Improving the Stability of Labyrinth Gas Seals

1997 ◽  
Vol 123 (2) ◽  
pp. 383-387 ◽  
Author(s):  
K. Kwanka
Keyword(s):  
Stability Limit ◽  
Labyrinth Seal ◽  
Flexible Rotor ◽  
Labyrinth Seals ◽  
Continuous Growth ◽  
Rotating Speed ◽  
Dynamic Coefficients ◽  
Flow Through ◽  
Gas Seals ◽  
The Stability

The flow through labyrinth seals of turbomachinery generates forces which can cause self-excited vibrations of the rotor above the stability limit. The stability limit is reached at a specific rotating speed or power. The continuous growth of power density and rotating speed necessitates an exact prediction of the stability limit of turbomachinery. Usually the seal forces are described with dynamic coefficients. A new, easy-to-handle identification procedure uses the stability behavior of a flexible rotor to determine the dynamic coefficients. Systematic measurements with a great number of labyrinth seal geometries lead to reasonable results and demonstrate the accuracy and sensitivity of the procedure. A comparison of the various methods used to minimize the excitation indicates which seal is more stable and will thus improve the dynamic behavior of the rotor.

Rotordynamic Impact of Swirl Brakes on Labyrinth Seals With Smooth or Honeycomb Stators

1997 ◽  
Author(s):  
K. Kwanka
Keyword(s):  
Stability Limit ◽  
Labyrinth Seal ◽  
Flexible Rotor ◽  
Labyrinth Seals ◽  
Identification Procedure ◽  
Continuous Growth ◽  
Rotating Speed ◽  
Dynamic Coefficients ◽  
Flow Through ◽  
The Stability

The flow through labyrinth seals of turbomachinery generates forces which can cause self-excited vibrations of the rotor above the stability limit. The stability limit is reached at a specific rotating speed or power. The continuous growth in of power density and rotating speed necessitates an exact prediction of the stability limit of turbomachinery. Usually the seal forces are described with dynamic coefficients. A new, easy-to-handle identification procedure uses the stability behavior of a flexible rotor to determine the dynamic coefficients. Systematic measurements with a great number of labyrinth seal geometries lead to reasonable results and demonstrate the accuracy and sensitivity of the procedure. A comparison of the various methods used to minimize the excitation indicates which seal is more stable and will thus improve the dynamic behavior of the rotor.

Dynamic Coefficients of Labyrinth Gas Seals: A Comparison of Experimental Results and Numerical Calculations

2000 ◽  
Author(s):  
K. Kwanka ◽  
J. Sobotzik ◽  
R. Nordmann
Keyword(s):  
Stokes Equations ◽  
Labyrinth Seal ◽  
Navier Stokes ◽  
Labyrinth Seals ◽  
Navier Stokes Equations ◽  
Rotating Speed ◽  
Dynamic Coefficients ◽  
Flow Through ◽  
Gas Seals ◽  
Good Agreement

Non-contacting labyrinth seals are still the most common constructive elements used to minimize leakage losses in turbomachinery between areas with high pressure and areas with low pressure. Unfortunately, the leakage flow through the labyrinth seal generates forces which can have a great impact on the dynamics of the turborotor. Particularly in cases of instability, the turbomachinery is restricted in its power or rotating speed because of violent self-excited vibrations of the rotor. The occurrence of self-excited rotor vibrations due to lateral forces must definitely be excluded. To consider the labyrinth forces in Finite-Element prediction, a set of preferably exact dynamic coefficients is required. Numerical approaches used to calculate the coefficients are based on Navier-Stokes equations. A comparison with experimental data is essential for a validation of the calculation. The experimental identification is difficult, because of the littleness of the forces to be measured in gas seals. Especially the non-conservative coefficients, cross-coupled stiffness and direct damping, show a good agreement in both magnitude and trend depending on the entry swirl of the seal.

