Investigating the effect of aerodynamic damping on an axial flow fan blade

Axial ow fans used in air-cooled condensers experience a variation in blade loading due to distorted inlet ow patterns and obstacles in close proximity to the fan blades. The fan blades vibrate at their natural frequencies when excited. Observations made during the operation of these fans have shown that the vibrational response exhibited by the blades varies with angle of attack. This phenomenon is suspected to be due to aerodynamic damping. The purpose of this study was to investigate the in uence of aerodynamic damping on the vibrational behaviour of a generic flat plate axial flow fan blade. The blade's structural vibrational properties were determined at vacuum conditions in a purpose designed and built vacuum chamber. Experiments were subsequently conducted in a large wind tunnel at various ow velocities, with the blade's setting angle altered between data sets. The blade's measured damped frequency and damping ratio was shown to be proportional to air density. The total damping was further shown to be proportional to the air ow velocity at which the blade was excited, with a greater increase noted at large setting angles. Phenomena noted during the investigation were validated numerically using a dynamic Euler-Bernoulli beam model. The model performed well for instances of static atmospheric conditions, but was unable to replicate results obtained from wind tunnel experiments. Trends noted during the vacuum chamber experiments were successfully replicated. Examination of the trends exhibited by the experimental results suggest a complex uid interaction with the blade. It is therefore recommended that a fluid-structure-interaction analysis be performed.National Research Foundatio