Analysis and full scale testing of an aspirating face seal with improved flow isolation (1998)

An enhanced aspirating face seal is being evaluated as a possible replacement for labyrinth seals in aircraft engines for reduced leakage and improved engine efficiency. A 36 diameter aspirating seal, designed to fit in the GE90 engine, has undergone dynamic testing in a rig designed to mimic the pressure, rotor speed, rotor runout, and seal tilt conditions expected in actual service. Seal/rotor axial clearances, as well as leakage rates, are monitored during testing. To date, tests have been conducted at pressures as high as 100 psid and rotor speeds of up to 2400 RPM, with a rotor axial runout as great as 0.010 TIR. Results indicate that by isolating the seal air dam flow from the hydrostatic bearing flow, a 0.0015-0.003 seal/rotor air gap is formed, with consistently low leakage rates. The results of 3-D Computational Fluid Dynamics (CFD) analyses support the test results, and underscore the importance of flow isolation with respect to seal performance. This paper covers the following: the full scale test facility, the test plan, test results for the original seal, analytical results, test results for the enhanced seal, and future plans. Introduction Conventional labyrinth seals are typically designed with a rotor radial clearance that increases proportionally with diameter. Aspirating face seals, as described by Hwang, are non-contacting seals that are designed to establish an equilibrium position within close proximity (typically 0.0015-0.003) of the rotor surface regardless of the seal diameter. As such, they have a potentially significant performance advantage over conventional labyrinth seals, particularly at large diameters. In addition, aspirating seals are inherently not prone to wear, owing to their non-contacting nature, so their performance is not expected to degrade over time. A 36 diameter aspirating seal, for application to the GE90 low pressure aft outer seal location, was designed and fabricated by the Stein Seal Company'. A cross-section of the seal design, enhanced by the presence of a flow deflector on the rotor face, is shown in Fig. 1, and major seal components are listed in Table 1. A test plan has been Copyright© 1998 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. 1 American Institute of Aeronautics and Astronautics established to evaluate seal performance under a variety of conditions that the seal would be subjected to in the GE90 aircraft engine application. The tests are being executed on a full scale rotary test rig, and analyses have been performed using 3-D Computational Fluid Dynamics (CFD) in order to validate test results. This paper focuses on test results for the original seal design, analytical validation of the test results and use of CFD analysis to evaluate an enhanced design, and test results for the enhanced seal/rotor configuration. Figure 1 Cross section of the enhanced 36 diameter aspirating face seal 1 2 3 4 5 6 7 8 9 10 11 12 Primary face seal ring Secondary piston ring Radial garter spring Axial compression springs Seal housing Primary seal dam Vent Groove Hydrostatic gas bearing surface Vent slots Aspirator tooth Retraction coil springs Flow Deflector (enhanced seal only) The Test Facility The full scale test rig was originally designed for testing 50 diameter brush seals and has been modified to test the 36 diameter aspirating seal. The assembled rig cross-section with the installed aspirating seal consists primarily of a pressure vessel built around a 36 diameter rotor, as shown in Fig 2. In order to balance thrust forces on the rotor, a 36 diameter brush seal is used on the side of the rotor opposite the aspirating seal. The rig configuration is such that air enters radially inward of both seals, and exits radially outward to an exhaust plenum that is common to both seals. The rig is capable of testing at pressures up to 100 psid, and speeds up to 2400 RPM. Aspirating seal Rotor Brush seal Table 1 Aspirating face seal components Figure 2 The Full Scale Test Rig At pressures less than 15 psig, the rotor thrust force is reduced sufficiently to enable static testing with the brush seal half of the pressure vessel completely removed. With this arrangement, seal movement during closure can be visually observed, air gap measurements can be American Institute of Aeronautics and Astronautics taken with feeler gauges, and the seal can be manually perturbed from its equilibrium position. Static tests run at low pressures with this configuration have proven to be good indicators of seal behavior at elevated pressures with rotation. In order to monitor the seal/rotor clearance during closed vessel testing, two axial proximity probes have been mounted to the seal, approximately 90 degrees apart. The probes provide a direct measurement of the air gap between the face of the aspirating seal and the adjacent rotor sealing surface. Also, in order to ensure that the seal is operating reliably and repeatably prior to rotation, the aspirating seal has been electrically isolated from the rig and equipped with six electrical contact probes, equally spaced around the perimeter of the seal. These probes, like the axial proximity probes, are mounted to the outer flange of the aspirating seal itself. The probe tips are set to contact the rotor when the aspirating seal face is <0.001 from the rotor surface, and trigger an audible alarm when contact occurs. To evaluate seal performance in the presence of a I/rev, axial rotor runout, the rotor face itself is machined with the desired runout of 0.000, 0.005 or 0.010. Finally, the rig can be configured with the aspirating seal mounted on a tilting mechanism that provides angular seal misalignment up to ± 0.27 degrees. The entire seal assembly is tilted relative to the rotor, mimicking the relative seal/rotor orientation during maneuvering.