Numerical Comparison of Rotordynamic Characteristics for a Fully Partitioned Pocket Damper Seal and a Labyrinth Seal With High Positive and Negative Inlet Preswirl

Abstract

Pocket damper seals (PDSs) are used as replacements for labyrinth seals in highpressure centrifugal compressors at the balancepiston location or center seal location to enhance rotordynamic stability. A concern exists that this enhanced stability will be lost at high positive inlet preswirl. Numerical results of frequencydependent rotordynamic force coefficients and leakage flow rates were presented and compared for a fully partitioned PDS (FPDS) and a labyrinth seal at high positive and negative inlet preswirl, using a proposed transient computational fluid dynamics (CFD) method based on the multifrequency elliptical orbit whirling model. The negative preswirl indicates a fluid swirl in a direction opposite to rotor rotation at seal inlet. Both seals have identical diameter and sealing clearance. The full 3D concentric CFD model and mesh were built for the labyrinth seal and FPDS, respectively. The accuracy and availability of the present transient CFD numerical method were demonstrated with the experiment data of frequencydependent rotordynamic coefficients of the labyrinth seal and FPDS at zero and high positive preswirl conditions. The numerical boundary conditions include two high positive preswirl, two high negative preswirl, and a zero preswirl. Numerical results show that the effect of inlet preswirl on the direct force coefficients is weak, but the effect on the crosscoupling stiffness and effective damping is dramatic. Both seals possess negative effective damping at lower excitation frequencies due to positive preswirl, and the crossover frequency of effective damping term increases with increasing positive preswirl. Negative preswirl produces negative crosscoupling stiffness and positive effective damping over the whole excitation frequency range. Increasing negative preswirl is a stabilizing factor for annular gas seals, which results in a significant increase in the effective damping and a decrease in the crossover frequency. It is desirable to reduce the inlet preswirl to zero or even negative through applications of negativeswirl brakes and negative injection devices.