Research Papers

Highly Resolved Large Eddy Simulation Study of Gap Size Effect on Low-Pressure Turbine Stage

[+] Author and Article Information
R. Pichler

Department of Mechanical Engineering,
University of Melbourne,
Melbourne 3010, Australia
e-mail: richard.pichler@unimelb.edu.au

V. Michelassi

GE Oil and Gas,
Florence 50127, Italy

R. Sandberg

Department of Mechanical Engineering,
University of Melbourne,
Melbourne 3010, Australia

J. Ong

GE Global Research,
Munich 85748, Germany

Contributed by the International Gas Turbine Institute (IGTI) of ASME for publication in the JOURNAL OF TURBOMACHINERY. Manuscript received August 19, 2017; final manuscript received September 18, 2017; published online November 14, 2017. Editor: Kenneth Hall.

J. Turbomach 140(2), 021003 (Nov 14, 2017) (11 pages) Paper No: TURBO-17-1123; doi: 10.1115/1.4038178 History: Received August 19, 2017; Revised September 18, 2017

Blade-to-blade interactions in a low-pressure turbine (LPT) were investigated using highly resolved compressible large eddy simulations (LESs). For a realistic setup, a stator and rotor configuration with profiles typical of LPTs was used. Simulations were conducted with an in-house solver varying the gap size between stator and rotor from 21.5% to 43% rotor chord. To investigate the effect of the gap size on the prevailing loss mechanisms, a loss breakdown was conducted. It was found that in the large gap (LG) size case, the turbulence kinetic energy (TKE) levels of the stator wake close to the rotor leading edge were only one third of those in the small gap (SG) case, due to the longer distance of constant area mixing. The small time-averaged suction side separation on the blade, found in the LG case, disappeared in the SG calculations, confirming how stronger wakes can keep the boundary layer attached. The higher intensity wake impinging on the blade, however, did not affect the time-averaged losses calculated using the control volume approach of Denton. On the other hand, losses computed by taking cross sections upstream and downstream of the blade revealed a greater distortion loss generated by the stator wakes in the SG case. Despite the suction side separation suppression, the SG case gave higher losses overall due to the incoming wake turbulent kinetic energy amplification along the blade passage.

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Fig. 1

Sketch of computational grid for illustrative purposes

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Fig. 2

Nondimensional vane wall grid spacing: (a) along blade and in span and (b) normal to blade

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Fig. 3

Nondimensional blade wall grid spacing: (a) along blade and in span and (b) normal to blade

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Fig. 4

Time-averaged ratio of subgrid scale to molecular viscosity

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Fig. 5

Phase-averaged TKE for the SG for phase p1 (a), p6 (b), p11 (c), and p16 (d) right

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Fig. 6

Phase-averaged TKE for the LG for phase p1 (a), p6 (b), p11 (c), and p16 (d)

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Fig. 7

Instantaneous isosurfaces of the imaginary part of the eigenvalues of the velocity gradient tensor (λci=10) around the rotor (shaded) with greyscale denoting contour levels of spanwise vorticity component (−30 to 30): leading-edge region in presence of wake (a), trailing edge region (b), and trailing edge region when the wake apex moves by (c)

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Fig. 8

Phase-averaged production of TKE and for the SG at phase p11 (a) and the LG at phase p1 (b)

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Fig. 9

Vane load: (a) time averaged, (b) SG phase averaged, and (c) LG phase averaged

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Fig. 10

Blade load: (a) time averaged, (b) SG phase averaged, and (c) LG phase averaged

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Fig. 11

Time-averaged vane wall shear stress (a) and space time plots of phase-averaged vane wall shear stress on the suction side for SG (b) and LG (c)

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Fig. 12

Time-averaged blade wall shear stress (a) and space time plots of phase-averaged rotor wall shear stress on the suction side for SG (b) and LG (c)

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Fig. 13

Wall normal boundary layer profiles along the blade surface (s) at phase t/T = 0.75, SG case. A solid black line indicates the boundary layer thickness at this phase, and the dashed black lines indicate the boundary layer thickness of the mean flow. The two solid lines starting at (0.85, 0.0) and (0.92, 0,0) are contour lines at zero tangential velocity. The arrows represent the defect velocity vector, and the contours represent TKE.

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Fig. 14

Time-averaged kinetic losses of the stator (a) and rotor (b)

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Fig. 15

Stagnation pressure in the vane–blade gap: (a) SG and (b) LG

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Fig. 16

Residual vane wake just upstream of the blade, i.e., (x = −0.05) in terms of time mean (μt) and standard deviation (σt) for both gap sizes. The colored band spans the area μt±σt.

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Fig. 17

Vane wake maturity

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Fig. 18

Blade loss breakdown

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Fig. 19

Time-averaged turbulent kinetic energy

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Fig. 20

Blade mixed-out losses as a function of incoming wake maturity



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