On the Thermodynamic Process in the Bulk-Flow Model for the Estimation of the Dynamic Coefficients of Labyrinth Seals

2017 ◽  
Vol 140 (3) ◽  
Author(s):  
Filippo Cangioli ◽  
Paolo Pennacchi ◽  
Giuseppe Vannini ◽  
Lorenzo Ciuchicchi ◽  
Andrea Vania ◽  
...  
Keyword(s):  
Gas Turbines ◽  
Energy Equation ◽  
Flow Model ◽  
Experimental Tests ◽  
Labyrinth Seal ◽  
Bulk Flow ◽  
Labyrinth Seals ◽  
State Problem ◽  
Rotordynamic Coefficients ◽  
Steady State Problem

The impact of sealing equipment on the stability of turbomachineries is a crucial topic because the power generation market is continuously requiring high rotational speed and high performance, leading to the clearance reduction in the seals. The accurate characterization of the rotordynamic coefficients generated by the seals is pivotal to mitigate instability issues. In the paper, the authors propose an improvement of the state-of-the-art one-control volume (1CV) bulk-flow model (Childs and Scharrer, 1986, “An Iwatsubo-Based Solution for Labyrinth Seals: Comparison to Experimental Results,” ASME J. Eng. Gas Turbines Power, 108(2), pp. 325–331) by considering the energy equation in the steady-state problem. Thus, real gas properties can be evaluated in a more accurate way because the enthalpy variation, expected through the seal cavities, is evaluated in the model. The authors assume that the enthalpy is not a function of the clearance perturbation; therefore, the energy equation is considered only in the steady-state problem. The results of experimental tests of a 14 teeth-on-stator (TOS) labyrinth seal, performed in the high-pressure seal test rig owned by GE Oil&Gas, are presented in the paper. Positive and negative preswirl ratios are used in the experimental tests to investigate the effect of the preswirl on the rotordynamic coefficients. Overall, by considering the energy equation, a better numerical estimation of the rotordynamic coefficients for the tests with the negative preswirl ratio has been obtained (as it results from the comparison with the experiments). Finally, the numerical results are compared with a reference bulk-flow model proposed by Thorat and Childs (2010, “Predicted Rotordynamic Behavior of a Labyrinth Seal as Rotor Surface Speed Approaches Mach 1,” ASME J. Eng. Gas Turbines Power, 132(11), p. 112504), highlighting the improvement obtained.

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 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.

Rotordynamic Stability Predictions for Centrifugal Compressors Using a Bulk-Flow Model to Predict Impeller Shroud Force and Moment Coefficients

2010 ◽  
Vol 132 (9) ◽  
Author(s):  
Manoj K. Gupta ◽  
Dara W. Childs
Keyword(s):  
Gas Turbines ◽  
Flow Model ◽  
Reaction Force ◽  
Labyrinth Seal ◽  
Bulk Flow ◽  
Hole Injection ◽  
Pump Impeller ◽  
Stability Margins ◽  
Rotordynamic Coefficients ◽  
Model Predictions

An analysis is developed for a compressible bulk-flow model of the leakage path between a centrifugal-compressor impeller’s shroud and its housing along the impeller’s front and back sides. This development is an extension of analyses performed first by Childs (1989, ASME J. Vib. Acoust., Stress, Reliab. Des., 111, pp. 216–225) for pump impellers. The bulk-flow model is used to predict reaction force and moment coefficients for the impeller shroud. A labyrinth seal code developed by Childs and Scharrer ( 1986, ASME Trans. J. Eng. Gas Turbines Power, 108, pp. 325–331) is used to calculate the rotordynamic coefficients developed by the labyrinth seals in the compressor stage and also provides a boundary condition for the shroud calculations. Comparisons between the measured shroud moment coefficients by Yoshida et al. (1996, Proceedings of the 6th International Symposium on Transport Phenomena and Dynamics of Rotating Machinery, 2, pp. 151–160) and model predictions show reasonable agreements for the clearance flow and reaction moments. For the conditions considered, low Mach number flow existed in the shroud clearance areas and compressible-flow and incompressible-flow models produced similar predictions. Childs’ model predictions for the direct damping and cross-coupled stiffness coefficients of a pump impeller produced reasonable agreement; hence the present model was validated to the extent possible. A rotor model consisting of an overhung impeller stage supported by a nominally cantilevered rotor was analyzed for stability using the present bulk-flow model and an API standard Wachel–von Nimitz formula model (1981, J. Petrol. Technol., pp. 2252–2260). The bulk-flow model predicted significantly higher onset speeds of instability. Given that some compressors have been predicted to be comfortably stable using API standard Wachel–von Nimitz formula but have been unstable on the test stand, these results suggest that unidentified destabilizing forces and or moments are present in compressors. Seal rub conditions that arise from surge events and increase the seal clearances are simulated, showing that enlarged clearances increase the preswirl at the seals, thus increasing these seal’s destabilizing forces and reducing stability margins. These results are consistent with field experience. Predictions concerning the back shroud indicate that shunt-hole injection mainly acts to enhance stability by changing the flow field of the division wall or balance piston seals, not by influencing the back-shroud’s forces or moments. Effective swirl brakes at these seals also serve this purpose.

