Measured Rotordynamic and Leakage Characteristics of a Tooth-on-Rotor Labyrinth Seal With Comparisons to a Tooth-on-Stator Labyrinth Seal and Predictions

2015 ◽  
Author(s):  
Stephen P. Arthur ◽  
Dara W. Childs
Keyword(s):  
Cavity Length ◽  
Labyrinth Seal ◽  
Diameter Ratio ◽  
Radial Clearance ◽  
Rotor Speed ◽  
Labyrinth Seals ◽  
Rotordynamic Coefficients ◽  
Leakage Characteristics ◽  
Identical Diameter ◽  
Length To Diameter Ratio

Rotordynamic and leakage data are presented for a see-through tooth-on-rotor (TOR) labyrinth seal with comparisons to a see-through tooth-on-stator (TOS) labyrinth seal. Measurements for both seals are also compared to predictions from XLLaby. Both seals have identical diameter and can be considered as relatively long labyrinth seals. The TOR seal has a length-to-diameter ratio of 0.62, whereas the TOS seal is longer and has a length-to-diameter ratio of 0.75. Both seals also differ by number of teeth, tooth height, and tooth cavity length. TOR labyrinth tests were carried out at an inlet pressure of 70 bar-a (1,015 psia), pressure ratios of 0.4, 0.5, and 0.6, rotor speeds up to 20,200 rpm, a radial clearance of 0.1 mm (4 mils), and three preswirl ratios. For comparison, TOS labyrinth tests were run at identical conditions as the TOR tests but for only one positive preswirl ratio. TOR labyrinth measurements show a pronounced dependence of rotordynamic coefficients on rotor speed, especially when compared to prior documented TOS labyrinth seal tests run at a radial clearance of 0.2 mm (8mils). The TOR labyrinth cross-coupled stiffness is higher in magnitude and increases at a higher rate than that of the TOS labyrinth across all test speeds. However, the TOR labyrinth effective damping was determined to be greater due to higher measurements of direct damping. Measured leakage rates for the TOR labyrinth were approximately 5–10% less than the TOS labyrinth. XLLaby underpredicted the rotordynamic coefficients for both seals. However, as with measurements, it predicted the TOR labyrinth to have higher effective damping than the TOS labyrinth.

Measurements of the Leakage and Rotordynamic Performance of Interlocking Labyrinth Seals

2018 ◽  
Author(s):  
Keith Gary ◽  
Dara W. Childs ◽  
Mauricio A. Ramirez
Keyword(s):  
Dynamic Pressure ◽  
Labyrinth Seal ◽  
Magnetic Bearings ◽  
Radial Clearance ◽  
Labyrinth Seals ◽  
Pressure Transducers ◽  
Rotordynamic Coefficients ◽  
Frequency Independent ◽  
Improved Stability ◽  
Reaction Forces

Leakage and rotordynamic measurements are presented for an interlocking-labyrinth gas seal. Magnetic bearings and differential-pressure transducers are used to measure dynamic forces in the labyrinth seal following the approach of Wagner et al. in 2009. Magnetic bearings precess 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. Each cavity has an axial length of 6 mm, and all teeth have a 5 mm height. Teeth on the rotor and stator, respectively, create a 0.2 mm radial clearance with respect to the stator and rotor. 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 bar, and pressure ratios vary between 0.5 ∼ 0.8. Static results are presented for leakage, inlet preswirl, and cavity pressure. Dynamic results are presented for 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. The seal shows 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.

