Abstract
BackgroundThis paper presents the experimental and numerical studies of last-stage low-pressure (LP) mistuned steam turbine bladed discs during run-down.MethodsThe natural frequencies and mode shapes of the turbine bladed disc were calculated using an FE model. The influence of the shaft on the modal properties, such as natural frequencies and mode shapes, was considered. The tip-timing method was used to find the mistuned bladed disc modes and frequencies.ConclusionsThe experimental results from the tip-timing analysis show that the mistuning in combination with shaft coupling suppresses pure nodal diameter type blade vibrations associated with the fundamental mode shape of a cantilevered blade. Vibration modes emerge when even a single blade is vibrating due to the well-known mode localization caused by mistuning. The numerical results confirm this.
Highlights
In the most commonly used low-pressure steam turbines, during certain summer exploitation conditions, a problem concerning high-level vibrations in unshrouded last-stage low-pressure blades was reported
A numerical study of a steam turbine bladed disc with real, small mistuning showed that nodal diameters do not appear in the first mode family associated with the fundamental mode shape of a cantilevered blade and only individual blades vibrate
A LP last stage mistuned bladed disc on a rotor was tested in a vacuum spin chamber to measure blade vibration
Summary
In the most commonly used low-pressure steam turbines, during certain summer exploitation conditions, a problem concerning high-level vibrations in unshrouded last-stage low-pressure blades was reported. A numerical study of a steam turbine bladed disc with real, small mistuning showed that nodal diameters do not appear in the first mode family associated with the fundamental mode shape of a cantilevered blade and only individual blades vibrate. In this paper, a shaft with mistuned bladed discs will be considered to analyse numerically and experimentally measured multistage coupling and the. The numerical results will be compared with the tip-timing measurements of mistuned bladed disc frequencies and modes. The above studies analysed only a few mistuned bladed discs, which is insufficient for multistage coupling investigation. A proper analysis of multistage coupling requires investigating tuned bladed discs on a shaft. The forced vibration of eight tuned bladed discs with different numbers of blades on a shaft was analysed by Rzadkowski and Drewczynski [16]. Simulated combustion chamber excitations were used by Rzadkowski and Maurin to study the effect of multistage coupling on the compressor stages and turbine stage in [20]
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