Research Status and Development Trend of Cylindrical Gas Film Seals for Aeroengines

Abstract

High-performance aeroengine design is an important component of modern industry, and advanced sealing technology is a key technology to meet the engine fuel consumption rate, thrust-to-weight ratio, pollutant emission, durability, and lifetime. Reducing the internal airflow leakage of the engine through a sealing technology can improve the performance and efficiency of the engine. In this paper, the typical sealing technology for an aeroengine is introduced in more detail, including the structural characteristics and use limitations of the labyrinth seal, brush seal, honeycomb seal, gas film face seal, and cylindrical gas film seal. It focuses on the development history, typical structure type, working principle, basic technology research method, steady-state performance, dynamic characteristics, multi-physical field coupling, structural deformation, experimental testing, processing technology. Finally, it summarizes the problems and future development trends of the current application of the cylindrical gas film seal in aeroengines, and points out that the seal performance test and evaluation based on advanced composite sensor technology and the innovative design of the seal based on new material, a new principle, and a new structure will be the new research direction.

1. Introduction

In an aeroengine, combustible gases are burned at high temperatures and pressures, generating energy that drives the turbine to rotate. The sealing structure is a key condition essential to maintaining normal engine rotation. The aeroengine is the “heart” of the aircraft, directly affecting the service life of the aircraft and operational reliability, its structural innovation and technological development can be regarded as a measure of the level of a country’s basic industry, military capability, and the overall national strength of the important indicators of the status of major power and the strategic guarantee of national security. Dynamic sealing technology for the aeroengine has become the basic key technology that affects the performance of the whole aeroengine [1,2,3].

For an aeroengine high-speed compliant rotor sealing system, in addition to the typical characteristics of an ordinary fluid rotating machinery rotor system, it also needs to adapt to the static “three highs” and dynamic vibration displacement deformation and other harsh conditions [4,5]. The static “three highs” refer to the high interfacial slip velocity (250 m/s), high ambient temperature (650 °C), and high differential seal boundary pressure (5 MPa) on the sealing sub-surface. Specifically, the high interface slip speed and high ambient temperature are more favorable to the formation of a dynamic pressure sealing gas film, the actual application of the gas film thickness is generally about 10 μm or even thinner, this thin gas film can be adapted to the high boundary pressure difference, can still maintain a small leakage, and at the same time avoid high frictional heat and wear and tear, reducing the aircraft fuel consumption to improve the performance of the whole aircraft and yielding a beneficial effect. Compared with the static “three high” conditions, the dynamic vibration conditions of the aeroengine have a more prominent impact on the sealing performance. Compared with other ground equipment, the structure of the aeroengine is more prone to deformation under the harsh conditions of large vibration, and the frequent start–stop and speed changes will cause large rotor displacement. The existing studies show that the radial displacement is 0.2~0.5 mm, and the axial displacement can be up to 0.125 mm or even more, which is far more than the average gas film thickness. Therefore, the study of the sealing technology mechanism for an aeroengine, mastering the design and manufacture of sealing products with excellent performance, the research and development of a new generation of aeroengine technology, and the strategic development of our country are of great significance [6,7,8,9,10]. Sealing materials and processing accuracy greatly affect the development of advanced aeroengine sealing technology. Aeroengines are facing increasingly complex working environments, and the range of working conditions is gradually expanding, which puts forward higher requirements for the performance and service life of engine sealing structures.

Dynamic sealing technology is one of the key technologies of an aeroengine, which has an important impact on the overall performance of the engine. As shown in Figure 1, there are dozens of fluid dynamic seals on a modern aeroengine, mainly including the main flow channel seals, gas system secondary flow seals, main bearing oil cavity seals, and transmission accessory output shaft seals in the accessory transmission magazine. Figure 2 illustrates the classification of different fluid seals. From the concept of less leakage and better control of secondary flow, high performance seals offer significant opportunities to improve engine efficiency.

Research on the aeroengine sealing mechanism, mastering the design and manufacture of sealing products with excellent performance, the research and development of a new generation of aeroengine technology, strategic development are of great significance. Aeroengine development technology has been developed to a very high level, however, in aerodynamic performance, heat transfer and other aspects have been difficult to achieve a major breakthrough, but with some improvements in certain structural design, the components and engine efficiency can be greatly improved. Modern aeroengine technology has been highly developed, and to further improve the aeroengine impeller efficiency, the most important thing is to improve the sealing effect between the rotor and the magazine of the impeller machine. With the military engine service environment becoming increasingly harsh, civilian engines with low energy consumption, low noise, and high efficiency and other aspects of the requirements continue to improve, a NASA study pointed out that improving the performance of the sealing device reduces the amount of leakage, to ensure that, in the worse service environment, the service life of the engine is extended and the rate of fuel consumption is reduced. The cylindrical gas film seal structure has a large compliant or floating characteristic, this compliant component can actively bear the dynamic displacement, to ensure the necessary gas film thickness, to avoid the sealing sub-surface contact friction and wear. Applying the cylindrical gas film sealing technology to the aeroengine, it is found that not only the sealing effect is improved, but also the thrust of the engine is increased by 2~2.5%, and the fuel consumption rate is reduced by 1~2%, which makes this sealing form of better value in saving energy and improving stability.

This paper summarizes and analyzes the current research and development status of cylindrical gas film seals for aeroengines from the aspects of the structure type and working principle of the cylindrical gas film seal, the development history of a typical cylindrical gas film seal structure, the research method of the basic technology of the cylindrical gas film seal, the stability and dynamic performance of a gas film seal, and experimental validation of a gas film seal, etc. It also points out the main problems in the current research of cylindrical gas film seal and further research directions and development trends. As a new type of high-efficiency sealing form, the cylindrical gas film seal should not only be innovative in theory, but also carry out in-depth research on structural design and material selection.

2. Comparison of the Characteristics of Different Gas Sealing Forms

At this stage, researchers have summarized and analyzed the different forms of seals used in rotating machinery such as aircraft engines, gas turbines, compressors, turbines, etc. The following are several widely used forms of seals, as shown in Figure 3, and the scope of application and limitations of each form of seal are further elaborated.

2.1. Labyrinth Seal

Labyrinth seal [11,12,13,14,15] has a non-contact sealing structure, as shown in Figure 4. It mainly consists of a series of sealing tooth gaps and expansion cavities, and its sealing efficiency depends on the radial gap between the rotor parts and static parts and the number of grate teeth. Labyrinth sealing technology has been widely used in the sealing devices of various types of steam engines, gas turbines, and aviation engines due to the existence of a certain sealing gap between the rotor shaft and the shaft bore, and the sealing gap provides enough space for thermal expansion, which can be adapted to a variety of extreme and complex working conditions. In aviation engines, it is widely used to block fluid leakage between high- and low-pressure chambers. When the unit is running, aviation gas flows from the high-pressure side to the low-pressure side, when the gas flows through the throttle gap, the process of fluid flow is approximated to the process of adiabatic expansion, the pressure decreases, the flow rate increases, and the pressure energy is converted into kinetic energy; when the gas flows into the sealing cavity through the throttle gap, the space of gas flow suddenly becomes larger, the gas in the sealed cavity can form a strong vortex, and the speed is almost reduced to zero, which is similar to the isobaric expansion process of the gas, part of the kinetic energy of the gas is converted into internal energy, and the other part Is converted into the vortex energy, so as to realize the dissipation of the gas energy [16]. However, in practice, the labyrinth seal needs to design a large number of sealing teeth to form an expansion cavity, thus increasing the design and installation space, in the design of the gap between the sealing teeth and the rotor, if the gap design is too large this will lead to a sharp rise in leakage, with sealing effect deterioration. Similarly, if the gap design is too small, although there is effective control of leakage, with the high-speed operation of the rotor and seal chamber temperature and pressure changes, the rotor and sealing teeth undergo serious scraping and abrasion, a resonance phenomenon [17,18,19]. This friction can lead to permanent deformation of the seal teeth, reduce the service life of the seal, and seriously lead to seal failure, jeopardizing the safety of the whole machine system [20,21,22,23,24,25,26].

2.2. Brush Seal

Brush seal is a new type of contact zero clearance dynamic seal with highly efficient damping performance, as shown in Figure 5. This structure is mainly composed of a brush filament, front plate, back plate, and runway, which has a very wide range of applications in aerospace, shipbuilding, electric power, petrochemical, metallurgy, and other fields [14,27,28,29,30,31,32]. Brush wire is sandwiched between the front plate and the back plate, and then the welding process is to weld the three into one, the front plate is located on the high-pressure side to play the role of a sandwiched brush wire, the back plate is located on the low-pressure side of the role of supporting the brush wire, to withstand the role of the differential pressure. However, the structure of the brush seal is more complex, the brush substrate manufacturing process is more cumbersome, the assembly of high precision requirements (back plate gap should be maintained in any operating conditions and radial deviation, the back plate cannot be in contact with the surface of the rotor), and the use of brush special materials relies on imports to limit the development of its application. The working pressure of a single-stage brush seal generally does not exceed 0.5 Mpa, and several single-stage brush seals can be connected in series to form a multi-stage brush seal structure, which improves the pressure resistance of the whole machine to a certain extent. When the unit is running at high speed, the rotor and brush filament will produce friction, and the constant friction will cause the brush filament to accumulate a large amount of frictional heat; when the rotor undergoes radial offset, the friction and wear between the brush filament and the rotor will be more intense, resulting in the chattering of the brush filament, which will reduce the stability of the seal [33,34,35,36,37,38,39,40]. With the increasing temperature of the brush tip, when the maximum temperature is close to the melting point of the brush, the metal mechanical properties of the material in the brush tip area decrease, resulting in a sharp increase in leakage and shortening of the service life, which directly restricts the application of brush seals in shaft-end seals and the interstage seals of large-scale rotating machinery [41,42,43].

