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Research Papers

Numerical Investigations of an Axial Exhaust Diffuser Coupling the Last Stage of a Generic Gas Turbine

[+] Author and Article Information
Marius Mihailowitsch

ITSM—Institute of Thermal Turbomachinery
and Machinery Laboratory,
University of Stuttgart,
Stuttgart D-70569, Germany
e-mail: mihailowitsch@itsm.uni-stuttgart.de

Markus Schatz

ITSM—Institute of Thermal Turbomachinery
and Machinery Laboratory,
University of Stuttgart,
Stuttgart D-70569, Germany
e-mail: schatz@itsm.uni-stuttgart.de

Damian M. Vogt

ITSM—Institute of Thermal Turbomachinery
and Machinery Laboratory,
University of Stuttgart,
Stuttgart D-70569, Germany
e-mail: vogt@itsm.uni-stuttgart.de

Manuscript received June 22, 2018; final manuscript received June 29, 2018; published online November 1, 2018. Editor: Jerzy T. Sawicki.

J. Eng. Gas Turbines Power 141(3), 031025 (Nov 01, 2018) (9 pages) Paper No: GTP-18-1305; doi: 10.1115/1.4040769 History: Received June 22, 2018; Revised June 29, 2018

It is well known that the last stage of a turbine and the subsequent diffuser should be viewed at and designed as a coupled system rather than as single standalone components. The turbine outlet flow imposes the inlet conditions to the diffuser, whereas the recovered dynamic pressure in the diffuser directly controls the turbine back pressure. With changing operating point, the turbine outflow can vary significantly. This results consequently in large variations of the diffuser performance. A major role in the coupled system of turbine and diffuser can be attributed to the tip leakage flow. While it is desirable to minimize the tip leakage with regard to the turbine, a higher leakage mass flow can often be beneficial for the diffuser performance. As there is currently a trend toward aggressive and hence shorter diffusers which are particularly prone to separation, the question arises where the optimum for this tradeoff problem lies. To investigate the performance in the coupled turbine/diffuser system, a generic last stage with shrouded rotor and axial exhaust diffuser has been designed. The components are representative for heavy duty stationary gas turbine applications. Results are presented for three different operating points representing part-load (PL), design-load (DL), and over-load (OL) condition. Three different seal gap widths are taken into account to control the leakage flow. The results indicate that an operating point-dependent optimum gap width can be found for the coupled system efficiency, whereas the maximum turbine performance is always achieved with a minimum gap width.

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Figures

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Fig. 2

Axial diffuser design: (a) meridional view of the diffuser with supporting strut and corresponding area ratio distribution, (b) annular diffuser in design chart, after [16], and (c) conical diffuser in design chart, after [16]

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Fig. 1

Meridional view of the last stage with shrouded rotor and evaluation planes

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Fig. 6

Intersection of isosurface cax=0 ms−1 and annular diffuser casing for unshrouded and medium gap width configuration at DL, the separated flow is indicated by the hachures: boundary layer separation (w/o)/strut separation (δ2)

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Fig. 3

Isentropic turbine efficiency: (a) dependence on the flow coefficient and (b) dependence on the relative leakage mass flow rate

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Fig. 4

Radial distribution of circumferential averaged flow quantities at rotor outlet: (a) flow angle (absolute) at different operating points and (b) total pressure (normalized) at different operating points

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Fig. 5

Dimensionless pressure recovery of the diffuser: (a) dependence on the flow coefficient and (b) dependence on the relative leakage mass flow rate

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Fig. 7

Pressure recovery along the diffuser: (a) PL, (b) DL, and (c) OL

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Fig. 8

Intersection of isosurface cax=0  ms−1 and annular diffuser casing for unshrouded and medium gap width configuration at OL, the separated flow is indicated by the hachures: boundary layer separation (w/o and δ2)

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Fig. 9

Intersection of isosurface cax=0  ms−1 and annular diffuser casing for unshrouded and medium gap width configuration at PL, the separated flow is indicated by the hachures: strut separation (w/o and δ2)

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Fig. 10

Radial distribution of the circumferential averaged flow angle (absolute) at rotor outlet for different configurations at PL

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Fig. 11

Isentropic system efficiency of turbine and diffuser

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Fig. 12

Dimensionless diffuser coefficients: (a) total pressure loss and (b) kinetic energy

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