The structural integrity and reliability of impeller–shaft assembly in a centrifugal air compressor or any turbo machinery is of at most importance for the trouble free operation. It is thus necessary to avoid any excitation that can cause a resonance for the impeller–shaft system. Considering above, a study on the resonance due to excitation from impeller–stator interaction is undertaken.
This paper deals with the construction of impeller interference diagram from the nodal diameters predicted from the cyclic symmetric model of a prototype impeller using ABAQUS. The excitations frequencies arising from impeller–diffuser and impeller–scroll tongue interactions are identified and calculated for the given impeller, diffuser and scroll system. These excitation frequencies are validated for a similar impeller through noise testing and concluded as potential excitation to be considered in design. Stress analysis was carried out to study stresses caused due to centrifugal forces & aerodynamic forces. A nonlinear Static analysis was carried out to account for dynamic stiffening due to centrifugal forces, prior to natural frequency extraction. The Campbell and SAFE diagrams are constructed and the interfering frequencies are identified from the plots constructed in a spread sheet. The nodal diameter versus harmonic force matrix is constructed to understand the forces that can excite a particular nodal diameter for both impeller and impeller–diffuser. From this analysis, it was inferred that diffuser pass frequency was exciting the impeller nodal diameter. Further, analysis was performed to evaluate dynamic stress by carrying out harmonic analysis.
Study was carried out to shift natural frequencies of impeller without significantly affecting aerodynamic performance. Iteratively disk back face design, disk thickness, blade thickness & blade geometry were modified to shift frequencies. Frequency extraction procedure was automated by developing user defined macro in ABAQUS.
After carrying out study and evaluating possible design Iterations, modifying the impeller blade geometry or altering frequency of source excitation by decreasing the number of diffuser blades were two possible solutions. The effect of both is studied in this paper.