According to the current stage of research based on the brush seal, the future of the brush seal should focus on the following aspects:

(1)

In the flow, friction, deformation of the brush seal, in addition to the study of the leakage flow characteristics and mechanical properties of the brush seal, should also be considered in a comprehensive manner, such as the deformation of the brush filament and other fluid–solid coupling effects.

(2)

In terms of the friction heat effect and heat transfer characteristics of the brush seal, it is necessary to study the brush seal in depth using experimental measurements and numerical simulations, to find out the influence laws of the geometrical structural parameters and effect of the operating conditions on the friction heat effect and heat transfer characteristics, and to improve the structure of the brush seal to avoid, as far as possible, the high temperature of the brush wire of the brush seal when it is in operation.

(3)

In the study of dynamic characteristics of the brush seal rotor, based on the experimental study of the dynamic characteristics of the brush seal rotor, develop a fast and accurate numerical prediction method for evaluating the dynamic characteristics of the brush seal rotor.

(4)

In the structural design of the brush seal, based on the leakage control mechanism of the brush seal, the friction and heat transfer mechanism, and the contact force and the deformation mechanism of the brush filaments, reduce the unfavorable effects of the brush filament abrasion, friction, and heat effects between the brush filaments and the rotor and the hysteresis effects on the sealing performance through the improvement of the structure and form of the brush seal.

2.3. Honeycomb Seal

Honeycomb seal [44] is an advanced sealing technology developed by the aerospace industry for the special working conditions of high temperature and high-pressure difference to stop the leakage of mechanical lubrication medium; the U.S. space shuttle, U2 fighter aircraft, and so on use honeycomb seals, and China’s civilian engines and new fighter aircraft have increased their average efficiency by 10% after adopting honeycomb sealing technology. As shown in Figure 6, the honeycomb seal mainly consists of a sealing body and a honeycomb band, and the honeycomb band consists of small hexagonal honeycomb holes [45]; the hexagonal honeycomb holes are generally in the range of 0.8–6 mm from opposite sides, and the depth of the honeycomb is generally in the range of 1.6–6 mm. Due to the traditional comb seal in the installation to ensure that there is a certain gap, when running in over critical speed conditions, the sealing teeth will intensify the wear and tear of the sealing gap in a short period and these will increase rapidly, the leakage is also increased, while the structural strength of the honeycomb seal is better, and at the same time has good abrasion resistance, in the use as a seal, the shaft end of the seal can be used for a long time to maintain a small sealing gap, to ensure that the sealing of the shaft end of the performance is stable. Honeycomb seals can absorb fluid kinetic energy to produce damping in operation, thus suppressing the self-excited vibration of the gas and reducing the sub-synchronous vibration [46]. With the increase of rotational speed, although the sealing material has good abrasion resistance, it is very easy to form collision friction with the high-speed rotation of the rotor shaft, and the leakage increases, and in serious cases, sparks will be generated, and there are certain safety hazards [47,48].

2.4. Gas Film Face Seal

Gas film face seal [49,50,51,52,53,54] is a new type of non-contact seal that uses one or more pairs of dynamic and static sealing rings with preload and fluid area together with a dynamic pressure sealing structure, as shown in Figure 7. The gas film face sealing structure is mainly composed of a spring, a card ring, a spring seat, an O ring, a push ring, a dynamic ring seat, a static ring, and a dynamic ring, etc., and the end face of the dynamic ring is open to a dynamic pressure groove of different shapes. The rotary axis rotates, and the pumping and fluid dynamic pressure effect of the dynamic pressure groove makes the medium gas continuously pressurized, and the pressure drop triggered by the step effect between the non-groove area and the groove area makes the pressure maximized in the groove root diameter so that a layer of gas film is formed on the end face of the movable and static ring to maintain a non-contact state [55]. There are many types of dynamic compression grooves, such as spiral grooves, T-shaped grooves, V-shaped grooves, tree-shaped grooves, and various types of bionic grooves, among which the classical spiral grooves are the most widely used, and their structure schematic diagrams are shown in Figure 8. Gas film face seal technology has undergone more in-depth research at this stage, especially the dynamic pressure, static pressure gas film face sealing technology is relatively mature, and there has been a wide range of successful application experienced on the ground equipment [56,57,58].

According to the aforementioned content, in all kinds of aeroengine dynamic seal, the labyrinth seal is the most widely used, but the labyrinth seal and the rotor shaft have a gap between the labyrinth seal and the radial movement of the rotor shaft surface under the influence of the labyrinth teeth that is very easy to wear so that the leakage increases, which limits the prospects for the application of the labyrinth seal; brush seal can keep the leakage lower than for the labyrinth seal, but there are contact friction heat, brush wear, breakage of the brush filament seal, etc., and the cost is expensive, compared to the form of these seals, the end of the face of the gas film seals have the advantages of small abrasion, leakage is small, there is low energy consumption, and long service life and so on [59,60,61]. It is considered to be one of the most promising types of dynamic seals in aeroengines and is expected to replace labyrinth seals or brush seals in specific areas of aeroengines. However, with the deepening of the research, in the harsh working conditions of the aircraft engine, the many efforts of the gas film face seal research, due to the rotor system vibration and thermal deformation of the rotor system being too large, or contact wear, leakage, and failure to work, the conventional gas film face seal is difficult to adapt to the end of the face runout and other issues, and at the same time, the structure of the gas film face seals is more complex, increasing the cost of equipment production and post maintenance. The gas film face seal does not perform well in response to unit displacement and vibration, and the large axial and radial displacement runout causes dry friction of the dynamic and static rings, resulting in frictional heating, severe wear of the sealing surface, and even the “shaft-holding” phenomenon, which restricts the application of its application in aeroengine rotor systems [62]. Although the suction face seal technology and foil gas film seal technology have unique technical advantages, there have been few public reports on their practical application in aeroengines so far.

To summarize, for the increasingly harsh working conditions of petrochemical equipment and the growing service environment of high-parameter rotating machinery, the following Figure 9 shows the history of different gas seal forms.

According to the aforementioned research basis of each fluid dynamic sealing technology, for coping with the large vibration displacement of the aeroengine, only the brush seal shows good flexibility and has been successfully applied, but the brush seal belongs to the contact seal, and it is difficult to form a hydrodynamic gas film, and there is inherent frictional wear of the metal and vibration of the brush filament. With the continuous development of aeroengine technology, the requirements for service conditions are also more demanding, at this stage, the existing fluid sealing technology cannot meet the aeroengine sealing performance requirements in some extreme conditions, so researchers in the 1990s designed a variety of different structures of the cylindrical gas film seal; the cylindrical gas film sealing structure of the aeroengine axial displacement of the changes in the aeroengine has good adaptability, and therefore has received a lot of attention. Aeroengine rotor in-flow system and rotor leaf tip at the seal in the working process are not subject to rotor support reaction force, a cylindrical gas film seal can provide better compliant floating support. In the aeroengine rotor, intense vibration and thermal deformation can still maintain the gas film lubrication, and in overcoming the vibration of the large displacement has an excellent potential for application. Structures such as the compliant foil cylindrical seal and the thin leaf plate seal (the specific structure and working principle are described in detail in Section 3) have passed the tests of the simulation tester and the gas turbine on the ground, which demonstrates that the large flexibility of the cylindrical foil seal structure ensures the formation of hydrodynamic lubricating gas film at the journal, avoiding the direct contact of metal, thus reducing the risk of collision and frictional wear. This sealing structure still has its shortcomings, but its better compliant adaptability and large displacement structure make it show excellent potential in overcoming obstacles such as large vibrations under the harsh conditions of aeroengines.

3. Cylindrical Gas Film Seal Structure Type and Working Principle

3.1. Structural Characteristics and Groove Structure of Cylindrical Gas Film Seal

Cylindrical gas film sealing technology is based on the principle of dynamic and static pressure of gases, through the relative rotation between the seals to produce a layer of micron-sized dynamic pressure fluid film, to achieve the sealing and lubrication effect. In this process, the non-contact state of the sealing interface is ensured by the extremely thin fluid film, which is characterized by low leakage, long life, and high stability. As shown in Figure 10 [63], the mechanical device of the cylindrical gas film seal mainly includes a seal support, a floating ring, a seal cavity, a rotating shaft, an end cap, a positioning pin, a shaft sleeve, and a compression spring. The rotating shaft is mounted on the bearing corresponding to the support, and the sleeve and the rotating shaft form an interference fit, so that the sleeve can rotate synchronously with the rotating shaft, and the surface of the sleeve is provided with a groove that can enhance the dynamic pressure effect of the fluid. Common forms of dynamic pressure grooves include various spiral grooves, rectangular grooves, trapezoidal grooves, T-type grooves, and the like, as shown in Figure 11 [64,65]. The floating ring is mounted on the sleeve and installed inside the sealing cavity, maintaining a certain clearance with the sleeve. One end of the floating ring is connected with the end cap by a positioning pin to realize positioning coordination, which on the one hand can play a role in restricting the circumferential rotation of the floating ring, to keep the floating ring in a relatively stable position in the circumferential direction. On the other hand, the positioning pin can determine the radial movement displacement of the floating ring, ensuring the stability of the floating ring in the operation process, ensuring that no friction and collision occur between the sealing chamber and the shaft sleeve, and prolonging the service life. This can effectively prevent the leakage of high-pressure medium gas to the low-pressure side of the shaft end through the seal cavity and reduce the risk of secondary leakage.