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

2018 ◽  
Author(s):  
Mauricio Ramirez ◽  
Dara Childs ◽  
Keith Gary
Keyword(s):  
Pressure Wave ◽  
Dynamic Pressure ◽  
Labyrinth Seal ◽  
Circumferential Direction ◽  
Radial Clearance ◽  
Pressure Measurements ◽  
Test Rig ◽  
Test Procedures ◽  
Labyrinth Seals ◽  
Rotordynamic Coefficients

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. 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. The see-through seal has eight stator teeth and a smooth rotor. The seal has a 75 mm radius, 3.5 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, making way for interlocking seal tests in the future. Experience showed that differential pressure transducers, a well-machined stator, and precise alignment are required for accurate measurements. Two sensors 180° apart from each other in a representative cavity demonstrate the development of a correct pressure wave inside the seal. All results are repeatable. Static pressure measurements in the seal cavities show pressure drops linearly across the seal’s cavities. Results show frequency-independent rotordynamic coefficients. As remarked 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. Negative values of direct stiffness K and cross-coupled damping c confirm the lagging dynamic pressure wave is behind the rotor, pushing it radially into the stator. Positive values of cross-coupled stiffness k (where Kxy = −Kyx = k) and direct damping C counteract each other in the circumferential direction, the former being destabilizing. Effective damping Ceff combines the stabilizing impact of direct damping C and cross-coupled stiffness k. Measured cavity Ceff 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.

Prediction of Rotordynamic Coefficients for Short Labyrinth Gas Seals Using Computational Fluid Dynamics

2012 ◽  
Vol 134 (6) ◽  
Author(s):  
Alexander O. Pugachev ◽  
Ulrich Kleinhans ◽  
Manuel Gaszner
Keyword(s):  
Fluid Dynamics ◽  
Computational Fluid Dynamics ◽  
Turbulence Modeling ◽  
Labyrinth Seal ◽  
Radial Clearance ◽  
Labyrinth Seals ◽  
Rotordynamic Coefficients ◽  
Processing Power ◽  
Gas Seals ◽  
Stiffness And Damping

The analysis is presented for the computational fluid dynamics (CFD)-based modeling of short labyrinth gas seals. Seal leakage performance can be reliably predicted with CFD for a wide operating range and various sealing configurations. Prediction of seal influence on the rotordynamic stability, however, is a challenging task requiring relatively high computer processing power. A full 3D eccentric CFD model of a short staggered three-tooth-on-stator labyrinth seal is built in ANSYS CFX. An extensive grid independence study is carried out showing influence of the grid refinement on the stiffness coefficients. Three methods for the prediction of stiffness and damping coefficients as well as the effect of turbulence modeling, boundary conditions, and solver parameters are presented. The rest of the paper shows the results of a parameter variation (inlet pressure, preswirl, and shaft rotational speed) for two labyrinth seals with a tooth radial clearance of 0.5 mm and 0.27 mm, respectively. The latter was compared with experimental data in Pugachev and Deckner, 2010, “Analysis of the Experimental and CFD-Based Theoretical Methods for Studying Rotordynamic Characteristics of Labyrinth Gas Seals,” Proceedings of ASME Turbo Expo 2010, Paper No. GT2010-22058.

Impact of Frequency Dependence of Gas Labyrinth Seal Rotordynamic Coefficients on Centrifugal Compressor Stability

2010 ◽  
Author(s):  
Giuseppe Vannini ◽  
Manish R. Thorat ◽  
Dara W. Childs ◽  
Mirko Libraschi
Keyword(s):  
Molecular Weight ◽  
Numerical Model ◽  
Control Volume ◽  
Labyrinth Seal ◽  
Labyrinth Seals ◽  
Frequency Dependent ◽  
Rotordynamic Coefficients ◽  
Frequency Independent ◽  
Rotor Surface ◽  
Independent Model

A numerical model developed by Thorat & Childs [1] has indicated that the conventional frequency independent model for labyrinth seals is invalid for rotor surface velocities reaching a significant fraction of Mach 1. A theoretical one-control-volume (1CV) model based on a leakage equation that yields a reasonably good comparison with experimental results is considered in the present analysis. The numerical model yields frequency-dependent rotordynamic coefficients for the seal. Three real centrifugal compressors are analyzed to compare stability predictions with and without frequency-dependent labyrinth seal model. Three different compressor services are selected to have a comprehensive scenario in terms of pressure and molecular weight (MW). The molecular weight is very important for Mach number calculation and consequently for the frequency dependent nature of the coefficients. A hydrogen recycle application with MW around 8, a natural gas application with MW around 18, and finally a propane application with molecular weight around 44 are selected for this comparison. Useful indications on the applicability range of frequency dependent coefficients are given.