Rotordynamic Characterization of Labyrinth Seals in Steam Turbines: Effects of Thermal and Mechanical Loads

2018 ◽  
Author(s):  
Filippo Cangioli ◽  
Steven Chatterton ◽  
Paolo Pennacchi ◽  
Leonardo Nettis ◽  
Lorenzo Ciuchicchi ◽  
...  
Keyword(s):  
Flow Model ◽  
High Pressures ◽  
Labyrinth Seal ◽  
Bulk Flow ◽  
Critical Conditions ◽  
Accurate Estimation ◽  
Transfer Model ◽  
Steam Turbines ◽  
Labyrinth Seals ◽  
The Stability

Over the last few decades, the increasing demand on efficiency and performance for steam turbines has resulted in OEMs operating machines near critical conditions of their structural and thermal capabilities. Consequently, a more accurate estimation of the dynamic behavior of the machine has become mandatory as well as the stability assessment. Steam turbines are subjected to high temperatures, high pressures and centrifugal forces that could change the nominal geometry, especially the clearance profile in correspondence of the sealing components, occasionally generating a convergent or divergent annulus. In this paper, a new thermo-elasto bulk-flow model for labyrinth seals has been introduced. The model includes the bulk-flow model for estimating the dynamic coefficients, heat transfer model for evaluating the temperature distribution in the rotating and stationary parts and structural-mechanics model for calculating the radial growth. By considering a staggered labyrinth seal installed in the balancing drum of a steam turbine, different inlet pre-swirl ratios, as well as the stability of the seal are investigated in this paper. The model can be extremely useful for the dynamic characterisation of a wide class of labyrinth seals considering the effect of the surrounding environment on the rotordynamic coefficient prediction.

Measured Comparison of Leakage and Rotordynamic Characteristics for a Slanted-Tooth and a Straight-Tooth Labyrinth Seal

2013 ◽  
Vol 136 (1) ◽  
Author(s):  
Naitik J. Mehta ◽  
Dara W. Childs
Keyword(s):  
Inclination Angle ◽  
Flow Model ◽  
Control Volume ◽  
Labyrinth Seal ◽  
Bulk Flow ◽  
Inlet Pressure ◽  
Radial Clearance ◽  
Rotordynamic Coefficients ◽  
Seal Design ◽  
Number Of Teeth

Measured results are presented to compare rotordynamic coefficients and leakage of a slanted-tooth labyrinth seal and a straight-tooth labyrinth seal. Both seals had identical pitch, depth, and number of teeth. The teeth inclination angle of the teeth on the slanted-tooth labyrinth was 65 deg from the normal axis. Experiments were carried out at an inlet pressure of 70 bar-a (1015 psi-a), pressure ratios of 0.4, 0.5, and 0.6, rotor speeds of 10.2, 15.35, and 20.2 krpm, and a radial clearance of 0.2 mm (8 mils). One zero and two positive inlet preswirl ratios were used. The results show only minute difference in the rotordynamic coefficients between the two seals. The slanted-tooth labyrinth seal consistently leaked approximately 10% less at all conditions. Predictions were made using a one control volume bulk-flow model (1CVM) which was developed for a straight-tooth labyrinth seal design. 1CVM under-predicted the rotordynamic coefficients and the leakage.