3.2. Working Principle of Cylindrical Gas Film Seal

As shown in Figure 12, according to the theory of gas lubrication [66], the working principles of non-contact gas film seals can be roughly categorized into three types, i.e., dynamic seal, hydrostatic seal, and extrusion seal. The working principle of the cylindrical gas film seal is similar to that of a gas radial bearing, which belongs to the first category of gas film sealing principle. When the bushing is rotating at high speed, the gas on the high-pressure side will be fed into the groove through the microgroove pump. The gas is continuously compressed and produces gas film pressure, in the process, the dynamic pressure effect is gradually enhanced, and a micron level dynamic pressure gas film is formed between the floating ring and the bush to realize the seal of the main leakage channel. At the same time, due to the eccentric installation between the sleeve and the floating ring, i.e., the centerline of the floating ring and the centerline of the sleeve have a certain eccentric distance e (as shown in Figure 13), the thickness of the gas film in the convergent wedge-shaped space formed between the floating ring and the sleeve is inconsistent, so that the gas film in the sealing gap is a wedge-shaped convergence of the distribution of the wedge-shaped effect, as shown in Figure 12a. The class of the principle of the gas film seal to enhance the seal of the dynamic effect of the seal significantly increases the buoyant force of the gas film. On the other hand, a very thin film of dynamic pressure gas formed between the floating ring and the shaft sleeve separates the floating ring from the shaft sleeve, resulting in a reduction of frictional wear on the solid surfaces in relative motion and ensuring that the temperature rise during operation does not change significantly.

4. Overview of the Development of a Typical Cylindrical Gas Film Seal Structure

Adapting to the sealing requirements under the special working conditions of the compliant rotor system of the aeroengine, the research of the cylindrical gas film seal is based on the proposal of a new type of sealing system structure form, and two basic points are mainly considered in this process:

(1)

From the overall system structure, through the design of the relevant elastic parts or floating parts, so that the system can have enough compliant support, this compliant support structure can absorb the vibration displacement of the rotor system, through the system’s being adaptive to ensure that there is enough gas film thickness, so that the gas film sealing maintains a non-contact state.

(2)

The structural form design should be fully considered, with an adaptive large compliant support structure form that should be easy to quantitatively design and undergo structural analysis.

4.1. Compliant Shaft Cylindrical Gas Film Seal Structure

For the structural design and research of the cylindrical gas film seal, the earliest can be traced back to the end of the last century. In 1992, NASA Laboratory [67,68] put forward a new concept of the cylindrical gas film seal structural form that can be applied to the sealing of the shaft end of the aeroengine—Compliant Shaft Seal (CSS), whose structural schematic diagram is shown in Figure 14 [69], which is the prototype of the structural form of the cylindrical micro-groove gas film seal. As a typical form of elastic floating seal, its main working principle is when the shaft is running steadily, due to the role of fluid dynamic pressure effect, resulting in a thin layer of fluid film, sealing the element metal sheet in the fluid film to realize the elasticity of floating, which can effectively reduce the static parts and rotating parts of the sealing vice of the friction between the scraping, touching abrasion, wear and tear. At the same time, this new type of seal structure design is more compact, can be realized by compliant support to control the seal shaft having a certain radial displacement (radial displacement within 0.381 mm) [70,71,72].

This structure eliminates the radial displacement of the shaft through the elastic design and provides an important guiding direction for the subsequent structural design of other types of cylindrical gas film seals. In recent years, several sealing technology patents have been disclosed [73,74,75,76] drawing on the form of compliant shaft sealing, the use of a metal sheet, or thin-walled beam structure to increase the flexibility of the sealing structure, combined with the structural form of the fingertip seals, labyrinth seals, the formation of different types of sealing structure that can be adapted to large deformations, but this type of structure, due to the quantitative design being more difficult, only stays in the theoretical stage of design, in addition to the relevant patents, and has not been seen in the experimental results of the public research reports.

4.2. Compliant Foil Cylindrical Gas Film Seal Structures

Compliant foil seal structure first appeared at the beginning of this century, in 2000, Salehi and Heshmat et al. [77,78,79] of Mohawk Innovative Technology (MIT) in the United States, based on the research foundation related to the compliant foil gas radial bearings, for the first time, proposed a sealing structure that can be reliably operated under the condition of large rotor deflections—Compliant Foil Seal (CFS). As shown in Figure 15, the compliant foil seal structure primarily includes a smooth compliant top foil, a compliant raised waveform foil with stiffness, and standoffs. The resilient top foil is in the form of a thin ring, and subsequent studies have found that it is also possible to provide micro-molding on the sealing surface in contact with the medium, with the flat foil at the low-pressure side end being free, and with the flat foil at the high-pressure side end having extensions curved in the direction of the outer diameter, and with the curved extensions extending partly outside of the sealing cavity and being clamped between the sealing end cap and the sealing cavity. The bump foil has a concave–convex corrugated shape and is sandwiched between the flat foil and the sealing chamber to limit its movement in the radial direction, and the annular corrugated foil can be secured either free at both ends or at one end with the sealing chamber or the back side of the flat foil.

When the compliant foil seal is in normal operation, the rotor rotates, the fluid film with the rotor rotation occurs in a synchronized circular motion, and due to the rotor eccentric installation, media pressure difference, fluid viscosity, and sealing face deformation and other factors, the friction between the vice will form a stiffness of the microscale wedge-shaped fluid film. This layer of rigidity of the film will keep the rotating shaft and the sealing surface of the sealing surface from each other to realize the non-contact lubrication sealing; the waveform foil acts as a compliant support structure to offset the sealing journal of the vibration displacement.

The specific structure of the foil sealing surface Is shown In Figure 16. The top of the compliant foil sealing waveform foil is in mutual contact with the bottom of the supporting flat foil, and when a pressure load is applied to the surface of the flat foil, the flat foil undergoes a displacement change, which will be transferred to the waveform foil, extruding the action of the waveform foil so that it also undergoes an elastic deformation. The bottom of the waveform foil is in contact with the sealing chamber, and the flat foil and the waveform foil can slip slightly from their respective fixed ends to their free ends.

The sealing surface of the combination of an elastic waveform foil and supporting flat foil undergoes adaptive deformation in response to environmental changes, as shown in Figure 17. This adaptive deformation makes the sealing structure have better inclusiveness to radial displacement, and can spontaneously adjust the sealing gap to ensure the effective reset of the equilibrium position, to improve the stability of the shaft system dynamics.

Salehi et al. [77,80] carried out system simulation calculations and bench tests on the compliant foil seal structure and brush seal structure, respectively, and the results showed that the leakage rate of the compliant foil seal structure was much smaller than that of the brush seal structure, and there was no obvious friction and wear phenomenon between the sealing sub-surfaces, realizing the non-contact surface of the cylindrical gas film sealing sub-surfaces. Then Salehi [81] applied the metal waveform foil to the compliant foil seal structure, and studied the influence of rotational speed and differential pressure on the sealing performance under the working condition of 649 °C. The test results show that the leakage of the compliant foil seal structure increases with the increase of differential pressure, but it is still within a reasonable range, which to a certain extent proves that the compliant foil seal can be applied to the working conditions with large differential pressure. NASA Glenn Research Center [82] in the United States, based on the research of foil gas radial bearing technology announced by MIT, conducted relevant high-speed performance tests on a compliant foil gas seal structure with a diameter of 215.9 mm at room temperature, and after the test was completed under the operating parameters of a rotational speed of 30,000 r/min and differential pressure of 0.103 Mpa, the removal of the seals revealed that severe wear of the moving ring and a large number of shaft cracks in the rotor coating were detected using a fluorescent anti-seepage agent, but the exact cause of the accident was not given in the report.

Munson et al. [83] proposed the compliant foil face seal (CFFS) structure for the first time, which can be used in large aerospace, new hydrogen centrifuges, compressors, etc. Heshmat et al. [84] conducted simulation tests on the compliant foil face seal (CFFS) structure and evaluated its performance under various test conditions, which showed that this structure has an excellent performance in controlling leakage with a low leakage rate; and then, it was simulated and evaluated under various test conditions. CFFS structure was simulated and evaluated under various test conditions, the test results show that the compliant foil face seal (CFFS) structure outperforms the brush seal in overall performance under severe conditions of up to 1000 F, up to 145 kPa differential pressure, and surface velocities of up to 425 m/s, and that the structure accommodates large-scale system thermo-mechanical excursions in all three dimensions. Finally, static sealing performance tests show that increasing radial path length and axial preload can effectively reduce leakage flow, and the report also describes high-temperature seal design techniques using KorolonTM coatings and high-temperature superalloy substrates at temperatures of up to 800 °C. Munson et al. [85] summarized and described the current state of development of hybrid designs of foil gas thrust bearings and conventional face seals, and the results indicated that, although compliant foil seals exhibit excellent resistance to deformation compared to conventional face seals, leakage requirements are sacrificed by the compliant foil seals for this purpose.

In recent years, some researchers have compliant foil sealing system installed in a small turbine engine simulator for performance tests, through the test study of compliant foil seal leakage, measurement found that, when the temperature is 20 °C, the differential pressure ratio is 6, the shaft diameter is φ72 mm, the rotor speed is 40,000 rpm, the measured leakage of the compliant foil seal for the brush seal leakage is about 30%. No serious contact friction and obvious wear phenomena were observed during the operation of the compliant foil seal [70,86], and this research result has not been tested by the harsh working conditions such as the actual large vibration displacement of aerospace gas turbines.