Segmentation Effects on Brush Seal Leakage and Rotordynamic Coefficients

2015 ◽  
Author(s):  
Alexander O. Pugachev ◽  
Manuel Gaszner ◽  
Christos Georgakis ◽  
Paul Cooper
Keyword(s):  
Electrical Discharge ◽  
Electrical Discharge Machining ◽  
Labyrinth Seal ◽  
Performance Characteristics ◽  
Radial Clearance ◽  
Single Plane ◽  
Rotordynamic Coefficients ◽  
Damping Coefficients ◽  
Brush Seal ◽  
Stiffness And Damping

This paper studies the effect of brush seal segmentation on the seal performance characteristics. A brush-labyrinth sealing configuration arranged of one brush seal downstream and two labyrinth fins upstream is studied experimentally and theoretically. The studied brush seal is of welded design installed with zero cold radial clearance. The brush seal front and back rings as well as the bristle pack are segmented radially in a single plane using the electrical discharge machining technique. The segmentation procedure results in loss of bristles at the site of the cuts altering the leakage flow structure in the seal and its performance characteristics. Two test rigs are used to obtain leakage, as well as rotordynamic stiffness and damping coefficients of the seal at different pressure ratios. The CFD-based model is used to predict the seal performance and to study in detail local changes in the flow field due to the segmentation. A back-to-back comparison of the performance of non-segmented and segmented brush seals, as well as baseline labyrinth seal is provided. The obtained results demonstrate that the segmentation in general negatively affects the performance of the studied brush-labyrinth sealing configuration. However, the segmented brush seal shows increased direct damping coefficients.

Numerical Comparison of Leakage Flow and Rotordynamic Characteristics for Two Types of Labyrinth Seals With Baffles

2020 ◽  
Vol 142 (9) ◽  
Author(s):  
Yuanqiao Zhang ◽  
Jun Li ◽  
Zhigang Li ◽  
Xin Yan
Keyword(s):  
Rotational Speed ◽  
Flow Resistance ◽  
Labyrinth Seal ◽  
Leakage Flow ◽  
Numerical Comparison ◽  
Vibration Frequencies ◽  
Labyrinth Seals ◽  
Rotordynamic Coefficients ◽  
Brush Seal ◽  
Swirl Intensity

Abstract Cavity separation baffles can decrease the circumferential swirl intensity of labyrinth seals and increase the seals' rotordynamic characteristics. Compared with conventional baffles, the bristle packs of brush seal baffles can contact the rotor directly, thereby further reducing the swirl intensity of the seal cavity. This paper, using the numerical model combining a multifrequency elliptical whirling orbit model, a porous medium model, and transient Reynolds-averaged Navier–Stokes (RANS) solutions, compares the leakage flow and rotordynamic characteristics of a labyrinth seal with brush-seal baffles (LSBSB) and a labyrinth seal with conventional baffles (LSCB). Ideal air flows into the seal at an inlet preswirl velocity of 0 m/s (or 60 m/s or 100 m/s), total pressure of 690 kPa, and temperature of 14 °C. The outlet static pressure is 100 kPa and the rotational speed is 7500 r/min (surface speed of 66.8 m/s) or 15,000 r/min (surface speed of 133.5 m/s). Numerical results show that the LSBSB possesses the slightly less leakage flow rate than the LSCB due to the flow resistance of the bristle pack to the fluid. Compared with the LSCB, the LSBSB shows a higher positive effective stiffness (Keff) at all considered vibration frequencies and a higher effective damping (Ceff) for most vibration frequencies. What is more, the crossover frequency (fc0) of the LSBSB is significantly lower than that of the LSCB, which means that the LSBSB has a wider frequency range offering positive effective damping. The increasing inlet preswirl velocity and rotational speed only slightly affect the Keff for both seals. The Ceff of two seals decreases as the inlet preswirl velocity rises, especially for the LSCB. The Ceff of the LSCB slightly decreases because of the increasing rotational speed. In contrast, the Ceff of the LSBSB is not sensitive to the changes in rotational speed. In a word, the LSBSB possesses superior rotordynamic performance to the LSCB. Note that this work also investigates the leakage flow and rotordynamic characteristics a labyrinth seal with inclined baffles (LSIB) under the condition of u0 = 60 m/s and n = 15,000 r/min. The inclined baffles of the LSIB are same as the backing plates of LSBSB baffles. The LSIB has rotordynamic coefficients almost equal to the LSCB. Hence, the reason why the LSBSB possesses better rotordynamic performance than that of the LSCB is the flow resistance of bristle packs of brush seal baffles, not the inclination direction variation of baffles.

