Leaf Seal Research Articles

Preliminary Wear Investigations With Pressure Actuated Leaf Seals for Superheated Steam Turbine Application

Abstract Pressure actuated leaf seals (PALS) are a promising technology for the modern load-follow operation of power plants and turbines. Their strength lies in their radial adaptivity, which enables a significantly smaller radial gap height than conventional labyrinth seals. In this paper, which is a continuation of ISUAAAT16-022 from 2022, the operating behavior with compressed air and shaft rotation up to 3000 rpm is investigated, including an analysis of the wear caused by frictional contact between the shaft and the seal leaves. This includes both concentric and eccentric contact behavior. The test seal with a nominal diameter of 300 mm is designed in such a way that it can be used in a similar design in subsequent industry-related superheated steam tests. In addition to the seal performance before, during, and after the wear-in process, the frictional torques, temperatures in the flow and on the shaft-contacting leaves as well as the material removal on leaves and shaft running surface are analyzed. The results show a basically robust operating behavior of the seal with moderate performance deterioration, which can be directly attributed to the increase in the clear gap. However, the shaft wear is higher than for other contact seals, so further investigations are necessary before use in practice is feasible.

Dynamic Characterization of an Adaptive Film-Riding Seal

Abstract Shaft seals control the leakage of fluid between areas of high pressure and low pressure around rotating components inside turbomachinery. Static seals are often subject to damaging rubs with the shaft, caused by assembly misalignments and rotordynamic vibrations during operation. Adaptive seals aim to reduce leakage flows whilst minimizing wear. The film riding pressure actuated leaf seal (FRPALS) is one such design which utilizes a large installation clearance and is blown down toward the shaft under pressure. This paper presents a numerical model which can be used in the design and development of adaptive shaft seals, validated by experimental data from the literature. The model uses a modified version of the Reynolds equation to predict the dynamic, frequency-dependent stiffness and damping coefficients of the fluid film. The dynamic coefficients have been solved for different operational clearances and pressure differences to generate coefficient maps. These maps have been incorporated into a blow down model with compliant mechanical leaves to predict the transient translational and angular displacement paths of the FRPALS when subject to an increasing pressure drop. The blow down model has been compared against experimental measurements collected from a specially designed test facility for the characterization of shaft seal performance. Eddy current probes were used to measure the displacement paths of the FRPALS with the experimental values showing that the model can accurately predict the dynamic movement of the seal when subject to a pressure difference.

Rotordynamic Behavior of Leaf Seals: Measurement of Frequency-Dependent Force Coefficients for Varying Inlet Parameters

Abstract While brush seals can be found in various applications for turbomachines today, leaf seals are a further development in compliant seal technology and have a lower level of maturity. Among the purported advantages are greater axial rigidity when subject to higher pressure differences and the potential for noncontacting operation due to lift-up. However, especially their rotordynamic behavior is little investigated in the literature so far. In this paper, measured rotordynamic force coefficients of a leaf seal are presented for varying inlet pressures, preswirl velocities, and excitation frequencies. The leaf pack of the tested leaf seal has zero rotor cold clearance, and its coverplates are designed for facilitating a lift-up effect when pressurizing the seal. Experiments were performed on a dynamic test rig with whirling rotor using active magnetic bearing (AMB) technology and evaluated in the frequency domain based on the impedance method. Test results for the leaf seal reveal positive direct stiffness and an advantageous rotordynamic behavior due to significant levels of direct damping and negative cross-coupled stiffness throughout the operating parameter range. Leaf seal results are compared to brush and labyrinth seal data from previous studies for varying inlet pressures and preswirl velocities. Additional computational fluid dynamics (CFD) simulations were carried out to predict the leaf deflection moment, which supports the findings regarding hydrostatic lift-up from the experimental results.

