Fatigue Endurance Optimization of a Steam Turbine Blade Curved Fir-Tree Root

Document Type : Original Research Paper

Authors

1 Mechanical Rotary Equipment Research Department, Niroo Research Institute (NRI), Tehran, Iran

2 Steam Engineering Unit, Turbomachinery Department, MAPNA Turbine Engineering and Manufacturing Company (TUGA), Karaj, Alborz, Iran

10.22084/jrstan.2023.27108.1228

Abstract

 In this paper, the guide curve of a fir-tree root is parametrically modeled for
the optimization of a newly designed free-standing low-pressure last-stage
blade airfoil of a steam turbine. These geometrical parameters include the
location of the center point of the curve root centerline and the radius of
the root curvature, while the axial length and the pitch are kept fixed. The
mechanical safety factor is computed at the upper section of a predefined root
profile for each considered design parameter in a commercial finite element
software. For high cycle fatigue failure analyses, the maximum alternating
stresses are calculated from harmonic responses under the dynamic fluid
loading obtained from computational fluid dynamics (CFD) simulation at
the maximum continuous rating condition. Preliminary modal analysis is
performed to estimate the natural frequencies for the principal modes near
the low engine order excitation. On this basis, dynamic stresses are calculated
at corresponding frequencies prone to low order engine modes of excitement.
Moreover, harmonic response is obtained at nozzle passing frequency for
each set of design parameters. In addition, the total strain is computed at
the upper section of the root from elastic-plastic analysis under centrifugal
loading resulting from 110% of nominal operating speed. Therefore, a
multi-objective optimization genetic algorithm is used to optimize the root
design parameters with objectives of maximizing safety factors obtained from
both strain-based low cycle fatigue and stress-based high cycle fatigue analyses.

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