Leakage Degradation of Straight Labyrinth Seal Due to Wear of Round Tooth Tip and Acute Trapezoidal Rub-Groove

2017 ◽  
Vol 139 (7) ◽  
Author(s):  
Yahya Dogu ◽  
Mustafa C. Sertçakan ◽  
Koray Gezer ◽  
Mustafa Kocagül ◽  
Ercan Arıcan ◽  
...  
Keyword(s):  
Pressure Ratio ◽  
Labyrinth Seal ◽  
Operating Conditions ◽  
Groove Width ◽  
Axial Position ◽  
Rotor Speed ◽  
Wall Angle ◽  
Round Shape ◽  
Number Of Teeth ◽  
Cfd Analyses

In this paper, labyrinth seal leakage is numerically quantified for an acute trapezoidal rub-groove accompanied with a rounded tooth, as a function of rub-groove sizes and tooth-groove axial positions. Analyses parameters include clearance, pressure ratio, number of teeth, and rotor speed. Labyrinth seals wear during engine transients. Radial incursion and axial movement of the rotor–stator pair cause the labyrinth teeth to rub against the unworn stator surface. The labyrinth teeth and/or stator wear depending on their material hardness. Wear damage in the form of material loss or deformation permanently increases seal clearance, and thus, leakage. This leakage is known to be dependent on the shape and geometry of the worn tooth and the stator rub groove. There are two types of reported tooth tip wear. These can be approximated as a mushroom shape and a round shape. The stator rub-groove shapes can be approximately simulated in five forms: rectangle, trapezoid (isosceles and acute), triangle, and ellipse. In this paper, the acute trapezoidal rub-groove shape is specifically chosen, since it is the most similar to the most commonly observed rub-groove form. The tooth tip is considered to be rounded, because the tooth tip wears smoothly and a round shape forms during rub-groove formation. To compare the unworn tooth, the flat stator is also analyzed as a reference case. All analyzed parameters for geometric dimensions (groove width, depth, wall angle, and tooth-groove axial position) and operating conditions (flow direction, clearance, pressure ratio, number of teeth, and rotor speed) are analyzed in their practical ranges. Computational fluid dynamics (CFD) analyses are carried out by employing a compressible turbulent flow solver in a 2D axisymmetrical coordinate system. CFD analyses show that the rounded tooth leaks more than an unworn sharp-edged tooth, due to the formation of a smooth and streamlined flow around the rounded geometry. This smooth flow yields less flow separation, flow disturbance, and less of vena contracta effect. The geometric dimensions of the acute trapezoidal rub-groove (width, depth, wall angle) significantly affect leakage. The effects of clearance, pressure ratio, number of teeth, and rotor speed on the leakage are also quantified. Analyses results are separately evaluated for each parameter.

Leakage Degradation of Straight Labyrinth Seal due to Wear of Round Tooth Tip and Acute Trapezoidal Rub-Groove

2016 ◽  
Author(s):  
Yahya Dogu ◽  
Mustafa C. Sertçakan ◽  
Koray Gezer ◽  
Mustafa Kocagül ◽  
Ercan Arıcan ◽  
...  
Keyword(s):  
Pressure Ratio ◽  
Labyrinth Seal ◽  
Operating Conditions ◽  
Groove Width ◽  
Axial Position ◽  
Rotor Speed ◽  
Wall Angle ◽  
Round Shape ◽  
Number Of Teeth ◽  
Cfd Analyses