Analysis of Leaf Seal Leakage Performance Under the Influence of Manufacturing Variations Using Experiment and Computational Fluid Dynamics

Abstract In this paper, test data are combined with results from two different computational fluid dynamics (CFD) models to investigate the leakage performance of leaf seals. Experimental data are gathered for centric rotor position using a rotating test rig at various rotational speeds, inlet pressures, and preswirl velocities. The test results are compared to brush and labyrinth seal leakage data from previous studies and reveal elevated leakage rates of the leaf seal. As the tested leaf seals are subject to thermal leaf deformation from welding during the manufacturing process, the influence of geometry variations within the leaf pack on leakage performance is investigated with the help of numerical simulations. Both a fully resolved leaf model and a modeling approach based on porous media are used. The CFD models are validated based on pressure measurements within the up- and downstream coverplate gaps at three different radii. Both CFD models show good agreement with test data for different inlet parameters. A variation of cold clearance shows moderate influence on leakage and small clearances can be brought into context with hydrodynamic lift-up indicated by experimental leakage data. Much higher sensitivity on leakage mass flow is predicted for variations in leaf spacing at the leaf root and leaf tip. The latter is discussed as an explanation for the measured leakage of the test seal with its manufacturing variations, while the first quantitatively shows optimization potential at the design stage of leaf seals.

Computational Fluid Dynamics and Thermal Analysis of Leaf Seals for Aero-engine Application

Aero-engine gas turbine performance and efficiency can be improved through the application of compliant shaft seal types to certain sealing locations within the secondary air system. Leaf seals offer better performance than traditional labyrinth seals, giving lower leakage flows at design duties. However, for aero-engine applications, seal designs must be able to cope with relatively large off-design seal closures and closure uncertainties. The two-way coupling between temperatures of seal components and seal closures, through the frictional heat generated at the leaf–rotor interface when in contact, represents an important challenge for leaf seal analysis and design. This coupling can lead to leaf wear and loss, rotor overheating, and possibly to unstable sealing system behavior (thermal runaway). In this paper, we use computational fluid dynamics (CFD), finite element (FE) thermal analysis, and experimental data to characterize the thermal behavior of leaf seals. This sets the basis for a study of the coupled thermomechanical behavior. CFD is used to understand the fluid-mechanics of a leaf pack. The leaf seal tested at the Oxford Osney Laboratory is used for the study. Simulations for four seal axial Reynolds number are conducted; for each value of the Reynolds number, leaf tip-rotor contact, and clearance are considered. Distribution of mass flow within the leaf pack, distribution of heat transfer coefficient (HTC) at the leaf surface, and swirl velocity pick-up across the pack predicted using CFD are discussed. The experimental data obtained from the Oxford rig is used to develop a set of thermal boundary conditions for the leaf pack. An FE thermal model of the rig is devised, informed by the aforementioned CFD study. Four experiments are simulated; thermal boundary conditions are calibrated to match the predicted metal temperatures to those measured on the rig. A sensitivity analysis of the rotor temperature predictions to the heat transfer assumptions is carried out. The calibrated set of thermal boundary conditions is shown to accurately predict the measured rotor temperatures.

Analytical Modeling and Experimental Validation of Heating at the Leaf Seal/Rotor Interface

The secondary air system of a modern gas or steam turbine is configured to satisfy a number of requirements, such as to purge cavities and maintain a sufficient flow of cooling air to key engine components, for a minimum penalty on engine cycle efficiency and specific fuel consumption. Advanced sealing technologies, such as brush seals and leaf seals, are designed to maintain pressures in cavities adjacent to rotating shafts. They offer significant reductions in secondary air parasitic leakage flows over the legacy sealing technology, the labyrinth seal. The leaf seal comprises a series of stacked sheet elements which are inclined relative to the radial direction, offering increased axial rigidity, reduced radial stiffness, and good leakage performance. Investigations into leaf seal mechanical and flow performance have been conducted by previous researchers. However, limited understanding of the thermal behavior of contacting leaf seals under sustained shaft contact has led to the development of an analytical model in this study, which can be used to predict the power split between the leaf and rotor from predicted temperature rises during operation. This enables the effects of seal and rotor thermal growth and, therefore, implications on seal endurance and rotor mechanical integrity to be quantified. Consideration is given to the heat transfer coefficient in the leaf pack. A dimensional analysis of the leaf seal problem using the method of extended dimensions is presented, yielding the expected form of the relationship between seal frictional power generation, leakage mass flow rate, and rotor temperature rise. An analytical model is derived which is in agreement. Using the derived leaf temperature distribution formula, the theoretical leaf tip temperature rise and temperature distributions are computed over a range of mass flow rates and frictional heat values. Experimental data were collected in high-speed tests of a leaf seal prototype using the Engine Seal Test Facility at Oxford University. These data were used to populate the analytical model and collapsed well to confirm the expected linear relationship. In this form, the thermal characteristic can be used with predictions of mass flow rate and frictional power generated to estimate the leaf tip and rotor temperature rise in engine operation.

