Session: 23-10 Labyrinth Seals

Paper Number: 153388

Rotordynamic Characteristics for a Tooth-On-Rotor Long Labyrinth Seal Using Computational Fluid Dynamics 

Tooth on rotor (TOR) labyrinth seals (LS) are widely used in centrifugal compressors (CC) to reduce leakage in the flow-path. Long LS, with slenderness ratio L/D > 0.3, act as balance piston seal in a straight-through CC or as a center seal/division wall seal in a back-to-back CC. LS directly affect CC rotordynamic stability, with TOR type seals having more potential for destabilization due to the presence of swirl flow at the seal inlet. Bulk flow models (BFM) and Computational Fluid Dynamics (CFD) models are available to predict the rotordynamic coefficients of annular clearance seals. However, prediction using the simple though fast BFM is less compelling than that of high fidelity CFD tools.

The paper (will) compares experimentally derived dynamic force coefficients for a TOR LS against   predictions obtained from URANS (Unsteady Reynolds Averaged Navier-Stokes) CFD simulations and a one-control volume (1CV) BFM. With 14 teeth and radial clearance c = 0.102 mm, the test LS has diameter D = 114.3 mm and overall length L = 0.62 D, as stated by Arthur and Childs [1]. The seal, supplied with pressurized air at inlet pressure P_I= 50 bar and ambient temperature, operates with rotor speed of 20.2 krpm (surface speed = 108 m/s), and is set to an exit to inlet pressure ratio (P_E/P_I) = 0.4. The CFD analysis applies a simultaneous multiple-frequency excitation method to displace the rotor, produces the resulting dynamic reaction forces, and extracts force coefficients, stiffness (K) and damping (C), as detailed by Thorat and Hardin [2]. The CFD predicted K, C show good correlation with the experimental force coefficient, albeit the measured LS leakage is ~14.5% lower than the CFD result. Note the experimental set-up includes long sections upstream and downstream of the test seal, and which having the same clearance (c), affect considerably the LS force coefficients, especially the cross-coupled stiffness and direct damping.  

As compared to the experimental results, the CFD analysis produces with high fidelity the whole flow field and reveals the actual boundary conditions upstream of the seal inlet plane. Their measurement is cumbersome as it demands complex techniques and sensors. The study further compares the results of a 1CV BFM against the CFD predictions and experimental results.  The simple BFM underpredicts the seal cross-coupled stiffness and direct damping even when implementing the inlet swirl condition derived from the CFD flow field. The need for improving the accuracy of bulk-flow models remains despite their apparent computational efficiency.

References:

1.       Arthur, SP, & Childs, DW. "Measured Rotordynamic and Leakage Characteristics of a Tooth-on-Rotor Labyrinth Seal with Comparisons to a Tooth-on-Stator Labyrinth Seal and Predictions." Proceedings of the ASME Turbo Expo 2015: Turbine Technical Conference and Exposition. Volume 7A: Structures and Dynamics. Montreal, Quebec, Canada. June 15–19, 2015. V07AT31A016. ASME. https://doi.org/10.1115/GT2015-43242

2.       Thorat, M. R., and Hardin, J. R. (January 8, 2020). "Rotordynamic Characteristics Prediction for Hole-Pattern Seals Using Computational Fluid Dynamics." ASME. J. Eng. Gas Turbines Power. February 2020; 142(2): 021004. https://doi.org/10.1115/1.4044760

Presenting Author: Manish Thorat Ebara Elliott Energy

Presenting Author Biography: Manish Thorat is a Senior Engineer and Machinery Dynamics & Acoustics Group Lead, R&D at Ebara Elliott Energy. He has been with Elliott since 2010. His areas of expertise include rotordynamics, vibration analysis, testing and evaluation of seal designs, squeeze film dampers and fluid film journal bearing design. Mr. Thorat holds two U.S. patents and has co-authored five publications in ASME journals. Mr. Thorat is a member of ASME and STLE. Mr. Thorat earned his MS degree in Mechanical Engineering from Texas A&M University (2010) and BE in Mechanical Engineering from Mumbai University (2007).

Authors:

Manish Thorat Ebara Elliott Energy
James Hardin Ebara Elliott Energy
Luis San Andrés Texas A&M University