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Research Papers

The Influence of Boundary Conditions on Tip Leakage Flow

[+] Author and Article Information
John D. Coull, Nicholas R. Atkins

Whittle Laboratory,
University of Cambridge,
1 J. J. Thomson Avenue,
Cambridge CB3 0DY, UK

The heat transfer coefficient and hence Nusselt number is related to the wall shear stress; as such the comparison provides validation data for both heat transfer and aerodynamics.

As the potential field strongly depends on Mach number, some care would be required to reproduce these effects in low speed testing.

Note that the rotor-only efficiency measure used in this paper is more sensitive to tip leakage losses than stage efficiency (Ref. [15]).

Contributed by the International Gas Turbine Institute (IGTI) of ASME for publication in the JOURNAL OF TURBOMACHINERY. Manuscript received July 16, 2014; final manuscript received September 19, 2014; published online December 11, 2014. Editor: Ronald Bunker.

J. Turbomach 137(6), 061005 (Jun 01, 2015) (10 pages) Paper No: TURBO-14-1152; doi: 10.1115/1.4028796 History: Received July 16, 2014; Revised September 19, 2014; Online December 11, 2014

Much of the current understanding of tip leakage flow has been derived from detailed cascade studies. Such experiments are inherently approximate since it is difficult to simulate the boundary conditions that are present in a real machine, particularly the secondary flows convecting from the upstream stator row and the relative motion of the casing and blade. The problem is further complicated when considering the high pressure turbine rotors of aero engines, where the high Mach numbers must also be matched in order to correctly model the aerodynamics and heat transfer of the leakage flow. More engine-representative tests can be performed on high-speed rotating turbines, but the experimental resolution achievable in such setups is limited. In order to examine the differences between cascade and engine boundary conditions, this paper presents a numerical investigation into the impact of inlet conditions and relative casing motion (RCM) on the leakage flow of a high-pressure turbine rotor. The baseline calculation uses a simplified inlet condition and no relative endwall motion, in typical cascade fashion. Only minor changes to the leakage flow are induced by introducing either a more realistic inlet condition or RCM. However, when both of these conditions are applied simultaneously, the pattern of leakage flow is significantly altered, with ingestion of flow over much of the early suction surface. The paper explores the physical processes driving the changes, the impact on performance and the implications for future experimental investigations.

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Figures

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Fig. 1

Simple schematic of tip leakage flow, with cut planes showing low speed behavior over the front of the tip (A) and high speed over the rear (B)

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Fig. 2

Calculation domain and mesh, τ* = 1%, from Ref. [15]

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Fig. 3

High-speed cascade heat transfer at three tip gaps: top, experiments [12]; bottom, computations with a perfectly sharp tip edge. Reproduced from Ref. [15].

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Fig. 4

TF and SC inlet conditions in the absolute frame: total pressure, whirl angle (5 deg increments) and total temperature

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Fig. 5

TF and SC inlet conditions in the relative frame: total pressure, whirl angle (20 deg increments) and total temperature

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Fig. 6

Radially integrated leakage mass flux vectors for different boundary conditions; τ* = 1%

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Fig. 7

Radially averaged leakage mass flux vectors for different combinations of SC and TF inlet conditions, with RCM; τ* = 1%

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Fig. 8

Slices of relative-frame total pressure loss with streamlines for each boundary condition. The thick grey streamlines have been released inside the tip gap at midheight; the thin blue lines are released at the inlet plane close to the casing.

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Fig. 9

Streamlines and contours of tangential velocity (positive in the direction of rotation) at a radial distance of τ/2 inwards from the casing

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Fig. 10

Over-tip driving pressure

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Fig. 11

Tangential (cross-passage) velocity contours in the axial plane indicated by the dashed line in Fig. 9(a)

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Fig. 12

Heat flux contours with surface streamlines

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Fig. 13

Lift-off (E, G) and impingement (F) lines induced by vortex interaction, over the aft portion of the blade

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