Research Papers

Numerical Study of Purge and Secondary Flows in a Low-Pressure Turbine

[+] Author and Article Information
Jiahuan Cui

CFD Laboratory,
Department of Engineering,
University of Cambridge,
Cambridge CB2 1PZ, UK
e-mail: jc763@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 July 3, 2016; final manuscript received August 27, 2016; published online October 4, 2016. Editor: Kenneth Hall.

J. Turbomach 139(2), 021007 (Oct 04, 2016) (10 pages) Paper No: TURBO-16-1142; doi: 10.1115/1.4034684 History: Received July 03, 2016; Revised August 27, 2016

The secondary flow increases the loss and changes the flow incidence in the downstream blade row. To prevent hot gases from entering disk cavities, purge flows are injected into the mainstream in a real aero-engine. The interaction between purge flows and the mainstream usually induces aerodynamic losses. The endwall loss is also affected by shedding wakes and secondary flow from upstream rows. Using a series of eddy-resolving simulations, this paper aims to improve the understanding of the interaction between purge flows, incoming secondary flows along with shedding wakes, and mainstream flows on the endwall within a stator passage. It is found that for a blade with an aspect ratio of 2.2, a purge flow with a 1% leakage rate increases loss generation within the blade passage by around 10%. The incoming wakes and secondary flows increase the loss generation further by around 20%. The purge flow pushes the passage vortex further away from the endwall and increases the exit flow angle deviation. However, the maximum exit flow angle deviation is reduced after introducing incoming wakes and secondary flows. The loss generation rate is calculated using the mean flow kinetic energy equation. Two regions with high loss generation rate are identified within the blade passage: the corner region and the region where passage vortex interacts with the boundary layer on the suction surface. Loss generation rate increases dramatically after the separated boundary layer transitions. Since the endwall flow energizes the boundary layer and triggers earlier transition on the suction surface, the loss generation rate close to the endwall at the trailing edge (TE) is suppressed.

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Fig. 1

Sketch of endwall flow and possible external disturbances from upstream rotor

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Fig. 2

Snapshots of instantaneous flow fields for all the test cases (LBL, WS, purge, WSP, purge with no blade and LBL (full span)). The vorticity magnitude contours are plotted at the inlet and on the endwall for WS case. The isosurfaces of Q-criterion are plotted for other cases.

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Fig. 3

Mean boundary layer velocity profile on the endwall at the inlet compared with the measurements [9]

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Fig. 4

x–y plane view of the computational domain with boundary conditions

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Fig. 5

y–z view of the computational domain with boundary conditions for the cases with purge flows

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Fig. 13

Mass-averaged total pressure loss coefficient

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Fig. 12

Velocity triangles for flows at the midspan (triangle I), close to the endwall after the purging slot without incoming secondary flow (triangle II) and with incoming secondary flows (triangle III)

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Fig. 11

Pitchwise-averaged profiles upstream of the leading edge: (a) dimensionless boundary layer velocity profile and (b) swirl angle

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Fig. 10

Q isosurface of time-averaged flow solution of LBL case

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Fig. 9

The parameters at the midspan for the half and full span resolved cases compared with available measurements: (a) pressure distribution on the blade, (b) boundary layer velocity profile on the suction surface near the trailing edge, and (c) boundary layer integral parameters on the suction surface

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Fig. 8

A snapshot of the Q-criterion isosurface (Q = 1000) contoured by velocity magnitude for the LBL (full span) case

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Fig. 7

Mesh resolution in Kolmogorov units (χ) at (a) x/Cx=0.8 and (b) x/Cx=1.2

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Fig. 6

Mesh resolution in wall units on the blade surface: (a) Δy+ isoline, (b) Δx+ isoline, and (c) Δz+ isoline

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Fig. 14

The interaction between endwall flows and purge flows: (a) loss from purge (no blade) case compared with the loss generated due to purge flows in the cases with blade and (b) pitchwise-averaged endwall boundary layer velocity profile at x/Cx=0.16

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Fig. 17

Total pressure loss and loss generation rate of term VII in Eq. (5)

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Fig. 15

Pitchwise-averaged (a) loss and (b) velocity angle at the exit (x/Cx=1.3)

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Fig. 16

Exit angle contours for (a) purge case and (b) WSP case

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Fig. 18

Averaged mesh resolution on the pressure surface, suction surface, and endwall: (a) 85 × 106 mesh and (b) 40 × 106 mesh

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Fig. 19

Pitchwise-averaged turbulent kinetic energy profiles at (moving from left to right) x/Cx = 0.5, 0.6, and 0.8: ——– 40 × 106 mesh, – – – – 85 × 106 mesh



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