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Research Papers: Multiphase Flows

Modeling Viscous Oil Cavitating Flow in a Centrifugal Pump

[+] Author and Article Information
Wen-Guang Li

Professor
Department of Fluid Machinery,
Lanzhou University of Technology,
287 Langongping Road,
Lanzhou, Gansu 730050, China
e-mail: liwg40@sina.com

Contributed by the Fluids Engineering Division of ASME for publication in the JOURNAL OF FLUIDS ENGINEERING. Manuscript received August 16, 2014; final manuscript received May 6, 2015; published online August 20, 2015. Assoc. Editor: Olivier Coutier-Delgosha.

J. Fluids Eng 138(1), 011303 (Aug 20, 2015) (12 pages) Paper No: FE-14-1454; doi: 10.1115/1.4031061 History: Received August 16, 2014

Properly modeling cavitating flow in a centrifugal pump is a very important issue for prediction of cavitation performance in pump hydraulic design optimization and application. As a first trial, the issue is explored by using computational fluid dynamics (CFD) method plus the full cavitation model herein. To secure a smoothed head-net positive suction head available (NPSHa) curve, several critical techniques are adopted. The cavitation model is validated against the experimental data in literature. The predicted net positive suction head required (NPSHr) correction factor for viscosity oils is compared with the existing measured data and empirical correlation curve, and the factor is correlated to impeller Reynolds number quantitatively. A useful relation between the pump head coefficient and vapor plus noncondensable gas-to-liquid volume ratio in the impeller is obtained. Vapor and noncondensable gas concentration profiles are illustrated in the impeller, and a “pseudocavitation” effect is confirmed as NPSHa is reduced. The effects of exit blade angle on NPSHr are presented, and the contributions of liquid viscosity and noncondensable gas concentration to the increase of NPSHr at a higher viscosity are identified.

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Figures

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Fig. 1

Fluid domain of the model pump (a), mesh structure in the midspan plane of volute (b), and mesh structure in horizontal plane through the shaft axis of pump (c)

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Fig. 3

Pump head coefficient against cavitation number at three flow coefficients and five viscosities: (a) φ = 0.220, (b) φ = 0.379, and (c) φ = 0.523, cavitation numbers for NPSHr at various viscosities are listed in the legend

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Fig. 2

Pump head coefficient–flow coefficient curve at five viscosities and head coefficient–impeller Reynolds number curve at the three flow coefficients, L, B, and H denote the low flow coefficient 0.220, BEP flow coefficient 0.379, and high flow coefficient 0.523 at which the cavitating flows in the pump were simulated: (a) head coefficient–flow coefficient curve and (b) head coefficient–Reynolds number curve

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Fig. 4

Vapor and gas-to-liquid volume ratio in the impeller in terms of cavitation number at three flow coefficients and five viscosities: (a) φ = 0.220, (b) φ = 0.379, and (c) φ = 0.523

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Fig. 5

Pump head coefficient against vapor and gas-to-liquid volume ration in the impeller at three flow coefficients and five viscosities: (a) φ = 0.220, (b) φ = 0.379, and (c) φ = 0.523

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Fig. 6

Cavitation number σr and NPSHr correction factor KNPSHr in terms of impeller Reynolds number Req and compared with the empirical correlation in Ref. [1] and the experimental data in Ref. [3], as well as vapor–liquid volume ratio in the impeller as a function of σr : (a) σr – Req, (b) KNPSHr at BEP, (c) KNPSHr at low and high flow coefficients, and (d) Vc/VL at BEP

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Fig. 7

Fluid domain of the experimental pump in Refs. [36] and [37] (a), and predicted and measured head–NPSHa curves at three flow rates: Q  = 168, 210, and 226.8 m3/hr (b)

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Fig. 8

Blade cavitation coefficient in terms of impeller Reynolds number at three flow coefficients, the solid line is for the case that the NPSHr predicted by CFD is modified by multiplying 0.7907, the dashed line is the case that λ is altered by multiplying 0.7082

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Fig. 9

Vapor and noncondensable gas volume fraction contours in the impeller at BEP under NPSHr condition for different viscosities: (a) 1 cS, (b) 24 cS, (c) 48 cS, (d) 60 cS, and (e)120 cS, the variable in legend is vapor and gas volume fraction, αv+αg

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Fig. 10

Correction factor in terms of impeller Reynolds number and compared with the empirical correlation in Ref. [1] for 15 ppm and 40 ppm noncondensable gas concentrations under φ = 0.379 condition for two impellers with 20 deg and 44 deg exit blade angles: (a) comparison of KNPSHr between 15 ppm and 40 ppm and (b) correlations of KNPSHr to Req for two impellers at 40 ppm

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Fig. 11

Pressure coefficient profile on blade in midspan surface at five viscosities and BEP(φ = 0.379) for two impellers with 20 deg and 44 deg exit blade angles under 3% head drop condition: (a) β2  = 20 deg and (b) β2  = 44 deg

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