Compliant Foil Research Articles

Thermal-Gas-Elastic Coupling Modeling and Performance Analysis of Foil Cylindrical gas Film Seal Considering Speed Slip, Temperature Jump and Shaft Misalignment

Abstract The Compliant foil cylindrical gas film seal is an advanced sealing technology designed for high-speed and high-temperature conditions in a jet engine. Its design involves using a compliant foil to create a cylindrical sealing surface, achieving adaptive sealing in environments with high pressure differences and rotational speeds. For high-speed and high-temperature operation, a speed slip flow model was established by considering film slip flow, wall temperature jump, foil deformation, and gas viscosity-temperature thermal effects. The variations in the lubrication performance of the foil cylindrical gas film seal with changes in shaft misalignment directions, misalignment angles, and operating parameters were investigated. The speed slip leads to a decrease in gas film lift and an increase in leakage. The wall temperature jump phenomenon caused by speed slip leads to a 2.56% decrease in film temperature. The shaft misalignment angles on the high-pressure and low-pressure sides have different impacts on the flow field, with film pressure and temperature differing by 66.78% and 11.19%, respectively.

Transient elasto-aerodynamically coupling behavior and performance analysis of bump-type compliant foil face gas seal at ultra-high speeds

Transient elasto-aerodynamically coupling behavior and performance analysis of bump-type compliant foil face gas seal at ultra-high speeds

Study on Sealing Characteristics of Compliant Foil Face Gas Seal under Typical Hypervelocity Gas Effects

In order to investigate the influence of typical hypervelocity gas effects on the gas lubrication performance of compliant foil face gas seal (CFFGS) end surfaces, the gas–elastic coupling lubrication theory model of CFFGS is modified by considering the choked flow and inertia effect, and the lubrication performance is solved using the finite difference method. Based on the choking effect, the effect of hypervelocity choked flow on the pressure field and velocity field of the seal is analyzed, and the influence of operating parameters on the choked flow and the mechanism of choked flow on the change in dynamic lubrication and sealing performance are explored. Furthermore, based on the inertia effect, the effect of gas inertia force on the flow field, and the correlation law between the pressure field and velocity field under the influence of operating parameters are studied. Then, the relationship between the inertia effect and sealing performance are analyzed. The results show that choked flow increases sealing outlet pressure significantly, from 0.1 MPa to a maximum of 14.25 MPa, and the sealing outlet flow velocity decreases by up to 50 times. The increase in medium pressure and balance film thickness aggravate the choking effect, resulting in a 20% maximum increase in opening force and a 99.6% maximum decrease in leakage rate. In addition, the inertia effect causes obvious centrifugal movement of the gas flow. As result, the radial flow velocity reduces by up to 50%, and the pressure distribution varies widely. Especially under high rotational speed and high medium pressure, the inertia effect is enhanced to clearly reduce the opening force (max. decrement of 3.5%) and leakage rate (max. decrement of 23%).

Numerical modelling of compliant foil structure in gas foil bearings: Comparison of four top foil models with and without radial injection

In the literature, there exists no consensus on a best practice for modelling the compliant foil structure of Gas Foil Bearings (GFBs). This paper focuses on the top foil modelling and its original contribution to the modelling problem is a comparison of steady state analysis efficiency, and transient and steady state accuracy between four top foil models with and without gas injection. The bump foil is modelled using the Simple Elastic Foundation Model (SEFM), and the four top foil models are (a) neglecting the top foil, i.e. the foil structure is modelled as a basic SEFM, (b) Euler–Bernoulli (EB) beam elements, (c) non-curved shell elements, and (d) curved shell elements. The theoretical analysis is carried out using a multi-domain numerical model and exemplified using a rigid rotor supported by three-pad GFBs. Comparing steady state journal eccentricities, low discrepancies are seen between almost all investigated models. However, the basic SEFM is found to be insufficient for cases with injection. Using the curved model as a benchmark, it is found that the predictions of steady state journal eccentricities, transient response, and eigenvalues made by the model with non-curved shell elements are less accurate than those of the model with EB beam elements. Thus, despite a higher degree of simplification, the model with EB beam elements is found to be both more computationally efficient as well as more accurate for steady state analysis when compared to the model with non-curved shell elements. While the radial foil deformation for the curved model exhibits dependency on the axial coordinate, this dependency is significantly overestimated by the model using non-curved shell elements. This is most pronounced for the case without injection, and it could explain the inferiority of the model using non-curved shell elements for estimating steady state journal eccentricities. A stress analysis indicates that this is caused by the non-curved shell element model not accounting for the membrane stresses in the top foil.

