Rotordynamic Force Coefficients of Volutes and Diffusers for Prediction of Turbomachinery Vibration

Abstract

The American Petroleum Institute (API) level II vibration stability analysis for impellers requires higher fidelity models to predict the dynamic forces of the whirling impeller. These forces are in turn required to predict the vibration stability, critical speeds, and steady-state vibration response of the shaft-bearing-seal-impeller system. A transient computational fluid dynamics (CFD)-based approach is proposed which is applicable to nonaxisymmetric turbomachinery components, such as the volute and/or diffuser vanes, unlike its predecessor models like the bulk-flow or the quasi-steady model. The key element of this approach is the recent advancements in mesh deformation techniques which permit less restrictive motion boundary conditions to be imposed on the whirling impeller. The results quantify the contributions of the volute and/or the diffuser to the total forces which guides the analyst on whether to include these components in the model. The numerical results obtained by this approach are shown to agree well with experimental measurements and to be superior to the earlier quasi-steady alternative in terms of accuracy. Furthermore, several volute shapes were designed and analyzed for the sensitivity of the solution to the geometrical properties of the volute. The design flow rotordynamic forces show a significant dependence on the presence of the volutes in the model, with the specific shape of the volute having a lesser influence. The dimensionless forces are shown to be almost independent of the spin speed.