Investigation of Rotordynamic Coefficients of Gas Labyrinth Seals Using a Steady State CFD Approach

2020 ◽  
Author(s):  
Casey Palanca ◽  
Abraham Engeda ◽  
Mike Cave
Keyword(s):  
Steady State ◽  
Turbulence Modeling ◽  
Driving Forces ◽  
Labyrinth Seal ◽  
Labyrinth Seals ◽  
Rotordynamic Coefficients ◽  
Cfd Models ◽  
Area Of Interest ◽  
The Status ◽  
The Stability

Abstract Labyrinth seals were one of the first seal configurations used in modern turbomachinery and continue to be one of the most frequently used clearance seal configurations today. Their primary purpose is to control internal leakage between the rotating and stationary components of centrifugal compressors. However, when fulfilling this objective, labyrinth seals have been shown to be a potential source of instability within the rotor-stator system. Driving forces inside the leakage flow path of the cavities often induce destabilizing vibrations on the rotor. The forces are characterized by stiffness and damping coefficients which describe the stability behavior of the seal. Therefore, accurately predicting these rotordynamic coefficients remains an important area of interest in gas compressor design. This paper reviews the status of current methods of obtaining rotordynamic coefficients. The objective of this work is to verify the accuracy of current steady state CFD models used to predict rotordynamic coefficients in dry gas labyrinth seals. For this purpose, a full 3D eccentric CFD model is conducted for three different labyrinth seal geometries. In this approach, the rotordynamic coefficients are predicted from the regression of the radial and tangential impedances as a function of whirl frequencies. For comparison, two seals are compared with experimental data available in literature, and a third seal is compared to bulk flow and numerical CFD results also found in the literature. Furthermore, the influence of pre-swirl entering the labyrinth seal and turbulence modeling are also considered in this work.

Rotordynamic Characterization of a Staggered Labyrinth Seal: Experimental Test Data and Comparison With Predictions

2018 ◽  
Author(s):  
Filippo Cangioli ◽  
Giuseppe Vannini ◽  
Paolo Pennacchi ◽  
Lorenzo Ciuchicchi ◽  
Leonardo Nettis ◽  
...  
Keyword(s):  
Gas Turbines ◽  
Flow Model ◽  
Labyrinth Seal ◽  
Accurate Estimation ◽  
Stability Assessment ◽  
Labyrinth Seals ◽  
Rotordynamic Coefficients ◽  
Working Principle ◽  
The Stability

As well known, the stability assessment of turbomachines is strongly related to internal sealing components. For instance, labyrinth seals are widely used in compressors, steam and gas turbines and pumps to control the clearance leakage between rotating and stationary parts, owing to their simplicity, reliability and tolerance to large thermal and pressure variations. Labyrinth seals working principle consists in reducing the leakage by imposing tortuous passages to the fluid that are effective on dissipating the kinetic energy of the fluid from high-pressure regions to low-pressure regions. Conversely, labyrinth seals could lead to dynamics issues. Therefore, an accurate estimation of their dynamic behavior is very important. In this paper, the experimental results of a long-staggered labyrinth seal will be presented. The results in terms of rotordynamic coefficients and leakage will be discussed as well as the critical assessment of the experimental measurements. Eventually, the experimental data are compared to numerical results obtained with the new bulk-flow model (BFM) introduced in this paper.

Rotordynamic Performance of a Negative-Swirl Brake for a Tooth-on-Stator Labyrinth Seal

2015 ◽  
Vol 138 (6) ◽  
Author(s):  
Dara W. Childs ◽  
James E. Mclean ◽  
Min Zhang ◽  
Stephen P. Arthur
Keyword(s):  
Labyrinth Seal ◽  
Test Results ◽  
Labyrinth Seals ◽  
Rotordynamic Coefficients ◽  
Stiffness Coefficients ◽  
Shaft Rotation ◽  
Computational Fluid Dynamics Cfd ◽  
The Cross ◽  
Inlet Conditions ◽  
The Stability

