Research Papers: Fundamental Issues and Canonical Flows

Particle Image Velocimetry Investigation of the Leakage Flow Through Clearance Gaps in Cambered Hydrofoils

[+] Author and Article Information
Sailesh Chitrakar

Department of Energy and Process Engineering,
Norwegian University of
Science and Technology,
Trondheim 7491, Norway
e-mail: sailesh.chitrakar@ntnu.no

Hari Prasad Neopane

Department of Mechanical Engineering,
Kathmandu University,
Kavre 45200, Nepal
e-mail: hari@ku.edu.np

Ole Gunnar Dahlhaug

Department of Energy and Process Engineering,
Norwegian University of
Science and Technology,
Trondheim 7491, Norway
e-mail: ole.g.dahlhaug@ntnu.no

1Corresponding author.

Contributed by the Fluids Engineering Division of ASME for publication in the JOURNAL OF FLUIDS ENGINEERING. Manuscript received December 6, 2016; final manuscript received March 6, 2017; published online June 7, 2017. Assoc. Editor: Hui Hu.

J. Fluids Eng 139(9), 091201 (Jun 07, 2017) (8 pages) Paper No: FE-16-1802; doi: 10.1115/1.4036269 History: Received December 06, 2016; Revised March 06, 2017

In Francis turbines, which are normally designed at a reaction ratio of 0.5, the available pressure energy in the fluid is converted into 50% kinetic energy before entering the runner. This causes high acceleration of the flow in guide vanes (GVs), which adds to the unsteadiness and losses in the turbine. In sediment-affected power plants, the hard sand particles erode and gradually increase the clearance gap between the GV and facing plates, which causes more disturbances in downstream turbine components. This study focuses on investigating the flow through the clearance gap of the GV with cambered hydrofoil shapes by using particle image velocimetry (PIV) technique. The measurements are carried out in one GV cascade rig, which produces similar velocity fields around a GV, as compared to the real turbine. The investigation is done in two cases of cambered GV National Advisory Committee for Aeronautics (NACA) profiles, and the comparison of the velocity and pressure distribution around the hydrofoil is done with the results in symmetric profile studied earlier. It is seen that the pressure distribution around the hydrofoil affects the velocity field, leakage flow, and characteristics of the vortex filament developed inside the cascade. NACA4412, which has flatter suction side (SS) than NACA2412 and NACA0012, is seen to have smaller pressure difference between the two adjacent sides of the vane. The flow inside the clearance gap of NACA2412 enforces change in the flow angle, which forms a vortex filament with a rotational component. This vortex along with improper stagnation angle could have greater consequences in the erosion of the runner inlet (RIn) and more losses of the turbine.

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Fig. 1

(a) Marks of the horseshoe vortex on facing plate, (b) erosion at GV ends, and (c) erosion at the runner inlet [1]

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Fig. 2

(a) Orientation of GVs and velocity and pressure distribution and (b) vorticity along GV span in one GV rig [20]

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Fig. 3

Description of the GV test specimen

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Fig. 4

Experimental layout in the lab

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Fig. 5

Arrangement of pressure measurement through back cover plate of the test rig

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Fig. 6

PIV measurement planes

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Fig. 7

Position of vectors and circumferential location corresponding to the real turbine

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Fig. 8

Pressure distribution around GVs

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Fig. 9

Average velocity contour in the clearance gap and midspan planes for: (a) NACA2412, (b) NACA4412, and (c) NACA0012 hydrofoils

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Fig. 10

Velocity normal to chord length, Vy inside clearance gap

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Fig. 11

Velocity vectors above the clearance gap

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Fig. 12

Average velocity at planes normal to chord from GV TE

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Fig. 13

Contours of the angle (α) at the planes of GV outlet and runner inlet




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