In this paper, labyrinth seal leakage is numerically quantified for an acute trapezoidal rub-groove accompanied with a rounded tooth, as a function of rub-groove sizes and tooth-groove axial positions. Analyses parameters include clearance, pressure ratio, number of teeth, and rotor speed. Labyrinth seals wear during engine transients. Radial incursion and axial movement of the rotor-stator pair cause the labyrinth teeth to rub against the unworn stator surface. The labyrinth teeth and/or stator wear depending on their material hardness. Wear damage in the form of material loss or deformation permanently increases seal clearance, and thus, leakage. This leakage is known to be dependent on the shape and geometry of the worn tooth and the stator rub groove. There are two types of reported tooth tip wear. These can be approximated as a mushroom shape and a round shape. The stator rub-groove shapes can be approximately simulated in five forms: rectangle, trapezoid (isosceles and acute), triangle, and ellipse. In this paper, the acute trapezoidal rub-groove shape is specifically chosen, since it is the most similar to the most commonly observed rub-groove form. The tooth tip is considered to be rounded, because the tooth tip wears smoothly and a round shape forms during rub-groove formation. To compare the unworn tooth, the flat stator is also analyzed as a reference case. All analyzed parameters for geometric dimensions (groove width, depth, wall angle, tooth-groove axial position,) and operating conditions (flow direction, clearance, pressure ratio, number of teeth, rotor speed) are analyzed in their practical ranges. CFD analyses are carried out by employing a compressible turbulent flow solver in a 2-D axi-symmetrical coordinate system. CFD analyses show that the rounded tooth leaks more than an unworn sharp-edged tooth, due to the formation of a smooth and streamlined flow around the rounded geometry. This smooth flow yields less flow separation, flow disturbance, and less of vena contract a effect. The geometric dimensions of the acute trapezoidal rub-groove (width, depth, wall angle) significantly affect leakage. The effects of clearance, pressure ratio, number of teeth, and rotor speed on the leakage are also quantified. Analyses results are separately evaluated for each parameter.

Labyrinth Seal Leakage Degradation due to Various Types of Wear

2016 ◽  
Author(s):  
Yahya Dogu ◽  
Mustafa C. Sertçakan ◽  
Koray Gezer ◽  
Mustafa Kocagül ◽  
Ercan Arıcan ◽  
...  
Keyword(s):  
Pressure Ratio ◽  
Tooth Wear ◽  
Labyrinth Seal ◽  
Operating Conditions ◽  
Labyrinth Seals ◽  
Engine Design ◽  
Acute Triangle ◽  
Compressible Turbulent Flow ◽  
Cfd Analyses ◽  
Transient Operations

This paper systematically presents a complete leakage comparison for various types of wear experienced by labyrinth seals. Labyrinth seals used in turbine engines are designed to work at a clearance during steady-state engine operations. The tooth tip rubs the stator and wears either itself or the stator surface during transient operations, depending on the material properties of the tooth and stator. Any type of wear that increases clearance or deforms the tooth tip will cause permanent and unpredictable leakage degradation. This negatively affects the engine’s overall efficiency, durability, and life. The teeth have been reported to wear into a mushroom profile or into a rounded profile. A rub-groove on the opposing surface may form in several shapes. Based on a literature survey, five rub-groove shapes are considered in this work. They are: rectangle, trapezoid (isosceles and acute), triangle, and ellipse. In this work, leakage degradation due to wear is numerically quantified for both mushroomed and rounded tooth wear profiles. It also includes analyses on rounded teeth with the formation of five rub-groove shapes. All parameters are analyzed at various operating conditions (clearance, pressure ratio, number of teeth, rotor speed). CFD analyses are carried out by employing compressible turbulent flow in a 2-D axi-symmetrical coordinate system. CFD analyses show that the following tooth-wear conditions affect leakage from least to greatest: unworn, rounded, and mushroomed. These are for an unworn flat stator. It is also observed that rub-groove shapes considerably affect the leakage depending on the clearance. Leakage increases with the following groove profiles: triangular, rectangular, acute trapezoidal, isosceles trapezoidal, and elliptical. The results show that any type of labyrinth seal wear has significant effects on leakage. Therefore, leakage degradation due to wear should be considered during the engine design phase.

CFD Investigation of Labyrinth Seal Leakage Performance Depending on Mushroom Shaped Tooth Wear

2015 ◽  
Author(s):  
Yahya Doğu ◽  
Mustafa C. Sertçakan ◽  
Ahmet S. Bahar ◽  
Altuğ Pişkin ◽  
Ercan Arıcan ◽  
...  
Keyword(s):  
Pressure Ratio ◽  
Tooth Wear ◽  
Labyrinth Seal ◽  
Performance Degradation ◽  
Permanent Teeth ◽  
Tooth Surface ◽  
Limited Information ◽  
Rotor Speed ◽  
Vena Contracta ◽  
Number Of Teeth

