Blade vibratory stresses in a multi-stage axial-flow compressor

Abstract

A detailed investigation of the stresses in the early stator blade rows of an axial-flow compressor, which had previously suffered blade failures during its initial performance tests, is described. These vibration tests show that stall cell excitation is present in the first and second blade rows and causes blade resonances over a fairly wide part-load condition. Resonances of the fundamental flexural modes of first and second row stator blades with the fourth and fifth stall cell harmonics give rise to stress peaks in the region of ± 10 to ± 20 tons/sq in. The inlet guide row also has a stress peak of about ± 10 tons/sq in. attributable to excitation associated with the fifth stall cell harmonic. Away from these resonant conditions stalling flutter (of moderate stress) occurs in the first and second stator row blades over about 30 per cent to 70 per cent design speed range, the compressor design being such that these stages are stalled up to about 80 per cent design speed. Under the forced vibration conditions, the blade phasing, in a row, is consistent with anti-phase vibration of adjacent blades indicating the effect of mechanical coupling. Analysis of the initial test conditions indicates agreement between the distribution failures and the number of cycles of high stress probably experienced by the blades.