Measurements of the Leakage and Rotordynamic Performance of Interlocking Labyrinth Seals

Annular-labyrinth seals restrict leakage in turbomachinery but may provide unwanted destabilizing forces. This work provides measurements of an interlockinglabyrinth gas seal to further the understanding of leakage characteristics and destabilizing forces. A method using magnetic bearings and differential-pressure transducers is used to measure dynamic forces in the labyrinth seal. Magnetic bearings excite the rotor creating a dynamic pressure wave that is measured and integrated to find the reaction forces. The interlocking seal has 3 teeth on the stator and 2 teeth on the rotor creating 4 cavities. All teeth have a 5 mm height, and the rotor has a 75 mm radius that creates a radial clearance of 0.2 mm with respect to the stator. All tests are conducted at ~ 167 Hz (10 krpm) rotor speed with and without swirl brakes for a range of precession frequencies from 10 – 50 Hz forward and backward. Inlet pressure is varied between 2.75 ~ 4.83 bars, and pressure ratios vary between 0.5 ~ 0.8. Static results are presented for leakage, inlet preswirl, and pressure. Dynamic results are presented for radial and circumferential gas reaction forces on the rotor as well as rotordynamic coefficients. Dynamic results show behavior that is unique to each cavity and are presented for the entire seal as well as for each cavity individually. Cross-coupled stiffness of the entire seal increases with increasing precession frequency, yet all other rotordynamic coefficients are frequency independent and show improved stability via increased effective damping with the use of swirl brakes when considering the entire seal. Negative direct damping values are seen in all but the third cavity. These measurements are useful to industry in validating Computational Fluid Dynamics (CFD) codes for interlocking seals