Comparison of a Liquid Annular Seal for Pump Applications With and Without a Swirl Brake

Abstract

An annular seal is an annular clearance between a rotor and a stator within a turbomachine that restricts leakage flow, arising due to the seal’s pressure differential. Annular seals are important for consideration in turbomachinery in that they affect the rotodynamics and stability of the turbomachine. Data were available for a smooth liquid annular seal that had been previously tested with no swirl brakes. The seal was modified by adding slots at the inlet to produce a swirl brake seal. Tests produced static and dynamic data for the swirl brake seal. The swirl brake and unmodified seal data are compared to demonstrate how swirl brakes impact the seal’s rotordynamic performance. Adding a swirl brake to a liquid annular seal increases direct stiffness, decreases cross-coupled stiffness, modestly increases direct damping, reduces cross-coupled damping, and decreases both the direct and cross-coupled virtual mass terms. The measured drop in cross-coupled stiffness via the addition of swirl brakes shows why swirl brakes are effective in remedying rotordynamic instabilities. Results show that varying inlet pre-swirl, or fluid rotation, on a swirl brake seal has little effect on the seal’s dynamic performance characteristics. A notable phenomenon was observed with the direct stiffness. At certain test points the direct stiffness would abruptly increase and decrease when increasing either pressure or running speed. The behavior could be largely explained by transitioning the laminar/transitional/turbulent boundaries.