Conventional labyrinth seal applications in turbomachinery encounter a permanent teeth tip damage and wear during transitional operations. This is the dominant issue that causes unpredictable seal leakage performance degradation. Since the gap between the rotor and the stator changes depending on engine transitional operations, labyrinth teeth located on the rotor/stator wear against the stator/rotor. This wear is observed mostly in the form of the labyrinth teeth becoming a mushroom shape. It is known that as a result of this tooth tip wear, leakage performance permanently decreases, which negatively affects the engine’s overall efficiency. However, very limited information about leakage performance degradation caused by mushroom wear is available in open literature. This paper presents a study that numerically quantifies leakage values for various radii of mushroom shaped labyrinth teeth by changing tooth-surface clearance, pressure ratio, number of teeth, and rotor speed. Analyzed parameters and their ranges are mushroom radius (R=0–0.508mm), clearance (cr=0.254–2.032mm), pressure ratio (Rp=1.5–3.5), number of teeth (nt=1–12), and rotor speed (n=0–80krpm). CFD analyses were carried out by employing compressible turbulent flow in 2-D axi-symmetrical coordinate system. CFD leakage results were also compared with well-known labyrinth seal semi-empirical correlations. Given a constant clearance, leakage increases with the size of the mushroom radius that forms on the tooth. This behavior is caused by less flow separation and flow disturbance and the vena contracta effect for flow over the smoothly shaped mushroom tooth tip compared to the sharp-edged tooth tip. This leakage increase is higher when the tooth tip wear is considered as an addition to the unworn physical clearance, since the clearance dominates the leakage. The leakage affected by the number of teeth was also quantified with respect to the mushroom radius. The rotational effect was also studied as a secondary parameter.

Computational Fluid Dynamics Investigation of Labyrinth Seal Leakage Performance Depending on Mushroom-Shaped Tooth Wear

2015 ◽  
Vol 138 (3) ◽  
Author(s):  
Yahya Dogu ◽  
Mustafa C. Sertçakan ◽  
Ahmet S. Bahar ◽  
Altuğ Pişkin ◽  
Ercan Arıcan ◽  
...  
Keyword(s):  
Fluid Dynamics ◽  
Computational Fluid Dynamics ◽  
Pressure Ratio ◽  
Labyrinth Seal ◽  
Performance Degradation ◽  
Permanent Teeth ◽  
Tooth Surface ◽  
Limited Information ◽  
Rotor Speed ◽  
Number Of Teeth

Conventional labyrinth seal applications in turbomachinery encounter a permanent teeth tip damage and wear during transitional operations. This is the dominant issue that causes unpredictable seal leakage performance degradation. Since the gap between the rotor and the stator changes depending on engine transitional operations, labyrinth teeth located on the rotor/stator wear against the stator/rotor. This wear is observed mostly in the form of the labyrinth teeth becoming a mushroom shape. It is known that, as a result of this tooth tip wear, leakage performance permanently decreases, which negatively affects the engine's overall efficiency. However, very limited information about leakage performance degradation caused by mushroom wear is available in open literature. This paper presents a study that numerically quantifies leakage values for various radii of mushroom-shaped labyrinth teeth by changing tooth-surface clearance, pressure ratio, number of teeth, and rotor speed. Analyzed parameters and their ranges are mushroom radius (R = 0–0.508 mm), clearance (cr = 0.254–2.032 mm), pressure ratio (Rp = 1.5–3.5), number of teeth (nt = 1–12), and rotor speed (n = 0–80 krpm). Computational fluid dynamics (CFD) analyses were carried out by employing compressible turbulent flow in 2D axisymmetrical coordinate system. CFD leakage results were also compared with well-known labyrinth seal semi-empirical correlations. Given a constant clearance, leakage increases with the size of the mushroom radius that forms on the tooth. This behavior is caused by less flow separation and flow disturbance, and the vena contracta effect for flow over the smoothly shaped mushroom tooth tip compared to the sharp-edged tooth tip. This leakage increase is higher when the tooth tip wear is considered as an addition to the unworn physical clearance, since the clearance dominates the leakage. The leakage affected by the number of teeth was also quantified with respect to the mushroom radius. The rotational effect was also studied as a secondary parameter.

