Self-tuning impact dampers have been proposed as improved means for suppressing vibrations in the rotor blades of turbomachines (turbines and compressors). Dampers are needed because aeroelastic blade/flow interactions in a turbomachine cause the blades to vibrate intensely, with resultant accumulation of damage in the blades and rotor over time. When sufficient damage has accumulated, the rotor and/or blades can fail explosively during operation. By reducing the amplitudes of vibrations and thereby reducing damage, improved vibration dampers could increase the safety and reliability and prolong the service life of the turbomachine.
The figure is a partly schematic depiction of turbomachine blades containing three types of dampers: a simple impact damper, a simple tuned-mass damper, and a self-tuning impact damper. A simple impact damper functions by dissipating energy each time its mass strikes the walls of a cavity within the blade. The shortcoming of a traditional impact damper is that in the presence of the high centrifugal force that occurs at high rotational speed, misalignment or friction can immobilize the impactor mass. A simple tuned-mass damper functions by absorbing kinetic energy from the vibrating blade into the tuned mass, which, in turn, sheds its kinetic energy through friction and/or viscosity. When driven at resonance, the tuned-mass damper becomes an impact damper because, at resonance, the damper mass undergoes maximum mass excursions and thus strikes the cavity wall.
A self-tuning damper according to the proposal features a ball in a spherical trough bounded by impact walls. Such a damper functions similarly to a simple tuned-mass damper except that its natural vibrational frequency can be made proportional to the speed of rotation and thus to the frequencies of the vibrations excited by the operation of the turbomachine. In other words, its resonance can be made to follow (at least approximately) an integer multiple of the speed of rotation. When the blade frequency equals the excitation frequency, the damper mass can undergo large excursions (thereby providing strong damping). The radii of the ball and spherical trough can be chosen to tune the resonance to the frequency of a vibrational excitation at a specified multiple of the rotation speed.
Theoretical calculations and experiments have shown that a self-tuning impact damper can be expected to perform more robustly than does a simple tuned damper: A simple tuned-mass damper is effective only when the tuned-mass and blade resonance frequencies are nearly identical, but a self-tuning impact damper can be effective as long as the tuned-mass resonance frequency is less than the blade resonance frequency.
Experiments have also shown that a self-tuning impact damper works better than does a simple impact damper. Moreover, the ball-in-spherical-trough design should make the self-tuned impact damper less susceptible to impairment by friction or misalignment.
This work was done by Kirsten P. Duffy of the Ohio Aerospace Institute, and Gerald Brown and Oral Mehmed of Glenn Research Center and Ronald L. Bagley of the University of Texas.
Inquiries concerning rights for the commercial use of this invention should be addressed to
NASA Glenn Research Center
Commercial Technology Office
Attn: Steve Fedor
Mail Stop 4-8
21000 Brookpark Road
Refer to LEW-16833.