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Research Papers: Multiphase Flows

Cavitation Phenomena and Performance Implications in Archimedes Flow Turbines

[+] Author and Article Information
Jacob D. Riglin

Department of Mechanical
Engineering and Mechanics,
P.C. Rossin School of Engineering and
Applied Science,
Lehigh University,
Bethlehem, PA 18015
e-mail: jar611@lehigh.edu

William C. Schleicher

Department of Mechanical
Engineering and Mechanics,
P.C. Rossin School of Engineering and
Applied Science,
Lehigh University,
Bethlehem, PA 18015
e-mail: wcs211@lehigh.edu

Alparslan Oztekin

Department of Mechanical
Engineering and Mechanics,
P.C. Rossin School of Engineering and
Applied Science,
Lehigh University,
Bethlehem, PA 18015
e-mail: alo2@lehigh.edu

1Corresponding author.

Contributed by the Fluids Engineering Division of ASME for publication in the JOURNAL OF FLUIDS ENGINEERING. Manuscript received February 21, 2014; final manuscript received September 9, 2015; published online December 23, 2015. Assoc. Editor: Olivier Coutier-Delgosha.

J. Fluids Eng 138(3), 031303 (Dec 23, 2015) (9 pages) Paper No: FE-14-1090; doi: 10.1115/1.4031606 History: Received February 21, 2014; Revised September 09, 2015

Cavitation produces undesirable effects within turbines, such as noise, decreases in efficiency, and structural degradation of the device. Two microhydro turbines incorporating Archimedean spiral blade geometries were investigated numerically for cavitation effects using computational fluid dynamics (CFD). Separate blade geometries, one with a uniform blade pitch angle and the other with a 1.5 power pitch, were modeled using the Schnerr–Sauer cavitation model. The method used to determine where cavitation occurs along the blade and within the flow involved varying inlet flow rates and the rotation rate of the blade. Cavitation analysis was conducted locally as well as globally, using both cavitation number and Thoma number. The cavitation number was used to correlate the single-phase to the multiphase results for rotation rates of 250 and 500 rpm, allowing for the single-phase simulations to be used to determine where the onset of cavitation occurs. It was determined that cavitation occurred at the exit of the blade at a flow coefficient of approximately 0.33 for the 1.5 pitch blade geometry, while the uniform blade geometry had a value of 1.35. When the rotation rate was reduced to 250 rpm, cavitation occurred at a flow coefficient of 0.72. From the simulations at both rotation rates, it was determined that both geometry and rotation rate have a significant effect on the onset of cavitation and water vapor inception within the flow field. As the rotation rate of the turbine decreases, the onset of cavitation will be prolonged to larger flow coefficients. As the flow coefficient increased beyond the value at which the onset of cavitation occurs, the intensity of cavitation increases and efficiency drops of up to 20% were experienced by the turbines. Based on the net positive suction head required in the system and the available head in the system, the cavitation results were validated. It was determined that the inception cavitation number, Cai, where the onset of cavitation occurs is approximately −1.51.

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References

Figures

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Fig. 6

Pressure contour with vortex core region based on a swirl intensity of 676.3 s−1 at a flow coefficient of 1.09 for multiphase flow (a) at a specific speed of 26 and single-phase flow (b) at a specific speed of 42

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Fig. 5

Efficiency as a function of flow coefficient for both uniform and variable (m = 1.5) pitch geometries

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Fig. 4

Computational fluid domain (a), cross section of mesh near the blade exit (b), and mesh along the blade and outer boundary (c) used for numerical modeling

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Fig. 3

Efficiency (a) and power (b) as a function of flow rate for variable pitch Archimedes spiral design [22]

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Fig. 2

Uniform pitch (a) blade geometry and 1.5 pitch, nonuniform (b) blade geometry

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Fig. 1

Operating ranges for various types of hydraulic turbines

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Fig. 7

Vapor fraction along blade on uniform geometry with flow coefficient of 2.70 and a specific speed of 178

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Fig. 8

Cavitation along blade of 1.5 pitch geometry operating at flow coefficients of 0.41 (a), 0.46 (b), 0.55 (c), and 0.67 (d)

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Fig. 9

Volume fraction exiting the turbine for the 1.5 pitch geometry at a flow coefficient of 0.55

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Fig. 10

Power coefficient (a), head coefficient (b), and efficiency (c) as a function of flow coefficient for the 1.5 pitch geometry

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Fig. 11

Cavitation number (a) and volume fraction (b) contours along the blade for the multiphase simulation with flow coefficient of 1.09 and specific speed of 26

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Fig. 12

Cavitation number along blade for single-phase simulation with flow coefficient of 1.09 and specific speed of 42

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Fig. 13

Head Coefficient as a function of flow coefficient predicting the onset of cavitation

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