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Forced Response and Stability of Flexible Rotor-Bearing Systems With Squeeze Film Dampers

He, Feng
Thesis/Dissertation; Online
He, Feng
Rockwell, Robert
Wood, Houston
Since 1960s, Squeeze Film Dampers (SFDs) have been widely applied to rotating machinery, including ground-based turbines, compressors and aero-engines. The reason behind the popularity of SFDs is their ability to attenuate system vibrations and improve system stability. Many efforts have been made to understand their properties at both component and system level. However, the discrepancies between predictions and experiments and the deficiencies of theoretical models to satisfy industrial requirements still exist in those studies. For the analysis of SFDs at component level, the major assumptions in the previous studies include open-end with full leakage, perfect seals with no leakage and ideal supply/discharge conditions. These presumptions fail to give reliable predictions of damping capacities in industrial applications. Another limitation of the component level analysis in the previous work is the constraint on the journal motion, which requires the journal to precess synchronously within either circular centered orbits or off-centered circular orbits having small amplitude. These kinds of presumptions prevent accurate predictions for the performances of SFDs in practical rotating systems. Besides the limitations of the component level analysis, the methods for predictions at the system level fail to handle complicated operating conditions for the practical rotating machinery with SFDs. The force coefficient method using traditional iteration procedures is sensitive to the variation of operating conditions and can not deal with rotating systems having excessive dynamic loads. Additionally, when using the direct force methods, most of the past studies consider simple rotating systems on SFDs, and all of them apply extensively the full end-leakage model. To meet the needs of industrial applications with SFDs, this work focuses on considering realistic damper configurations and improving the efficiency and robustness of prediction tools for rotordynamic analysis. One of the common sealing conditions, the piston ring seals, is investigated with two different supply/discharge methods, the central groove and direct holes. The finite difference method is used to obtain the pressure distribution and damping capacity of these complicated dampers. To incorporate these kinds of dampers into the rotordynamic analysis of complex rotating systems, this work also utilizes the ideal of homotopy and predictor-corrector procedure to improve the traditional methods for the analysis at system level. The parametric studies on a flexible rotor-bearing system and an aero-engine are discussed using the developed methods. The results of this study show that SFDs with piston ring seals can generate three times more damping than that of open-end SFDs. Different from the analysis using simplified models, the current work finds that the pressure within the supply groove is not always zero, but dependent on the groove depth. The influence of supply pressure, which cannot be captured by the simplified damper models, is observed to increase the effective damping and improve system stability. The developed curve intersection method enables effective parametric study on flexible rotor-bearing systems with excessive unbalance forces. The improved harmonic balance method results in more than five times faster computation speed than the nonlinear transient analysis in obtaining steady state responses. The application of the developed methods to the aero-engine shows the effects of dampers with practical configurations on the rotordynamics of complicated systems for the first time. The simplified damper model is found to under predict the responses about 30% than these from the finite difference model. Additionally, the prediction methods and the results from current work pave the base for the optimized design of industrial rotating systems on SFDs with complex configurations.
University of Virginia, Department of Mechanical and Aerospace Engineering, PHD, 2013
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