On the Thermodynamic Process in the Bulk-Flow Model for the Estimation of the Dynamic Coefficients of Labyrinth Seals

2017 ◽  
Author(s):  
Filippo Cangioli ◽  
Paolo Pennacchi ◽  
Giuseppe Vannini ◽  
Lorenzo Ciuchicchi ◽  
Andrea Vania ◽  
...  
Keyword(s):  
Energy Equation ◽  
Flow Model ◽  
State Of The Art ◽  
Control Volume ◽  
Zero Order ◽  
The State ◽  
Bulk Flow ◽  
Thermodynamic Process ◽  
Labyrinth Seals ◽  
Stiffness Coefficients

The influence of sealing components on the stability of turbomachinery has become a key topic because oil and gas market is increasingly requiring high rotational speed and high efficiency, which implies the clearance reduction in the seals. The accurate prediction of the effective damping of the seals is critical to avoid instability issues. In recent years, “negative-swirl” swirl brakes have been employed to reverse the circumferential direction of inlet flow, changing the sign of the cross-coupled stiffness coefficients and generating stabilizing forces. Industries started to investigate, by experiments, the dynamical behavior of labyrinth seals. The experimental results of a 14 teeth-on-stator labyrinth seal with nitrogen, performed in the high-pressure seal test rig owned by GE Oil&Gas, are presented in the paper. Both experimental tests with positive and negative pre-swirl values were performed in order to investigate the pre-swirl effect on the cross-coupled stiffness coefficients. Concerning with the dynamic characterization of the seal, the fluid-structure interaction into the seal can be modelled by the bulk-flow numeric approach that is still more time efficient than computational fluid dynamics (CFD). Dealing with the one-control volume bulk-flow model, the thermodynamic process in the seal is considered isenthalpic, despite an expected enthalpy variation along the seal cavities, both for gas and steam applications. In this paper, the authors improve the state-of-the-art one-control volume bulk-flow model [1], by introducing the effect of the energy equation in the zero-order solution. In this way, the real gas properties are evaluated in a more accurate way because the enthalpy variation, expected through the seal cavities, is taken into account in the model. The authors, considering the energy equation only in the zero-order solution, assume that the enthalpy is not a function of the clearance perturbation (i.e. of the rotor perturbed motion). The energy equation links the continuity and the circumferential momentum equations. The density, in the leakage correlation, depends on the enthalpy, which is calculated (in the energy equation) on the basis of the circumferential velocity and of the fluid/rotor shear stress. Therefore, the leakage mass-flow rate and the fluid thermodynamic properties depend, indirectly, on the shear stresses. This fact is proved in the literature by several CFD simulations that investigate the leakage in the straight-through labyrinth seals, hence, the energy equation allows to better characterize the physics of the problem. Overall, by taking into account the energy equation, a better estimation of the coefficients in the case of negative pre-swirl ratio has been obtained (as it results from the comparison with the experimental benchmark tests). The numerical results are also compared to the state-of-the-art bulk-flow model developed by Thorat and Childs (2010), highlighting the improvement obtained.

Influence of Hole-Pattern Stator on Leakage Performance of Labyrinth Seals

2018 ◽  
Author(s):  
Xin Yan ◽  
Xinbo Dai ◽  
Kang Zhang ◽  
Jun Li ◽  
Kun He
Keyword(s):  
Experimental Tests ◽  
Labyrinth Seal ◽  
Leakage Rate ◽  
Control Performance ◽  
Labyrinth Seals ◽  
Rotordynamic Coefficients ◽  
Honeycomb Seal ◽  
Leakage Control ◽  
Stability Enhancement ◽  
The Cost

The honeycomb seal shows promising characteristics in many turbine machines for the leakage control and rotor stability enhancement. However, the cost of honeycomb seal is relative high due to its complexities in manufacture and installation process. The hole-pattern seal has a very close leakage and rotordynamic performance with honeycomb seal, and also the manufacture and installation of hole-pattern seal are easier than the honeycomb seal, which attract the researchers and designers in recent years. In the published literature, there have been many papers dealt with the rotordynamic coefficients measurements, but very few researchers concentrated on the leakage control performance for the labyrinth seal with hole-pattern land. In this paper, the experimental tests were carried out to obtain the leakage rates versus pressure ratios at four clearances for the straight-through labyrinth seal with smooth stator, straight-through labyrinth seal with hole-pattern stator, stepped labyrinth seal with smooth stator and the stepped labyrinth seal with hole-pattern stator. The flow fields in the seal chambers were also visualized at different clearances and pressure ratios. Moreover, the CFD tool was also implemented to predict the leakage performance in labyrinth seals, and the numerical results were compared with the measurements. The results show that, the stepped labyrinth seal with hole-pattern land performs better leakage control characteristic than the straight-through labyrinth seal with hole-pattern land, and the stepped labyrinth seal with smooth land performs better leakage control than the straight through labyrinth seal with smooth land. For the stepped labyrinth seal, holes in the stator increase the effective clearance thus increase the leakage rate in seal. However, the straight-through labyrinth seal with hole-pattern land almost has the identical leakage performance with the smooth configuration.

