Flow-induced vibrations and failure of axial fan blades in electric generators

Research Project, 2019 – 2021

Axial ventilation by use of axial fans was the most common cooling concept for hydropower generators in Sweden during the 50-70’s when the main part of the Swedish hydropower plants where built. Axial ventilation by use of axial fans is still used for new produced machines especially for high speed machines where the small diameter limits the choice of ventilation concept. Axial fans are also common after renovation of old machines when the rotating parts are reused. The design of the fans which is used in the main part of hydropower generators are made by bended plates which are
welded on a fan ring or segments. 

The working principle of an axial fan blade is very similar as an airplane wing profile where a high pressure side occurs below the fan blade and a low pressure side occurs above the fan blade. Vortex
separations may occur at the low pressure side which under some circumstances can results in vibration problem of the fan blades (stalling). Such effects are most likely more severe under some conditions that will more frequently be passed due to an increase in regulation of the electric grid. Cracks and broken fan blades due to fatigue can be a consequence of the vibration which may cause serious damages of the machine due the fact that the fans are located close to the rotating poles and the stator winding.

In order to prevent this kind of problems a frequency analysis of the fan blades is carried out during the design phase. It is very important that the flow-induced frequencies of the forces acting on the fan blades, due to e.g. vortex separation, do not coincide with the eigenfrequency of the blades. That requires a detailed understanding of the flow around fan blades with designs particularly used in axially cooled hydropower generators.