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Research Papers

Investigation of Three-Dimensional Unsteady Flows in a Two-Stage Shrouded Axial Turbine Using Stereoscopic PIV—Kinematics of Shroud Cavity Flow

[+] Author and Article Information
Yong Il Yun

School of Mechanical and Aerospace Engineering,  Seoul National University, Seoul 151-742, Koreayongil.yun@samsung.com

Luca Porreca

Turbomachinery Laboratory,  Swiss Federal Institute of Technology Zurich, Zurich CH-8092, Switzerlandluca.porreca@ch.manturbo.com

Anestis I. Kalfas

Turbomachinery Laboratory,  Swiss Federal Institute of Technology Zurich, Zurich CH-8092, Switzerlandkalfas@lsm.iet.mavt.ethz.ch

Seung Jin Song

School of Mechanical and Aerospace Engineering,  Seoul National University, Seoul 151-742, Koreasjsong@snu.ac.kr

Reza S. Abhari

Turbomachinery Laboratory,  Swiss Federal Institute of Technology Zurich, Zurich CH-8092, Switzerlandabhari@lsm.iet.mavt.ethz.ch

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Presently at Samsung Engineering Co. Ltd., Samsung SEI Tower 467-14, Dogok-2Dong, Gangnam-Gu, Seoul 135-856, Korea.

J. Turbomach 130(1), 011021 (Jan 28, 2008) (9 pages) doi:10.1115/1.2720873 History: Received June 03, 2006; Revised October 09, 2006; Published January 28, 2008

This paper presents an experimental study of the behavior of leakage flow across shrouded turbine blades. Stereoscopic particle image velocimetry and fast response aerodynamic probe measurements have been conducted in a low-speed two-stage axial turbine with a partial shroud. The dominant flow feature within the exit cavity is the radially outward motion of the main flow into the shroud cavity. The radial migration of the main flow is induced by flow separation at the trailing edge of the shroud due to a sudden area expansion. The radially outward motion is the strongest at midpitch as a result of interactions between vortices formed within the cavity. The main flow entering the exit cavity divides into two streams. One stream moves upstream toward the adjacent seal knife and reenters the main flow stream. The other stream moves downstream due to the interaction with the thin seal leakage flow layer. Closer to the casing wall, the flow interacts with the underturned seal leakage flow and gains swirl. Eventually, axial vorticity is generated due to these complex flow interactions. This vorticity is generated by a vortex tilting mechanism and gives rise to additional secondary flow. Because of these fluid motions combined with a contoured casing wall, three layers (the seal leakage layer, cavity flow layer, and main flow) are formed downstream of the shroud cavity. This result is different from the two-layer structure, which is found downstream of conventional shroud cavities. The seal leakage jet formed through the seal clearance still exists at 25.6% axial chord downstream of the second rotor. This delay of complete dissipation of the seal leakage jet and its mixing with the cavity flow layer is due to the contoured casing wall. Time-averaged flow downstream of the shroud cavity shows the upstream stator’s influence on the cavity flow. The time-averaged main flow can be viewed as a wake flow induced by the upstream stator whose separation at the shroud trailing edge induces pitchwise non-uniformity of the cavity flow.

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Copyright © 2008 by American Society of Mechanical Engineers
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Figures

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Figure 1

Partially shrouded rotor

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Figure 2

Schematic of stereoscopic PIV setup

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Figure 3

Measurement planes within the cavity and downstream of the second rotor

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Figure 4

PIV errors at a typical measurement point within the cavity

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Figure 5

Meridional view of time and pitchwise averaged velocity within the cavity

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Figure 6

Time and pitchwise averaged velocity triangles within the cavity

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Figure 7

Time-averaged axial velocity fields within and downstream of the cavity

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Figure 8

Pitchwise velocity profiles downstream of the blade trailing edge within the cavity

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Figure 9

Meridional velocity fields within the cavity

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Figure 10

Vortex interactions within the cavity

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Figure 11

Absolute velocity fields on blade-to-blade planes within the cavity

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Figure 12

Vorticity kinematics of the exit cavity flow

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Figure 13

Time-averaged flow velocities downstream of the shroud cavity

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Figure 14

Upstream stator effect on time-averaged cavity flow

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