Rotordynamic Stability Predictions for Centrifugal Compressors Using a Bulk-Flow Model to Predict Impeller Shroud Force and Moment Coefficients

2006 ◽  
Author(s):  
Manoj K. Gupta ◽  
Dara W. Childs
Keyword(s):  
Flow Model ◽  
Reaction Force ◽  
Labyrinth Seal ◽  
Bulk Flow ◽  
Hole Injection ◽  
Pump Impeller ◽  
Stability Margins ◽  
Flow Models ◽  
Rotordynamic Coefficients ◽  
Model Predictions

An analysis is developed for a compressible bulk-flow model of the leakage path between a centrifugal-compressor impeller’s shroud and its housing along the impeller’s front and back sides. This development is an extension of analyses performed first by Childs [15] for pump impellers. The bulk-flow model is used to predict reaction force and moment coefficients for the impeller shroud. A labyrinth seal code developed by Childs and Scharrer [21] is used to calculate the rotordynamic coefficients developed by the labyrinth seals in the compressor stage and also provides a boundary condition for the shroud calculations. Comparisons between the measured shroud moment coefficients by Yoshida et al. [18] and model predictions show reasonable agreements for the clearance flow and reaction moments. For the conditions considered, low Mach number flow existed in the shroud clearance areas and compressible-flow and incompressible-flow models produced similar predictions. Childs’ model predictions for the direct damping and cross-coupled stiffness coefficients of a pump impeller produced reasonable agreement; hence the present model was validated to the extent possible. A rotor model consisting of an overhung impeller stage supported by a nominally cantilevered rotor was analyzed for stability using the present bulk-flow model and an API standard Wachel-formula model [10]. The bulk-flow model predicted significantly higher onset speeds of instability. Given that some compressors have been predicted to be comfortably stable using API standard Wachel-formula but have been unstable on the test stand, these results suggest that unidentified destabilizing forces and or moments are present in compressors. Seal rub conditions that arise from surge events and increase the seal clearances are simulated, showing that enlarged clearances increase the preswirl at the seals, thus increasing these seal’s destabilizing forces and reducing stability margins. These results are consistent with field experience. Predictions concerning the back shroud indicate that shunt-hole injection mainly acts to enhance stability by changing the flow field of the division wall or balance piston seals, not by influencing the back-shroud’s forces or moments. Effective swirl brakes at these seals also serves this purpose.

Analysis of Labyrinth Seal Flow Patterns to Improve Bulk Flow Code Predictions

2019 ◽  
Author(s):  
Nathaniel Gibbons ◽  
Cori Watson-Kassa ◽  
Christopher Goyne ◽  
Houston Wood
Keyword(s):  
Film Thickness ◽  
Flow Behavior ◽  
Labyrinth Seal ◽  
Operating Conditions ◽  
Bulk Flow ◽  
Governing Equations ◽  
Labyrinth Seals ◽  
Cross Sectional ◽  
Rotordynamic Coefficients ◽  
Pressure Profiles

Abstract Non-contacting annular seals are frequently used in turbomachinery to reduce leakage of a fluid through a section with a large pressure differential. A typical type of non-contacting seal is the labyrinth seal, where circumferential grooves are cut into the rotor, stator or both. Using a tortuous path, labyrinth seals reduce leakage by dissipating the fluid’s kinetic energy through viscous forces caused by the formation of vortices in each seal groove. Due to a lower cost when compared to experimental measurements, bulk flow codes are frequently used for predicting seal contributions to rotordynamic performance. Existing seal codes use constant or linear values for the fluid film thickness at different seal sections and display inaccuracies in their prediction of velocity and pressure profiles and rotordynamic coefficients for labyrinth seals when compared to experimental data. The primary objective of this study is to determine the effect of implementing an effective film thickness into the governing bulk flow equations on the code prediction of axial velocity and pressure profiles. Simulations were run using ANSYS CFX with cross-sectional models of individual seal grooves. Seal parameters, including inlet circumferential velocity and rotor speed, were varied to better understand the behavior of the film thickness under various operating conditions. Streamlines were used to determine the maximum film thickness and an effective film thickness profile that can be used in the modified bulk flow code. Modified governing equations were developed, and predictions for the axial profiles resulting from the modified code solutions for the zeroth order governing equations are compared to CFD results and previous code predictions for improved accuracy. Preliminary results for a set of cases indicate far higher accuracy when an effective film thickness is used and represent the first results from a seal bulk flow code that implements a nonlinear effective film thickness. Improvement in code prediction of flow behavior across the seal, and subsequently in the seal codes accurate prediction of rotordynamic coefficients, allows for the design of more efficient and effective seals and machine systems.

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