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Research Papers: Flows in Complex Systems

Investigation of Separation Phenomena in a Radial Pump at Reduced Flow Rate by Large-Eddy Simulation

[+] Author and Article Information
Antonio Posa

Department of Mechanical
and Aerospace Engineering,
The George Washington University,
Washington, DC 20052
e-mail: aposa@gwu.edu

Antonio Lippolis

Professor
Dipartimento di Meccanica,
Matematica e Management,
Politecnico di Bari,
Viale Japigia 182,
Bari 70126, Italy
e-mail: antonio.lippolis@poliba.it

Elias Balaras

Professor
Department of Mechanical
and Aerospace Engineering,
The George Washington University,
Washington, DC 20052
e-mail: balaras@gwu.edu

1Corresponding author.

Contributed by the Fluids Engineering Division of ASME for publication in the JOURNAL OF FLUIDS ENGINEERING. Manuscript received May 7, 2015; final manuscript received April 25, 2016; published online August 3, 2016. Assoc. Editor: D. Keith Walters.

J. Fluids Eng 138(12), 121101 (Aug 03, 2016) (13 pages) Paper No: FE-15-1318; doi: 10.1115/1.4033843 History: Received May 07, 2015; Revised April 25, 2016

Turbopumps operating at reduced flow rates experience significant separation and backflow phenomena. Although Reynolds-Averaged Navier–Stokes (RANS) approaches proved to be usually able to capture the main flow features at design working conditions, previous numerical studies in the literature verified that eddy-resolving techniques are required in order to simulate the strong secondary flows generated at reduced loads. Here, highly resolved large-eddy simulations (LES) of a radial pump with a vaned diffuser are reported. The results are compared to particle image velocimetry (PIV) experiments in the literature. The main focus of the present work is to investigate the separation and backflow phenomena occurring at reduced flow rates. Our results indicate that the effect of these phenomena extends up to the impeller inflow: they involve the outer radii of the impeller vanes, influencing significantly the turbulent statistics of the flow. Also in the diffuser vanes, a strong spanwise evolution of the flow has been observed at the reduced load, with reverse flow, located mainly on the shroud side and on the suction side (SS) of the stationary channels, especially near the leading edge of the diffuser blades.

Copyright © 2016 by ASME
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Figures

Grahic Jump Location
Fig. 1

Turbopump geometry: suction pipe (A), impeller blades (B), shroud (C), hub (D), diffuser walls (E), diffuser blades (F), volute (G), and casing (H)

Grahic Jump Location
Fig. 2

Details of the meridional section of the simulated pump geometry, together with an instantaneous vorticity field in the impeller vane (i), diffuser vane (d), and volute (v). The areas indicated by s were tagged as outside of the fluid domain. Arrows show the main flow direction. The clearance between impeller shroud/hub and casing was not simulated.

Grahic Jump Location
Fig. 3

Locations of the suction and pressure sides of the impeller and diffuser blades: rotor blade suction side (RBSS), rotor blade pressure side (RBPS), stator blade suction side (SBSS), and stator blade pressure side (SBPS). The rotation of the impeller is counterclockwise. The impeller blades were cut in the plane of representation. PIV measurements were carried out in the diffuser vanes by Boccazzi et al. [26] (white box in the figure).

Grahic Jump Location
Fig. 4

Top: passage-averaged LES fields of the velocity magnitude in the r−ϑ plane (only radial and tangential components) at 77% of the diffuser span from the hub side for the design (a) and off-design (b) conditions, normalized using the tangential velocity of the impeller blades at their trailing edge, u2. Bottom: passage-averaged profiles of the velocity component, ut, tangential to the PS of blade 5 along the line f in Fig. 3 for the design (c) and the off-design (d) conditions (•: experiments by Boccazzi et al. [26]; : present computations). The horizontal axis refers to the distance, xn, from the PS of blade 5, scaled by the local width of the diffuser channel, h.

