Research Papers: Flows in Complex Systems

Investigation of Flow Separation in a Diffuser of a Bulb Turbine

[+] Author and Article Information
Pierre Duquesne

Hydraulic Machine Laboratory (LAMH),
Department of Mechanical Engineering,
Laval University,
Québec, QC G1V 0A6, Canada
e-mail: pierre.duquesne.1@ulaval.ca

Yvan Maciel

Hydraulic Machine Laboratory (LAMH),
Department of Mechanical Engineering,
Laval University,
Québec, QC G1V 0A6, Canada
e-mail: Yvan.Maciel@gmc.ulaval.ca

Claire Deschênes

Hydraulic Machine Laboratory (LAMH),
Department of Mechanical Engineering,
Laval University,
Québec, QC G1V 0A6, Canada
e-mail: Claire.Deschenes@gmc.ulaval.ca

Contributed by the Fluids Engineering Division of ASME for publication in the JOURNAL OF FLUIDS ENGINEERING. Manuscript received September 9, 2014; final manuscript received July 7, 2015; published online September 3, 2015. Assoc. Editor: Bart van Esch.

J. Fluids Eng 138(1), 011102 (Sep 03, 2015) (9 pages) Paper No: FE-14-1499; doi: 10.1115/1.4031254 History: Received September 09, 2014; Revised July 07, 2015

A three-dimensional unsteady flow separation in the straight diffuser of a model bulb turbine is investigated using tuft visualizations, unsteady wall pressure sensors, and particle image velocimetry (PIV). Experimental results reveal a link between the flow separation zone extension and the sudden drop in turbine performances. The flow separation zone grows as the flow rate increases past the best efficiency operating point (OP). It starts on the bottom wall and expands azimuthally and upstream. It deviates and perturbs the flow far upstream. Despite high unsteadiness, a global separation streamline pattern composed of a saddle point and a convergence line emerges.

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Grahic Jump Location
Fig. 1

Test bench schematic

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Fig. 2

The four PIV measurement zones. Zones with dashed lines are further away from the wall.

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Fig. 3

Wall distance normalized by Dref for B1, B2, S3, and S4 on (a), (b), (c), and (d), respectively. The thicker black contour is the measurement plane limit.

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Fig. 4

Normalized efficiency hill chart for runner blade angle at 30.2 deg, showing the region for which flow separation was detected. The black dashed line is the line of maximal efficiency for a fixed N11. Black dots are the OPs chosen for the PIV measurements. All values are normalized with corresponding values at best efficiency point.

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Fig. 5

Performance and pressure recovery curves at N11 = 170 rpm for OP 2 to OP 5: (a) efficiency and pressure recovery curves and (b) power and pressure recovery curves. All values are normalized with the corresponding values at blade angle 22.5 deg, the BEP.

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Fig. 6

Radial profiles of average velocity components measured with LDV at Z = 0.13 Ldt, just downstream of the hub: (a) CZ, axial component and (b) Cθ, circumferential component. Adapted from Ref. [19].

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Fig. 7

Flow separation zone extent as a function of the OP from tuft visualizations. White arrows represent the expansion of the separation zone with increasing flow rate, black arrows represent the general direction of the near-wall flow inside the separation zone, and contour lines show the approximate average limit of observed backflow for each OP.

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Fig. 8

Plane S3 instantaneous pseudo-streamlines at OP 5. Gray contours correspond to backflow velocity in the measurement referential uS3¡≤0.

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Fig. 9

Average pseudo-streamlines in PIV planes. OPs 2, 4, and 5 represent the gray line, dark gray line, and black line with arrow, respectively. (a) Plane B1, (b) plane B2, (c)plane S3, and (d) plane S4.

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Fig. 10

Contours of the RMS of u fluctuations normalized by the local mean of u. Contour values are 0.3, 0.7, and 1.2 for all measurement planes. OPs 2, 4, and 5 are shown with gray line, dark gray line, and black line, respectively. (a) Plane B1, (b) plane B2, (c) plane S3, and (d) plane S4.

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Fig. 11

Contours of the time average of the backflow occurrence function for the downstream portion of the four measurement planes. Contour values are 2%, 5%, 10%, and 15%. OPs 4 and 5 are shown in dark gray and black lines, respectively. (a) Plane B1, (b) plane B2, (c) plane S3, and (d) plane S4.

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Fig. 12

Conditionally averaged pseudo-streamlines with 20–30% of vectors in backflow for planes B1, B2, and S3




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