Prediction of fatigue life of mistuned steam turbine blades subjected to variations in blade geometry.
Mashiachidi, Makgwantsha
Mashiachidi, Makgwantsha
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Abstract
Many power stations are suffering from failure of the steam turbine blades owing to fatigue. The steam turbine blades are also subjected to steam flow bending, centrifugal loading, vibration response, and structural mistuning. These mentioned factors significantly contribute to the fatigue failure of the steam turbine blades. Low-Pressure steam turbines also experience premature blade and disk failure due to the stress concentrations at the blade root area of its bladed disk. Driven by the problems encountered by the steam power plant electricity generating utilities with regards to the failure of steam turbine blades owing to fatigue, this study of the mistuned steam turbine blades which are subjected to variations in the geometry of the blades will be of great significance to the electricity generation industry. However, only a limited number of studies have considered the effects of uncertainty regarding the geometry of the steam turbine blades. Hence, inspired by the problems encountered with steam turbine blades, this research presents a numerical methodology for the prediction of the life of mistuned steam turbine blades subjected to variations in blade geometry as such studies have not yet been contemplated. A simplified, scaled-down mistuned steam turbine bladed disk model was developed using ABAQUS finite element software. Acquisition of the vibration characteristics and steady-state stress response of the disk models was performed through finite element analysis. Thereafter, numerical stress distributions were obtained, and the model was subsequently exported to Fe-Safe software for fatigue life calculations based on centrifugal and harmonic sinusoidal pressure loading. Subsequently, Monte Carlo Simulation was utilised in Companion software to determine the probabilistic fatigue life for 20 cases based on the accepted scatter manufacturing percentage of steam turbine bladed disks. This was carried out by designating mistuning percentages (geometry variations) as the random variables. The results obtained were validated by literature. The fatigue life results showed that the fatigue life is most sensitive to changes in blade length, followed by the width, and then the thickness, in this order. The correlation was achieved between the probabilistically predicted life and the discrete fatigue life was calculated using the Fe-Safe software. The predictions of the fatigue lives correlated highly well compared with the literature. Hence, the developed methodology was proven to be reliable and can be successfully employed to predict the actual fatigue life of mistuned steam turbine blades.
Description
A dissertation submitted in partial fulfilment of the requirements for the degree, Master of Engineering in the Department of Mechanical and Mechatronics Engineering in the Faculty of Engineering and the Built Environment at the Tshwane University of Technology.
Date
2024-07-01
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Tshwane University of Technology
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Keywords
Fatigue, Steam turbine, Steam turbine blades