Effect of Tooth Bending Damage on the Leakage of Straight-Through Labyrinth Seals

2005 ◽  
Author(s):  
Jinming Xu ◽  
Matthew S. Ambrosia ◽  
David L. Rhode
Keyword(s):  
Flow Pattern ◽  
High Reynolds Number ◽  
Labyrinth Seal ◽  
Quantitative Information ◽  
Radial Clearance ◽  
Labyrinth Seals ◽  
Wall Functions ◽  
Broad Variety ◽  
Volume Algorithm ◽  
High Reynolds

Unavoidable rotordynamic impacting on labyrinth seal teeth sometimes occurs when centrifugal compressors, for example, undergo transients. Consequently, the labyrinth seal teeth are damaged or disfigured in various ways when the surface opposite to the teeth is non-abradable. Thus far, no quantitative information concerning the effect on seal leakage is available. The present work focuses on the effect of seal leakage due to such permanently bent labyrinth seal teeth. The investigation was done numerically by solving the 2-D, axisymmetric RANS equations with a finite-volume algorithm. The high-Reynolds number k-ε turbulence model was used with standard wall functions. A broad variety of tooth seal bending was studied by varying the bending curvature and the length of bending, as well as the after-bend tooth radial clearance. The results show that the bending damage drastically affects the leakage as well as the flow pattern. This is due largely to the altered clearance caused by the bending. However, other bending factors, such as the bending curvature and the percentage of tooth length that is bent, also contribute to the change of leakage and flow pattern.

Leakage and Cavity Pressures in an Interlocking Labyrinth Gas Seal: Measurements vs. Predictions

2019 ◽  
Author(s):  
Luis San Andrés ◽  
Tingcheng Wu ◽  
Jose Barajas-Rivera ◽  
Jiaxin Zhang ◽  
Rimpei Kawashita
Keyword(s):  
Mass Flow ◽  
Pressure Ratio ◽  
Labyrinth Seal ◽  
Secondary Flows ◽  
Tip Clearance ◽  
Inlet Pressure ◽  
Steam Turbines ◽  
Rotor Speed ◽  
Labyrinth Seals ◽  
Gas Seals

Abstract Gas labyrinth seals (LS) restrict secondary flows (leakage) in turbomachinery and their impact on the efficiency and rotordynamic stability of high-pressure compressors and steam turbines can hardly be overstated. Amongst seal types, the interlocking labyrinth seal (ILS), having teeth on both the rotor and on the stator, is able to reduce leakage up to 30% compared to other LSs with either all teeth on the rotor or all teeth on the stator. This paper introduces a revamped facility to test gas seals for their rotordynamic performance and presents measurements of the leakage and cavity pressures in a five teeth ILS. The seal with overall length/diameter L/D = 0.3 and small tip clearance Cr/D = 0.00133 is supplied with air at T = 298 K and increasing inlet pressure Pin = 0.3 MPa ∼ 1.3 MPa, while the exit pressure/inlet pressure ratio PR = Pout/Pin is set to range from 0.3 to 0.8. The rotor speed varies from null to 10 krpm (79 m/s max. surface speed). During the tests, instrumentation records the seal mass flow (ṁ) and static pressure in each cavity. In parallel, a bulk-flow model (BFM) and a computational fluid dynamics (CFD) analysis predict the flow field and deliver the same performance characteristics, namely leakage and cavity pressures. Both measurements and predictions agree closely (within 5%) and demonstrate the seal mass flow rate is independent of rotor speed. A modified flow factor Φ¯=m.T/PinD1-PR2 characterizes best the seal mass flow with a unique magnitude for all pressure conditions, Pin and PR.

