Research Papers

On the Prediction and Theory of the Temperature Increase of Low Pressure Last Stage Moving Blades During Low Volume Flow Conditions, and Limiting it Through Steam Extraction Methods

[+] Author and Article Information
Adam Beevers

Alstom Power (Switzerland),
Baden 5400, Switzerland
e-mail: adam.beevers@power.alstom.com

Said Havakechian, Benjamin Megerle

Alstom Power (Switzerland),
Baden 5400, Switzerland

Contributed by the International Gas Turbine Institute (IGTI) of ASME for publication in the JOURNAL OF TURBOMACHINERY. Manuscript received October 16, 2014; final manuscript received March 17, 2015; published online May 12, 2015. Assoc. Editor: Ronald Bunker.

J. Turbomach 137(10), 101002 (May 12, 2015) (11 pages) Paper No: TURBO-14-1271; doi: 10.1115/1.4030258 History: Received October 16, 2014

During extreme low volume flow conditions, the last stages of a low pressure steam turbine operate in ventilation conditions that can cause a significant temperature increase of critical regions of the last stage moving blade (LSB). Under some conditions, the blade temperature may rise above a safe operating temperature, requiring the machine to be shut down. Limiting the heating effect on the LSB increases the allowable operating range of the low pressure turbine. One common method is to spray water droplets into the low pressure exhaust. As the length of LSBs continues to increase, this method reaches its limit of practical operating effectiveness due to the amount of water required and its impact on the erosion of the LSB. An investigation into complimentary solutions to limit the temperature increase was conducted using CFD. An appropriate CFD setup was chosen from a sensitivity study on the effects from geometry, mesh density, turbulence model, and time dependency. The CFD results were verified against steam turbine data from a scaled test facility. The proposed solutions include low temperature steam extraction, targeted for critical regions of the moving blade. From the test turbine and CFD results, the drivers of the temperature increase during ventilation conditions are identified and described.

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Fig. 1

LP flow structure during very low flow conditions

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Fig. 2

CFD geometry for single passage cases (1 and 2), including the computational walls (shown as shaded) placed at the exhaust outlet of case 2

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Fig. 3

CFD geometry for full annulus cases (3–6)

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Fig. 4

Temperature measurement traverse positions

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Fig. 5

Circumferential position of upstream and downstream temperature measurements

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Fig. 6

Location of thermocouple mounted on the LSB (circled)

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Fig. 7

Comparison of upstream flow temperatures for cases 1 and 2

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Fig. 8

Comparison of downstream flow temperatures for cases 1 and 2

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Fig. 9

Comparison of vortices in the diffuser and exhaust when the swirl component of the flow is reduced

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Fig. 10

Comparison of blade temperatures for cases 1 and 2

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Fig. 11

Comparison of measured and CFD blade temperatures for cases 1 and 2

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Fig. 12

Comparison of measured blade temperature and CFD prediction (case 1) for a range of operating points

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Fig. 13

Comparison of upstream and downstream CFD data sampling (solid) with the measurement (dashed)

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Fig. 14

Measured and computed flow temperatures upstream of LSB for case 3

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Fig. 15

Measured and computed flow temperatures downstream of LSB for case 3

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Fig. 16

LSB blade temperatures profiles for case 3

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Fig. 17

LSB circumferential and time and circumferentially averaged blade temperatures for case 5

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Fig. 18

Comparison of upstream flow temperatures of all the CFD cases and measurement data

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Fig. 19

Comparison of downstream flow temperatures of all the CFD cases and measurement data

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Fig. 20

Comparison of the difference between measured and computed blade temperatures at the thermocouple on the pressure surface

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Fig. 21

Comparison of spanwise blade temperatures for the range of CFD cases

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Fig. 22

Predicted maximum temperatures for a range of LSBs. Length normalized by the length of the shortest LSB.

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Fig. 23

Meridional velocity vectors of flow in the upper spans of the last stage

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Fig. 24

Streamlines showing angular momentum in the upper span regions of the last stage

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Fig. 25

Circumferentially averaged angular momentum in the last stage. Last stage blading is outlined.

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Fig. 26

Blade to blade plot of turbulence kinetic energy and velocity vectors in the last stage at 95% span

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Fig. 27

Entropy generated at the trailing edge of the last stage guide. High entropy regions (dark regions near casing) represent the stall cells present.

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Fig. 28

Comparison of spanwise difference in LSB maximum temperature between baseline and the extraction cases

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Fig. 29

Streamline plots showing absolute total temperature for cases A, C, and D. Extraction positions shown in shaded boxes. (a) Baseline, (b) case A, (c) case C, and (d) case D.

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Fig. 30

Comparison of LSB LE relative flow angle and delta axial velocity for the baseline and extraction cases A and C




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