Research Papers: Gas Turbines: Turbomachinery

The Influence of Inlet Asymmetry on Steam Turbine Exhaust Hood Flows

[+] Author and Article Information
Zoe Burton

e-mail: zoe.burton@durham.ac.uk

Simon Hogg

e-mail: simon.hogg@durham.ac.uk

Grant L. Ingram

e-mail: g.l.ingram@durham.ac.uk
School of Engineering and Computing Sciences,
Durham University,
South Road,
Durham DH1 3LE, UK

1Corresponding author.

Contributed by the Turbomachinery Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received November 4, 2013; final manuscript received November 8, 2013; published online December 19, 2013. Editor: David Wisler.

J. Eng. Gas Turbines Power 136(4), 042602 (Dec 19, 2013) (9 pages) Paper No: GTP-13-1398; doi: 10.1115/1.4026003 History: Received November 04, 2013; Revised November 08, 2013

It has been widely recognized for some decades that it is essential to accurately represent the strong coupling between the last stage blades (LSB) and the diffuser inlet, in order to correctly capture the flow through the exhaust hoods of steam turbine low pressure cylinders. This applies to any form of simulation of the flow, i.e., numerical or experimental. The exhaust hood flow structure is highly three-dimensional and appropriate coupling will enable the important influence of this asymmetry to be transferred to the rotor. This, however, presents challenges as the calculation size grows rapidly when the full annulus is calculated. The size of the simulation means researchers are constantly searching for methods to reduce the computational effort without compromising solution accuracy. However, this can result in excessive computational demands in numerical simulations. Unsteady full-annulus CFD calculation will remain infeasible for routine design calculations for the foreseeable future. More computationally efficient methods for coupling the unsteady rotor flow to the hood flow are required that bring computational expense within realizable limits while still maintaining sufficient accuracy for meaningful design calculations. Research activity in this area is focused on developing new methods and techniques to improve accuracy and reduce computational expense. A novel approach for coupling the turbine last stage to the exhaust hood employing the nonlinear harmonic (NLH) method is presented in this paper. The generic, IP free, exhaust hood and last stage blade geometries from Burton et al. (2012. “A Generic Low Pressure Exhaust Diffuser for Steam Turbine Research,”Proceedings of the ASME Turbo Expo, Copenhagen, Denmark, Paper No. GT2012-68485) that are representative of modern designs, are used to demonstrate the effectiveness of the method. This is achieved by comparing results obtained with the NLH to those obtained with a more conventional mixing-plane approach. The results show that the circumferential asymmetry can be successfully transferred in both directions between the exhaust hood flow and that through the LSB, by using the NLH. This paper also suggests that for exhaust hoods of generous axial length, little change in Cp is observed when the circumferential asymmetry is captured. However, the predicted flow structure is significantly different, which will influence the design and placement of the exhaust hood internal “furniture.”

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Fig. 1

Diagram of exhaust hood vortices

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Fig. 3

P variations downstream of rotor trailing edge with cell count

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Fig. 2

Flow properties applied at stator inlet plane

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Fig. 5

P contours at rotor-hood interface (Pa)

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Fig. 4

Hood static pressure recovery coefficient variations with number of harmonics modeled

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Fig. 8

Diagram of exhaust hood back wall locations

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Fig. 9

Static pressure recovery coefficient versus back wall location

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Fig. 6

P at the exhaust hood half-joint plane (Pa)

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Fig. 7

Swirl angle at the hood inlet plane (deg)

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Fig. 10

Rotor outlet Pdyn and P RMS (Pa)

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Fig. 11

Diffuser outlet Pdyn RMS (Pa)



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