Research on the operation mechanism and performance of bump-type compliant foil face gas seal

Research on the operation mechanism and performance of bump-type compliant foil face gas seal

The Seal Performance of Compliant Foil Gas Seal Based on Multi-Scale Analysis

Compliant foil gas seal is a non-contact and high-efficiency sealing technology. The dynamic performance of compliant foil gas seal with different structure parameters was analyzed in this paper. These parameters include the seal diameter, gas film thickness and the ratio of groove. Compared with the rigid film, the advantage of compliant film is analyzed. The stability performance and dynamic performance with the different structures are obtained. The results show that the larger diameter is a disadvantage for the stability performance. However, the increase of seal length can decrease the leakage. Otherwise, the increase of gas thickness improves the dynamic characteristics with the leakage rising and gas force dramatically decreasing. While the groove length ratio is around 0.6, the seal performance and dynamic characteristics are best. The compliant structure benefits the improvement of the sealing performance.

Flow field and sealing performance analysis of compliant foil face gas seal

Based on the theory of gas lubrication and elastic mechanics, a theoretical model of gas-elastic coupling lubrication of compliant foil face gas seal (CFFGS) is presented. The correlation between the film pressure and thickness of CFFGS and its effect on the flow field are investigated, and the differential effect of pressure conditions on the flow field and sealing performance is analyzed. Also explored the influence law of operating and structural parameters on sealing performance. Results show that CFFGS have stable and excellent leakage control performance, and the pressure forms has a differential effect on the coordination mechanism of film pressure-deformation-film thickness and the sealing performance. Moreover, compliant foil structure has a positive impact on the end-face adaptive capability and leakage control performance of the seal.

Performance of the Compliant Foil Gas Seal with Surface Micro-Textured Top Foil

In various fields, micro-textures have been successfully applied to the surface of friction pairs to effectively improve flow field and friction performance. This paper aims to investigate how different textures affect the sealing performance of compliant foil gas film seals. In theoretical simulations, a facile method for characterizing the shape of micro-textures is proposed, and the equilibrium relationship between the gas film pressure, gas film thickness, and foil deformation is established. The transient Reynolds equation considering the eccentric convergence problem and abrupt Rayleigh step changes is solved to analyze the static and dynamic characteristics. The results show that (i) compared with the directionality of the texture, the gas volume accommodated by the texture has a greater impact on the sealing performance, and a convergent texture can effectively control the leakage rate; (ii) when the texture depth exceeds 9 μm, the sealing system may be unstable; (iii) the compliant foil seal is well suited to higher-speed service conditions, and the inverted triangular texture shows the best comprehensive sealing performance.

Design guidelines for bump-type compliant conical foil bearing

The integral bump foil strip cannot optimize the performance for the compliant conical foil bearing (CFB) as the uneven distribution of structural stiffness. To maximize the bearing characteristics, this paper proposed different bump foil schemes. Firstly, the anisotropy of CFB was studied based on the nonlinear bump stiffness model, and the circumferentially separated foil structure was proposed. Moreover, an axially separated bump foil structure with the variable bump length was introduced to make the axial stiffness distribution more compliant with the gas pressure. In addition, the effect of foil thickness was also discussed. The results show that CFB with integral bump foil exhibits obvious anisotropy, and the suggested installation angle for largest load capacity and best dynamic stability are in the opposite position. Fortunately, a circumferential separated bump foil can improve this defect. The characteristics of CFB with axial separated foil structure can be improved significantly, especially for that with more strips and the variable bump half-length design. The suitable bump and top foil thickness should be set considering the improved supporting performance and proper flexibility. The results can give some guidelines for the design of CFB.

