A survey of the alternating pressures exciting high frequency vibrations in gas turbines

Abstract

A survey of the predominant harmonic components of the alternating pressures is made for three centrifugal compressors, three axial-flow compressors and a single-stage turbine to ascertain the forces exciting vibration in a compressor or turbine stage. The predominant harmonic components of the alternating pressures in the casing are the first-order components of the impulse from the rotor blades or impeller vanes. Harmonics up to the seventh of these impulses can be detected in parts of the speed range, The alternating pressures in a compressor annulus have components similar to those observed in the casing for the stage but in addition have components of greater magnitude excited by dissimilar flows in individual blade passages. The largest orders are those which excite the fundamental flexural mode of vibration of the blades and the corresponding blade movements amplify the alternating pressures. The amplitude of the alternating pressures is dependent on the work done in a blade or impeller vane passage, the incidence at entry to the stage, the exit conditions and the general flow conditions and there is usually a relative reduction when operating near the design point. There is a reduction in the alternating pressures with distance from the source particularly across a blade row. There are three other sources of excitation at part load : (a) Stalling flutter at high positive incidence can occur in a.,dal-flow compressors for blades of low stiffness over wide ranges of operation provided the incidence and mass flow are greater than the boundary conditions. (b) Alternating pressures can be excited gear the surge by one or more stall cells rotating at some fraction of compressor speed (c) At the stalling incidence of a stage, if a proportion of the stage stalls due to dissimilar geometry or flow and the remainder of the stage and compressor as a whole is operating at relatively high efficiency, large alternating pressures of all forcing orders can be excited. This effect is the more severe the lower the natural frequencies of the blades.