Variation of Fluid Flow Forces in Seals With Rotor Bending

1995 ◽  
Author(s):  
K. Kwanka
Keyword(s):  
Stability Limit ◽  
Mode Shapes ◽  
Flexible Rotor ◽  
Labyrinth Seals ◽  
Identification Procedure ◽  
Dynamic Coefficients ◽  
Exciting Forces ◽  
Direct Stiffness ◽  
The Stability ◽  
Turbomachine Rotor

Abstract Fluid-induced forces in labyrinth seals can cause unstable self-excited vibrations of the turbomachine rotor. Generally, a linear approach employing dynamic coefficients is used to describe these forces. A new procedure for the identification of the coefficients which uses two excitation sources placed on a flexible rotor is presented. The change in the stability limit and vibrational frequency caused by the investigated labyrinth gas seal contains the dynamic coefficients. It is important that problems which may also occur in the real turbomachine are considered by the identification procedure. The conservative dynamic coefficients, such as the direct stiffness, influence the bending of the mode shapes and thus affect indirectly the stability limit. The magnitude of the exciting forces depends on the axial positioning of the excitation source and also on the mode shape bending. These two dependencies are investigated by experiment and considered in the identification procedure.

scholarly journals Numerical validation of an analytical seal flutter model

2021 ◽  
Vol 5 ◽  
pp. 191-201
Author(s):  
Michele Greco ◽  
Roque Corral
Keyword(s):  
Analytical Model ◽  
Fluid Dynamic ◽  
Stability Limit ◽  
Labyrinth Seal ◽  
Analytical Models ◽  
Mode Shapes ◽  
Labyrinth Seals ◽  
Low Frequencies ◽  
Wide Range ◽  
The Stability

An analytical model to describe the flutter onset of straight-through labyrinth seals has been numerically validated using a frequency domain linearized Navier-Stokes solver. A comprehensive set of simulations has been conducted to assess the stability criterion of the analytical model originally derived by Corral and Vega (2018), “Conceptual Flutter Analysis of Labyrinth Seals Using Analytical Models - Part I: Theoretical Support,” ASME J. Turbomach., 140 (12), pp. 121006. The accuracy of the model has been assessed by using a simplified geometry consisting of a two-fin straight-through labyrinth seal with identical gaps. The effective gaps and the kinetic energy carried over are retained and their effects on stability are evaluated. It turns out that is important to inform the model with the correct values of both parameters to allow a proper comparison with the numerical simulations. Moreover, the non-isentropic perturbations included in the formulations are observed in the simulations at relatively low frequencies whose characteristic time is of the same order as the discharge time of the seal. This effect is responsible for the bending of the stability limit in the <inline-formula><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><mml:mn>0</mml:mn><mml:mi>t</mml:mi><mml:mi>h</mml:mi></mml:math></inline-formula> ND stability map obtained both in the model and the simulations. It turns out that the analytical model can predict accurately the stability of the seal in a wide range of pressure ratios, vibration mode-shapes, and frequencies provided that this is informed with the fluid dynamic gaps and the energy carried over to the downstream fin from a steady RANS simulation. The numerical calculations show for the first time that the model can be used to predict accurately not only the trends of the work-per-cycle of the seal but also quantitative results.

Dynamics of Labyrinth Gas Seals With Air Injection

2001 ◽  
Author(s):  
K. Kwanka
Keyword(s):  
Stability Limit ◽  
Air Injection ◽  
Design Stage ◽  
Early Design ◽  
Early Design Stage ◽  
Dynamic Coefficients ◽  
Exciting Forces ◽  
Gas Seals ◽  
The Stability ◽  
The Impact

Exciting forces generated in gas seals and blading of turbo-machinery can lead to vibrations of the rotor with unacceptably high amplitudes (Thomas [1958], Alford [1965]). It is important to predict in an early design stage the stability limit of the rotor so as to avoid the occurrence of self-excited vibrations. In the field, there are only few possibilities to stabilize the rotor. One way minimizing the cross-forces is by injecting air into the seal and counteracting the exciting mechanisms. The paper will present experimental dynamic coefficients of a gas seal of the teeth on stator type with eight cavities. The coefficients will be compared to the values obtained with a tangential injection of air into the first cavity in co-rotating and in counter-rotating direction. It seems that the impact of air injection is not as effective as swirl brakes or honeycomb stators.

Dynamic Coefficients of Stepped Labyrinth Gas Seals

1999 ◽  
Author(s):  
K. Kwanka
Keyword(s):  
Experimental Data ◽  
High Density ◽  
Labyrinth Seals ◽  
Running Speed ◽  
Identification Procedure ◽  
Damping Coefficients ◽  
Dynamic Coefficients ◽  
Flow Through ◽  
Gas Seals ◽  
Reliable Design

Rotor-fluid interactions can cause self-excited shaft vibrations of high density turbomachinery. Often the amplitude of the vibrations reaches unacceptably high amplitudes and the scheduled power or running speed cannot be achieved. One of the most important sources of excitation is the flow through labyrinth seals. For a reliable design it is necessary to predict these forces exactly, including not only stiffness but also damping coefficients. As the forces in labyrinth gas seals are rather small only minimal experimental data is available for the comparison and validation of calculations. Meanwhile a new and easy-to-handle identification procedure enables the investigation of numerous seal geometrys. The paper presents dynamic coefficients obtained with a stepped labyrinth and the comparison with other seal concepts.