Experimental Characterization of Rotor Convective Heat Transfer Coefficients in the Vicinity of a Leaf Seal

Leaf seals are filament seals for use at static to rotating interfaces in the engine secondary air system. They offer reduced leakage rates and better off-design performance over conventional labyrinth seals. If compared with advanced brush seals, leaf seals are more compliant due to their lower stiffness and can withstand higher axial pressure differences. Although leaf seals can exhibit hydrodynamic air-riding, this is not always the case and seal–rotor contact can occur. As a result, friction between the leaf tips and the rotor causes heat generation and wear. To predict the diameter of the rotating shaft and the seal life, the shaft and seal interface temperature needs to be estimated. In the steady state, this is determined by the ratio of convective heat transfer through the seal to that through the shaft. To that end, the convective heat transfer characteristics of the flow over the shaft around the seal are required to build accurate thermal models. In this paper, the convective heat transfer coefficient (HTC) distribution in the close vicinity of a typical leaf seal is investigated in a new test facility. The experimental setup and test method are described in detail, and accuracy considerations are included. The methodology employed to derive HTC is explained with reference to an analogous computational fluid dynamics (CFD) model. The importance of the choice of an appropriate driving gas temperature is demonstrated. Experimental HTC maps are presented for a blow-down seal geometry operating over a range of engine representative pressure ratios. Insight is gained into the flow field characteristics and heat transfer around the seal.

High-Speed Characterization of a Prototype Leaf Seal on an Advanced Seal Test Facility

Advanced contacting seals, such as leaf seals or brush seals, can offer reduced leakage during engine operation when compared to conventional labyrinth seals. The flexible elements of these seals provide better compliance with the rotor during flight maneuvers. The functionality and performance retention attributes of an engine-scale prototype leaf seal have been investigated on a seal test facility at Rolls-Royce that achieves engine-representative pressures and speeds and allows dynamic control of the seal position relative to the rotor, both concentric and eccentric. In this paper, the experimental setup and the test method are described in detail, including the quantification of the measurement uncertainty developed to ASME standard PTC 19.1. Experimental data are presented that show the variations in leakage and torque over typical variations of the test parameters. Insight is gained into the interactions between the operating pressure and speed and the concentric and eccentric movements imposed on the seal.

PALS - An Auspicious New Gas Turbine Seal

This article highlights various aspects of a new gas turbine shaft seal called the pressure activated leaf seal (PALS). The paper in detail discusses the architecture and working of a gas turbine seal. PALS is designed to use changing pressure drop forces across the seal to eliminate rub. The seal elements stay clear of the rotor seal surface during start-up and shutdown transients, and subsequently close to a small, non-contacting, steady-state running clearance. During start-up or shutdown, when the axial pressure difference across PALS is small, the leaves are in a relaxed open position, providing a general clearance gap for possible rotor seal surface eccentricities. At operating speeds, the resulting axial pressure difference causes the leaf element to elastically deflect and close, reaching the design clearance when they contact the support member. The test results show that the PALS concept provides for a potentially viable, robust, low leakage seal for gas turbine applications.

