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Research Papers

An Investigation of Nonlinear Flow Oscillations in a High-Pressure Centrifugal Pump

[+] Author and Article Information
Claudio Lettieri

MIT Gas Turbine Laboratory,
Massachusetts Institute of Technology,
Cambridge, MA 02139
e-mail: lettieri@mit.edu

Jeff Defoe

MIT Gas Turbine Laboratory,
Massachusetts Institute of Technology,
Cambridge, MA 02139
e-mail: jdefoe@uwindsor.ca

Zoltán S. Spakovszky

MIT Gas Turbine Laboratory,
Massachusetts Institute of Technology,
Cambridge, MA 02139
e-mail: zolti@mit.edu

1Corresponding author.

2Current position: Assistant Professor of Mechanical Engineering, University of Windsor, Windsor, ON N9B 3P4, Canada.

Contributed by the International Gas Turbine Institute (IGTI) of ASME for publication in the JOURNAL OF TURBOMACHINERY. Manuscript received November 12, 2014; final manuscript received August 4, 2015; published online September 2, 2015. Assoc. Editor: Stephen W. T. Spence.

J. Turbomach 137(11), 111004 (Sep 02, 2015) (9 pages) Paper No: TURBO-14-1294; doi: 10.1115/1.4031250 History: Received November 12, 2014; Revised August 04, 2015

High-pressure multistage pumps and their coupled piping systems, typically used in the process and power generation industry, can experience dangerous system-level instabilities. This can occur at flow coefficients well away from the surge limit and in the absence of cavitation. Such a pumping system and a related new kind of instability are the focus of this paper. A system-wide instability was observed at 0.05 times rotor frequency for flow coefficients near maximum head rise but at negative slope, thus on the stable side of the head rise characteristic. A previous study based on system-level experiments concluded that this instability differs from classical surge, cavitation surge, rotating stall, and rotating cavitation, but the underlying mechanism and necessary flow conditions remain unknown. This paper investigates the root cause of the system-wide pump instability, employing a systematic analysis of the impact of geometry changes on pump stability and performance. It is found that the upstream influence of the unsteady flow separation in the return channel leads to a time-varying incidence angle change on the volute tongue which causes periodic ingestion of low-stagnation pressure fluid into the diffuser passages. This sets up a limit cycle, promoting the system-wide instability. With the instability mechanism determined, the pump is redesigned to remove the flow separation while maintaining performance at design conditions. Unsteady numerical simulations demonstrate improved efficiency and pressure recovery at low flow coefficients. A time accurate calculation also indicates stable operation at all relevant flow conditions. The paper resolves a long-standing pump stability problem and provides design guidelines for reliable and improved performance, important to the chemical processing and power generation industry.

Copyright © 2015 by ASME
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References

Figures

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Fig. 1

Measured head rise characteristic of two-stage pump

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Fig. 2

Grid sensitivity analysis—velocity profiles at diffuser inlet for four meshes with increasing refinement levels

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Fig. 3

Datum stage geometry

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Fig. 4

Geometries used for datum stage calculations: (a) full stage, (b) single-passage, static components only, (c) single-passage, static components with return channel removed, and (d) single-passage, static components with vaneless space and volute tongue removed

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Fig. 5

Time-averaged flow field at midspan in static component calculation. Left: nondimensional velocity field and right: vorticity field. In each image, the portion to the left of the discontinuity is at midspan in the volute; the portion at right is at midspan in the deswirler.

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Fig. 6

Low-stagnation pressure fluid (red arrow) shed from volute tongue due to incidence swings enters adjacent diffusing passage

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Fig. 7

Tongue boundary layer pathline, showing long residence time in the vaneless space. Pathline shown both upstream and downstream of the point indicated by the cross. Flow direction is counterclockwise as shown.

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Fig. 8

Low-stagnation pressure fluid (red arrow) interacts with boundary layer in the diffusing passage, leading to vortex shedding (purple arrow). Shed vortices interact with the return vane, leading to bluff-body separation with upstream influence.

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Fig. 9

Time trace of loss coefficient at stage exit, ω = (pt1−p¯t2M)/0.5ρUT2

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Fig. 10

(a) Single passage of static components, (b) static components with return channel removed, and (c) static components with vaneless space and volute tongue removed

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Fig. 11

Midspan velocity field: (a) all static components present, (b) return channel removed, and (c) vaneless space and volute tongue removed. Vortex shedding in the diffusing passage is inhibited when the return channel is removed. Vortex shedding is eliminated when the vaneless space and tongue are removed.

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Fig. 18

Time trace of loss coefficient at stage exit, ω = pt1−p¯t2M/0.5ρUT2: black—redesign stage and gray—datum stage

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Fig. 17

Contour of normalized velocity at 50% span at the design flow coefficient of pump redesign. Velocities are relative in the rotating frame of the impeller and absolute for the stationary components. (a) Impeller and diffuser and (b) return channel.

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Fig. 16

Polytropic efficiency versus flow coefficient

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Fig. 15

Computed stage total-to-total head rise coefficient without leakage flow for original design and stage redesign. Experimental data (with leakage flows) are also shown.

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Fig. 14

Effect of b2* on mass-averaged diffuser inlet flow angle and total-to-total head rise coefficient

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Fig. 13

To maintain fixed work input coefficient (and thus cuT), the redesign with an increased, constant-width flow path leads to increased trailing edge blade angle and increase absolute flow angle at impeller exit

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Fig. 12

Conventional flow path design with volute (left) versus new design with constant-width flow path and vaned diffuser (right)

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