Development and Validation of Test Rig for Measurements of Leakage and Rotordynamic Performance of Interlocking Gas Labyrinth Seals

Understanding the rotordynamic and leakage characteristics of gas annular seals is important for the design of efficient and reliable turbomachinery. This work sets the foundation to advance the understanding of gas interlocking labyrinth seals. Data for such seals are vague and scarce in literature. A test rig for interlocking seals is developed and validated via tests for a see-through tooth-on-stator labyrinth seal. Test procedures by Millsaps in 1994 and Wagner et al. in 2009 are adapted. Dynamic forces are integrated from the perturbed pressure fields created inside the seal cavities while precessing the rotor using magnetic bearings. Measurements are made by adapting a test rig at the Turbomachinery Lab at Texas A&M University, previously used in 2006 for a different purpose. The see-through seal has eight stator teeth and a smooth rotor. The rotor has a 75 mm radius, 3 mm tooth height, and 0.2 mm radial clearance. Tests at a rotor speed of 10 kRPM are performed for a range of inlet pressures, pressure ratios, and precession frequencies between 15 and 50 Hz. Forward and backward precessions are imposed. Measured leakage values and rotordynamic coefficients validate the performance of the test rig. Experience showed that differential pressure transducers, a well-machined stator, and precise centering and alignment are required for correct measurements. Two sensors 180° apart from each other in a representative cavity demonstrate the development of a theoretically correct pressure wave inside the seal. All results are repeatable. Static pressure measurements in the seal cavities show pressure dropping linearly across the seal’s cavities. Results show frequency-independent rotordynamic coefficients. As noted above, only one of the seal’s cavities was provided with dynamic pressure measurements, and the following rotordynamic coefficients apply for that cavity, not the entire seal. Measured negative values of direct stiffness K and cross-coupled damping c confirm the lagging pressure wave is behind the rotor, pushing it radially towards the stator. Positive values of cross-coupled stiffness k and direct damping C counteract each other in the circumferential direction, the former being iii destabilizing. Effective damping Cveff combines the impact of direct damping C and cross-coupled stiffness k. For the cavity chosen, Cveff values are negative, and thus destabilizing in the circumferential direction. Future work involves testing interlocking labyrinth seals and using the test results to benchmark CFD codes