Influence of Annular/Pocket Groove on the Static and Rotordynamic Characteristics of Hole-Pattern Seals
Authors: Li Zhigang , Fang Zhi , Li Jun 
Published: May 2020

Abstract
Annular damper seals, such as hole-pattern seals, are widely used to control leakage and enhance rotordynamic stability in turbomachinery, especially, for the balance-piston seal in the straight-through multistage centrifugal compressor, and the center seal in the back-to-back compressor. To avoid negative static stiffness (zero frequency), annular grooves on seal stator have been used to increase the direct static stiffness of hole-pattern seals by dividing a long seal to several shorter seal sections. However, few literatures are available for understanding the influences of grooves on seal static and rotordynamic characteristics. To comprehensively understand the effects of grooves on the static and rotordynamic characteristics of annular damper seals, a proposed three-dimensional (3D) transient CFD-based method was used to predict the rotordynamic characteristics of annular damper seals, based on the multifrequency one-dimensional rotor whirling model and mesh deformation technique. Moreover, a 3D steady CFD-based method based on the mesh deformation technique was also proposed to predict the static characteristics of annular damper seals. The accuracy and reliability of the present transient CFD-based method were demonstrated with the experimental data of frequency-dependent rotordynamic force coefficients for an experimental hole-pattern seal at three inlet preswirl conditions (μ0 = −0.244, 0, 0.598). Numerical results such as leakage flowrates, static and rotordynamic force coefficients were presented and compared for a conventional straight-through hole-pattern seal (without grooves, HPS) and two types of grooved hole-pattern seals (one with annular groove on stator, HPS-AG; one with pocket groove on stator, HPS-PG) with three groove positions (20%, 40%, 60% of seal axial length), at zero (μ0 = 0) and positive (μ0 = 0.598) inlet preswirl conditions. These seals are long (L/D = 0.75), with a diameter of 114.5 mm and a radial clearance of 0.2 mm. The effects of groove types (annular and pocket) and groove positions on seal's static and rotordynamic force coefficients were numerically discussed. Results show that compared to the conventional HPS seal, two types of grooves (annular and pocket) both produce a significant increase (20–150%, especially for the larger rotor eccentric ratios) in static direct stiffness, and the HPS-PG seal possesses the relatively optimal static stability. Two types of grooves (annular and pocket) both result in a slight increase (less than 5%) in seal leakage. The annular groove will significantly weaken the seal dynamic stiffness capability, however, the pocket groove shows only very weak influences. Compared to the conventional HPS seal, the HPS-PG seal possesses the similar increasing effective damping and decreasing crossover frequency with the HPS-AG seal. This suggests that the pocket groove is a more suitable design to improve the seal static and rotordynamic characteristics. The rotordynamic force coefficients show a strong dependence on the groove location for the HPS-AG seal, but which is insensitive for the HPS-PG seal. The optimal location of annular groove is strongly related to the inlet preswirl conditions.