Acoustic Fatigue of Impellers of Rotating Machinery

Abstract

Impellers of air, gas, or steam moving rotating machinery are exposed to acoustic pressure loading (pressure pulsations) from acoustic waves generated in the gaseous medium from flow within the machine casing. The acoustic waves and the impeller structure contain a number of natural frequencies and corresponding mode shapes. The acoustic waves are expressed by pressure pulsations which typically consist of 1) fluctuating wall pressures, 2) plane waves, and 3) higher-order acoustic modes. Out of the three excitation phenomena, the higher-order acoustic modes are the most efficient excitation sources of impeller vibration. The reason for this is the interaction of these modes with impeller structural modes when they become coincident. Of importance is the coincidence of acoustic modes and structural modes with matching wave numbers. The most severe dynamic loading of the impeller occurs at complete coincidence in which in addition to a match in wave numbers (mode shapes) also matching of acoustic and structural frequencies occurs. The paper will provide the theoretical background of the physical phenomena governing the fluid-structure interaction which may lead to extreme loading of the impeller in an acoustically structurally coupled system. Such loading may result in vibration and structural fatigue (acoustic fatigue) and possibly in the destruction of the impeller.