Session: 07-04-02: Fluid Structure Interaction / Marine Electromechanical Systems and Ocean Mechatronics

Paper Number: 96957

96957 - Effect of Whirling Motion on the Fluid Induced Instability Force in Axial Compressor Rotors 

In axial flow compressors, tip clearance distribution along the circumferential direction is designed to be uniform. However, during the operation of the compressor rotor, vibration and deflection of the rotor are inevitable thus results in a non-uniform tip clearance distribution. As the local performance is closely corelated to the local tip clearance height, pressure force on all the blades and hub is not balanced and a net force will be generated due to the non-uniform tip clearance distribution. This net force can be seen as cross-coupled force since it has a force component that is perpendicular to the rotor deflection. This type of fluid induced cross-coupled force was previously recognized as Alford/Thomas force which was considered as a potential instability source in rotordynamics modeling.

Whirling motion of the rotor is one of the major factors that causes non-uniform tip clearance distribution in axial compressors. In addition, whirling motion makes the distribution of the clearance unsteady if the whirling speed is not synchronized with blade speed. A set of full 3D unsteady CFD simulations are presented to predict the fluid induced cross-coupled force on the rotor under the influence of rotor whirling motion. Motions of rotor whirl and rotor self-spin are superimposed to prescribe the rigid body motion of the compressor rotor.  Because the tip clearance distribution is time dependent, mesh morphing solver is implemented before the flow field calculation of each time step in order to achieve remeshing process for the blade tip clearance meshing adjustment due to the unsteady motion of the blade. Cell qualities can be maintained as good as that before mesh morphing activated and no cell distortion is found according to the mesh quality report. Both pressure forces on blade and compressor hub are measured in a rotating coordinate which is fixed to the rotor center and rotates with the rotor whirling. Summation of the tangential force on all the blades yields a net blade force. The addition of the net blade force and the hub force in the direction which is perpendicular to the instantaneous rotor deflection is accounted for the cross-coupled force. Three axial compressor models are used to evaluate the cross-coupled force. Across all three models, simulations show that whirling motion significantly affect the cross-coupled force, which results in a stabilizing effect for the rotordynamics model. Results of the simulations indicate that the cross-coupled force approximately have a linear relationship with the whirling speed, and the force decreases with the growing whirling speed. This kind of trend had not been reported in the previous studies as they treated the rotor as only being statically deflected but without the presence of whirling motion.

Presenting Author: Xia Sheng University of Cincinnati

Presenting Author Biography: Xia Sheng is currently a Ph.D student at the department of Mechanical Engineering in the University of Cincinnati. His research includes rotordynamics in turbomachinery, structural vibration and fluid dynamics. He obtained his B.S. of Mechanical Engineering and M.S. of Mechanical Engineering in Donghua University in Shanghai, China.

Authors:

Xia Sheng University of Cincinnati
Tandalam Shashikant Indraneel University of Cincinnati
Jay Kim University of Cincinnati