Labyrinth Seal Leakage Degradation Due to Various Types of Wear

2017 ◽  
Vol 139 (6) ◽  
Author(s):  
Yahya Dogu ◽  
Mustafa C. Sertçakan ◽  
Koray Gezer ◽  
Mustafa Kocagül ◽  
Ercan Arıcan ◽  
...  
Keyword(s):  
Pressure Ratio ◽  
Tooth Wear ◽  
Labyrinth Seal ◽  
Operating Conditions ◽  
Labyrinth Seals ◽  
Engine Design ◽  
Acute Triangle ◽  
Computational Fluid Dynamics Cfd ◽  
Compressible Turbulent Flow ◽  
Cfd Analyses

This paper systematically presents a complete leakage comparison for various types of wear experienced by labyrinth seals. Labyrinth seals used in turbine engines are designed to work at a clearance during steady-state engine operations. The tooth tip rubs the stator and wears either itself or the stator surface during transient operations, depending on the material properties of the tooth and stator. Any type of wear that increases clearance or deforms the tooth tip will cause permanent and unpredictable leakage degradation. This negatively affects the engine's overall efficiency, durability, and life. The teeth have been reported to wear into a mushroom profile or into a rounded profile. A rub-groove on the opposing surface may form in several shapes. Based on a literature survey, five rub-groove shapes are considered in this work. They are rectangle, trapezoid (isosceles and acute), triangle, and ellipse. In this work, leakage degradation due to wear is numerically quantified for both mushroomed and rounded tooth wear profiles. It also includes analyses on rounded teeth with the formation of five rub-groove shapes. All parameters are analyzed at various operating conditions (clearance, pressure ratio, number of teeth, and rotor speed). Computational fluid dynamics (CFD) analyses are carried out by employing compressible turbulent flow in a 2D axisymmetrical coordinate system. CFD analyses show that the following tooth-wear conditions affect leakage from least to greatest: unworn, rounded, and mushroomed. These are for an unworn flat stator. It is also observed that rub-groove shapes considerably affect the leakage depending on the clearance. Leakage increases with the following groove profiles: triangular, rectangular, acute trapezoidal, isosceles trapezoidal, and elliptical. The results show that any type of labyrinth seal wear has significant effects on leakage. Therefore, leakage degradation due to wear should be considered during the engine design phase.

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.

Experimental Analysis of Abradable Labyrinth Seal Leakage With Simulated Groove: Part 2

2013 ◽  
Author(s):  
Luca Innocenti ◽  
Stefania Ricupero ◽  
Rajeev Kumar Pandit ◽  
Nuo Sheng
Keyword(s):  
Pressure Ratio ◽  
Test Sample ◽  
Labyrinth Seal ◽  
Axial Position ◽  
Radial Clearance ◽  
Pressure Drops ◽  
Flow Physics ◽  
Main Requirement ◽  
Numerical Predictions ◽  
Mass Leakage

In the secondary flow path of centrifugal compressors, abradable seals are in high demand when higher efficiency is the main requirement. This is because abradable seals can maintain very tight clearances between static and rotating components compared to other sealing technology. However, due to the rubbing of the teeth into the abradable material, some grooves can form. The flow physics and mass leakage of abradable seals are strongly dependent on the presence of these grooves and their shape. For cases where no grooves are present, seal leakage is mainly a function of inlet pressure, pressure ratio across the seal and tooth radial clearance. Once grooves are formed, the flow physics and seal leakage also are a function of groove dimensions, tooth clearance and tooth axial position inside the groove. The scope of the present paper is to describe the experimental campaign that has been performed to validate the numerical analysis of part 1 of the paper [1]. The experimental test matrix investigates the groove and teeth positioning effects on seal leakages. To achieve higher accuracy, the key geometric parameters, such as radial and axial gaps, were controlled in the test sample during the tests. For cases where grooves are present the experimental measurements reveal that flow field strongly depends on groove dimensions, tooth radial clearance and tooth axial position. The authors, finally, found generally a good agreement between numerical predictions and measured data, both in terms of leakage and pressure drops across the teeth.