In terms of theoretical studies, in 2000, Salehi M et al. [87] carried out a thermal and fluid flow analysis of a compliant foil seal structure, and the results showed that most of the heat in the fluid flow process is transferred by heat conduction, and the use of a modified Couette flow approximation for the working fluid gas is more helpful in analyzing the temperature changes. Subsequently, Salehi M et al. [88] established a thin-film pressure control equation considering turbulence effects and analyzed the sealing performance of a compliant foil seal under high-speed operating conditions using a continuous over-relaxation iterative method, and the relaxation factor was found to be the key to the convergence of the pressure control equation during the solution process. Kim et al. [89] combined the perturbation method of the floating ring seal with the finite element method of the compliant foil seal to analyze the characteristic law, and found that, with the increase of the thickness of the waveform foil, the eccentricity decreases while the direct stiffness and damping coefficients increase. Lee et al. [90] designed a floating ring seal structure, which is shown in Figure 18. The structure uses an arch foil support ring, and the eddy current stability experiments proved that the structure remains stable at high speeds.

In 2015, Sashidhar et al. [91] detailed the machining of waveform foils and flat foils with single-stage and three-stage machining of the top foil parts in a compliant foil sealing device, and fabricated a prototype seal with one layer of flat foil and two layers of flat foil, respectively, as shown in Figure 19. It is proved by a test that the machining process and machining accuracy of the top foil will have a significant effect on the sealing performance, and then the variation relationship between the design parameters and the sealing performance is analyzed, and the result points out that the radial clearance is an important factor affecting the sealing work. In addition, the extended bending part of the top flat foil is prone to large friction, which is prone to wear leading to shaft damage. At the same time, it is pointed out that the biggest problem in the design of a compliant foil sealing structure is that the parameters of the compliant elastic support structure of the waveform cannot be quantitatively designed according to the actual working conditions, which leads to an inability to accurately predict its sealing performance, and therefore it has not been widely popularized and applied.

In recent years, the design and theoretical research of compliant-supported gas film sealing structures have gradually received more attention from domestic scholars. Zhejiang University of Technology [92] staggered the honeycomb sealing unit in the sealing cavity, and in this way designed a cylindrical sealing structure that can autonomously regulate the sealing gap, as shown in Figure 20. Kunming University of Science and Technology [93,94], to improve the impact resistance of the gas film seal and enhance the opening force of the gas film, put forward the use of metal bubbles and a segmented fixed arch foil support of the compliant foil seal structure and so on, as shown in Figure 20, respectively.

In 2020, Wang X et al. [95,96] obtained the pressure distribution in the compressible flow field of a compliant foil cylindrical gas film seal and analyzed the influence law of seal structure and operating parameters on seal performance and dynamic parameters, and the comparison found that the introduction of the compliant structure is conducive to the improvement of the overall sealing performance, and the effect is best at the slot-length ratio of 0.6, these findings can be used to predict the operational performance of aeroengines. In the same year, Kunming University of Science and Technology [97,98,99] used ANSYS software to carry out the thermal coupling analysis of the flexure system and found that the thermal coupling role played a dominant role. They also used this method to analyze the bubble-type supported cylindrical seal, optimizing it to obtain the preferred range of bubble radius of 0.9–1.5 mm and the thickness of floating ring of 0.6–1.0 mm. Chen Shuda [100] used a combination of the finite element method and a Newton–Raphson iterative method for the sealing of high-pressure cylinders of steam turbines, using high-temperature and high-pressure water vapor as the sealing medium, to analyze the effects of arch foil thickness, eccentricity, etc., on the static characteristics of the compliant foil seal. Wang Xueliang et al. [101] analyzed the sealing characteristics of compliant foil cylindrical gas film seals with different surface roughness values, and tested the influence of surface roughness values on the start–stop cycling performance of high-speed compliant foil seals, and the results showed that the improvement of interfacial surface properties can effectively attenuate the hysteresis effect of the seal. Sun Junfeng et al. [102] proposed a new compliant support foil gas film seal structure that can absorb vibration and reduce thermal deformation of the rotor system, as shown in Figure 21. Jie Xu et al. [103,104] used a wave foil stiffness model considering the Coulomb friction effect, applied the small perturbation method combined with the equilibrium relationship between the pressure and foil deformation, and analyzed the influences of the working condition parameters, foil structural parameters, and the distribution position of the linear dynamic pressure groove on the static and dynamic characteristics of the seal. At the same time, the rough flat foil surface with triangular structure is characterized by a three-dimensional W-M fractal function, and the pressure control equations for the synchronous rotation of the wedge-shaped gas film and the moving rotor are established to analyze the influence of fractal parameters on the lubrication state of the flow field and the sealing characteristics by considering the microchannel scale effect, the step effect of the interface between the fluid–solid interface, and the deformation of the elastic surface.

According to the above content, compared with foreign countries, the domestic research on compliant sealing surfaces or the compliant support seal is relatively late, more concentrated on the structural design, and most of the experimental testing has not been carried out; the theoretical analysis of the last two years needs more research.

4.3. Leaf and Wafer Seals

With the continuous innovation of structural design by researchers, Mitsubishi Heavy Industries as well as some scholars proposed a new structure of the gas film seal on the cylindrical lamella plate [71,105,106], which is called the leaf seal, and this structure is an improvement on the structure of the wafer seals proposed by Steinetz and Sirocky et al. [107], and the leaf seal and lamella seal have a similar encapsulation, and its structure and working principle are shown in Figure 22 [71]. In this sealing structure, several 5~10 mm wide compliant thin leaf plates are regularly arranged along the circumferential direction and can be relatively free to move in the radial direction, leaving a small gap between adjacent leaf plates, and the stiffness of the compliant leaf plates is controlled by adjusting the thickness of the thin leaf plates, the radial height, the mounting tilt angle, the housing gap, and the connecting points, etc., and specific modeling is shown in Figure 23. When the seal is in a static state, the tip of the compliant thin leaf and the rotor journal contact each other, when the rotor begins to rotate, with the acceleration of the rotating speed, the tip of the leaf due to the fluid dynamic pressure effect of the formation of the opening force makes the leaf and the rotor journal separate, this achieves the non-contact working condition. Its work is characterized by the compliant leaf having a certain axial width, the gap between the leaf plate being very small, the flow of gas will be blocked by the leaf plate, so that the leakage is reduced. The non-contact state of the seal effectively avoids frictional wear and heat generation in the working process, and the way of controlling the stiffness of the leaf plate by adjusting the structural size of the leaf plate makes the thin leaf plate seal applicable to a variety of different differential pressure environments [108,109,110]. Test trials in the area between the compressor and turbine of a ground-based turbine engine showed that the leakage of this seal structure was comparable to that of a brush seal structure, with a slight rubbing of the tip of the compliant leaf and very little wear.

Nakane et al. [71,111] conducted experimental tests on the leakage performance of leaf seals; when the leaf seals and labyrinth seals were mounted on the same test rotor and operated back to back, the leakage of the leaf seals was small, which was approximately equal to one third of that of the labyrinth seals, and the results of the theoretical calculations, experimental tests, and CFD (Computational Fluid Dynamics) approaches were in good agreement, and little wear was observed on the smooth coated rotor surfaces, and the leaf seals were then evaluated in terms of their leakage, stability, and life in an M501 G industrial gas turbine. Gardner et al. [112] proposed another form of leaf seal structure, as shown in Figure 24. This seal structure consists of several flexibly supported thin metal leaves that overlap each other and extend around the rotating shaft from the inlet of the seal, and the leaf within the cantilever forms a compliant support that allows for a certain radial deflection. The working principle of this structure is similar to the compliant foil bearing, at the same time it can also be adapted to hydrostatic pressure operation, that is, after the external pressure is applied, the leaf will make adaptive deformation to maintain the seal of the working state. This structure has excellent sealing performance, and at the same time, almost no frictional wear, but the structure has certain limitations in the handling of large radial offset and high system differential pressure without damaging the leaf. In 2010, Grondahl et al. [113] proposed a leaf seal structure with a segmented slideway, whose structure is shown in Figure 25. This structure can make automatic adaptive adjustments to the transient vibration of the rotating shaft operation, which can ensure that the sealing surface always remains in a non-contact state.

4.4. Other New Cylindrical Gas Film Seal Structures and Concepts

In the late 1980s, NASA first proposed a straight cylindrical sealing structure [114,115]. This sealing technology is still in the research and exploration stage, and its principle is that the gas turbine rotor system has a wedge-shaped gap between the dynamic ring and the static ring, and in the rotation process of the rotor, the hydrodynamic effect of the sealing interface makes the rotor and the sealing ring separate, which avoids the direct contact friction between the dynamic and static rings. In 2004, NASA gave several common groove types of straight cylindrical seal structures, as shown in Figure 26 [116], and the common ones are helical grooves, tapered angle types, Riley’s stepped grooves, etc. These groove types are generally designed on static or floating rings, and the study of groove types is of great significance for the development of the technology of the cylindrical gas film seal.

In recent years, the design and theoretical research of compliant-supported gas film sealing structures has gradually received more attention from domestic scholars. To improve the sealing performance of turbofan engines, Su Hua and Ma Gang et al. [117,118,119] proposed a large flexure floating ring sealing structure—a double rotor type compliant-supported cylindrical gas film sealing structure, as shown in Figure 27 and Figure 28. The structure is designed by utilizing a thin cantilever sheet arranged circumferentially to support a rigid floating ring or a thin-walled cylinder, and the rotor is supported in the same direction. This structure can greatly improve the sealing performance when the rotors rotate in the same direction by utilizing a circumferentially arranged cantilevered sheet to support a rigid floating ring or a thin-walled cylinder.