In the late 1970s, Benckert and Wachter (Technical University Stuttgart) tested labyrinth seals using air as the test media and measured direct and cross-coupled stiffness coefficients. They reported the following results: (1) fluid preswirl in the direction of shaft rotation creates destabilizing cross-coupled stiffness coefficients and (2) effective swirl brakes at the inlet to the seal can markedly reduce the cross-coupled stiffness coefficients, in many cases reducing them to zero. In recent years, “negative-swirl” swirl brakes have been employed, which attempt to reverse the circumferential direction of inlet flow, changing the sign of the cross-coupled stiffness coefficients and creating stabilizing stiffness forces. This study presents test results for a 16-tooth labyrinth seal with positive inlet preswirl (in the direction of shaft rotation) for the following inlet conditions: (1) no swirl brakes, (2) straight, conventional swirl brakes, and (3) negative-swirl swirl brakes. The negative-swirl swirl-brake designs were developed based on computational fluid dynamics (CFD) predictions. Tests were conducted at 10.2, 15.35, and 20.2 krpm with 70 bar of inlet pressure for pressure ratios of 0.3, 0.4, and 0.5. Test results include leakage and rotordynamic coefficients. In terms of leakage, the negative-swirl brake configuration leaked the least, followed by the conventional brake, followed by the no-brake design. Normalized to the negative-swirl brake configuration, the conventional-brake and no-brake configurations mass flow rate was greater, respectively, by factors of 1.04 and 1.09. The direct-stiffness coefficients are negative but small, consistent with past experience. The conventional swirl brake drops the destabilizing cross-coupled stiffness coefficients k by a factor of about 0.8 as compared to the no-brake results. The negative-swirl brake produces a change in sign of k with an appreciable magnitude; hence, the stability of forward precessing modes would be enhanced. In descending order, the direct-damping coefficients C are: no-swirl, negative-swirl, and conventional-swirl. Normalized in terms of the no-swirl case, C for the negative and conventional brake designs is, respectively, 0.7 and 0.6 smaller. The effective damping Ceff combines the effect of k and C. Ceff is large and positive for the negative-swirl configuration and near zero for the no-brake and conventional-brake designs. The present results for a negative-brake design are very encouraging for both eye-packing seals (where conventional swirl brakes have been previously employed) and division-wall and balance-piston seals, where negative shunt injection has been employed.

Rotordynamic Characterization of a Staggered Labyrinth Seal: Experimental Test Data and Comparison With Predictions

2018 ◽  
Vol 141 (1) ◽  
Author(s):  
Filippo Cangioli ◽  
Giuseppe Vannini ◽  
Paolo Pennacchi ◽  
Lorenzo Ciuchicchi ◽  
Leonardo Nettis ◽  
...  
Keyword(s):  
Gas Turbines ◽  
Flow Model ◽  
Labyrinth Seal ◽  
Accurate Estimation ◽  
Stability Assessment ◽  
Labyrinth Seals ◽  
Rotordynamic Coefficients ◽  
Working Principle ◽  
The Stability

As well known, the stability assessment of turbomachines is strongly related to internal sealing components. For instance, labyrinth seals are widely used in compressors, steam, and gas turbines and pumps to control the clearance leakage between rotating and stationary parts, owing to their simplicity, reliability, and tolerance to large thermal and pressure variations. Labyrinth seals working principle consists of reducing the leakage by imposing tortuous passages to the fluid that are effective on dissipating the kinetic energy of the fluid from high-pressure regions to low-pressure regions. Conversely, labyrinth seals could lead to dynamics issues. Therefore, an accurate estimation of their dynamic behavior is very important. In this paper, the experimental results of a long-staggered labyrinth seal will be presented. The results in terms of rotordynamic coefficients and leakage will be discussed as well as the critical assessment of the experimental measurements. Eventually, the experimental data are compared to the numerical results obtained with the new bulk-flow model (BFM) introduced in this paper.

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.

Rotordynamic Performance of a Negative-Swirl Brake for a Tooth-on-Stator Labyrinth Seal

2014 ◽  
Author(s):  
Dara W. Childs ◽  
James E. Mclean ◽  
Min Zhang ◽  
Stephen P. Arthur
Keyword(s):  
Labyrinth Seal ◽  
Test Results ◽  
Labyrinth Seals ◽  
Rotordynamic Coefficients ◽  
Stiffness Coefficients ◽  
Shaft Rotation ◽  
Direct Stiffness ◽  
The Cross ◽  
Inlet Conditions ◽  
The Stability

