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TECHNICAL PAPERS

On the Physics of Flow Separation Along a Low Pressure Turbine Blade Under Unsteady Flow Conditions

[+] Author and Article Information
Meinhard T. Schobeiri, Burak Öztürk

 Turbomachinery Performance and Flow Research Laboratory, Texas A&M University, College Station, TX 77843-3123

David E. Ashpis

National Aeronautics and Space Administration,  John H. Glenn Research Center at Lewis Field, Cleveland, OH 44135-3191

J. Fluids Eng 127(3), 503-513 (Feb 24, 2005) (11 pages) doi:10.1115/1.1905646 History: Received August 03, 2004; Revised February 24, 2005

The present study, which is the first of a series of investigations dealing with specific issues of low pressure turbine (LPT) boundary layer aerodynamics, is aimed at providing detailed unsteady boundary flow information to understand the underlying physics of the inception, onset, and extent of the separation zone. A detailed experimental study on the behavior of the separation zone on the suction surface of a highly loaded LPT-blade under periodic unsteady wake flow is presented. Experimental investigations were performed at Texas A&M Turbomachinery Performance and Flow Research Laboratory using a large-scale unsteady turbine cascade research facility with an integrated wake generator and test section unit. To account for a high flow deflection of LPT-cascades at design and off-design operating points, the entire wake generator and test section unit including the traversing system is designed to allow a precise angle adjustment of the cascade relative to the incoming flow. This is done by a hydraulic platform, which simultaneously lifts and rotates the wake generator and test section unit. The unit is then attached to the tunnel exit nozzle with an angular accuracy of better than 0.05°, which is measured electronically. Utilizing a Reynolds number of 110,000 based on the blade suction surface length and the exit velocity, one steady and two different unsteady inlet flow conditions with the corresponding passing frequencies, wake velocities and turbulence intensities are investigated using hot-wire anemometry. In addition to the unsteady boundary layer measurements, blade surface pressure measurements were performed at Re=50,000, 75,000, 100,000, and 125,000 at one steady and two periodic unsteady inlet flow conditions. Detailed unsteady boundary layer measurement identifies the onset and extent of the separation zone as well as its behavior under unsteady wake flow. The results presented in ensemble-averaged and contour plot forms contribute to understanding the physics of the separation phenomenon under periodic unsteady wake flow. Several physical mechanisms are discussed.

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Copyright © 2005 by American Society of Mechanical Engineers
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Figures

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Figure 1

Turbine cascade research facility with the components and the adjustable test section

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Figure 2

Cascade geometry and stagger angle are listed in Table 1. Number of blades=5, SPB-1 and SPB-2 are blades with static pressure taps, HFB is instrumented with surface mounted hot films to be used for future investigations.

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Figure 3

Static pressure distribution at four different Re-numbers and reduced frequencies Ω=0,1.59,3.18, (no rod, 160mm, 80mm), Start=Separation start for steady and unsteady cases, SE=Separation end for steady case, UE=Separation end for unsteady cases.

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Figure 4

Distribution of time averaged velocity (a) and fluctuation rms velocity (b) along the suction surface for steady case Ω=0(SR=∞) and unsteady cases Ω=1.59(SR=160mm) and Ω=3.18(SR=80mm) at Re=110,000. Note the changes in y scale.

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Figure 5

Ensemble averaged velocity as a function of time for (a) steady flow case Ω=0(SR=∞) and (b) unsteady case Ω=1.59(SR=160mm) at s∕s0=0.0208 and Re=110,000

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Figure 6

Distribution of the ensemble averaged velocity development along the suction surface for different s∕s0 with time t∕τ as parameter for Ω=1.59(SR=160mm) and Re=110,000, note scale change in (f)

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Figure 7

Contour plot of the ensemble averaged velocity distribution showing the effect of periodic wakes on the separation zone at different streamwise positions and Re=110,000

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Figure 8

Wake velocity and fluctuation rms distribution, Re=110,000

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Figure 9

Separation zone, definition of contraction begin, contraction end, suppression, and regeneration

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Figure 10

Separation contraction, suppression, and regeneration, Re=110,000

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Figure 11

Ensemble-averaged relative momentum thickness distribution along the suction surface for Ω=1.59(SR=160mm) and Re=110,000

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Figure 12

Ensemble averaged relative form parameter distribution along the suction surface for Ω=1.59(SR=160mm), Re=110,000

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