Single Nodal Diameter Excitation of Turbine Blades: Experimental and Theoretical Study

Validating simulation results of vibrating turbine blades relies usually on measurements of realistic
or academic specimens on special test rigs. In real operation the blades are excited mainly by
aerodynamic forces. Stator blade vanes introduce wakes and vortices that excite the rotor blade
rows. The circumferential spectrum of the aerodynamic forces contains many engine orders which
are multiples of the engine speed. Each engine order forces the bladed disk to vibrate approximately
in a certain nodal diameter mode shape. Additionally to the circumferential spectrum there may also
exist non-synchronous excitation mechanisms. Particularly during run-up or run-down the flow
through the turbine is instationary. Altogether the excitation forces during operation are highly
complex and difficult to quantify. Therefore, for measurements of blade vibration on special test
rigs, the excitation should be well known to be able to reproduce the excitation spectrum applied in
simulations. It is desirable to use excitation spectra that consists of only a few engine order
excitations. Especially for nonlinear systems unwanted excitation orders can possibly lead to
nonlinear effects which may interfere with the measurement. An easy way to achieve this is the use
of a number of permanent magnets that pull the blades each time they pass over a magnet. The main
engine order can be controlled by varying the number of magnets. This concept has the drawback
that besides the main engine order also other engine orders get excited as the magnets introduce
force pulses in the structure. The use of discrete electromagnets is also common. It offers a more
flexible operation as besides the number of magnets, the coil current can also be varied. The
disadvantage of force pulses still persists. At the Institute of Dynamics and Vibration Research in
Hannover an innovative excitation device was developed and patented to overcome the
aforementioned problems, which is analyzed in this paper. It resembles a toroidal horseshoe
electromagnet. The excitation force spectrum is controlled by a variable air gap over the
circumference between device and blade. As the air gap can be manufactured arbitrarily, any desired
engine order excitation can be realized. Additionally, by varying the devices coil current frequency
sweeps can be performed. In this paper an extensive study of the excitation force spectrum of the
device is conducted. Therefore, theoretical investigations of the expectable spectrum are given
under simultaneous variation of air gap geometry and excitation current. These predictions are then
validated by experiments on a rotation test rig that features a small, academic bladed disk. The
magnetic excitation and vibrations of the blades are measured. The device promises to create well
predictable and controllable excitation force spectra which will improve the validation strategy in
particular non-linear simulation tools for the vibrations of turbine blades.