In 2011, Wang Hong et al. [120,121] first proposed a cylindrical seal structure with a metal rubber outer ring, as shown in Figure 29. This structure realizes elastic support by adding a metal rubber ring between the static sub and the rigid floating ring, and it is a cylindrical gas film seal structure with functional damping (Gas Film Seal Damper, GFSD). During operation, a dynamic pressure gas film is formed between the rigid floating ring and the rotor journal, while the metal rubber acts as an elastic support structure, which not only has a certain stiffness of its own, but also allows for a certain radial displacement. The authors analyzed the relationship between the compliant support and the gas film by quantitatively designing the stiffness of the metal rubber, numerically simulated the steady-state characteristics of the gas film seal damping structure, and compared it with the through grate sealing, which showed that the leakage was much smaller than that of the through grate sealing structure if the structure was reasonably designed. Meanwhile, the quantitative design method of this kind of compliant support structure is also provided, but it is not widely used due to its insufficient high-temperature resistance.

Lu Junjie [63,122] also designed a new floating cylindrical gas film seal structure with a similar structure. Due to the difficulty of machining the groove pattern on the inner wall of the floating ring, domestic scholars chose to machine the groove pattern on the shaft sleeve, and this structure improves the lubrication performance of the floating seal by machining the helical groove on the shaft sleeve. Then the multi-objective parameter optimization model of the floating cylindrical gas film seal structure was established to obtain the groove structure with high accuracy and the gas film opening force. Xuexing Ding realized the groove engraving on the surface of the cylindrical rotating ring by using a 3D laser marking machine, verified the validity of the cylindrical gas film seal through tests, and investigated the flow field characteristics and sealing mechanism in the cylindrical gas film sealing structure by numerical simulation [123,124,125].

5. Research Methodology for Cylindrical Gas Film Seals

5.1. Main Research Methods

The establishment of a quantitative analysis method of the seal rotor system under specific working conditions, the study of seal performance change law, the parameters of the seal system quantitative coordination design, have an important position in the seal system test and application. According to the foregoing, the working principle of cylindrical gas film seal is similar to that of radial gas bearing, and there are two main methods for the current research of cylindrical gas film seals:

(1)

By establishing the geometric model of the cylindrical gas film seal, dividing the mesh, and simulating the flow field of the cylindrical gas film seal by using a finite element analysis software such as Fluent, the pressure distribution of the gas film on the sealing surface is derived, and then the performance changes of the seal structure and other aspects are investigated.

(2)

According to the operating conditions and assumptions of the seal, it is possible to establish an appropriate Reynolds equation [126], using the finite difference method, finite element method, analytical method, etc., to solve the calculation, to obtain the pressure distribution on the sealing surface, and then obtain the working condition parameters, structural parameters, etc., of the steady-state performance of the cylindrical gas film seal of the influence of the law. Similarly, it is possible, after perturbation of the Reynolds equation, to set up the solution to obtain the seal dynamic performance change rule.

The Finite Difference Method (FDM) Is the earliest method used In numerical simulation and is still widely used today. The method replaces the continuous solution domain with a finite number of grid nodes by dividing the solution domain into a difference grid. The method is based on the Taylor series expansion of the derivatives in the control equations. The finite difference method is discretized by replacing the derivatives in the governing equations with the quotients of the differences of the function values at the grid nodes by methods such as Taylor series expansion, which creates a system of algebraic equations with the values at the grid nodes as the unknowns.

The finite element method (FEM) Is based on the variational principle and the weighted margin method, and its basic solution idea is to divide the computational domain into a finite number of non-overlapping cells, and in each cell, select the appropriate node as the interpolation point of the solution function, rewriting the variables in the differential equation as a linear expression consisting of the node values of the variables or their derivatives, and the selected interpolation function, and then discretizing the differential equations using the variational principle or the weighted margin method. The differential equations are solved discretely by dividing the computational domain into a finite number of interconnected and non-overlapping computational units, selecting the basis functions in each computational unit, approximating the analytic solution in the unit by a linear combination of the unit basis functions, and then integrating the basis functions of each unit approximately into the overall basis function of the entire computational domain.

The key issues to be noted in the process of solving the above system of equations using the finite element method include the following:

(1)

The system of variational equations is composed of a steady-state equation and four mutually coupled perturbed linear equations, which must be solved by association, and the dynamic pressure in each perturbed equation is related to the perturbed frequency, which needs to be associated with the perturbed equations of motion of the sealing ring, and then solved by a numerical iteration method.

(2)

The solution domain is divided into triangular cells, the cells and nodes are numbered sequentially, and the nodes to be solved are numbered preferentially; the number of boundary nodes is recorded first and then introduced point by point according to the boundary conditions; according to the structural form of the sealing interface, attention is paid to the slots and platforms boundaries falling on the boundaries of the cell mesh.

The analysis methods for the dynamic characteristics of gas film seals mainly include linearization and nonlinearization methods. The linearization method is based on the assumption of linearity and mainly includes the small perturbation method and the step method. The small perturbation method assumes that the film thickness and pressure change of the gas film near the equilibrium position are linear, and the gas film force under the perturbation condition is calculated by applying a small perturbation to the steady state, and then the stiffness coefficient and damping coefficient describing the dynamic characteristics of the gas film are obtained, which is generally applied in the study of dynamic characteristics of the end-face gas film seals; the stepping method assumes that the gas film’s response to a small successive step of each degree of freedom is linear, and the gas film’s response to a step of each degree of freedom is calculated by the stepping method. The step method assumes that the response of the membrane is linear to small successive steps in each degree of freedom, and describes the dynamic characteristics of the membrane through the response of the membrane to the step perturbation in each degree of freedom. The nonlinearization method usually adopts direct numerical simulation, through the coupled solution of the transient Reynolds equations and the system dynamics equations containing the time term, to obtain the trajectory of the center of the sealing ring and the law of motion and then studying the dynamic characteristics of the system law. The nonlinear direct numerical simulation method can provide the relevant motion information of the seals in the system motion, calculate the sealing characteristics of the sealing system in the process of the motion, and better predict and analyze the dynamic characteristics, which is the only method to describe the dynamic characteristics of the gas membrane completely. However, at present, most of the domestic studies on the dynamic characteristics of gas film seals use the linearization method, and the results of the research using the nonlinear method are rarely seen.

5.2. Establishment and Solution of the Reynolds Equation

The model diagram of the cylindrical gas film seal is shown in Figure 30. According to the aforementioned structural characteristics and working principle of the cylindrical gas film seal, when the axle sleeve rotates with the rotating shaft to bring the medium gas into the converging gap and into the microgroove, the gas film carrying capacity is balanced with the load of the floating ring, and its equilibrium position is skewed to one side. Based on the fluid lubrication theory, the Reynolds equation is solved to obtain the pressure distribution law in the fluid-lubricated film. Based on the following assumptions, the Reynolds equation is derived to obtain the Reynolds equation for a cylindrical gas-film seal under isothermal conditions (the viscosity and density of the gas do not change with temperature), and the following assumptions are used in the derivation in addition to the isothermal conditions:

(1)

The gas lubrication is a Newtonian fluid, the effect of volume forces is neglected, and there is no sliding of the fluid on the solid interface.

(2)

In the direction along the thickness of the lubricating film, the change of pressure is not counted.

(3)

Neglect the change of velocity direction caused by the surface curvature.

(4)

The flow state is laminar, ignoring the effect of inertial force.

With the above assumptions, the Reynolds equation for a cylindrical gas film seal can be written as:

$$\frac{1}{R^2}\frac{\partial }{\partial \theta }\left(\frac{p{h}^3}{{\mu }_{g}T}\frac{\partial p}{\partial \theta }\right)+\frac{\partial }{\partial z}\left(\frac{p{h}^3}{{\mu }_{g}T}\frac{\partial p}{\partial z}\right)=6\omega \frac{\partial }{\partial \theta }\left(\frac{ph}{T}\right)+12\frac{\partial }{\partial t}\left(\frac{ph}{T}\right)$$

(1)

6. Progress in the Study of Stability, Dynamic Performance, and Multi-Physics Field Coupling of Cylindrical Gas Film Seal

In the development history of the cylindrical seal, due to the influence of technological embargoes and barriers, there are fewer public reports on seals for aeroengines that can be found in the existing research. This section is based on the sealing technological needs of aeroengines, and summarizes and analyzes the current research status of the cylindrical gas film seals in terms of theoretical research (steady-state characterization, dynamic characterization parameter, multi-physical field coupling, and structural deformation), experimental test analyses, and machining technologies.

