Abstract
The rotordynamic (RD) fluid force generated in seals affects the stability of turbomachinery. RD coefficients have been determined using bulk-flow or computational fluid dynamics analyses, and the effect of seals on the stability has been evaluated. Plain annular seals and convergent tapered annular seals have been used to improve stability, and their dynamic characteristics have been studied. On the other hand, research has also been conducted on divergent tapered annular seals, because the seals can become divergent tapered due to operating conditions, pressure deformation, and manufacturing error. These studies do not provide a comprehensive picture of the effect of divergent tapered annular seals on stability, because some show that divergent tapered annular seals reduce stability, whereas others show that they may increase stability. This study analyses a plain liquid annular seal, divergent tapered annular seals, and convergent tapered annular seals to clarify the effect of the taper angle on the stability of a vertical rotor-seal system. The analysis was carried out by continuously changing the taper angle for both the divergent tapered and convergent tapered annular seals, the RD coefficients were calculated in each case, and the onset speed of instability (OSI) was calculated by eigenvalue analysis. The change in OSI, or stability, with a change in taper angle is then investigated. The change in OSI with a change in taper angle was investigated experimentally and compared with the analytical results to demonstrate the validity of the analytical results. The analytical and experimental results were in qualitative and quantitative agreement.
