Fatigue cracks from high cycle vibratory loading have developed in a centrifugal fan impeller after a relatively short period of operation. Based on vibration tests of the impeller and acoustic analysis of the internal spaces of the fan, structural-acoustic coupling with matching frequencies involving two-nodal diameter modes (structural and acoustic) was identified as the cause of the vibration problem. This paper gives a description of the problem and identifies available remedies for its elimination. Special attention is given in Case 1 to the acoustics of the system, where changes to the fan casing utilizing a decoupling acoustic chamber attached to the casing was investigated, and in Case 2, where changes needed to the structural characteristics of the impeller by utilizing a modified impeller with a substantially increased natural frequency (by 44%) of the two-nodal diameter vibratory mode shape were studied. Experimental tests confirmed that the structural-acoustic coupling and vibration were significantly reduced in Case 1 and were fully eliminated in Case 2.

Issue Section:
Technical Briefs
Keywords:
dynamic testing, fans, fatigue cracks, impellers, structural acoustics, vibrations
Topics:
Acoustics, Impellers, Vibration, Mode shapes, Strain gages, Fatigue
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