Research Papers

Flow Patterns in High Pressure Steam Turbines During Low-Load Operation

[+] Author and Article Information
Matthias Binner

MTU Maintenance Hannover GmbH,
Muenchner Strasse 31,
Langenhagen 30855, Germany
e-mail: Matthias.Binner@mtu.de

Joerg R. Seume

Institute of Turbomachinery and Fluid Dynamics,
Leibniz University Hannover,
Appelstrasse 9,
Hannover 30167, Germany
e-mail: Seume@tfd.uni-hannover.de

Contributed by the International Gas Turbine Institute (IGTI) of ASME for publication in the JOURNAL OF TURBOMACHINERY. Manuscript received July 20, 2012; final manuscript received June 30, 2013; published online November 21, 2013. Assoc. Editor: Dilip Prasad.

J. Turbomach 136(6), 061010 (Nov 21, 2013) (12 pages) Paper No: TURBO-12-1153; doi: 10.1115/1.4025162 History: Received July 20, 2012; Revised June 30, 2013

Due to the legislative efforts of promoting renewable energy sources, electricity from these sources is preferentially fed into the electrical grid. This requires more frequent part- and low-load operation of peak- and even of base-load power plants to compensate for the varying energy output of renewable energy sources. These requirements ultimately lead to an increased part- and low-load operation not only of low pressure (LP) steam turbines but also of high pressure (HP) steam turbines, putting them at risk of damage due to windage, i.e., strongly separated flow with associated heat generation. For the first time measurements of the steady-state flow field in a 7-stage model air turbine with a modern HP steam turbine blading are conducted in order to extend the understanding of the part- and low-load operation from LP to HP steam turbines. In comparison with LP steam turbines, similar flow fields develop during windage. However, differences exist especially concerning the vortex development in front of the turbine vane rows. The present, geometrically realistic 7-stage turbine, unlike other turbines investigated before, does not show these vortices, which is explained by the shape of the vane passages of this turbine blading. Furthermore, steady-state flow field measurements at different rotor speeds show that the flow coefficient can be used as a nondimensional parameter for maintaining flow field similarity even in part- or low-load operation. Additionally, unsteady circumferential pressure measurements show the existence of pressure perturbations moving circumferentially in front of the stage 7 blades. Seven pressure perturbations moving at 60% of the rotor speed are identified. Due to the shrouded design of the HP steam turbine blading used, the pressure perturbations are not due to tip leakage vortices. Hence, they are identified as features which are similar to “Rotating Stall” cells known from compressors.

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Fig. 1

Flow through a LP turbine during windage operation [3]

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Fig. 2

Meridional stream traces through the 2-stage turbine during windage operation at design rotor speed and 9% of the design mass flow (φdesign=0.78) [8]

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Fig. 3

Schematic of the windage flow field in a LP steam turbine [9]

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Fig. 4

Predicted flow field development of the 7-stage model air turbine at design rotor speed and different mass flows [18]

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Fig. 5

7-stage model air turbine with modern HP steam turbine blading

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Fig. 6

Map of the 7-stage model air turbine

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Fig. 7

Five-hole probe (all dimensions in mm)

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Fig. 8

Flow field in the 7-stage turbine during design- and low-load operation

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Fig. 9

Velocity triangles during design- and part-load operation of a turbine

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Fig. 10

Change of angle of attack due to decreasing mass flow

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Fig. 11

Stream traces in stages 5 and 6 of the 7-stage model air turbine during windage as predicted by Herzog [18]

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Fig. 12

Turbine vane passage of the 2-stage turbine [8] (a) and the 7-stage turbine (b)

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Fig. 13

Schematic of the growth of the pressure-side separation bubble on vanes and resulting influence on available flow area

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Fig. 14

Schematic of windage flow fields with (a) and without (b) vortex in front of the vane row

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Fig. 15

Part-load flow fields of the 7-stage turbine at different rotor speeds but equal flow coefficients

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Fig. 16

Frequency spectrum of the static pressure in stage 7 during low-load operation

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Fig. 17

Graphical scheme for determination of number and circumferential speed of pressure perturbations in stage 7 (CC X/Y = cross correlation of transducers X to Y, AC = autocorrelation)



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