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Research Papers: Gas Turbines: Controls, Diagnostics, and Instrumentation

Computational Fluid Dynamics Modeling of Low Pressure Steam Turbine Radial Diffuser Flow by Using a Novel Multiple Mixing Plane Based Coupling—Simulation and Validation

[+] Author and Article Information
Peter Stein

Alstom, Power,
Baden 5401, Switzerland
e-mail: peter.stein@power.alstom.com

Christoph Pfoster

Alstom, Power,
Baden 5401, Switzerland
e-mail: christoph.pfoster@power.alstom.com

Michael Sell

Alstom, Power,
Baden 5401, Switzerland
e-mail: michael.sell@power.alstom.com

Paul Galpin

ANSYS Canada Ltd.,
Waterloo, ON N2J 4G8, Canada
e-mail: paul.galpin@ansys.com

Thorsten Hansen

ANSYS Germany GmbH,
Otterfing 83624, Germany
e-mail: thorsten.hansen@ansys.com

1Corresponding author.

Contributed by the Controls, Diagnostics and Instrumentation Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received July 16, 2015; final manuscript received July 21, 2015; published online October 21, 2015. Editor: David Wisler.

J. Eng. Gas Turbines Power 138(4), 041604 (Oct 21, 2015) (10 pages) Paper No: GTP-15-1332; doi: 10.1115/1.4031388 History: Received July 16, 2015; Revised July 21, 2015

The diffuser and exhaust of low pressure steam turbines show significant impact on the overall turbine performance. The amount of recovered enthalpy leads to a considerable increase of the turbine power output, and therefore a continuous focus of turbine manufacturers is put on this component. On the one hand, the abilities to aerodynamically design such components are improved, but on the other hand a huge effort is required to properly predict the resulting performance and to enable an accurate modeling of the overall steam turbine and therewith plant heat rate. A wide range of approaches is used to compute the diffuser and exhaust flow, with a wide range of quality. Today, it is well known and understood that there is a strong interaction of rear stage and diffuser flow, and the accuracy of the overall diffuser performance prediction strongly depends on a proper coupling of both domains. The most accurate, but also most expensive method is currently seen in a full annulus and transient coupling. However, for a standard industrial application of diffuser design in a standard development schedule, such a coupling is not feasible and more simplified methods have to be developed. The paper below presents a computational fluid dynamics (CFD) modeling of low pressure steam turbine diffusers and exhausts based on a direct coupling of the rear stage and diffuser using a novel multiple mixing plane (MMP). It is shown that the approach enables a fast diffuser design process and is still able to accurately predict the flow field and hence the exhaust performance. The method is validated against several turbine designs measured in a scaled low pressure turbine model test rig using steam. The results show a very good agreement of the presented CFD modeling against the measurements.

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Figures

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Fig. 1

Pressure recovery and the effect on the rear stage power conversion, without diffuser (left) and with diffuser (right)

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Fig. 2

Typical pressure distribution in an exhaust box

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Fig. 3

Two-dimensional depiction of a mixing plane interface, which is implemented as a series of annular rings, the control surfaces, over which the fluxes are balanced, mixed out, and the frame of reference is changed

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Fig. 4

Low pressure steam turbine module CFD setup of rear stage and diffuser. In this configuration, four independent rear stages are connected to the four corresponding sectors around the circumference of the diffuser inlet region.

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Fig. 5

CFD mesh of stator vane (left), rotor blade (mid), and exhaust (right)

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Fig. 6

Mach number (axial) development through the rotor–stator interface

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Fig. 7

Comparison of enthalpy recovery for different rotor–stator interface types

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Fig. 8

Pressure and Mach number distribution downstream of the rotor–stator interface, including diffuser lip (grey) and flow separation at diffuser lip (black)

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Fig. 9

Mach number distribution at a vertical split section

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Fig. 10

Low pressure steam turbine test rig

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Fig. 11

Selection of available diffuser related measurement positions for validation

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Fig. 12

Validation case design comparison

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Fig. 13

Validation of CFD against experiment—pressure recovery

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Fig. 14

Validation of CFD against experiment at section S1—traverses—case1

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Fig. 15

Validation of CFD against experiment at section S2—traverses—case1

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Fig. 16

Validation of CFD against experiment at section S3—traverses—case1

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