Rotating Machinery Acoustics: A Complete Coupling Contributes to the Impeller Fatigue Failure

Complex dynamic fields such as pulsation, vibration and noise are generated in turbo-machinery systems. These fields are created by the acoustic waves propagating in gaseous media and interacting with the turbo-machine casing and its components. The acoustic loads result from the flows nonuniformity, flow path obstacles or the machine kinematics. The acoustic waves frequently interact with the structural modes inside the turbo-machine. Such interactions may contribute significantly to structural failures and therefore are of primary interest to a machine designer. High ‘efficiency’ couplings usually lead to structural failures while low ‘efficiency’ couplings are not important. Thus, assessment of the coupling ‘efficiency’ is crucial in the calculation of failure probabilities. This knowledge could also be used during the root cause failure analysis or in the machine re-design process. The most ‘efficient’ coupling takes place when the so-called joint acceptance function, representing spatial coupling ‘efficiency’ and the transfer function, representing time coupling ‘efficiency’, describe full coincidence of the acoustic and structural modes. Partial couplings are usually unimportant due to their low ‘efficiency’. This paper attempts to explain partial and complete coupling concepts based on example of a failed compressor impeller. Some practical design remedies to avoid impeller failures are given while predicting the complete coupling.