6.1. Steady-State Performance Analysis of Cylindrical Gas Film Seals

The gas film seal was developed based on the theory of the gas bearing. Whipple [127] in the 1940s put forward the basic theory and working principle of the spiral groove gas bearing, and then Muijderman [128,129] further developed and improved Whipple’s theory to obtain the analytical solution of the Reynolds equation for the spiral groove gas bearing. The theory was later called the “narrow groove theory”. In 2004, NASA [130], in a public research report, introduced GCYLT, a program that can be applied to different interfaces, and then used this program to calculate and analyze the performance parameters of cylindrical gas film seals with different groove structures. In addition, the report summarizes and introduces the structure types and working principles of several common cylindrical gas film seals, which are of great significance to the subsequent research on cylindrical gas film seals. Mel’nik V.A et al. [131,132,133] elaborated the working mechanism of the floating ring seal, systematically calculated and analyzed the performance parameters of the floating ring seal, designed and manufactured the related components of the floating ring seal, and improved the numerical calculation method, which made the calculation of the performance parameters of the cylindrical seal faster and more accurate. Hirs [134] derived a zero-equation turbulence model based on the Bulk Flow theory using relevant experimental data, and Nelson [135] improved the turbulence calculation model based on the Bulk Flow theory proposed by Hirs, and successfully applied this model to the performance analysis of cylindrical gas film seals. Childs et al. [136,137] also adopted the Bulk Flow model proposed by Hirs in the numerical analysis of the flow field of the cylindrical gas film seal. Then, the k-ε turbulence model in CFD software was used to calculate the leakage of the cylindrical gas film seal and this was compared with the experimental data, and it was found that the theoretical calculations and the experimental data were in good agreement. Chen et al. [138] summarized the theory of the cylindrical gas film seal, the calculation method for solving the gas film pressure, the structure and working principle, and the numerical solution model, etc., and compared several commonly used numerical calculation methods to obtain the applicable conditions of each method as well as the computational errors between each method. Gang Ma et al. [139], through the establishment of a three-dimensional spiral groove cylindrical gas film seal structure model, the use of Fluent software to analyze the performance of the spiral groove cylindrical seal with the change rule of the structural parameters to obtain the pressure distribution of the gas film flow field and the change rule of the performance parameters. The results show that sealing the different structural parameters of the sealing performance of the impact of different structural parameter changes will almost not affect the friction torque. LI et al. [140] took a floating ring seal and labyrinth seal as an example, and simulated the sealing performance of seals for aeroengines in different hot and cold states through experiments, and the results showed that, no matter what the sealing form, its critical pressure ratio in the cold state is significantly larger than that in the hot state and the leakage coefficient in the hot state is larger than that in the cold state; however, in general, the performance of the floating ring seal is superior to that of the labyrinth seal. Zhao et al. [141] proposed a new multiphase coupling method to investigate the interfacial slip of the cylindrical seal, established the interfacial local slip theory, and used the JFO cavitation theory dynamic pressure model to investigate the influence law of the slip flow on the seal ring performance under high operating parameters, and the research results showed that the area of the slip zone increases with the increase of the rotational speed. Zhang et al. [142] established an analytical model of the floating ring seal to analyze the stability of the floating ring seal rotor system under the full consideration of the effect of the axial pressure gradient and the fluid Lomakin effect, and through the study, it was found that the increase of the sealing width of the floating ring seal as well as the use of laser machining technology for the engraving of micro-groove structure in the micro-interface of the cylindrical can significantly increase the dynamic pressure effect of the cylindrical seal to improve the buoyancy force. Zhang et al. [143] designed a new radial annular seal (RARS) to slow down the fluid leakage and reduce the fluid-induced force of the traditional turbomachinery tip seal and compared it with the traditional labyrinth seal, and the results showed that this seal changed the direction of the leakage from axial to radial direction, which greatly reduced the leakage rate. ANDRÉS et al. [144] investigated the leakage performance of three different forms of annular seals in a high-temperature environment, and the results showed that hybrid brush seals (HBS) do not produce localized thermal deformation and are free from wear and scoring, which allows them to be used in gas turbine shaft end seals. Scharrer et al. [145,146] first derived the incompressible flow equations for an annular seal of an inclined rotor based on the assumption that the fluid is a completely turbulent flow in the axial and circumferential directions, and then solved the momentum and continuity equations by using the Fast Fourier Transform method, and obtained the velocity and pressure distributions; and then, based on the above, they derived the incompressible flow equations of an annular seal with a rough surface by utilizing the Bulk Flow theory proposed by Hirs, and carried out an integral operation of the equations to obtain the perturbed pressure distributions, and compute the related dynamic coefficients.

Due to the late start in China, the theoretical research now remains at the stage of numerical simulation; with the development of numerical computing technology, more and more researchers use CFD technology to analyze the flow field of gas film seals engraved with different groove shapes. Su Zehui et al. [147] studied the performance of a cylindrical gas film seal under an eccentric structure; the results showed that the change of eccentricity will have a large impact on the sealing performance, and comparison of the sealing performance under different eccentricities showed that the sealing performance is optimal when the eccentricity is at 0.7, and this eccentricity structure was used to study the rotational speed of the sealing performance of the influence of the law. The results show that the dynamic pressure effect of the fluid is not obvious at low speeds, and with the increase of rotational speed, the dynamic pressure effect gradually increases and eventually tends to stabilize. Junhua Ding et al. [148] investigated the sealing performance of a cylindrical gas film seal with and without grooves using a combination of Fluent simulation and experimental validation, and found that the cylindrical gas film seal with grooves has a better dynamic pressure effect and a lower leakage rate; regardless of grooves or no grooves, the leakage rate increases with the increase of the eccentricity and the differential pressure but the rotational speed has a lesser effect on the sealing performance. Liu Hong et al. [149] studied the performance of a cylindrical spiral groove seal under gas–liquid two-phase flow using Fluent’s two-phase flow Mixture model, and the results show that, under the same working conditions, the performance of the cylindrical seal with gas–liquid two-phase flow is better than that of the cylindrical seal with a pure gas phase, which is because, with the entry of the liquid-phase fluid, the gravitational force of the liquid phase will force the liquid phase to gather at the root of the spiral groove, which effectively prevents the gas from leaking out; and it was also found that the increase of rotational speed, differential pressure, and the ratio of liquid and gas is conducive to the enhancement of the performance of the cylindrical seal. Based on the PH linearization method, He Zhenhong et al. [125,150] solved the steady-state Reynolds equation of the microscale cylindrical seal considering the slip boundary condition, obtained the approximate analytical solution of the gas film pressure on the cylinder, and analyzed the influences of the working condition parameters and geometrical parameters on the steady-state performance of the cylindrical seal. The results showed that the influence of the sealing differential pressure on the sealing performance was more prominent compared with that of the eccentricity, and at the same time, combined with the analysis of the steady-state performance, optimal structural parameters were proposed. Miao Chunhao et al. [151], based on fully considering the microgap structure of the cylindrical gas film seal, used Solidworks to model the cylindrical gas film seal and simulated the three-dimensional flow field of the cylindrical seal using CFD software to obtain the change rule of the steady-state performance (buoyancy force and leakage) of the seal under different working conditions of the operating conditions. It is also found that, only when the eccentricity exists does the cylindrical seal produce the dynamic pressure effect, i.e., the surface groove and eccentric structure are the key to the design of the cylindrical gas film seal. Wang Shipeng et al. [152] obtained the distribution maps of the gas film pressure and gas film thickness by establishing the analytical model of a helical groove cylindrical gas film seal, discretizing the equations by using the central difference method and solving the calculations by using the Newton–Raphson iterative method, and then discussed the change rule of the steady-state performance of the seal under different operating parameters. Using CFD software, Wang Ting et al. [153] quantitatively analyzed the steady-state performance of a one-slot cylindrical gas film seal, and concluded that the influence of the structural parameters, operating parameters, and slot parameters on the sealing performance should be comprehensively considered in the actual seal design. Dai Di et al. [154] simulated the flow field and steady-state characteristics of a flexibly supported cylindrical gas film seal by CFD software and verified it by experiments, and found that the sealing differential pressure, fluid viscosity, and working condition parameters would have a large impact on the sealing performance. Lu, J. et al. [155] analyzed the influence law of slip flow on the cylindrical gas film seal and investigated the intrinsic correlation between the gas slip flow effect and the seal operating parameters. The results show that the presence of slip flow reduces the gas film pressure of the cylindrical seal, and at small film thicknesses, slip flow reduces the gas flow rate between the floating ring and the rotating ring. Sun Junfeng et al. [156] analyzed the static pressure, flow rate, and shear stress distribution law of two types of cylindrical gas film seals without grooves and with surface T-grooves by using CFD software, and it was found that the surface grooves could obtain better sealing performance. Ma Gang et al. [157,158,159] used the Fluent software to simulate the flow field of an end-cylindrical combined gas film seal under turbulent flow, and analyzed the influence law of the cylindrical groove-length ratio on the sealing performance; and then, based on the finite element method, they established its system characteristic analysis model, solved the pressure distribution of the gas film, and discussed the influence of the sealing geometric structure parameter on the sealing steady-state performance. Chen Tao et al. [160] used numerical simulation of the gas film pressure and steady-state performance of the cylindrical gas film seal, and compared with other references where the results have a certain degree of credibility, and they then analyzed the rule of change of the gas film reaction force and deflection angle with the operating parameters. Qiu-Fa Wei, Zehui Su et al. [161,162,163] investigated the factors affecting the film stiffness, floating force, and leakage rate of T-slot cylindrical gas film seals, and optimized the geometrical parameters of the T-slot to obtain better sealing performance. Ma Gang et al. [64] took the herringbone spiral groove cylindrical gas film seal as the research object, calculated the pressure field of the seal gas film based on the finite element method, and analyzed the influence law of the seal bearing capacity, friction torque, and leakage under the change of geometrical parameters. Lu, J. et al. [63] proposed an adaptive cylindrical gas film seal, which analyzed in depth the relationship between the gas film force and leakage with speed and pressure, and concluded that the dynamic pressure effect is more obvious under the effect of eccentric structure and surface grooving, and verified the accuracy of the theoretical analysis results through tests. Sun Dan et al. [164,165] proposed a new type of floating convergent pocket sealing structure and used it as a research object to obtain the circumferential pressure distribution law, which was experimentally verified to have adaptive concentric performance and good floating response characteristics. Ma Lijun et al. [166] described the steady-state performance of gas film floating ring seals under different conditions by combining theoretical and experimental methods, and the results showed that a larger eccentricity and throttling length can improve the floating force. Hao Muming et al. [167] comparatively analyzed the performance of ordinary floating rings, spiral groove floating rings without a dam area, and spiral groove floating ring seal with a dam area, and obtained the variation curves of the floating force and leakage with working condition parameters.

