Finite element, modal testing and modal analysis of a radial flow impeller

This paper is concerned with the finite element analysis and modal testing of an industrial radial flow impeller. The goal is to determine and verify the vibration characteristics of the impeller using both experimental and analytical techniques. The finite element model of the impeller with tapered blades was built using a 3D solid element. The convergence properties of the FE model was then verified by mesh refinement and mass distribution methods. Next, a pre-test plan was performed before conducting a modal test in order to select the proper suspension points, driving point(s) and response points on the impeller. Impact testing using hammer excitation and laser Doppler vibrometer (LDV) response measuring techniques were used to measure the vibration properties of the impeller. Using a scanning laser Doppler vibrometer enabled the backplate modes to be described in terms of their nodal diameter components. The nodal diameters versus natural frequencies graph was then compared with the same results obtained from FE and hammer testing and showed good agreement. Finally, a parametric study was conducted on disc thickness, blade thickness, blade trailing, leading edge profiles and blade mistuning. It was found that the effect of varying disc thickness on the lower modes of the impeller was not significant. However, significant natural frequency shifts were observed for the higher modes. It was also concluded that varying the blade leading edge position had a marked effect on the natural frequencies while the lower modes were somehow insensitive to the variation of trailing edge position.