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Research Papers: Flows in Complex Systems

The Effect of Impeller Cutback on the Fluid-Dynamic Pulsations and Load at the Blade-Passing Frequency in a Centrifugal Pump

[+] Author and Article Information
Raúl Barrio, Eduardo Blanco, José González

Área de Mecánica de Fluidos, Universidad de Oviedo, Campus de Viesques, 33271 Gijón, Spain

Jorge Parrondo

Área de Mecánica de Fluidos, Universidad de Oviedo, Campus de Viesques, 33271 Gijón, Spainparrondo@uniovi.es

Joaquín Fernández

Escuela de Ingenierías Industriales, Universidad de Extremadura, Avenida de Elvas s/n, 06071 Badajoz, Spain

J. Fluids Eng 130(11), 111102 (Sep 22, 2008) (11 pages) doi:10.1115/1.2969273 History: Received May 08, 2007; Revised June 11, 2008; Published September 22, 2008

A study is presented on the fluid-dynamic pulsations and the corresponding dynamic forces generated in a centrifugal pump with single suction and vaneless volute due to blade-volute interaction. Four impellers with different outlet diameters, obtained from progressive cutbacks (trimmings) of the greatest one, were successively considered in the test pump, so that the radial gap between the impeller and the volute ranged from 8.8% to 23.2% of the impeller radius. The study was based on the numerical computation of the unsteady flow through the machine for a number of flow rates by means of the FLUENT code, solving the 3D unsteady Reynolds-averaged Navier–Stokes equations. Additionally, an experimental series of tests was conducted for the pump with one of the impellers, in order to obtain pressure fluctuation data along the volute front wall that allowed contrasting the numerical predictions. The data collected from the numerical computations were used to estimate the dynamic radial forces and torque at the blade-passing frequency, as a function of flow rate and blade-tongue radial gap. As expected, for a given impeller diameter, the dynamic load increases for off-design conditions, especially for the low range of flow rates, whereas the progressive reduction of the impeller-tongue gap brings about corresponding increments in dynamic load. In particular, varying the blade-tongue gap within the limits of this study resulted in multiplying the maximum magnitude of the blade-passing frequency radial force by a factor of about 4 for low flow rates (i.e., below the nominal flow rate) and 3 for high flow rates.

Copyright © 2008 by American Society of Mechanical Engineers
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Figures

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Figure 1

(a) Scheme of the test pump with location of measurement points. (b) Detail of the tongue region (dimensions in mm).

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Figure 2

Test pump with pressure transducers installed on the volute

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Figure 3

Experimental pressure spectra at angular position φ=25deg, as a function of frequency and flow rate (d2=0.210m)

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Figure 4

Flow through pump and leakage from volute to impeller eye

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Figure 6

Details of the surface mesh. (a) Inlet duct, volute and diffuser. (b) Tongue separating volute and diffuser. (c) Impeller with shroud removed. (d) Suction duct with inlet guide vane.

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Figure 7

Effect of mesh refinement (total number of cells) on the numerical predictions for the d*=1 impeller operating at 160% of the nominal flow rate. From top to bottom: flow coefficient, head coefficient, and amplitude of fBP pressure fluctuation at angular position φ=25deg, all of them normalized by the values corresponding to the upper limit of number of cells.

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Figure 8

Experimental head-flow rate curve (d*=1) and numerical predictions for four impeller diameters

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Figure 12

Amplitude of pressure fluctuations predicted at fBP along the front wall of the volute, for four impeller diameters (one at each column) and five flow coefficients (one at each row). For impeller d*=1, the experimental data are also shown (△, experimental data; ●, numerical results).

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Figure 13

Orbit of the unsteady partial radial forces on the impeller obtained from integration of pressure fluctuations at fBP along the front wall of the volute for the d*=1 impeller and five flow coefficients (…, experimental data; —, numerical predictions)

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Figure 14

Orbit of the total unsteady radial force on the impeller predicted for four impeller diameters (one at each row) and five flow coefficients (one at each column)

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Figure 15

Prediction of the maximum magnitude of the total unsteady radial force as a function of the flow coefficient for the four impellers tested

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Figure 16

Prediction of the fBP unsteady torque as a function of flow coefficient for the four impellers tested

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Figure 11

Spectra of the numerical pressure fluctuations at angular position φ=25deg, as a function of frequency and flow coefficient (d*=1)

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Figure 10

Time evolution of the static pressure in the near-tongue region for the d*=1 impeller (at midspan of impeller outlet width) during one blade-passage period. Results for three flow coefficients, equivalent to 20%, 100%, and 160% of the nominal flow rate.

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Figure 9

Velocity vectors in the near-tongue region for the d*=1 impeller at midspan of impeller outlet width and three flow coefficients (equivalent to 20%, 100%, and 160% of the nominal flow rate)

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Figure 5

Pump numerical model. (a) Pump with inlet and outlet ducts. (b) Impeller surface. (c) Impeller-tongue gap for the impeller cutbacks tested.

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