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Research Papers

Numerical Investigation of Three-Dimensional Separation in an Axial Flow Compressor: The Influence of Freestream Turbulence Intensity and Endwall Boundary Layer State

[+] Author and Article Information
Ashley D. Scillitoe, Paul G. Tucker

CFD Laboratory,
Department of Engineering,
University of Cambridge,
Cambridge CB2 1PZ, UK

Paolo Adami

CFD Methods,
Rolls-Royce Deutschland,
Eschenweg 11,
Blankenfelde-Mahlow 15827, Germany

1Corresponding author.

Contributed by the International Gas Turbine Institute (IGTI) of ASME for publication in the JOURNAL OF TURBOMACHINERY. Manuscript received July 25, 2016; final manuscript received September 9, 2016; published online October 26, 2016. Editor: Kenneth Hall.

J. Turbomach 139(2), 021011 (Oct 26, 2016) (10 pages) Paper No: TURBO-16-1170; doi: 10.1115/1.4034797 History: Received July 25, 2016; Revised September 09, 2016

Regions of three-dimensional separations are an inherent flow feature of the suction surface-endwall corner in axial compressors. These corner separations can cause a significant total pressure loss and reduce the compressor's efficiency. This paper uses wall-resolved LES to investigate the loss sources in a corner separation, and examines the influence of the inflow turbulence on these sources. Different subgrid scale (SGS) models are tested and the choice of model is found to be important. The σ SGS model, which performed well, is then used to perform LES of a compressor endwall flow. The time-averaged data are in good agreement with measurements. The viscous and turbulent dissipation are used to highlight the sources of loss, with the latter being dominant. The key loss sources are seen to be the 2D laminar separation bubble and trailing edge wake, and the 3D flow region near the endwall. Increasing the freestream turbulence (FST) intensity changes the suction surface boundary layer transition mode from separation induced to bypass. However, it does not significantly alter the transition location and therefore the corner separation size. Additionally, the FST does not noticeably interact with the corner separation itself, meaning that in this case the corner separation is relatively insensitive to the FST. The endwall boundary layer state is found to be significant. A laminar endwall boundary layer separates much earlier leading to a larger passage vortex. This significantly alters the endwall flow and loss. Hence, the need for accurate boundary measurements is clear.

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Figures

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Fig. 1

Two-dimensional slice of computational grid for cascade 2, showing every fifth grid point

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Fig. 2

Endwall boundary layer profiles at inflow: (a) mean velocity profiles and (b) velocity fluctuations and shear stress profiles (nondimensionalized by uτ)

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Fig. 3

Iso-surfaces of positive Q-Criterion colored by streamwise vorticity for case 1-L-N (a) suction surface and (b) pressure surface

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Fig. 4

Spanwise-averaged Cf and Cp distributions on suction surface: (a) friction coefficient and (b) pressure coefficient

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Fig. 5

Contours of span-averaged SGS viscosity ratio near separation bubble

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Fig. 6

Wall normal maximums of streamwise and spanwise velocity fluctuations inside the suction and pressure surface boundary layers: (a) Streamwise, urms and (b) spanwise, wrms

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Fig. 7

Span-averaged Reθ along suction surface

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Fig. 8

Pressure surface Cf and Cp distributions: (a) Friction coefficient and (b) pressure coefficient

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Fig. 9

LES surface streamlines and experimental oilflow

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Fig. 10

Instantaneous iso-surfaces of positive Q-criterion colored by streamwise vorticity for case 2-L-TBL: (a) suction surface and (b) pressure surface

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Fig. 11

Cp distributions near endwall and midspan

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Fig. 12

Pitchwise mass-averaged exit angle and loss coefficient versus span at x/Cx = 1.5: (a) exit angle and (b) loss coefficient streamwise, urms2

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Fig. 13

Time-averaged iso-surfaces of vorticity magnitude colored by urms: (a) Case 2-L-TBL and (b) Case 2-L-LBL

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Fig. 14

Cf coefficient at midspan: (a) suction surface and (b) pressure surface

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Fig. 15

Wall normal maximums of streamwise and spanwise velocity fluctuations in the blade passage at midspan: (a) streamwise, urms and (b) spanwise, wrms

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Fig. 16

Area-averaged (in y-z plane) viscous and turbulent dissipation through blade passage: (a) 35–50% span and (b) 0–35% span

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Fig. 17

Contours of turbulent dissipation and loss coefficient near endwall: (a) Case 2-L-TBL And (b) Case 2-L-LBL

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Fig. 18

Cumulative integral of area-averaged viscous and turbulent dissipation through blade passage: (a) 35–50% span and (b) 0–35% span

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Fig. 19

Contours of velocity fluctuations at trailing edge, for LES case 2-L-TBL

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