Leakage and Windage Heating in Stepped Labyrinth Seals

2019 ◽  
Author(s):  
Kali Charan Nayak ◽  
Nomesh P. Kandaswamy ◽  
Syed Faheemulla
Keyword(s):  
Pressure Ratio ◽  
Labyrinth Seal ◽  
Vital Role ◽  
Step Height ◽  
Axial Position ◽  
Radial Clearance ◽  
Labyrinth Seals ◽  
Geometric Factors ◽  
Pitch Ratio ◽  
Computational Fluid Dynamics Cfd

Abstract Stepped labyrinth seals are used in multiple locations in the gas turbine with the intent to reduced leakage compared to straight labyrinth seals. However the selection of geometric factors in stepped labyrinth seals is critical to allow lower leakage in its operating envelope. Particularly the step height and axial position during the running condition play a vital role. The influence of these factors on the leakage, swirl development and windage heating in stepped labyrinth seal has not been thoroughly investigated in the previously published work. This paper focuses to study above effects with numerical simulations in a smooth four-fin stepped labyrinth seal. Specifically, a 2D axi-symmetric computational fluid dynamics (CFD) model is developed utilizing commercial finite volume-based software incorporating the standard k-ε turbulence model. Using this model, a broad parametric study is conducted by varying step height, axial position of the knife from the step, radial clearance and pressure ratio for a four-teeth stepped labyrinth seal. It has been observed that the seal leakage reduces with increase in step height to pitch ratio up to 0.35 and with further increase it tails off. The axial position of the tooth has strong influence on the flow structure and swirl development in the seal pocket.

Leakage and Dynamic Force Coefficients for Two Labyrinth Gas Seals: Teeth-on-Stator and Interlocking Teeth Configurations — A CFD Approach to Their Performance

2018 ◽  
Author(s):  
Luis San Andrés ◽  
Tingcheng Wu
Keyword(s):  
Pressure Ratio ◽  
Labyrinth Seal ◽  
Inlet Pressure ◽  
Dynamic Force ◽  
Rotor Speed ◽  
Frequency Range ◽  
Force Coefficients ◽  
Supply Pressure ◽  
Exit Pressure ◽  
Gas Seals

Labyrinth gas seals (LS) commonly used in turbomachines reduce secondary flow leakage. Conventional see-through labyrinth seal designs include either all Teeth-On-Stator (TOS) or all Teeth-On-Rotor (TOR). Experience shows that an interlocking labyrinth seal (ILS), with teeth on both stator and rotor, reduces gas leakage by up to 30% compared to the conventional see-through designs. However, field data for ILS rotordynamic characteristics is still vague and scarce in the literature. This work presents flow predictions for an ILS and a TOS LS, both seals share identical design features, namely radial clearance Cr = 0.2 mm, rotor diameter D = 150 mm, tooth pitch Li = 3.75 mm, and tooth height B = 3 mm. Air enters the seal at supply pressure Pin = 3.8, 6.9 bar (absolute) and temperature of 25 °C. The ratio of gas exit pressure to supply pressure ranges from 0.5 to 0.8, and the rotor speed is fixed at 10 krpm (surface speed of 79 m/s). The analysis implements a computational fluid dynamics (CFD) method with a multi-frequency-orbit rotor whirl model. The CFD predicted mass flow rate for the ILS is ∼21% lower than that of the TOS LS, thus making the ILS a more efficient choice. Integration of the dynamic pressure fields in the seal cavities, obtained for excitation frequency (ω) ranging from 12% to 168% of rotor speed (sub and super synchronous whirl), allows an accurate estimation of the seal dynamic force coefficients. For all the considered operating conditions, at low frequency range the TOS LS shows a negative direct stiffness (K < 0), frequency independent; whereas the ILS has K > 0 that increases with both frequency and supply pressure. For both seals, the magnitude of K decreases when the exit pressure/inlet pressure ratio increases. On the other hand, the cross-coupled stiffness (k) from both seals is frequency dependent, its magnitude increases with gas supply pressure, and the k for the ILS is more sensitive to a change in the exit/inlet pressure ratio. Notably, k turns negative for subsynchronous frequencies below rotor speed (Ω) for both the TOS LS and ILS. The direct damping (C) for the TOS LS remains constant for ω > ½ Ω and has a larger magnitude than the damping for the ILS over the frequency range up to 1.5Ω. An increase in exit/inlet pressure ratio decreases the direct damping for both seals. The effective damping coefficient, Ceff = (C-k/ω) whenever positive aids to damp vibrations, whereas Ceff < 0 is a potential source for an instability. For frequencies ω /Ω < 1.3, Ceff for the TOS LS is higher in magnitude than that for the ILS. From a rotordynamics point of view, the ILS is not a sound selection albeit it reduces leakage. Comparison of the CFD predicted force coefficients against those from a bulk flow model demonstrate the later simple model delivers poor results, often contradictory and largely indifferent to the type of seal, ILS or TOS LS. In addition, CFD model predictions are benchmarked against experimental dynamic force coefficients for two TOS LSs published by Ertas et al. (2012) and Vannini et al. (2014).

