Numerical Evaluation and High-Speed Rotating Test on Circular Arc Spring Dampers for Centrifugal Compressors

Abstract

Abstract A circular arc spring damper (CASD) is a recently proposed fluid-film damper that has two or more arc-shaped centering springs and dual radial clearances formed by wire electric discharge machining. CASD requires less space and weight than a conventional cage-centered squeeze film damper (SFD). It provides linear stiffness and stable damping force in rotor-bearing systems to attenuate vibration due to imbalance or to improve rotordynamic stability. The authors have been investigated the dynamic characteristics of CASD in component-level experiments. However, their performance and applicability to real machines have not been confirmed in system-level experiments. Additionally, a theoretical means of evaluation for CASD should be established to predict its dynamic coefficients, and to understand the mechanism of dynamic force generation. In the first part of this study, a numerical evaluation method using two-way fluid-structure interaction analysis and its theoretical background is presented. Transient structural analysis and fluid-film flow analysis with a simple homogeneous cavitation model are coupled in the commercial multi-physics platform ANSYS. The accuracy of the method was validated by comparing the damping and added-mass coefficients with results from previous experiments. Furthermore, several aspects of the force generation mechanism, and the difference from conventional SFD were studied numerically. The second part of the study addresses the application of CASD in a multi-stage centrifugal compressor. A combined 4-inch diameter, 5-Pad tilting pad journal bearing (TPJB) with 4-arc type CASD was newly designed and manufactured. To prove the applicability of the developed damper bearing, a series of rotating tests was conducted at a high-speed balancing facility with a full-scale dummy rotor with a critical speed ratio of approximately 3.1. The measured unbalance response showed a much lower amplification factor than that of the conventional TPJB without the damper, which infers a significant improvement in the stability. The measured responses agreed with the rotordynamic analysis, which uses the dynamic coefficients of CASD derived from the proposed numerical evaluation method.