Performance improvement of foil air journal bearing employing micro-pocket and textures on top compliant surface

Investigations to reduce the friction and improve the dynamic performance of a self-acting air foil journal bearing have been performed and presented herein employing the texture, micro-groove and micro-pocket on the top foil. Significant improvements in the static and dynamic performances have been found in presence of micro-pocketed/grooved and textured top foils in comparison to conventional plain foil. The micro-pocketed top foil yields better performance as compared to textured and conventional cases. In the presence of micro-pocket on top compliant foil, the bearing yielded a significant reduction in friction coefficient varying in the range of 8%–11% as compared to conventional one. Moreover, better dynamic stability has also been found even at lightly loaded conditions in the presence of texture and micro-pocket.

Challenges in Validating a Thermo-Hydrodynamic Gas Foil Bearing Model

Abstract Gas foil bearings (GFBs) are suitable for high speed and temperature applications where conventional lubricated bearing solution are not feasible. This requires the prediction of bearing temperatures and thus a thermal model considering the heat generation and heat flow paths in the bearing. The effects of two different bump foil stiffness (Iordanoff, I., 1999, “Analysis of an Aerodynamic Compliant Foil Thrust Bearing: Method for a Rapid Design,” ASME J. Tribol., 121(4), pp. 816–822; Le Lez, S., Arghir, M., and Frene, J., 2007, “A New Bump-Type Foil Bearing Structure Analytical Model,” ASME J. Eng. Gas Turbines Power, 129(4), pp. 1047–1057.) and heat transfer models (a simplified and a detailed one) are presented in respect to measured temperatures from literature (Radil, K., and Zeszotek, M., 2004, “An Experimental Investigation Into the Temperature Profile of a Compliant Foil Air Bearing,” Tribol. Trans., 47(4), pp. 470–479; Sim, K., and Kim, T. H., 2012, “Thermohydrodynamic Analysis of Bump-Type Gas Foil Bearings Using Bump Thermal Contact and Inlet Flow Mixing Models,” Tribol. Int., 48, pp. 137–148). The comparison is drawn over a wide range of operational conditions as well as measuring positions, which in such detail has not been shown before. While good agreement is found for some of the conditions and positions, only reasonable agreement is found for others. The deviations and difficulties in validating a thermal model against experiments are highlighted in a discussion about various temperature influencing parameters, especially concerning the change of clearance during operation. In conclusion, it is found that the models are able to predict temperatures reasonably well, but require delicate fine-tuning to achieve these results. Finally, the impact of temperature distribution on the maximum load capacity is evaluated by comparing predictions between an isothermal model and one including thermal effects.

Development of a simplified mathematical model for a foil gas bearing operation

A theoretical study of a gas flow process in a clearance, changing during operation between a compliant foil of a foil gas bearing and a rotor of a high-speed turbomachine, is presented. The approaches to mathematic simulation of the position of the compliant foil of the gas bearing, deforming under the influence of gas pressure on the outer surface and a system of distributed springs on the inner surface, are developed. An example of numerical simulation of the foil gas bearing in order to obtain an elastic line of the compliant foil under the influence of gas forces, acting in a wedge-shaped clearance, is considered.

The effect of sensory feedback topology on the entrainment of a neural oscillator with a compliant foil for swimming systems

The sensorimotor system of fish endows them with remarkable swimming performance that is unmatched by current underwater robotic vehicles. To close the gap between the capabilities of fish and the capabilities of underwater vehicles engineers are investigating how fish swim. In particular, engineers are exploring the sensorimotor systems of fish that control the motion of fins. It is generally accepted that specialized neural circuits (known as central pattern generators) within the sensorimotor system produce the periodic drive signal that is used to control the motion of fins. An important aspect of these circuits is that their output signal can be modified by sensory feedback. Specifically, the way in which sensory feedback signals are applied to a CPG (i.e. the sensory feedback topology) affects the CPG’s entrainment characteristics. This has been shown in simulation but has not been investigated in a robot interacting in the real-world. Furthermore, CPG-based control has only limitedly been applied to fish like robots and many questions remain as to how it should be applied to these types of systems. In this work we examine the effect of sensory feedback topology on the entrainment characteristics of a CPG-based neural oscillator driving three different foils swimming in flow. Additionally, we investigate how sensory feedback should be acquired from a foil and applied to a neural oscillator to promote beneficial swimming characteristics.