Experimental Investigation of a Leaf Seal Prototype at Engine-Representative Speeds and Pressures

The application of compliant filament seals to jet engine secondary air systems has been shown to yield significant improvements in specific fuel consumption and improved emissions. One such technology, the leaf seal, provides comparable leakage performance to the brush seal but offers higher axial rigidity, significantly reduced radial stiffness, and improved compliance with the rotor. Investigations were carried out on the Engine Seal Test Facility at the University of Oxford into the behavior of a leaf seal prototype at high running speeds. The effects of pressure, speed, and cover plate geometry on leakage and torque are quantified. Earlier publications on leaf seals showed that air-riding at the contact interface might be achieved. Results are presented which appear to confirm that air-riding is taking place. Consideration is given to a possible mechanism for torque reduction at high rotational speeds.

Improved Understanding of Stiffness in Leaf-Type Filament Seals

Filament seals, such as brush seals and leaf seals, are investigated as a potential improved seal for gas turbine applications. As these seals operate in contact with the rotor, a good understanding of their stiffness is required in order to minimize seal wear and degradation. This paper demonstrates that the filament and complete seal stiffness is affected in comparable magnitudes by mechanical and aerodynamic forces. In certain cases, the aerodynamic forces can also lead to an overall negative seal stiffness which may affect stable seal operation. In negative stiffness, the displacement of the seal or rotor into an eccentric position causes a resultant force, which, rather than restoring the rotor to a central position, acts to amplify its displacement. Insight into the forces acting on the seal filaments is gained by investigating a leaf seal, which consists of a pack of thin planar leaves arranged around the rotor, with coverplates on either side of the leaf pack, offset from the pack surfaces. The leaf seal is chosen due to its geometry being more suitable for analysis compared to alternative filament seals such as the brush seal. Data from two experimental campaigns are presented which show a seal exhibiting negative stiffness and a seal exhibiting a stiffness reduction due to aerodynamic effects. An empirical model for the forces acting on leaf filaments is developed based on the experimental data, which allows separation of mechanical and aerodynamic forces. In addition a numerical model is developed to analyze the flow approaching the leaf pack and the interleaf flow, which gives an insight into the causes of the aerodynamic forces. Using the empirical and numerical models together, a full picture of the forces affecting leaf stiffness is created. Validation of the models is achieved by successfully predicting seal stiffness for a further data set across the full range of operating conditions. The understanding of aerodynamic forces and their impact on filament and seal stiffness allows for their consideration in leaf seal design. A qualitative assessment of how they may be used to improve seal operation in filament seals is given.

Numerical simulation and performance analysis of leaf seal

In order to obtain the result of pressure and velocity distribution and leakage of the leaf seal system which is applied to high-speed rotor,a mathematical model of sealing zone flow field calculated numerically with SIMPLER method and finite difference method was established. A software based on the algorithm referred was also designed specially for the performance analysis of leaf seal system. The influence of different structure parameters and working conditions on the system's performance was discussed. Analysis result shows that the leakage increases linearly with the sealing pressure and decreases slightly with the increase of rotor speed. Increasing width,reducing spacing between front / back cover plate to rotor and decreasing length are beneficial in decreasing the leakage of the system. While changing installation angle direction of the leaf has little influence on leakage. Research on dynamic response between leaf and rotor will be carried out based on the results of the flow field.

Pressure Activated Leaf Seal Technology Readiness Testing

This paper continues the evaluation of pressure actuated leaf seals (PALSs) technology readiness for shaft and shroud sealing in power generation and aerospace applications. Seal designs tested are prototypical and constructed using processes appropriate for volume production. Results include both static and dynamic seal leakage measurements running against a 5.1 in. (130 mm) diameter smooth surface test rotor and another that simulates sealing against turbine blade shrouds. A further test was undertaken using a two-dimensional (2D) static rig that determined acoustic noise experienced during testing was attributed to leaves vibrating at their natural frequency as a result of interleaf gaps. The dynamic simulated shroud test includes steps, duplicating small discontinuities of adjacent shroud sealing surfaces and slots to inject air radially under the seal leaves as may occur between shrouds on blades with a high degree of reaction. Consistent seal performance over 15 h confirms suitability for turbine blade tip applications. Controlled deflection of PALS leaves with operating differential pressure is effective for startup rub avoidance in service as well as conformal wear-in sizing of leaf tips with the rotor. Tested leaf tip wear-in of approximately 0.010 in. (0.25 mm) against rotor disks without hard-face coating shows potential to eliminate seal misalignment and run-out contributions to operating seal clearance. PALS design features prevent further rubbing contact with the operating rotor after initial wear-in sizing, thereby sustaining a small effective seal clearance and prospects for long seal life. Measurements of rotor surface wear tracks from the wear-in process and endurance runs are included as well as rotor and leaf tip photos. Test results support the technology readiness of the PALS concept as a viable, robust, low leakage dynamic seal for select commercial application.