scholarly journals Rotordynamic Coefficient and Leakage Test Results for Interlock and Tooth-on-Stator Labyrinth Seals

1988 ◽  
Author(s):  
Dara W. Childs ◽  
David A. Elrod ◽  
Keith Hale
Keyword(s):  
Labyrinth Seal ◽  
Damping Coefficient ◽  
Test Results ◽  
Labyrinth Seals ◽  
Rotordynamic Coefficients ◽  
Damping Coefficients ◽  
Leakage Test ◽  
Stiffness And Damping ◽  
Rotordynamic Coefficient

Test results (leakage and rotordynamic coefficients) are presented for an interlock and tooth-on-stator labyrinth seals. Tests were carried out with air at speeds out to 16,000 cpm and supply pressures up to 7.5 bars. The rotordynamic coefficients consist of direct and cross-coupled stiffness and damping coefficients. Damping-coefficient data have not previously been presented for interlock seals. The test results support the following conclusions: (a) The interlock seal leaks substantially less than labyrinth seals. (b) Destabilizing forces are lower for the interlock seal. (c) The labyrinth seal has substantially greater direct damping values than the interlock seal. A complete rotordynamics analysis is needed to determine which type of seal would yield the best stability predictions for a given turbomachinery unit.

Leakage and Rotordynamic Coefficients of Brush Seals With Zero Cold Clearance Used in an Arrangement With Labyrinth Fins

2013 ◽  
Vol 135 (12) ◽  
Author(s):  
Manuel Gaszner ◽  
Alexander O. Pugachev ◽  
Christos Georgakis ◽  
Paul Cooper
Keyword(s):  
Fluid Dynamics ◽  
Computational Fluid Dynamics ◽  
Dynamic Test ◽  
Radial Clearance ◽  
Labyrinth Seals ◽  
Rotordynamic Coefficients ◽  
Damping Coefficients ◽  
Stiffness Coefficients ◽  
Brush Seal ◽  
Stiffness And Damping

A brush-labyrinth sealing configuration consisting of two labyrinth fins upstream and one brush seal downstream is studied experimentally and theoretically. Two slightly different brush seal designs with zero cold radial clearance are considered. The sealing configurations are tested on the no-whirl and dynamic test rigs to obtain leakage performance and rotordynamic stiffness and damping coefficients. The no-whirl tests allow identification of the local rotordynamic direct and cross-coupled stiffness coefficients for a wide range of operating conditions, while the dynamic test rig is used to obtain both global stiffness and damping coefficients but for a narrower operating range limited by the capabilities of a magnetic actuator. Modeling of the brush-labyrinth seals is performed using computational fluid dynamics. The experimental global rotordynamic coefficients consist of an aerodynamic component due to the gas flow and a mechanical component due to the contact between the bristle tips and rotor surface. The computational fluid dynamics (CFD)–based calculations of rotordynamic coefficients provide, however, only the aerodynamic component. A simple mechanical model is used to estimate the theoretical value of the mechanical stiffness of the bristle pack during the contact. The results obtained for the sealing configurations with zero cold radial clearance brush seals are compared with available data on three-tooth-on-stator labyrinth seals and a brush seal with positive cold radial clearance. Results show that the sealing arrangement with a line-on-line welded brush seal has the best performance overall with the lowest leakage and cross-coupled stiffness. The predictions are generally in agreement with the measurements for leakage and stiffness coefficients. The seal-damping capability is noticeably underpredicted.

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