Research Papers

Blade Tip Shape Optimization for Enhanced Turbine Aerothermal Performance

[+] Author and Article Information
C. De Maesschalck

e-mail: demaess@vki.ac.be

S. Lavagnoli

e-mail: lavagnoli@vki.ac.be

G. Paniagua

e-mail: paniagua@vki.ac.be
von Karman Institute for Fluid Dynamics,
Rhode Saint Genese B-1640, Belgium

Contributed by the International Gas Turbine Institute (IGTI) of ASME for publication in the JOURNAL OF TURBOMACHINERY. Manuscript received June 30, 2013; final manuscript received July 2, 2013; published online October 24, 2013. Editor: Ronald Bunker.

J. Turbomach 136(4), 041016 (Oct 24, 2013) (11 pages) Paper No: TURBO-13-1118; doi: 10.1115/1.4025202 History: Received June 30, 2013; Revised July 02, 2013

In high-speed, unshrouded turbines, tip leakage flows generate large aerodynamic losses and intense unsteady thermal loads over the rotor blade tip and casing. The stage-loading and rotational speeds are steadily increased to achieve higher turbine efficiency, and hence, the overtip leakage flow may exceed the transonic regime. However, conventional blade tip geometries are not designed to cope with supersonic tip flow velocities. A great potential lies in the modification and optimization of the blade tip shape as a means to control the tip leakage flow aerodynamics, limit the entropy production in the overtip gap, manage the heat-load distribution over the blade tip, and improve the turbine efficiency at high-stage loading coefficients. The present paper develops an optimization strategy to produce a set of blade tip profiles with enhanced aerothermal performance for a number of tip gap flow conditions. The tip clearance flow was numerically simulated through two-dimensional compressible Reynolds-averaged Navier–Stokes (RANS) calculations that reproduce an idealized overtip flow along streamlines. A multiobjective optimization tool, based on differential evolution combined with surrogate models (artificial neural networks), was used to obtain optimized 2D tip profiles with reduced aerodynamic losses and minimum heat transfer variations and mean levels over the blade tip and casing. Optimized tip shapes were obtained for relevant tip gap flow conditions in terms of blade thickness to tip gap height ratios (between 5 and 25) and blade pressure loads (from subsonic to supersonic tip leakage flow regimes), imposing fixed inlet conditions. We demonstrated that tip geometries that perform superior in subsonic conditions are not optimal for supersonic tip gap flows. Prime tip profiles exist, depending on the tip flow conditions. The numerical study yielded a deeper insight on the physics of tip leakage flows of unshrouded rotors with arbitrary tip shapes, providing the necessary knowledge to guide the design and optimization strategy of a full blade tip surface in a real 3D turbine environment.

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Fig. 1

Visualization of the novel design methodology

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Fig. 2

Assessment of the 2D overtip flow assumption using a 3D numerical calculation

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Fig. 3

Illustration of the investigated tip gap flow regimes and corresponding application to a 3D HP turbine blade

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Fig. 4

Schematic representation of the combined evaluation and optimization routine

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Fig. 5

The parameterization of the blade tip geometry (top) and possible 2D tip shapes in the design domain (bottom)

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Fig. 6

The computational domain and detail of the tip gap

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Fig. 7

Evaluation of the performance parameters

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Fig. 8

2D objective space for the optimization of the central case and zoom into the relevant area

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Fig. 9

Mach number and turbulent viscosity contours (a), midgap Mach number evolution (b), and performance budget (c) of optimal tip profiles for the central case

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Fig. 10

Specific entropy variation versus gap mass flow relative to the flat tip geometry

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Fig. 11

Mach number contours for the flat tip geometry and for an intermediate optimum shape (a). Performance budget of optimal tip profiles for the lower left case (b).

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Fig. 12

Mach number and turbulent viscosity contours (a) and the performance budget (b) of three optimal tip profiles of the upper left case

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Fig. 13

Mach number contours (a), shroud heat transfer evolution in the tip gap (b), and the performance budget (c) for three optimal tip profiles of the upper right case

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Fig. 14

The Mach number contour (a) and the budgeting compared to the flat tip (b) for the lower right case

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Fig. 15

Aerodynamic and heat transfer parameters for each tip flow regime relative to the flat tip case

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Fig. 16

Specific entropy rise versus gap mass flow, relative to the flat tip case for large blade width (w/h = 25) at low (Mexit = 0.6) and high (Mexit = 1.4) speeds




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