Kim, Eunseok (2016-10). CFD Based Prediction of Rotordynamic Coefficients and Stability Analysis for Centrifugal Pump/Compressor. Doctoral Dissertation.
Thesis
Fluid forces acting on the shrouded centrifugal pump/compressor impellers are evaluated using the computational fluid dynamics (CFD) approach. A face-seal impeller is selected to study for predicting the rotordynamic coefficients of the whirling impeller. The resultant impedances of the face-seal impeller can be modeled as a quadratic polynomial of whirling frequency to calculate the rotordynamic coefficients. However, the impedances of a conventional wear-ring seal impeller have some inflection points in the curves. Some possible sources that can affect the fluctuating impedance curves are investigated by varying design parameters such as shape of leakage path, inlet tangential velocity, flow rate of primary passage, and seal clearance. Effects of the peaks in the impedance curves of the wear-ring seal impeller are evaluated by performing stability analysis of a rotordynamic finite element (FE) model with the unconventional impedances. A linear curve-fit algorithm is developed to identify the complex shape of impedance curves of the wear-ring seal impeller. A Jeffcott rotor model with the impeller forces is modeled to apply and validate the developed curve-fit algorithm and the stability analysis has been performed. The analysis results indicate that the bump and dip in the impedance curves destabilize the Jeffcott rotor. Precessing motion of the shrouded centrifugal impeller is also an important vibration source that can cause the rotordynamic instability problem. By considering the tilted impeller, the reaction force and moment coefficients of the face-seal impeller are predicted utilizing CFD technique, and a full set of the rotordynamic coefficient matrices (4x4) is calculated. In order to evaluate the coupled motion of whirling and precessing for the face-seal impeller, a 3D impeller model with the dynamic eccentricity and the tilting angle is generated and the corresponding rotordynamic coefficients are obtained. The calculated results indicate that the precessing motion increases the tendency towards destabilization of the rotor. Lastly, influence of static eccentricity on the face-seal impeller is investigated. In actual operation of the rotor, misalignment may be destabilizing and cause serious vibration problems. A CFD modeling approach is developed to evaluate the forces for the statically eccentric face-seal impeller, and it has been found that the presence of static eccentricity on the face-seal impeller has an adverse effect on rotordynamic stability.
