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Research Papers

Loss Mechanisms of Interplatform Steps in a 1.5-Stage Axial Flow Turbine

[+] Author and Article Information
Robert Kluxen

Institute of Jet Propulsion and Turbomachinery,
RWTH Aachen University,
Aachen 52062, Germany
e-mail: robert.kluxen@mtu.de

Stephan Behre

Institute of Jet Propulsion and Turbomachinery,
RWTH Aachen University,
Aachen 52062, Germany
e-mail: behre@ist.rwth-aachen.de

Peter Jeschke

Institute of Jet Propulsion and Turbomachinery,
RWTH Aachen University,
Aachen 52062, Germany
e-mail: jeschke@ist.rwth-aachen.de

Yavuz Guendogdu

MTU Aero Engines,
Dachauer Strasse 665,
Munich 80955, Germany
e-mail: yavuz.guendogdu@mtu.de

1Present address: MTU Aero Engines, Munich 80995, Germany.

Contributed by the International Gas Turbine Institute (IGTI) of ASME for publication in the JOURNAL OF TURBOMACHINERY. Manuscript received September 6, 2016; final manuscript received September 22, 2016; published online November 16, 2016. Editor: Kenneth Hall.

J. Turbomach 139(3), 031007 (Nov 16, 2016) (14 pages) Paper No: TURBO-16-1226; doi: 10.1115/1.4034848 History: Received September 06, 2016; Revised September 22, 2016

In this paper, the detailed steady and unsteady numerical investigations of a 1.5-stage axial flow turbine are conducted to determine the specific influence of interplatform steps in the first stator—as caused by deviations in manufacturing or assembly. A basic first stator design and a design consisting of a bow and endwall contours are compared. Apart from step height, the position and geometry of the interplatform border are varied for the basic design. To create the steps, every third stator vane was elevated, together with its platforms at hub and shroud, such that the flow capacity is only little affected. The results show that the effects of steps on the platform borders in front and aft of the first stator can be decoupled from those occurring on the interplatform steps. For the latter, being the main contributor to the additional loss, the intensity of recirculation zones and losses increase substantially when the platform border is located close to the suction side. Using a relative step height of 1.82% span, the entropy production doubles when compared to a position close to the pressure side, which can be explained by differences in local flow velocity level. Regarding a circular-arc-shaped platform, the losses can be more than halved—mainly due to lower included angles between step and endwall flow streamlines. The findings can be explained by a nondimensional relation of the local entropy production using local values for step height and characteristic flow quantities. Furthermore, a reduction in step height leads to an attenuation of the otherwise linear relationship between step height and entropy production, which is mainly due to lower local ratio of step height and boundary layer thickness. In the case of laminar or transitional flow regions on the endwall, typical for turbine rigs with low inlet turbulence and low-pressure turbines under cruise conditions, the steps lead to immediate local flow transition and thus substantially different results.

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Figures

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Fig. 1

Basic and fully three-dimensional blading: (a) BSCV1 and BSCB1; (b) F3DV1 and EWCB1

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Fig. 2

Platform geometry and modifications: (a) basic geometry and parameters and (b) modifications

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Fig. 3

Step configuration

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Fig. 4

First vane numerical grid: (a) blade-to-blade view, hub and (b) grid detail F3D

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Fig. 5

V1 BSC endwall flow: (a) hub, hrel = 1.82%, (b) hub,−1.82%, and (c) shroud, 1.82%

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Fig. 6

First vane hub vorticity structures, BSC: (a) flat passage (view from vane C to A), (b) suction side passage (view from vane A to B), and (c) pressure side passage (view from vane B to C)

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Fig. 7

Local entropy production at the steps, BSC

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Fig. 8

V1 BSC hub surface flow, hrel = 1.82% (elevation): (a) EPRa transitional, (b) EPRa turbulent, and (c) intermittency

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Fig. 9

Entropy rise in V1 with step height: (a) integral loss, mass flow-averaged and (b) radial distribution, BSC

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Fig. 10

Entropy differences at interplatform and back steps with respect to the flat endwall flow: (a) axial distribution of entropy difference and (b) integral loss

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Fig. 11

Loss coefficients for two-dimensional step flows with varying h/δ: (a) log–log and (b) linear

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Fig. 12

Flow field (left) and entropy production (right) for 2D forward-facing step flows with varying h/δ: (a) h/δ = 5.88, (b) h/δ = 5.88, (c) h/δ, and (d) h/δ

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Fig. 13

Flow field (left) and entropy production (right) for 2D backward-facing step flows with varying h/δ: (a) h/δ = 8.16, (b) h/δ = 8.16, (c) h/δ, and (d) h/δ

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Fig. 14

Projection of hub entropy production rate for varying step heights: (a) hrel = 0.91%, (b) hrel = 1.82%, and (c) hrel = 3.64%

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Fig. 15

Axial distribution of 1D entropy production rate and characteristic flow quantities for the local step flow: (a) entropy production rate, (b) velocity, (c) included angle, and (d) height ratio

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Fig. 16

Projection of hub entropy production rate for varying platform geometry at hrel = 1.82%: (a) Δθ = 0 deg, (b) Δθ = −0.5 deg, (c) Δθ = 1.5 deg, and (d) CircArc

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Fig. 17

Axial distribution of 1D entropy production rate and characteristic flow quantities for the local step flow: (a) entropy production rate, (b) velocity, (c) included angle, and (d) height ratio

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Fig. 18

Prediction of entropy production along the interplatform steps using dimensional analysis

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