Author(s): S Afara; B Nennemann; A Gehrer

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Keywords: No Keywords

Abstract: During the startup of a horizontal high-head Francis turbine, significant axial vibrations occurred at sub-synchronous speeds. The issue was exasperated by the low axial stiffness of the bearings. The vibration amplitude increased with speed, suggesting resonance tuning rather than self-excitation. Further evidence of resonance came from the site measurements that showed a beat-like signal in the draft tube pressure pulsations and synchronised draft tube and casing pulsations. Additionally, the measured draft tube pressure pulsations exhibited spike-like shapes, which are indicative of large cavitation volume implosions. The source of the excitation was identified, through CFD simulations, as a rope-like vortex beneath the runner, which generated both small asynchronous and large synchronous pulsations. The corresponding frequency matched well with the excitation frequency measured at site thus supporting the hypothesis of hydraulic resonance. The same normalized frequency was also observed in the model test on the dynamic pressures providing further support of the hypothesis.

DOI: https://doi.org/10.1088/1755-1315/1561/1/012030

Year: 2025