Design approach for maximising contacting filament seal performance retention

Good sealing is a key requirement for modern efficient turbomachinery such as steam and gas turbines. A class of seals that promise better performance, compared to conventional labyrinth seals, are contacting filament seals such as brush, leaf, or finger seal. When new, these filament seals offer better performance; however, if poorly designed they wear excessively, resulting in leakages higher than a comparable labyrinth seal. This paper outlines a design methodology for selecting ideal contacting filament seal properties for a given operating cycle or set of operating cycles. Following this approach ensures the seal performs well, the seal retains its performance, and performance is retained if the operating cycle is altered. In the approach, the seals are described by four generic properties (stiffness, blow-down, cross-coupling, and build clearance), which are then used for a performance evaluation based on a number of test cycles. Once the ideal seal properties for a given operating cycle have been identified, a seal to match these can be designed. The approach is evaluated with a generic gas turbine cycle and recommendations for ideal contacting filament seal properties for this cycle are made.

Simulation Investigation on Application of Leaf Seal in Turbine Stage

Simulation Investigation on Application of Leaf Seal in Turbine Stage

G051015 スパイラル溝を用いた高速回転体の風損低減機構の開発([G051-01]流体工学部門,一般セッション : 流体力学の機器への応用)

Recently, it is required to reduce windage loss of rotors such as motors for cars as they are used at higher and higher speed and the windage loss becomes larger. At first, this paper shows that large diameter spiral grooves can reduce a pressure in the housing quickly and largely, and also shows that the gap between spiral grooves and rotor must be quite narrow such as less than 10 μm for the high-performance depressurization function. Next, this paper proposes "leaf spiral grooves pump (LSGP)" to keep the gap quite narrow even under the condition of severe vibration and large change of the temperature which often cause the unsteady gap. LSGP is formed by multi layered flexible leaves like a leaf seal. Three-dimensional numerical analysis shows that LSGP can reduce the pressure properly.

Comparative Analysis of Seal Structure and Aerodynamic Characteristics of Labyrinth and Leaf Seals

The structure feature,aerodynamic characteristics,sealing mechanisms and sealing effect of labyrinth seal and leaf seal are numerically studied under same condition,the difference of two kinds seals is compared.The reliability of commercial computational fluid dynamics computational code CFX is validated by using the experimental data from literature.The performance curves of computation has a good agreement with the experimental data,the numerical simulation method is feasible.Numerical simulation on the flow field of the labyrinth seal and leaf seal are carried out respectively by using the commercial software CFX.Through comparison analysis based on the calculation results of the two type seals,their sealing mechanisms is described,the sealing characteristics and the friction torque variation law of their own are given.The analysis results show that,the radial clearance of leaf seal is much smaller than the labyrinth seal,its leakage flow rate is about 1/3 labyrinth seal's,and the friction resistance torque on the rotation axis is lower than the labyrinth seal.Because of the superiority of the leaf seals in structure operation characteristics and aerodynamic performance,leaf seals may replace labyrinth seals in the near future.

Leaf seal

Leaf seal

Leaf seal

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Dresser-Rand to obtain seal technology from CMG Tech, LLC

Dresser-Rand Group Inc. has announced the signing of a technology license agreement with CMG Tech, LLC to produce low-leakage, pressure-activated leaf seals (PALS).