Investigation of Large-Scale Cooling System Fan Blade Vibration

Abstract

Fans operating at the edges of large-scale air-cooled steam condensers often do so under distorted inlet air flow conditions. These conditions create variations in the aerodynamic loads exerted on a fan blade during rotation which causes it to vibrate. In order to isolate the sources of the unsteady aerodynamic loads as well as their effects on blade vibration, a potential flow fluid dynamics code was written to determine the aerodynamic loads exerted on a fan blade as a function of its rotation. The lift and drag forces were exported to a finite element code approximating the fan blade as a cantilever beam. With these two sets of code the response of the blade when subjected to varying aerodynamic loads could be determined. Furthermore, the effect of changing certain parameters such as blade stiffness or damping can be investigated. It was found that the blade’s response closely resembles that which was measured at the full-scale facility and that slight changes to the blade’s stiffness can potentially reduce the vibrational amplitude but may also lead to resonance.

Axial Fan Blade Vibration Assessment under Inlet Cross-Flow Conditions Using Laser Scanning Vibrometry

“…Under the influence of inlet cross-flow, the blade-pass frequency and the sixth rotation-rate harmonic were excited strongly, while no significant amplification of the natural frequencies was measured. These findings agree with previously observed cross-flow excitation effects [17,18]. Integral fan blade vibration almost doubled at the low fan flow rate, and increased by 20% at the higher fan flow rate.…”

“…For future investigation, it is also advised to not only record the relative phase relation of the measured scanning points, but also to identify the azimuthal position in relation to the cross-flow. Such results could yield a better comparison to the findings of e.g., Muiyser et al [17,18,24]. Till Heinemann performed the experiments, analyzed the data and wrote the paper.…”

“…Up to this point, similar investigations have only been made numerically [15,16,18] or by using strain gauges on larger-diameter fans [17,24]. Using LSV in this setup, it was possible to gain more insight into the spatial resolution of the fan blade vibration under cross-flow influence, and to compare the resulting motion distributions to numerical mode analysis.…”

“…The blade load was increased dominantly by magnitudes of around 20% at the upwind side of the periphery fan; moderately higher loads were observed at the advancing blade side compared to the retreating blade side. Earlier potential flow computations by Muiyser et al [18] had already identified the sensitivity of the blade vibration to distorted inlet-flow conditions.…”

Analysis of Fan Blade Vibration with a Non-Contact Method

“…Several structural problems and health management approaches were reported for large fan blades installed in ground systems such as wind tunnels [10], cooling fans [6,13,14] or desulphurisation systems [19]. Applied methods included strain gauges, laser vibrometry, fast camera image processing [3], shaft torsional vibration analysis [11] and BTT.…”