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Research Papers

Experimental and Numerical Investigation of Optimized Blade Tip Shapes—Part II: Tip Flow Analysis and Loss Mechanisms

[+] Author and Article Information
Marek Pátý

Turbomachinery and Propulsion Department,
von Karman Institute for Fluid Dynamics,
Rhode Saint Genèse 1640, Belgium
e-mail: marek.paty@seznam.cz

Bogdan C. Cernat

Turbomachinery and Propulsion Department,
von Karman Institute for Fluid Dynamics,
Rhode Saint Genèse 1640, Belgium
e-mail: cernat@vki.ac.be

Cis De Maesschalck

Turbomachinery and Propulsion Department,
von Karman Institute for Fluid Dynamics,
Rhode Saint Genèse 1640, Belgium
e-mail: cis.demaesschalck@gmail.com

Sergio Lavagnoli

Turbomachinery and Propulsion Department,
von Karman Institute for Fluid Dynamics,
Rhode Saint Genèse 1640, Belgium
e-mail: lavagnoli@vki.ac.be

1Present address: CTU, Prague 166 36, Czech Republic.

2Corresponding author.

3Present address: Rolls-Royce plc, Derby DE21 7BB, UK.

Contributed by the International Gas Turbine Institute (IGTI) of ASME for publication in the JOURNAL OF TURBOMACHINERY. Manuscript received August 22, 2018; final manuscript received September 8, 2018; published online October 18, 2018. Editor: Kenneth Hall.

J. Turbomach 141(1), 011006 (Oct 18, 2018) (13 pages) Paper No: TURBO-18-1220; doi: 10.1115/1.4041466 History: Received August 22, 2018; Revised September 08, 2018

The leakage flows within the gap between the tips of unshrouded rotor blades and the stationary casing of high-speed turbines are the source of significant aerodynamic losses and thermal stresses. In the pursuit for higher component performance and reliability, shaping the tip geometry offers a considerable potential to modulate the rotor tip flows and to weaken the heat transfer onto the blade and casing. Nevertheless, a critical shortage of combined experimental and numerical studies addressing the flow and loss generation mechanisms of advanced tip profiles persists in the open literature. A comprehensive study is presented in this two-part paper that investigates the influence of blade tip geometry on the aerothermodynamics of a high-speed turbine. An experimental and numerical campaign has been performed on a high-pressure turbine stage adopting three different blade tip profiles. The aerothermal performance of two optimized tip geometries (one with a full three-dimensional contoured shape and the other featuring a multicavity squealer-like tip) is compared against that of a regular squealer geometry. In the second part of this paper, we report a detailed analysis on the aerodynamics of the turbine as a function of the blade tip geometry. Reynolds-averaged Navier-Stokes (RANS) simulations, adopting the Spalart–Allmaras turbulence model and experimental boundary conditions, were run on high-density unstructured meshes using the numecafine/open solver. The simulations were validated against time-averaged and time-resolved experimental data collected in an instrumented turbine stage specifically setup for the simultaneous testing of multiple blade tips at scaled engine-representative conditions. The tip flow physics is explored to explain variations in turbine performance as a function of the tip geometry. Denton's mixing loss model is applied to the predicted tip gap aerodynamic field to identify and quantify the loss reduction mechanisms of the alternative tip designs. An advanced method based on the local triple decomposition of relative motion is used to track the location, size and intensity of the vortical flow structures arising from the interaction between the tip leakage flow and the main gas path. Ultimately, the comparison between the unconventional tip profiles and the baseline squealer tip highlights distinct aerodynamic features in the associated gap flow field. The flow analysis provides guidelines for the designer to assess the impact of specific tip design strategies on the turbine aerodynamics and rotor heat transfer.

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Figures

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Fig. 1

Computational domain and tip gap control volume

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Fig. 2

Detail of the rotor tips meshes

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Fig. 3

Tip leakage loss generation inside the tip gap and by mixing with the passage flow [14]

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Fig. 6

From left to right: radially integrated tip leakage mass-flow vectors; contours of tip heat flux with surface streamlines; axial cuts coloured by relative total pressure and vortex boundaries from TDM; axial cuts with contours of relative Mach number. Plotted for the IN01D, AC06D, and AC07T tips.

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Fig. 5

Vortices identified by the TDM (green and blue patches) and by λ2 method (red isolines), in axial planes at 0.6 Cax,r, 0.9 Cax,r, and 1.1 Cax,r

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Fig. 4

Contours of residual vorticity (magnitude, radial and blade-to-blade components) in an axial plane at 0.7 Cax,r

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Fig. 12

Tracking of the TLV, UPV and LPV, plotted per blade geometry for the IN01D, AC06D and AC07T tips: (Top) spanwise location of vortex center, error bars making vortex strength, and (bottom) pitchwise location of vortex core

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Fig. 13

Tracking of the TLV, UPV and LPV, plotted per vortex parameters for the IN01D, AC06D and AC07T tips: (a) spanwise location, (b) pitchwise location, and (c) strength

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Fig. 7

Rotor static pressure distributions in the near-tip region

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Fig. 10

Entropy generation due to tip leakage flows (CFD) compared to predictions of the tip loss model

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Fig. 8

Tip leakage mixing loss distribution (data normalized by the maximum value reached by the design-gap flat tip case)

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Fig. 9

Entropy generation in the tip gap

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Fig. 11

Vortex boundaries from TDM method (red: clockwise, green: counter-clockwise) and pressure loss contours (top); heat flux contours with surface streamlines (bottom): (a) IN01D, (b) AC06D, and (c) AC07T

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Fig. 16

Average heat flux (left) and integral heat load on the blade tip (right)

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Fig. 14

Interaction of the tip leakage jet with the TLV. The position of the TLV and the jet velocity vectors are drawn to scale with the measured values.

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Fig. 15

Aerodynamic performance parameters

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