Influence of Unbalance and Differential Pressure on the Stability of Vertical Rotor-Seal System

Abstract

The rotordynamic (RD) fluid force generated in fluid elements such as seals in turbomachinery affects the stability of turbomachinery and causes shaft vibrations. Various studies have been conducted to clarify the effects of seals on the stability of rotor systems. Many studies have investigated the rotor dynamics of horizontal rotating shaft systems, considering the RD fluid force generated in the seals, and in these studies, the stability of horizontal rotating shaft systems has been assessed via eigenvalue analysis using RD coefficients. However, few studies have analyzed vertical rotating shaft systems. The dynamic behavior of vertical rotating shafts differs significantly from that of horizontal rotating shafts because the weight of the rotor does not act on the seal in a vertical rotating shaft system. Vertical rotating shaft systems are generally prone to instability because of the fluid film whirl, and the amplitude of the shaft whirl tends to be large. When the amplitude is large, the RD fluid force cannot be linearized around the equilibrium point using RD coefficients. Therefore, destabilization and stabilization phenomena that appear in vertical rotating shaft systems cannot be predicted using eigenvalue analysis. Fluid–structure interaction (FSI) analysis that considers the interaction between the shaft vibration and the RD fluid force generated in seals is required to predict such phenomena. This study used FSI analysis to investigate the effects of unbalance and differential pressure on the stability of a vertical rotating shaft system subjected to RD fluid force generated in the seal.