The effects of detuning upon the forced vibrations of bladed disks

Abstract

In the construction of steam and gas turbines it is not feasible to attain complete identity of all the blades mounted on one turbine disk due to limitations inherent in manufacturing processes. In this paper, a simplified model is used to study the implications of this real situation from a consideration of the vibration levels which these components might suffer in practice. It has been shown elsewhere that detuning—the presence of small differences between the blades on a bladed disk—gives rise to a natural frequency “splitting” phenomenon in which a pair of modes with close natural frequencies and similar characteristics are formed in place of an apparently single mode of vibration in the perfect, or tuned, system. The present work describes the dynamic characteristics of a system having such pairs of close natural frequency modes when it is forced to vibrate under typical operating conditions—with excitation being derived from the presence of obstructions in the flow. A simplified analysis suggests that the best system is one in which all the blades are identical. It is shown that with the random arrangements of typical blades which will occur in practice, some blades may suffer stress levels which are 20% higher than those of the equivalent tuned system. However, by careful arrangement of the same set of non-identical blades, it is possible to eliminate the risk of such an increase in the peak stress levels. An optimum arrangement of typical blades is proposed.