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Research Papers

Effect of Wakes and Secondary Flow on Re-attachment of Turbine Exit Annular Diffuser Flow

[+] Author and Article Information
David Kluß

Department for Fluid Energy Machines, Ruhr-Universität Bochum, 44801 Bochum, Germanydavid.kluss@rub.de

Horst Stoff

Department for Fluid Energy Machines, Ruhr-Universität Bochum, 44801 Bochum, Germany

Alexander Wiedermann

 MAN Turbo AG, 46145 Oberhausen, Germany

J. Turbomach 131(4), 041012 (Jul 06, 2009) (12 pages) doi:10.1115/1.3070577 History: Received August 22, 2008; Revised October 16, 2008; Published July 06, 2009

In this paper numerical results of wake and secondary flow interaction in diffuser flow fields are discussed. The wake and secondary flow are generated by a rotating wheel equipped with 30 cylindrical spokes with a diameter of 10 mm as a first approach to the turbine exit flow environment. The apex angle of the diffuser is chosen such that the flow is strongly separated according to the well-known performance charts of Sovran and Klomp (1967, “Experimentally Determined Optimum Geometries for Rectilinear Diffusers With Rectangular, Conical or Annular Cross-Section  ,” in Fluid Mechanics of Internal Flow, Elsevier, New York, pp. 272–319). This configuration has been tested in an experimental test rig at the Leibniz University Hannover (Sieker and Seume 2007, “Influence of Rotating Wakes on Separation in Turbine Exhaust Diffusers,” Paper No. ISAIF8-54). According to these experiments, the flow in the diffuser separates as free jet for low rotational speeds of the spoke-wheel, as expected by theory. However, if the 30 spokes of the upstream wheel rotate beyond the value of 500 rpm the measurements indicate that the flow remains attached to the outer diffuser wall. It will be shown by the present numerical analysis with the commercial solver ANSYS CFX-10.0 that only an unsteady approach using the elaborate scale adaptive simulation with the shear stress transport turbulence model is capable of predicting the stabilizing effect of the rotating wheel to the diffuser flow at larger rotational speeds. The favorable comparison with the experimental data suggests that the mixing effect of wakes and secondary flow pattern is responsible for the reattachment. As a result of our studies, it can be stated that the considerably higher numerical costs associated with unsteady calculations must be accepted in order to increase the understanding of the physical flow phenomena in turbine exit flow and its interaction with the downstream diffuser.

Copyright © 2009 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Diffuser test rig facility

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Figure 2

Measurement positions of pneumatic probes and LDV for the comparison of experiments with numerical data

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Figure 3

Computational domain

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Figure 4

Structured mesh topology at the meridional plane with ANSYS ICEM CFD

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Figure 5

Variation of interface distances downstream of the spoke-wheel

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Figure 6

Absolute velocity field in the meridional plane

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Figure 7

Static pressure recovery versus normalized interface position

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Figure 8

Interface plane position and definition of counter-rotating wall for the entire rotating domain covering spoke-wheel and annular diffuser

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Figure 9

Axial velocity distributions at 50% of the annular diffuser length (x/h1=0.89)

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Figure 10

Normalized turbulent kinetic energy distributions at 50% of the annular diffuser length (x/h1=0.89)

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Figure 11

Axial velocity distributions at 50% of the annular diffuser length (x/h1=0.89) with nonrotating wheel

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Figure 12

Axial velocity distributions at 10% of the conical diffuser length (x/h1=3.57) with nonrotating wheel

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Figure 13

Axial and tangential velocity distributions at 50% of the annular diffuser length (x/h1=0.89); rotational speed n=1500 rpm

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Figure 14

Axial and tangential velocity distributions at 10% of the conical diffuser length (x/h1=3.57); rotational speed n=1500 rpm

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Figure 15

Unsteady eddy-viscosity/molecular viscosity ratio versus time steps; rotational speed n=1500 rpm

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Figure 16

Instantaneous isosurface of Mach number colored by the turbulent length scale/spoke-wheel diameter; top: SST-URANS, bottom: SAS-SST

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Figure 17

Time-averaged axial and radial components of vorticity distributions and back-flow regions at casing; rotational speed n=1500 rpm

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Figure 18

Axial velocity fluctuations and time averaging at 50% of the annular diffuser length (x/h1=0.89) near casing versus time steps used for averaging; rotational speed n=1500 rpm

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Figure 19

Measured and computed axial velocity distributions at 50% of the annular diffuser length (x/h1=0.89); rotational speed n=1500 rpm

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Figure 20

Measured and computed axial velocity distributions at the outlet of the annular diffuser (x/h1=1.69); rotational speeds n=0, 1500, 2500 rpm

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Figure 21

Measured and computed axial velocity distributions at 10% of the conical diffuser length (x/h1=3.57); rotational speed with n=0, 1500, 2500 rpm

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Figure 22

Static pressure recovery coefficients versus flow coefficient

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Figure 23

Contour plots of time-averaged axial, radial, and tangential components of vorticity in stationary frame of reference and back-flow regions for rotational speeds n=0 rpm and n=500 rpm, respectively

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Figure 24

Contour plots of time-averaged axial, radial, and tangential components of vorticity in stationary frame of reference and back-flow regions for rotational speeds n=1500 rpm and n=2500 rpm, respectively

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Figure 25

Time-averaged absolute values of axial and radial vorticity at constant channel height versus normalized annular diffuser length

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Figure 26

Time-averaged axial and radial components of vorticity along a monitoring path at constant channel height versus normalized annular diffuser length

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Figure 27

Computed time-averaged turbulent kinetic energy distributions at constant channel height versus annular diffuser length

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