Research Papers

Numerical Investigation of Contrasting Flow Physics in Different Zones of a High-Lift Low-Pressure Turbine Blade

[+] Author and Article Information
Jiahuan Cui

CFD Laboratory,
Department of Engineering,
University of Cambridge,
Cambridge CB2 1PZ, UK
e-mail: jc763@cam.ac.uk

V. Nagabhushana Rao

CFD Laboratory,
Department of Engineering,
University of Cambridge,
Cambridge CB2 1PZ, UK
e-mail: nrv24@cam.ac.uk

Paul Tucker

CFD Laboratory,
Department of Engineering,
University of Cambridge,
Cambridge CB2 1PZ, UK
e-mail: pgt23@cam.ac.uk

Contributed by the International Gas Turbine Institute (IGTI) of ASME for publication in the JOURNAL OF TURBOMACHINERY. Manuscript received August 20, 2015; final manuscript received August 24, 2015; published online October 13, 2015. Editor: Kenneth Hall.

J. Turbomach 138(1), 011003 (Oct 13, 2015) (10 pages) Paper No: TURBO-15-1183; doi: 10.1115/1.4031561 History: Received August 20, 2015; Revised August 24, 2015

Using a range of high-fidelity large eddy simulations (LES), the contrasting flow physics on the suction surface, pressure surface, and endwalls of a low-pressure turbine (LPT) blade (T106A) was studied. The current paper attempts to provide an improved understanding of the flow physics over these three zones under the influence of different inflow boundary conditions. These include: (a) the effect of wakes at low and high turbulence intensity on the flow at midspan and (b) the impact of the state of the incoming boundary layer on endwall flow features. On the suction surface, the pressure fluctuations on the aft portion significantly reduced at high freestream turbulence (FST). The instantaneous flow features revealed that this reduction at high FST (HF) is due to the dominance of “streak-based” transition over the “Kelvin–Helmholtz” (KH) based transition. Also, the transition mechanisms observed over the turbine blade were largely similar to those on a flat plate subjected to pressure gradients. On pressure surface, elongated vortices were observed at low FST (LF). The possibility of the coexistence of both the Görtler instability and the severe straining of the wakes in the formation of these elongated vortices was suggested. While this was true for the cases under low turbulence levels, the elongated vortices vanished at higher levels of background turbulence. At endwalls, the effect of the state of the incoming boundary layer on flow features has been demonstrated. The loss cores corresponding to the passage vortex and trailing shed vortex were moved farther from the endwall with a turbulent boundary layer (TBL) when compared to an incoming laminar boundary layer (LBL). Multiple horse-shoe vortices, which constantly moved toward the leading edge due to a low-frequency unstable mechanism, were captured.

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Fig. 1

Sketch showing: (a) time-averaged separation bubble and receptive elements and (b) key features of the endwall flows

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Fig. 2

Computational domain and boundary conditions for the test cases concerning the flow over midspan

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Fig. 3

Mesh resolution in wall units along the suction side for the test case LF

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Fig. 4

Inflow characteristics: (a) Tu decay in the freestream for LF and HF cases, (b) normalized phase-averaged mean velocity variation, and (c) phase-averaged Tu variation for LFW and HFW cases

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Fig. 5

Streamwise variation of time- and span-averaged (a) pressure coefficient, Cp (lines: LES and symbols: experiments [19]) and (b) skin-friction coefficient, Cf

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Fig. 6

Phase-averaged Cp variation on the aft portion of the suction surface at ten different phases for (a) LFW and (b) HFW

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Fig. 7

Comparison of momentum thickness variation (a) on the suction surface for LF and HF, (b) at the trailing edge (S/S0≈0.98) for all test cases (symbols: experiments and lines: LES), and (c) at the trailing edge as a bar graph

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Fig. 8

Isosurfaces of Q contoured by velocity magnitude for the cases LFW and HFW (Z-vorticity in the background frame shows wake passing)

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Fig. 9

Görtler number (G = (Ublθ/ν)(θ/R)1/2) on pressure surface

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Fig. 10

Q isosurfaces close to wall (n/C < 0.02) are flooded by velocity magnitude over the pressure surface. The dashed lines: 0.1 < x/Cx < 0.9 with increment 0.1.

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Fig. 11

Development of mushroomlike velocity contours on the pressure surface for LF case. Inset plots: (a) velocity profiles at different streamwise stations (spanwise location of the extracted line is marked in contour plot), (b) location of extracted contour planes relative to blade, and (c) streamlines of fluctuating velocity showing counter-rotating vortices.

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Fig. 12

(a) Velocity profiles and (b) turbulent kinetic energy profiles of the LBL and TBL imposed at the inlet

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Fig. 13

(a) Comparison of pressure isoline superimposed over contours of turbulent kinetic energy extracted at Z/h = 0.13% and (b) line contour of total pressure loss superimposed over streamwise vorticity extracted at x/Cx = 1.3 for the cases with incoming LBL and TBL profiles

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Fig. 14

(a) Isosurfaces of Q, contoured with total velocity, showing the instantaneous flow features at the endwall. (b) Streamlines demonstrating multiple leading edge separations, their convection and merging, and the time-averaged streamlines of this phenomenon. (c) Surface streamlines showing the interaction of endwall flow with 2D-separation bubble.



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