Grahic Jump Location
Fig. 5

(a) Cross-stream sections where the ensemble-averaged distributions in (b) and (c) are evaluated; also the impeller blades (B), the shroud (C), and the hub (D) are shown; (b) radial evolution of the ensemble-averaged velocity w = −uz; and (c) radial evolution of the ensemble-averaged turbulent kinetic energy. The case at design conditions is represented by solid lines and that at off-design conditions by dashed lines. The arrows indicate the evolution along the streamwise direction. The profiles are normalized by the velocity of the impeller blades at their trailing edge, u2, and the local radius of the impeller, Ri.

Grahic Jump Location
Fig. 6

Fields of phase-averaged turbulent kinetic energy on the section d in Fig. 5(a) at design (a) and off-design (b) conditions. The values are normalized by u22.

Grahic Jump Location
Fig. 7

Local streamlines from seeding markers over the surface of the impeller and diffuser blades at design (a) and off-design (b) conditions. For visibility, only the blades, the hub, and the diffuser wall on the hub side are represented. Detail of the rotating channels.

Grahic Jump Location
Fig. 8

Azimuthal phase-averaged distribution at the impeller outlet radius of the angle of the flow, β, relative to the azimuthal direction. The horizontal dashed line represents the inlet angle of the diffuser vanes. The vertical lines indicate the locations of the trailing edges of the impeller blades (dashed) and leading edges of the diffuser blades (dotted–dashed). The fluid angles are represented by solid lines (design condition) and dashed lines (off-design condition): (a) 77% of the diffuser span (shroud side); (b) 23% of the diffuser span (hub side).

Grahic Jump Location
Fig. 9

Azimuthal phase-averaged distribution at the impeller outlet radius of the radial velocity component, ur, scaled by u2. The vertical lines indicate the locations of the trailing edges of the impeller blades (dashed) and leading edges of the diffuser blades (dotted–dashed). The radial velocities are represented by solid lines (design condition) and dashed lines (off-design condition): (a) 77% of the diffuser span (shroud side); (b) 23% of the diffuser span (hub side).

Grahic Jump Location
Fig. 10

Azimuthal distribution at the impeller outlet radius of the phase-averaged turbulent kinetic energy. The vertical lines indicate the locations of the trailing edges of the impeller blades (dashed) and leading edges of the diffuser blades (dotted–dashed). The values of turbulent kinetic energy are represented by solid lines (design condition) and dashed lines (off-design condition): (a) 77% of the diffuser span (shroud side); (b) 23% of the diffuser span (hub side). The vertical scales on the left and the right refer to the off-design and design conditions, respectively.

Grahic Jump Location
Fig. 11

Evolution of the skin-friction coefficient in the diffuser vanes along the PS (solid lines) and the SS (dashed lines) at design (b) and off-design (c) conditions. Ensemble averages in time and space. The horizontal axis refers to the location along the blade in percentage of the chord length, defined by means of projections along the chord (PPS and PSS) from probes on the PS and SS of the blades (NPS and NSS), as shown in (a).

Grahic Jump Location
Fig. 12

Evolution of the pressure coefficient in the diffuser vanes along the PS (solid lines) and the SS (dashed lines) at design (a) and off-design (b) conditions. Ensemble averages in time and space. The horizontal axis refers to the location along the blade in percentage of the chord length.

Grahic Jump Location
Fig. 13

Passage-averaged fields of pressure coefficient, averaged along the diffuser span. The impeller region is blanked out. (a) Design condition and (b) off-design condition.

Grahic Jump Location
Fig. 14

Evolution of the skin-friction coefficient along the SS of the diffuser blade 4 at 23% (dashed and dotted lines), 50% (dashed lines), and 77% (solid lines) of the span. Passage averages at design (a) and off-design (b) conditions. The horizontal axis refers to the location along the blade in percentage of the chord length.

Grahic Jump Location
Fig. 15

Evolution of the skin-friction coefficient along the SS (symbols) and PS (lines) of the diffuser blades 4 (○, solid lines), 6 (△, dashed lines), and 0 (, dashed and dotted lines). Passage averages at design (a) and off-design (b) conditions at the midspan of the diffuser vanes. The horizontal axis refers to the location along the blade in percentage of the chord length.

Grahic Jump Location
Fig. 16

Fields of passage-averaged velocity magnitude in the r−ϑ plane, urϑ, at the diffuser midspan, normalized by u2. (a) Design condition and (b) off-design condition.

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