 45deg

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Figure 10

Results of the proposed analysis applied to data from Fig. 9

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Figure 11

Relations between experimentally determinated values of Km and σ1m on the two tested valves

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Figure 12

Head loss curves in the tested butterfly valve for different valve positions and approximate validity bounds as a function of the downstream head pressure

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Figure 13

Explanation of the relation between the critical cavitation limit values determined by Tullis (3) and the head loss coefficient based on the influence of the pressure fluctuation level

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Figure 14

Explanation of the relation between the incipient cavitation limit values determined by Tullis (3) and the head loss coefficient based on the influence of the pressure fluctuation level

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