6.2. Dynamic Characterization of Cylindrical Gas Film Seal

Cylindrical gas film seal structure in the actual application of aeroengine working conditions will be subject to a variety of vibration interferences, so the dynamic characteristics of the sealing system determine the service life and working effect of the cylindrical seal. When the external vibration is too large, it will lead to the deterioration of the stability of the sealing film, which will further lead to the instability of the sealing system, and the seriousness will lead to the sealing end face touching abrasion and failure, so the stability of the sealing system gas film is a serious challenge for the sealing of aeroengines. At this stage, the research on the dynamics of the gas film seal has been extended to include unsteady-state dynamics, small perturbation linear dynamics, nonlinear dynamics, and other aspects. Balakh L.Y et al. [168,169,170] investigated the theoretical hydrodynamic characteristic relations of a cylindrical seal rotor between the moving and static rings while considering the kinetic characteristic relations of the rotor system with the floating ring during high-speed rotation, and discussed and studied in depth the parameters related to the sealing ring of a multi-mass rotor system. The results show that, when the rotational speed is aligned with the partial frequency of the floating ring, the floating ring acts as a damper in the rotor system, the rotor stiffness varies more drastically compared to the rotational speed in the hydrodynamic layer, and the hydrodynamics may be an important reason for the disappearance of the critical speed of the rotor. Xu et al. [171] proposed a novel transient CFD method for the prediction study of the dynamic characteristics of liquid annular seals, and then calculated the dynamic characteristics of liquid annular seals under different seal lengths, and the calculation results showed that the dynamic principal stiffness of the liquid annular seals would turn from a positive value to a negative value with the increase of the seal length, and that such a change might change the direction of the force of the annular seals, which further affects the support conditions of the rotor system. Ruan [172], based on fully considering the problems of rotor runout, static sub misalignment, thin gas effect, face contact, etc., by discretizing the transient Reynolds equations using the finite element method and calculating them using the iterative method, and at the same time, coupling the system dynamics equations and the face contact equations, analyzed the sealing system in the inverse form during the phases of starting and stopping under the consideration of the state of face contact, and the change rules of the rotor displacement as well as leakage volume were obtained. Mihai et al. [173] investigated the system stability of a cylindrical seal under nonlinear vibration, obtained the judgment method of seal instability, and also analyzed the distribution law of the seal ring’s motion trajectory under dynamic perturbation with different performance parameters. Volodymyr Y et al. [174], in the process of modifying the sealing system of a centrifugal compressor, attempted to use a cylindrical seal instead of the traditional sealing form and succeeded. Then the vibration state and dynamic characteristic parameters of the cylindrical seal were analyzed and studied, and the results showed that a better sealing performance could be obtained by using the cylindrical seal. Arghir et al. [175] obtained an analytical solution of the flow field of the cylindrical seal by establishing a numerical analysis model of the cylindrical seal, then numerically solved the dynamic characteristic parameters of the cylindrical seal and obtained the motion trajectory of the floating ring, and then analyzed the stability of the system of the cylindrical seal. Ha et al. [176] took the high-pressure floating ring seal for a liquid rocket engine turbopump as the research object, carried out an in-depth study on the dynamics and rotor vibration characteristics of the cylindrical seal, established the control analysis equations of the eccentric ring seal based on the “large flow rate model”, and used the Fast Fourier Transform method proposed by Nelson and Nguyen for the development of the solution program to analyze the dynamics and vibration coefficients of the rotor under the influence of the change of the working condition parameters and the geometrical parameters. Ha et al. [177] used CFD software to analyze the annular gas seal performance and proposed a method to determine the dynamic coefficients of the cylindrical seal rotor, and compared the results of numerical simulation based on the Bulk Flow theory with the experimental data, which showed that the results of the CFD calculations of the direct stiffness and cross-coupled stiffness were both in good agreement with the theoretical and experimental results. As shown in Figure 31, Wu [178] utilized CFD software to solve the flow field of a cylindrical seal under rotor perturbation to evaluate the rotor dynamic coefficients. Then, a new quasi-steady-state method and transient method were proposed based on the traditional steady-state method, and these three methods were utilized to calculate the rotor dynamic coefficients of the seal, respectively, and compared with the experimental data, and the results showed that this quasi-steady-state method was superior to the traditional steady-state method and the transient method was superior to the previous two methods, and the transient method could accurately predict the quality coefficients of the cylindrical seal, but this method sacrifices the computational resources and the calculation process is very time consuming. Then, the variation law of the dynamic coefficients of the cylindrical seal under non-uniform incoming flow is investigated by using the transient method, and it is found that this transient method has excellent performance in predicting the direct mass M of the seal, and the dynamic stiffness coefficients of the cylindrical seal change with the change of direction of the eccentricity when the incoming flow is non-uniform. Finally, based on the study of the dynamic coefficients of the cylindrical seal rotor under uniform incoming flow, a linearized fluid force formulation that can well explain the variable stiffness problem is proposed.

As shown in Figure 32, Wu [179] summarized the two main methods currently used to study the dynamic characteristics of the cylindrical seal: one is to numerically solve the transient flow equations by the perturbation method, and the other is to simulate the perturbed flow field of the cylindrical seal by using software such as CFD. On this basis, he proposed a new transient CFD method based on the rotor variable-speed vortex to improve the dynamic mesh delineation and computational time-consuming problems in the existing technology, which requires only two transient CFD simulations to obtain all the dynamic performance parameters of the seal, and the results obtained are compared with the experimental results and the traditional method, and the results show that this method can guarantee the computational accuracy, but also greatly shorten the computation time.

Li et al. [180] developed a new three-dimensional transient CFD perturbation method using a non-uniform Eulerian multiphase flow model in a humid gas environment, which was combined with the dynamic mesh technique to study the rotor dynamics characteristics and leakage performance of three different annular gas seals. The structures of these three annular gas seals are shown in Figure 33.

As shown in Figure 34, the trend of the dynamic coefficient will directly affect the stability of the sealing system. Domestic research is most concentrated in Beijing University of Aeronautics and Astronautics and Lanzhou University of Technology, etc. Ma Gang et al. [181] defined the dynamic characteristic coefficients of the cylindrical gas film seal by using the distribution parameter method, established the partial differential equations of the cylindrical gas film seal of the perturbed state, expressed the dynamic characteristic coefficients in the form of the complex variable, and numerically solved the dynamic characteristics of the downstream spiral groove cylindrical gas film seal based on the finite element method to obtain the change rules of the cylindrical gas film seal’s dynamic stiffness and dynamic damping coefficients. The results show that the cross-stiffness coefficient and cross-damping coefficient of the gas film show good axisymmetric characteristics, but the change rule of the forward stiffness coefficient and forward damping coefficient is more complicated. Under a certain critical frequency ratio, the increase in compressibility number has a bad effect on the sealing gas film stability. Ma Gang et al. [182] developed a calculation and analysis program for the stable and dynamic characteristics of cylindrical gas film seals based on the genetic idea and based on the particle swarm multidimensional optimization (GAPSO) to numerically analyze the inverted slanting T and double-layer slanting groove-type cylindrical gas film seals and to optimize the multidimensional parameters of the groove structure to improve the sealing characteristics and stability of the sealing system. Ma Gang et al. [183] proposed a design and analysis method for the quasi-dynamic equilibrium of equivalent circular feedthrough, investigated the characteristic law of a compliant supported cylindrical gas film sealing system, and obtained the variation curves of the influence of the gas film thickness on the seal characteristic parameters. Ma Gang et al. [184] realized the nonlinear numerical calculation of the dynamic characteristics of the cylindrical gas film seal by establishing a dynamic analysis model of the cylindrical gas film seal system and solving the transient Reynolds equations and the system dynamics equations by using the finite element method, and meanwhile obtained the center trajectory of the cylindrical seal ring and the response map under perturbation. Yang Shaoyu et al. [185] used Fluent software with a finite volume method to calculate the leakage and fluid excitation force of the liquid annular seal, and solved the dynamic characteristic coefficients of the liquid annular seal based on the equation of the excitation force and the rotor dynamics model, and compared the results with those of the experiments and the references, which were in good agreement.

Cylindrical gas film seals are less studied, and the existing results are focused on the study of dynamic characteristic coefficients; there are still a lot of dynamic problems that need to be solved by further research.

6.3. Multi-Physics Field Coupling and Structural Deformation Study of Cylindrical Gas Film Seals

According to the working principle of the aforementioned cylindrical gas film seal, it relies on the eccentric structure to generate a fluid dynamic pressure effect to realize the seal, so in practical applications, the choice of a reasonable structure to generate the stability and rigidity of the gas film has an important impact. The most common way in today’s development process is to groove the surface of the floating ring to produce a better fluid dynamic pressure effect.