Numerical Evaluation of Rotordynamic Coefficients for Compliant Foil Gas Seal

Compliant foil gas seal is one of the advanced cylindrical gas seal technologies and can be commonly used in the secondary flow system of an aero-engine. It can enhance the dynamic stability of the aero-engine by meeting the steady requirements of the aero-engine seal system. To evaluate the performance of compliant foil gas seal, the steady performance of the gas seal is firstly analyzed to predict the sealing efficiency and obtain the pressure distribution of the gas seal in the compressible flow field. Then, the effects of the operating parameters on the rotordynamic coefficients are analyzed using the finite differential method. It can be used to predict the operation performance of the aero-engine and prepare for the optimization and test rig of compliant foil gas seal on the T-shaped groove.

Experimental study of the aircraft air cycle machine rotor dynamics on gas foil bearings

In this paper the results of the experimental research of the dynamics of the rotor of a four-wheeled aircraft air cycle machine on gas foil supports under conditions of vibration. The description of the experimental unit, measuring system, and methods of results processing is given. The rotor of the turbomachine is mounted on compliant gas foil journal bearings of the second generation. The rotor speed varied from 20000 to 32000 rpm. The cooling turbine was exposed to broadband random vibration with a mean square value of Vibro-acceleration up to 5 g. The influence of vibration load level, air supply temperature, amplitude-frequency characteristics of the turbine cooler case, and vibration equipment on the rotor dynamics was assessed.

On the Design, Manufacture, and Premature Failure of a Metal Mesh Foil Thrust Bearing—How Concepts That Work on Paper, Actually Do Not

Oil-free microturbomachinery (OFT) implements compliant foil bearings because of their minute drag and ability to operate in extreme (high or low) temperature. Prominent to date, bump-foil thrust bearings integrate an underspring thin metal structure that provides resilience and material damping, and while the rotor is airborne, it acts in series with the stiffness and damping of the gas film. The design and manufacturing of foil bearings remain costly as it demands extensive engineering and actual experience. Alternative foil bearing configurations, less costly and easier to manufacture, are highly desirable to enable widespread usage of OFT. This paper details the design and manufacturing of a novel Rayleigh-step metal mesh foil thrust bearing (MMFTB) as well as its testing on a dedicated rig. Metal mesh structures offer significant material structural damping and can be easily procured at a fraction of the cost of a typical bump-foil strip layer. The MMFTB consists of a solid carrier, a number of stacked annular copper mesh sheets (wire diameter = 0.25, 0.3, and 0.41 mm), and a steel top foil (0.127 mm thick) that makes six pads (ID = 50.8 mm, OD =2 ID), each 45 deg in extent. A 3 μm polymer coats each pad, and a photochemical process etches a step 20 μm in height. Static and dynamic load measurements (without rotor speed) demonstrate that the MMFTB has structural stiffness and material damping similar to that of a publicized bump-type foil thrust bearing. A maiden test of the MMFTB with rotor speed of Ω = 15 krpm (∼80 m/s at bearing outer diameter (OD)) proved briefly the bearing operation when applying a tiny thrust load. Further tests with ambient air, a rotor speed of 40 krpm (∼212 m/s at bearing OD), and a very light load/area <7 kPa failed several of the prototype bearings, all exhibiting significant wear on one or more pads. The source of the failure is the inherent unevenness of the metal mesh stacked substructures, thus causing the pads to bulge toward the rotor collar surface before a load applies. A deficient anchoring method exacerbates the unevenness. Incidentally, a high rotor speed induced large windage that lifted the top foils pushing them against the spinning collar. As the bearing moved toward the rotating collar to begin applying thrust, the local high spots rubbed against the collar before a hydrodynamic wedge could form to separate the surfaces. Without a robust sacrificial coating, metal-to-metal contact quickly disfigured the contacting top foil pads, erasing the etched step, and leading to failure. In concept, and on paper, the mesh sheets and the top foil lay flat against the bearing carrier, giving a false sense of uniformity in the design process. In actuality, a designer must consider the manufactured states of the individual components and how they assemble. A redesign of the bearing intends to overcome the existing flaws (highlighted herein) by incorporating a thicker top foil that is well anchored (to better withstand the effects of windage), a robust sacrificial coating, and a hydrodynamic wedge accomplished via a circumferential taper on each pad.

