Effect of moment-related seal coefficients and system configuration on the stability

Abstract

The stability of rotor-bearing-seal (RBS) systems is critical for the safe and efficient operation of turbomachinery. This paper evaluates all the dynamic coefficients of the annular seal due to the cylindrical whirl motion of the rotor using the transient CFD method. The numerical findings are compared with past experimental results for different static eccentricity ratios and rotational speeds, demonstrating a strong consistency with the experimental data. The moment-related dynamic coefficients are evaluated for the static eccentricity ratio up to 0.8 and rotational speed up to 410 Hz, a significant parameter in evaluating onset speed on instability in the system, which has not been attempted in the past. Incorporating moment-related dynamic coefficients improves stability margins by up to 18.4 %, depending on system configuration, including seal position, shaft length, and bearing stiffness. This study highlights the limitations of traditional stability parameters such as effective damping and whirl frequency ratio. It emphasizes the integration of CFD-evaluated seal dynamic coefficients with finite element analysis as an essential design activity for accurately predicting instability onset and developing robust, high-speed rotor systems.