Leakage and Dynamic Force Coefficients for Two Labyrinth Gas Seals: Teeth-on-Stator and Interlocking Teeth Configurations. A Computational Fluid Dynamics Approach to Their Performance

2018 ◽  
Vol 141 (4) ◽  
Author(s):  
Tingcheng Wu ◽  
Luis San Andrés
Keyword(s):  
Gas Turbines ◽  
Pressure Ratio ◽  
Labyrinth Seal ◽  
Inlet Pressure ◽  
Dynamic Force ◽  
Rotor Speed ◽  
Frequency Range ◽  
Force Coefficients ◽  
Supply Pressure ◽  
Gas Seals

Labyrinth gas seals (LSs) commonly used in turbomachines reduce secondary flow leakage. Conventional see-through labyrinth seal designs include either all teeth-on-stator (TOS) or all teeth-on-rotor (TOR). Experience shows that an interlocking labyrinth seal (ILS), with teeth on both stator and rotor, reduces gas leakage by up to 30% compared to the conventional see-through designs. However, field data for ILS rotordynamic characteristics are still vague and scarce in the literature. This work presents flow predictions for an ILS and a TOS LS, both seals share identical design features, namely radial clearance Cr = 0.2 mm, rotor diameter D = 150 mm, tooth pitch Li = 3.75 mm, and tooth height B = 3 mm. Air enters the seal at supply pressure Pin = 3.8, 6.9 bar (absolute) and temperature of 25 °C. The ratio of gas exit pressure to supply pressure ranges from 0.5 to 0.8, and the rotor speed is fixed at 10 krpm (surface speed of 79 m/s). The analysis implements a computational fluid dynamics (CFD) method with a multi-frequency-orbit rotor whirl model. The CFD predicted mass flow rate for the ILS is ∼ 21% lower than that of the TOS LS, thus making the ILS a more efficient choice. Integration of the dynamic pressure fields in the seal cavities, obtained for excitation frequency (ω) ranging from 12% to 168% of rotor speed (sub and super synchronous whirl), allows an accurate estimation of the seal dynamic force coefficients. For all the considered operating conditions, at low frequency range, the TOS LS shows a negative direct stiffness (K < 0), frequency independent; whereas the ILS has K > 0 that increases with both frequency and supply pressure. For both seals, the magnitude of K decreases when the exit pressure/inlet pressure ratio increases. On the other hand, the cross-coupled stiffness (k) from both seals is frequency dependent, its magnitude increases with gas supply pressure, and k for the ILS is more sensitive to a change in the exit/inlet pressure ratio. Notably, k turns negative for subsynchronous frequencies below rotor speed (Ω) for both the TOS LS and the ILS. The direct damping (C) for the TOS LS remains constant for ω > ½ Ω and has a larger magnitude than the damping for the ILS over the frequency range up to 1.5 Ω. An increase in exit/inlet pressure ratio decreases the direct damping for both seals. The effective damping coefficient, Ceff = (C-k/ω), whenever positive aids to damp vibrations, whereas Ceff < 0 is a potential source for an instability. For frequencies ω/Ω < 1.3, Ceff for the TOS LS is higher in magnitude than that for the ILS. From a rotordynamics point of view, the ILS is not a sound selection albeit it reduces leakage. Comparison of the CFD predicted force coefficients against those from a bulk flow model demonstrates that the later simple model delivers poor results, often contradictory and largely indifferent to the type of seal, ILS or TOS LS. In addition, CFD model predictions are benchmarked against experimental dynamic force coefficients for two TOS LSs published by Ertas et al. (2012, “Rotordynamic Force Coefficients for Three Types of Annular Gas Seals With Inlet Preswirl and High Differential Pressure Ratio,” ASME J. Eng. Gas Turbines Power, 134(4), pp. 04250301–04250312) and Vannini et al. (2014, “Labyrinth Seal and Pocket Damper Seal High Pressure Rotordynamic Test Data,” ASME J. Eng. Gas Turbines Power, 136(2), pp. 022501–022509.)

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