During start-up and coast-down of the turbine train large last stage moving blades may show elevated amplitude levels. In cases where the excitation mechanism results from a rotor imbalance, it is well known that the occurring amplitude level can be influenced by the turbine set-up as well as the mode of operation during start-up and coast-down. For example, by increasing speed gradients the exposure time in which the blade row is operated at or close to speed-synchronous resonance is reduced and hence the excitation is minimized.

To evaluate the blade response during transient operation an analytical model is developed which allows for an accurate description of the exciting inertia forces resulting from rotor imbalance. In the present paper, the effect of a small-scale variation of the tuned blades’ resonance frequency and intentional mistuning of the blade row, (Siewert & Stüer, 2014), on the obtainable resonance amplitudes and the potentially accumulated fatigue is assessed. The results show a significant increase in robustness against transient excitation by application of a dedicated tuning, allowing for enhanced flexible operation of large last stage moving blades.

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