In the late 1970’s, Benckert and Wachter (Technical University Stuttgart) tested labyrinth seals using air as the test media and measured direct and cross-coupled stiffness coefficients. They reported the following results: (1) Fluid pre-swirl in the direction of shaft rotation creates destabilizing cross-coupled stiffness coefficients, and (2) Effective swirl brakes at the inlet to the seal can markedly reduce the cross-coupled stiffness coefficients, in many cases reducing them to zero. In recent years, “negative-swirl” swirl brakes have been employed that attempt to reverse the circumferential direction of inlet flow, changing the sign of the cross-coupled stiffness coefficients and creating stabilizing stiffness forces. This study presents test results for a 16-tooth labyrinth seal with positive inlet preswirl (in the direction of shaft rotation) for the following inlet conditions: (1) No swirl brakes, (2) Straight, conventional swirl brakes, and (3) Negative-swirl swirl brakes. The negative-swirl swirl-brake designs were developed based on CFD predictions. Tests were conducted at 10.2, 15.35, and 20.2 krpm with 70 bars of inlet pressure for pressure ratios of 0.3, 0.4, 0.5. Test results include leakage and rotordynamic coefficients. In terms of leakage, the negative-swirl brake configuration leaked the least, followed by the conventional brake, followed by the no-brake design. Normalized to the negative-swirl brake configuration, the conventional-brake and no-brake configurations mass flow rate were greater, respectively, by factors of 1.04 and 1.09. The direct stiffness coefficients are negative but small, consistent with past experience. The conventional swirl brake drops the destabilizing cross-coupled stiffness coefficients k by a factor of about 0.8 as compared to the no-brake results. The negative-swirl brake produces a change in sign of k with an appreciable magnitude; hence, the stability of forwardly-precessing modes would be enhanced. In descending order, the direct damping coefficients C are: no-swirl, negative-swirl, conventional-swirl. Normalized in terms of the no-swirl case, C for the negative and conventional brake designs are, respectively, 0.7 and 0.6 smaller. The effective damping Ceff combines the effect of k and C. Ceff is large and positive for the negative-swirl configuration and near zero for the no-brake and conventional-brake designs. The present results for a negative-brake design are very encouraging for both eye-packing seals (where conventional swirl brakes have been previously employed) and division-wall and balance-piston seals where negative shunt injection has been employed.

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.

A Theoretical Analysis of Floating Bush Journal Bearing With Axial Oil Film Rupture Being Considered

2002 ◽  
Vol 124 (3) ◽  
pp. 494-505 ◽  
Author(s):  
Kiyoshi Hatakenaka ◽  
Masato Tanaka ◽  
Kenji Suzuki
Keyword(s):  
Steady State ◽  
Reynolds Equation ◽  
Speed Ratio ◽  
Shaft Speed ◽  
Film Rupture ◽  
State Behavior ◽  
Oil Film ◽  
Rotordynamic Coefficients ◽  
The Stability ◽  
Very High

A new modified Reynolds equation is derived with centrifugal force acting on the hydrodynamic oil film being considered. This equation, together with a cavitation model, is used to obtain the steady-state equilibrium and calculate the rotordynamic coefficients of lightly loaded floating bush journal bearings operating at very high shaft speeds. The bush-to-shaft speed ratio and the linear cross-coupling spring coefficients of the inner oil film is found to decrease with the increase in shaft speed as the axial oil film rupture develops in the inner oil film. The present model can give reasonable explanation to the steady-state behavior and the stability behavior of the bearing observed in actual machines.

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.

Tip-Shroud Labyrinth Seal Effect on the Flutter Stability of Turbine Rotor Blades

2019 ◽  
Author(s):  
Roque Corral ◽  
Michele Greco ◽  
Almudena Vega
Keyword(s):  
Rotor Blade ◽  
Labyrinth Seal ◽  
Turbine Rotor ◽  
Rotor Blades ◽  
Stability Prediction ◽  
Labyrinth Seals ◽  
Turbine Rotor Blade ◽  
The Stability ◽  
Shrouded Rotor ◽  
Tip Shroud

Abstract The effect of the tip-shroud seal on the flutter onset of a shrouded turbine rotor blade, representative of a modern gas turbine, is numerically tested and the contribution to the work-per-cycle of the aerofoil and the tip-shroud are clearly identified. The numerical simulations are conducted using a linearised frequency domain solver. The flutter stability of the shrouded rotor blade is evaluated for an edgewise mode and compared with the standard industrial approach of not including the tip-shroud cavity. It turns out that including the tip shroud significantly changes the stability prediction of the rotor blade. This is due to the fact that the amplitude of the unsteady pressure created in the inter-fin cavity, due to the motion of the airfoil, is much greater than that of the airfoil. It is concluded that the combined effect of the seal and its platform tends to stabilise the rotor blade for all the examined nodal diameters and reduced frequencies. Finally, the numerical results are shown to be consistent with those obtained using an analytical simplified model to account for the effect of the labyrinth seals.

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.

Sign in / Sign up

Export Citation Format

Share Document