The Wing Foil: A Novel Compliant Radial Foil Bearing Design

To meet the challenging demands for high performance, affordable compliant foil bearings (CFBs), a novel compliant support element has been developed. This recently patented, novel support element uses a multidimensional array of multiple, formed, cantilever “wing foil” tabs. The wing foil bearing has all the features required to achieve state-of-the-art performance (Gen III for radial bearings). This paper describes two radial foil bearings using the wing foil and the unique design features. Test data for a 31.75 mm diameter bearing operating in air and in steam up to 42 krpm are presented to demonstrate the performance of this bearing. It is shown to have low subsynchronous vibration and reasonable damping through rigid shaft critical speeds.

Active bump-type foil bearing with controllable mechanical preloads

Abstract This paper presents an active bump-type foil bearing (ABFB), in which mechanical preloads can be actively adjusted by controlling the driving voltages of piezoelectric actuators (PZTs) to create a variable multiple-lobe clearance and enhance bearing performance. A theoretical model that couples active and compliant foil structures is proposed, and static load–deformation tests and measurements of airborne drag torque on the basis of a prototype bearing are conducted at different driving voltages. Tests results show that the proposed active structure is feasible and effective in controlling mechanical preload. Moreover, the static and dynamic performances of ABFB are discussed. This work lays foundation for the further study of active control methods in gas foil bearings.

Computational Analysis of Foil Air Journal Bearings Using a Runtime-Efficient Segmented Foil Model

This contribution is concerned with the development and the implementation of a foil air journal bearing model. For this purpose, the numerical procedure solving the Reynolds equation for compressible fluids has to be coupled to a compliant foil model. The presented beam-based approach is supposed to reproduce most of the experimentally known particularities in the mechanical behavior of the foil structure, while being at least as runtime-efficient as the commonly used simple elastic foundation model. The developed modeling approach will be validated by comparing simulation results to data found with a more complex reference model. In the analysis part, most notably, the top foil compliance is shown to deteriorate the load-carrying capacity of air bearings. Moreover, the influence of the top foil compliance on the dynamics of a rigid rotor supported by two foil air journal bearings will be discussed.

On the manufacturing of compliant foil bearings

The fabrication of foil bearings is challenging due to the dependence on sheet metal forming to produce the compliant structure. This paper is an attempt to shed light into the foil bearing manufacturing know-how. Design of experiments techniques are used to quantify the effects of the different manufacturing parameters, as well as defining an optimum manufacturing procedure. The effect of manufacturing noise on the static performance of foil bearings is quantified using a Monte Carlo simulation of a bump foil stiffness model. A non-intrusive optical measurement technique which has been developed to measure the formed bump foils is also presented. An uncertainty quantification was performed for the produced foils, showing large uncertainty in the bump dimensions, which significantly affect both nominal bearing clearance and compliance. Finite element simulations are used to model the bump foil forming process that would present potential problems during fabrication, suggesting sharp bends in the bump foil as the main driver for manufacturing deviations. Based on this outcome an improved design for the compliant bump foil with reduced curvature is proposed, manufactured and measured. The novel design allows to reduce springback error by 69% compared to classical bump foils and thus offers an equivalent yet more robust foil bearing design.