Dynamic Coefficients of Stepped Labyrinth Gas Seals

2000 ◽  
Vol 122 (3) ◽  
pp. 473-477 ◽  
Author(s):  
Klaus Kwanka
Keyword(s):  
Experimental Data ◽  
High Density ◽  
Labyrinth Seals ◽  
Running Speed ◽  
Identification Procedure ◽  
Damping Coefficients ◽  
Dynamic Coefficients ◽  
Flow Through ◽  
Gas Seals ◽  
Reliable Design

Rotor-fluid interactions can cause self-excited shaft vibrations of high density turbomachinery. Often the amplitude of the vibrations reaches unacceptably high amplitudes and the scheduled power or running speed cannot be achieved. One of the most important sources of excitation is the flow through labyrinth seals. For a reliable design it is necessary to predict these forces exactly, including not only stiffness but also damping coefficients. As the forces in labyrinth gas seals are rather small only minimal experimental data is available for the comparison and validation of calculations. Meanwhile a new and easy-to-handle identification procedure enables the investigation of numerous seal geometrys. The paper presents dynamic coefficients obtained with a stepped labyrinth and the comparison with other seal concepts. [S0742-4795(00)00903-0]

Influence of a Honeycomb Facing on the Flow Through a Stepped Labyrinth Seal

2000 ◽  
Vol 124 (1) ◽  
pp. 140-146 ◽  
Author(s):  
V. Schramm ◽  
K. Willenborg ◽  
S. Kim ◽  
S. Wittig
Keyword(s):  
Flow Field ◽  
Three Dimensional ◽  
Theoretical Work ◽  
Labyrinth Seal ◽  
Honeycomb Structure ◽  
Experimental Investigations ◽  
Labyrinth Seals ◽  
Numerical Predictions ◽  
Flow Through ◽  
Aero Engines

This paper reports numerical predictions and measurements of the flow field in a stepped labyrinth seal. The theoretical work and the experimental investigations were successfully combined to gain a comprehensive understanding of the flow patterns existing in such elements. In order to identify the influence of the honeycomb structure, a smooth stator as well as a seal configuration with a honeycomb facing mounted on the stator wall were investigated. The seal geometry is representative of typical three-step labyrinth seals of modern aero engines. The flow field was predicted using a commercial finite volume code with the standard k-ε turbulence model. The computational grid includes the basic seal geometry as well as the three-dimensional honeycomb structures.

A CFD Study on the Dynamic Coefficients of Labyrinth Seals

2012 ◽  
Author(s):  
Donghui Zhang ◽  
Chester Lee ◽  
Michael Cave
Keyword(s):  
High Speed ◽  
Fluid Dynamic ◽  
Labyrinth Seal ◽  
Leakage Flow ◽  
Labyrinth Seals ◽  
Dynamic Coefficients ◽  
Rotating Impeller ◽  
Coupling Stiffness ◽  
Direct Stiffness ◽  
Compressor Efficiency

Labyrinth seals are widely used in gas compressors to reduce internal leakage and increase the compressor efficiency. Due to the eccentricity between the rotating impeller and the stationary part as *well as the shaft whirling motion, forces are generated when the leakage flow passing through the cavities and the seals. For a lot of applications with high speed and pressure, these forces can drive the system unstable. Thus, predicting the forces accurately become a very important for compressor rotordynamic designs. A lot of research and studies has been done to the seals itself, including bulk flow method, computational fluid dynamic (CFD) and test measurement. The seal and leakage flow interaction forces can be predicted relatively accurate. But very few research treat the seal and cavities as one component interacting with the leakage flow and produce the forces. This paper presents results of CFD investigations on the dynamic coefficients of one typical impeller eye seal and front cavity. The CFD results show that large forces are generated in the front cavity due to circumferential uniform pressure distribution, which caused by the downstream labyrinth seal. The forces generated in the front cavity are more than in the front seal. It was found that the inertia, damping, and stiffness are proportional to average pressure. The cross-coupling stiffness increases with speed with power of 2 while the direct stiffness increases with speed with power of about 1.7.

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.

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