Enhanced Assessment of Compressor Rotor Stability: Using Computational Fluid Dynamics to Predict Impeller Destabilizing Forces, 18-9491

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Principal Investigators
J. Jeffrey Moore
Anthony J. Smalley
J.R. Hollingsworth
K. Brun

Inclusive Dates:  07/01/04 - 07/01/05

Background - This project will apply computational fluid dynamics (CFD) software to enhance SwRI's state-of-the-art for evaluating stability of high-performance centrifugal compressors. These compressors are critical to many energy-related processes - in particular the offshore production of oil and gas and the liquefaction of natural gas for ocean transportation. Numerous instances exist in the last 25 years where unexpected instability problems have delayed the commissioning of very expensive installations, such as the Ekofisk platform in the North Sea, the Marjan platform in offshore Saudi Arabia, and the LNG plant in Bonny Island, Nigeria. Although SwRI has been a leader in establishing effective methods for predicting rotor instability, its treatment of impeller forces now suffers from clear limitations, as do those of others. There is an unmet need for enhanced understanding and methodology.

Approach - During the last ten years, computational methods and computer speeds have reached levels that make it feasible to model the flow efficiently and accurately in a centrifugal compressor. Within the project proposed herein, a CFD model will be prepared for a real compressor, which has exhibited subsynchronous, aerodynamically excited vibrations. A leading manufacturer of small and mid-size gas turbines and centrifugal compressors has agreed to provide actual physical data and geometry information on such a compressor. A complete model of this impeller will be generated with a proven CFD program for which SwRI holds a license. The output results will be integrated to obtain rotordynamic coefficients. These coefficients will then be compared to the laboratory data collected by the turbine manufacturer and also against other data available in the public domain.

Accomplishments - A nondisclosure agreement with the turbine manufacturer was executed in early September. Drawings are being delivered to SwRI. Meetings were held to refine the goals and scope of the project including analytical approach to the modeling. CFD codes currently available at SwRI are being evaluated for use on the project. The methodology, developed by Dr. Moore for his PhD thesis, consists of a geometric perturbation of the centrifugal impeller and solving of the flow field in the whirling frame of reference. Successive computations at varying whirl frequencies result in rotordynamic impedances that produce the stiffness and damping coefficients compatible with most commercial rotordynamics programs. This method was already benchmarked against measurements for pump impellers and labyrinth seals with good results.

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