Alternating pressures and blade stresses in an axial-flow compressor

Abstract

The majority of blade failures in axial-flow compressors have occurred in the rotor blades of the lower pressure stages. These have been caused mainly by fatigue in the fundamental flexural mode of vibration in the presence of the steady centrifugal and gas bending stresses. An investigation has been made into the origin of the forces exciting vibration in an axial-flow compressor, and their magnitude relative to the calculated gas bending loads. The blade stresses resulting from these forces were measured, and a value obtained for the energy input to a blade when vibrating in the predominant modes under running conditions. The experimental data were obtained from pressure elements and strain-gauges fitted to the inlet guide vanes and first four stator-blade rows of an early compressor. The oscillating pressure reached peak values when the forcing frequencies in a stage coincided with the blade natural frequencies of that stage or of adjacent stages, and was predominant for modes approximating to the fundamental flexural mode of vibration. This indicated that vibrations of any one stage ‘modulated’ the stream, the pressure waves extending upstream and downstream. The magnitude of the alternating pressure was 40 per cent and 5 per cent of the stage pressure rise at 4,000 and 8,000 r.p.m. respectively. The ratio of the major harmonic components of the alternating stress to the calculated gas bending stress was 2:1.3 : 0.6 at rotor speeds of 4,000, 6,000 and 8,000 r.p.m. Values of input energy and kinetic energy obtained for resonances approximating to the fundamental flexural mode showed that the damping present in. a stator blade was high. Contributing factors to this high damping were the light root platform and loose fit of some blades. The prominence of blade vibration resulting from modulation of the air stream by the vibration of blades in adjacent stages is attributed to the high damping present.