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Flows in Complex Systems

Numerical Analysis of External Gear Pumps Including Cavitation

[+] Author and Article Information
D. del Campo

 Department of Aeronautics, Universitat Politecnica de Catalunya, Colom 11, 08222 Terrassa, Spaindavid.del.campo@upc.edu

R. Castilla

Department of Fluid Mechanics,  Universitat Politecnica de Catalunya, Colom 11, 08222 Terrassa, Spaincastilla@mf.upc.edu

G. A. Raush

Department of Fluid Mechanics,  Universitat Politecnica de Catalunya, Colom 11, 08222 Terrassa, Spaingustavo.raush@upc.edu

P. J. Gamez Montero

Department of Fluid Mechanics,  Universitat Politecnica de Catalunya, Colom 11, 08222 Terrassa, Spainpjgm@mf.upc.edu

E. Codina

Department of Fluid Mechanics,  Universitat Politecnica de Catalunya, Colom 11, 08222 Terrassa, Spainecodina@mf.upc.edu

Animations available in the online version of this paper.

J. Fluids Eng 134(8), 081105 (Aug 09, 2012) (12 pages) doi:10.1115/1.4007106 History: Received March 15, 2012; Revised June 07, 2012; Published August 09, 2012; Online August 09, 2012

Hydraulic machines are faced with increasingly severe performance requirements. The need to design smaller and more powerful machines rotating at higher speeds in order to provide increasing efficiencies has to face a major limitation: cavitation. The problem is inherently three-dimensional, due to the axial clearances, the relief and circumferential grooves, and to the circular pipes through which the fluid enters and exits the pump. A simplified two-dimensional numerical approach by means of computational fluid dynamics (CFD) has been developed for studying the effect of cavitation in the volumetric efficiency of external gear pumps. The assumptions employed prevent from predicting realistic values of the volumetric efficiency, but show to be valid to understand the complex flow patterns that take place inside the pump and to study the influence of cavitation on volumetric efficiency. A method for simulating the contact between solid boundaries by imposing changes in viscosity has been developed. Experiments of unsteady cavitation in water and oil performed by other authors have been numerically reproduced using different cavitation models in order to select the most appropriate one and to adjust its parameters. The influence of the rotational speed of the pump has been analyzed. Cavitation in the suction chamber very effectively damps the water hammer associated to the sudden change of the contact point position at the end of the gearing cycle. At high rotational speeds, the volume of air becomes more stable, reducing the flow irregularity. When cavitation takes place at the meshing region downstream from the contact point, the volume of air that appears acts as a virtual second contact point, increasing the volumetric efficiency of the pump.

Copyright © 2012 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Computational domain of the reference pump with the defined zones. Point P represents a control point for checking convergence.

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Figure 2

Cell number evolution in the gearing zone for the coarse and fine meshes

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Figure 3

Mesh in a region of inlet chamber/gearing zone interface after 10 gearing cycles. Up: coarse mesh. Down: fine mesh.

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Figure 4

Average equivolume skewness evolution for each pump

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Figure 5

Detail of original and deformed mesh after 10 gearing cycles

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Figure 6

Dynamic viscosity imposed in the contact point position in order to simulate its effect

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Figure 7

Mesh of the venturi nozzle

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Figure 8

Incipient and choking cavitation numbers versus Reynolds. Experiments from Yamaguchi [23] and numerical simulations with different sets of cavitation model parameters.

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Figure 9

Relative air volume in the orifice at Re = 4000 for three different cavitation numbers

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Figure 10

Simplifications made in the numerical simulations

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Figure 11

Gears and contact point position at each 1/10 of a gearing cycle

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Figure 12

Inlet flow rate at different rotational speeds with and without cavitation

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Figure 13

Outlet flow rate of the reference gear pump at different rotational speeds with and without cavitation

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Figure 14

Volume fraction of oil at 1000 rpm each 1/10 of a gearing cycle

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Figure 15

Volume fraction of oil at 3000 rpm each 1/10 of a gearing cycle

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Figure 16

Relative volume of air evolution at different rotational speeds

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Figure 17

Volume fraction of oil in the squeeze volume at 1500 rpm in the beginning of a gearing cycle

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Figure 18

Volumetric ratio against operating velocity with and without cavitation effects

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Figure 19

Volumetric ratio against operating velocity with and without cavitation effects and modified volumetric ratio (according to Eq. 4) subtracting the effect of the volume of air trapped in the squeeze volume at the beginning of the gearing cycle

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