Numerical investigations on the static and rotordynamic characteristics of hole-pattern seals with annular or pocket grooves on seal stator

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 compressor, and the center seal in the back-to-back compressor. To avoid or minimize negative static stiffness, annular grooves on seal stator have been used to increase direct static stiffness of hole-pattern seals by dividing one long seal to several shorter seal sections. However, few literatures are available for understanding the influences of annular grooves on seal static and rotordynamic characteristics. To understand the comprehensive effects of grooves on the static and rotordynamic characteristics of annular seals, a proposed three-dimensional (3D) transient CFD-based method was used for predictions of rotordynamic characteristics of hole-pattern seals, based on the multi-frequency one-dimensional rotor oscillating model and mesh deformation technique. Moreover, a 3D steady CFD-based method based on the mesh deformation technique was also utilized to predict static characteristics of hole-pattern seals. The accuracy and reliability of the present transient CFD-based method were demonstrated with experimental data of frequency-dependent rotordynamic coefficients of an experimental hole-pattern seal (HPS) at three inlet preswirl conditions (μ₀= -0.2441, 0, 0.598). The leakage flow rates, static and rotordynamic force coefficients were computed for three types of HPS (one without grooves - HPS, one with annular grooves on stator - HPS-AG, and one with pocket grooves on stator – HPS-PG) with three axial locations of grooves (20%, 40%, 60% of seal axial length) at zero and positive inlet preswirl conditions. The effects of groove types (annular and pocket grooves) and groove locations on the static and rotordynamic coefficients of HPS were numerically discussed. Numerical results show that the annular groove and pocket groove on the seal stator both produce a significantly increase in static stiffness, and the HPS-PG seal possesses relatively optimal static stiffness. The annular groove and pocket groove both result in slight increase (less than 5%) in leakage flow rate. The annular groove will significantly weaken the seal dynamic stiffness capability but weakly influence the seal net damping capability. However, the pocket groove shows weakly influences on the dynamic stiffness and damping characteristics. This suggests that the pocket groove is a more suitable design to improve the static and rotordynamic characteristic. The rotordynamic force coefficients show strong dependence on the annular groove location for the HPS-AG seal, but are insensitive to the pocket groove location for the HPS-PG seal. The optimal location of annular groove is strongly related to the inlet preswirl conditions. The increasing swirl velocity induced by the annular groove results in the decrease in stiffness and damping of the HPS-AG seal.