Influence of Shunt Injection on Predicted Rotordynamic Coefficients of a Labyrinth Seal Using Transient CFD

Shunt injection is one of the methods used in improving rotordynamic stability of centrifugal compressors. It involves eliminating swirl by injection of fluid in an intermediate cavity of a seal. The fluid injection is usually directed from the discharge volute or diffuser section of the compressor. In this paper, shunt injection is modeled for a center seal in a back-to-back compressor arrangement. The test case is from an actual machine which was shown to be unstable during a factory loaded mechanical test (as per API standard 617) without shunt injection and stable with shunt injection. The center seal is a tooth-on-rotor type of labyrinth seal. Impeller back-face cavity passages at the entrance and exit of the seal are also modeled. To investigate the influence on shunt injection, models with and without shunt injection geometry are considered.

Steady state Computational Fluid Dynamics (CFD) is used to study the influence on steady state characteristics such as leakage rate and swirl ratio. The CFD analysis shows a significant swirl reduction with shunt injection at the injection location. The estimated swirl ratio is 0.55 at the injection location without shunt injection and approximately 0 with shunt injection. Frequency dependent seal rotordynamic coefficients are evaluated using transient CFD. Both translational and moment coefficients are evaluated for the seal and impeller passages. For the case with shunt injection, CFD results show negative (destabilizing) effective damping from the seal at the rotor first damped natural frequency. Shunt injection case shows a positive (stabilizing) estimated effective damping at the frequency of interest. The CFD results are shown to be consistent with the factory test case observations.