Nagai et al. [186,187] carried out theoretical numerical analysis and experimental validation studies on the microgap static characteristics of a cylindrical floating ring seal by carving spiral groove microgrooves onto the surface of the cylindrical seal ring, and obtained the change rule of leakage and the pressure distribution of the flow field within the sealing microscale gap, and the results found that better sealing performance could be obtained by grooving the surface of the seal ring. Kyoung-Work et al. [188] carried out a structural device design of a compliant floating cylindrical seal to obtain the sealing characteristic parameters of the cylindrical seal under different gas film thicknesses and different eccentricities, and the results of the study showed that, when the structural device of the compliant floating cylindrical seal has a larger damping and eccentricity, it can have a more stable service environment, and the amount of wear is also reduced. Vinogradov et al. [189] considered the flow field distribution of the sealing ring under arbitrary deformation and combined it with the finite element method to establish a coupled flow–solid model, which proved that the solid deformation had a significant effect on the flow field, but the model did not take into account the coupled effect of the temperature field on the flow field and the solid field. Peng et al. [190] found that the structural changes of the cylindrical seal will have a large impact on the sealing performance of the rotor system, and gave a specific example; when the seal operates under high-pressure conditions, the elastic deformation of the floating ring will have an impact on the dynamic coefficient of the system and the amount of leakage, and the amount of leakage of elastic seals is less than that of rigid seals due to the elastic deformation of the floating ring which reduces the sealing gap. It is also found that eccentricity is an important factor affecting the variation of the dynamic coefficient of the rotor of the elastic seal. Ma Gang et al. [191] conducted a study on the optimization of structural parameters of gas film seals, and proposed a multidimensional optimization calculation method for the structural parameters of gas film seals based on the particle swarm intelligent optimization algorithm to obtain the optimal groove geometrical parameters by taking the multidimensional optimization of the sealing performance as an objective. The results show that the influence of each groove geometry parameter on the seal performance is not independent, which indicates that the best seal geometry groove results can only be obtained by using the multidimensional optimization method. Zhao et al. [192], to improve the bearing capacity and resistance to deformation of the compliant support of the cylindrical sealing structure, proposed a bubble-type support cylindrical gas film sealing structure by using a bubble foil instead of the traditional cylindrical floating ring, and the sealing performance was investigated under different bubble radii, bubble heights, and thicknesses of the foil, and the results showed that this type of bubble-type support structure can provide a larger equivalent force, which in turn can resist the deformation of the structure well, and the changes in the thickness of the foil has a larger impact on the sealing performance, and the changes in the radius of the bubble and the height of the bubble have a lesser impact on the sealing performance. Chen Tao et al. [193] combined the coordinate rotation method and the interval decreasing method and applied them to optimize the parameters of the groove type of the cylindrical gas film seal with the stiffness-to-leakage ratio as the objective function. Ran Zhang et al. [194] investigated the influence law of surface grooving on the static and dynamic characteristics of a cylindrical gas film seal by using a non-constant dynamic mesh technique and a multi-frequency elliptic vortex solver model. Bai Chaobin et al. [195] took the wave-foil type compliant support cylindrical gas film seal as the research object, and analyzed the effects of gas film thickness, eccentricity, and each relevant performance parameter of the compliant support material (Poisson’s ratio, modulus of elasticity, and several foils) on the deformation of the seal structure. Wang Jingjing et al. [196] studied the influence of working condition parameters, such as medium pressure and rotational speed, on the deformation of the cylindrical gas film seal by Fluent software, and found that the deformation is mainly extrusion deformation along the radial direction, with maximum deformation at the outer edge of the floating ring, and this deformation exceeds the average gas film thickness of the seal.

Due to the pressure distribution, deformation, and temperature distribution in the gap of the sealing structure interacting with each other, if the model only considers the role of the gas film and ignores the coupling between the gas film and the solid, it often fails to reflect the real working state of the sealing structure and accurately predict the sealing performance, so it is crucial to analyze the fluid–solid–thermal multi-physics field coupling of the gas film sealing structure. In the structural mechanism and multi-physical field coupling study of the gas film seal, based on the field synergy under the cylindrical seal behavior, research and performance prediction will be the focus of future research. The development of an aeroengine sealing special multi-physical field coupling calculation method, in-depth understanding of the sealing structure and the working principle, to enhance the sealing structure of the independent innovation design are the urgent needs of the cylindrical seal technology field.

6.4. Research on the Test and Processing Technology of Cylindrical Gas Film Sealing

With the in-depth development of the theoretical research on the gas film sealing technology and the need for innovation in the sealing principle and structure, the research on seal testing and evaluation technology based on advanced and composite sensing technology is equally important [197]. The continuous updating of sensors and the increasing accuracy of transmitters, coupled with the rapid development of Computer Assistant Test (CAT) technology, have led to the maturation of the gas seal test and testing technology [198,199,200,201]. Domestic and foreign researchers have conducted a large number of experimental studies on different types of cylindrical gas film seals, the aforementioned seal stability and dynamic characteristics, structural deformation, and other content have been studied most by scholars through tests to verify the accuracy of the theoretical calculation model, and will not be repeated here, as this part has a main focus on the sealing structure of the test of the technical means used in the sealing ring groove type machining process for elaboration. Dapeng Zhang et al. [124] used a 3D fiber laser marking machine to fabricate four different groove structures of the cylindrical gas film seal structure (streamlined slant groove, rectangular groove, optimized slant groove, and optimized rectangular groove), and the four different forms of seals were tested on a test rig, and the scouring of the surface of the bushing and the floating ring was observed. Zhao Yafei et al. [202] used multivariate linear regression and the least squares method to fit the runway groove parameters of the cylindrical gas film seal to study the change characteristics of the cylindrical gas film seal characteristics under different interfacial structures, and obtained the optimized structural parameters of the seal. Kasem et al. [203,204] combined a fiber-optic two-color pyrometer with an infrared camera to test the temperature and obtain the true friction area during the testing of a gas film seal structure. Jin et al. [205] measured the equilibrium film thickness, leakage, and gas film stiffness of the seal to obtain the optimized groove structural parameters and analyzed the effect of the structural parameters on the sealing performance. Ding et al. [123] tested different positions of the seal using temperature sensors and found that the highest heat of the gas film was found at the root of the groove at high-speed operating conditions.

As shown in Figure 35. Surface grooving technology is the key technology of the gas film sealing structure, at this stage the grooving technology and surface processing technology methods mainly include EDM, sandblasting method, photolithography processing, etc., and the use of laser processing methods in the existing technology is more common, the applicability of this method to the material is good, and the processing quality is good and has high efficiency [206,207].

7. Conclusions

Problems in the Study of Cylindrical Gas Film Seals and the Future Direction of Development

(1)

Along with the rapid development of industrial technology, aeroengines and other rotating machinery and equipment have put forward higher requirements for the sealing technology and sealing structure. Compared with the traditional labyrinth sealing, brush sealing, cellular sealing, and other aeroengine sealing technologies, gas film sealing, as a kind of non-contact sealing form with low leakage, low wear and tear, long service life, etc., has demonstrated great potential in the development of the aeroengine sealing technology. Since the concept of a gas film seal was put forward, through the continuous efforts of researchers, the end-face gas film seal has been successfully applied in all kinds of rotating machinery; however, the static “three highs” of the aeroengine and the dynamic strong vibration of the harsh conditions limit the engineering application of the gas film face seal, due to the large compliant structure of the cylindrical gas film seal, with a large axial degree of freedom, which can effectively reduce the friction and wear of the sealing structure due to the tilting of the rotor, and it has a very good prospect for application in the field of aeroengine sealing technology.

(2)

The compliant support structure of the cylindrical gas film seal is an important part of the sealing technology. Researchers have introduced compliant shafts, compliant foils, thin leaf plates, thin-walled beams, and other large compliant parts to increase the axial degree of freedom of the cylindrical sealing structure and to resist rotor vibration and inhomogeneous thermal deformation under the harsh conditions of aeroengines. Researchers proposed a new series of cylindrical gas film seal structures with a new form for solving the existing problems of aeroengine gas film seal structure ideas, but most of the compliant structures have quantitative design processing and manufacturing difficulties and other problems, so for the compliant support structure material selection and structure design, the gas film seal research needs to focus on the consideration.

(3)

Cylindrical gas film seal technology has been developed for decades, and researchers have made a lot of contributions to the structural design of cylindrical gas film seals. The existing research basis is mainly carried out based on the influence of steady-state performance, while the structural design method mostly adopts the control variable method and trial-and-error method, and less consideration is given to the multidimensional optimization algorithm and overall topology optimization. An aeroengine in actual operation needs to consider the dynamic characteristics, self-excited vibration of the gas film, and stability and other issues, so the design quantization of the cylindrical gas film seal structure needs a complete set of structural design theories.

(4)

Regarding the research on the steady dynamic performance and structural mechanism of the cylindrical gas film seal, with the deepening of the understanding of the sealing technology, the research on the dynamics of the gas film seal has been extended to include the unsteady-state dynamics, the linear dynamics of small perturbations, and the nonlinear dynamics and other aspects.

(5)

Due to the mutual influence of the pressure distribution, deformation, and temperature distribution in the gap of the sealing structure, it is crucial to analyze the heat–fluid–solid multi-physical field coupling of the gas film sealing structure. In the structural mechanism and multi-physical field coupling study of the gas film seal, based on the field synergy under the cylindrical seal behavior, research and performance prediction will be the focus of future research, the development of an aeroengine seal-specific multi-physical field coupling calculation method, in-depth understanding of the sealing structure and the working principle, to enhance the sealing structure of the independent design of innovation are the urgent needs of the field of cylindrical sealing technology.

(6)

With the deepening of sealing technology research for aviation engines, the continuous updating of sensors and the increasing accuracy of transmitters, coupled with the rapid development of computer-aided testing technology, the gas sealing test and testing technology is becoming more mature, in the future, seal test and evaluation based on an advanced and composite sensing technology will be the new direction, and sealing based on the new principles, new materials, and new structural innovations in the design of the seal will also shine.