Ferrofluid Seal Research Articles

Centrifugal failure mechanism analysis for ferrofluid seals based on a bidirectional coupled multi-physics model

In this paper, we propose a bidirectional coupled multi-physics (BCM) model to study the centrifugal failure mechanism of ferrofluid seals, employing a hybrid finite volume–finite element method. In this model, the evolution of the flow field is described by the two-phase incompressible Navier-Stokes (NS) equations with Kelvin force. The volume of fluid (VOF) method is used to capture the gas-ferrofluid interface. Magnetic scalar potential is introduced to solve for the magnetostatic field. We conducted secondary development on the OpenFOAM and Elmer software packages, which are based on the finite volume method (FVM) and finite element method (FEM), respectively, to calculate the flow field and magnetic field in ferrofluid seals. By developing interpolation methods between FEM and FVM grids based on the parallel bidirectional coupler EOF-Library[1], field information exchange between the flow field and magnetic field is achieved. The BCM model executes iterative computations on the flow and magnetic fields in an alternating way, continuously synchronizing the information of each, thus facilitating bidirectional coupling between the FVM and FEM frameworks. We employ the BCM model to investigate the failure mechanisms of ferrofluid seals under different conditions, including various load pressure and rotational velocity. The results indicate that the ferrofluid will leave the surface of the shaft along the pole teeth under the action of centrifugal forces, with the highest radial velocity occurring at the boundary of the liquid film. Under varying conditions of rotational velocity and pressure drop, the centrifugal failure modes of ferrofluid seals exhibit distinct differences. Moreover, two dimensionless numbers are proposed to quantitatively evaluate the effects of load pressure and centrifugal force, thereby augmenting the efficacy of seal design.

Experiment and Simulation on the Ferrofluid Boundary Deformation and Fluctuation Characters of a High-Speed Rotary Seal

Abstract A ferrofluid seal is a rotary seal widely used in the chemical industry, shipbuilding, aerospace, aviation, and other fields, demonstrating various advantages, such as high reliability, low wear, and low leakage rate. The reason for sealing is that the ferrofluid ring isolates the sealed gas from the atmosphere. Hence, boundary deformation and fluctuation are two significant factors to consider for the design of a ferrofluid seal. However, the ferrofluid boundary is wrapped in the shell and pole shoe, which poses a challenge in direct observation. In this paper, an experimental ferrofluid rotary seal device is presented, which can be used to monitor the boundary changes at different rotational speeds and pressure differences. According to the simulation result, the boundary fluctuation affects the pressure transfer and causes the sealing performance to deteriorate. The ferrofluid boundary obtained through the experiment is essential for an in-depth investigation into the rotary seal failure mechanism for improving the design of the ferrofluid rotary seal.

Experimental study of symmetrical ferrofluid seals with single magnetic source and small clearance

To enhance the sealing pressure of normal ferrofluid seals with single magnetic source and small sealing clearance, a symmetrical ferrofluid seal (SFFS) was designed. The study utilizes a combination of finite element simulation and experiments to investigate the influence of crucial factors, including sealing clearance, number of pole teeth, step height, and pole teeth position, on the sealing pressure of SFFS. The findings indicate that the experimental sealing pressure values for SFFS closely match the theoretical sealing pressure values. The sealing pressure of the SFFS decreases as the sealing clearances and step height increase. It increases with an increase in the number of pole teeth.

Numerical and experimental study on pole teeth interlaced divergent ferrofluid seals with single magnetic source

For the improvement of pressure resistance in ferrofluid seal with a small gap, a structure of divergent ferrofluid seal (FS) with alternating pole teeth (PT) was proposed in this paper. Distribution of the magnetic field in the seal gap (SG) is analyzed by using the magnetic field finite element method, and the theoretical pressure resistance (PR) is calculated by using the step ferrofluid seal PR theory. Effect of the number of pole teeth and seal gap on its sealing PR were studied experimentally. The PR of the interlaced divergent FS is compared to the experimental findings of a conventional ferrofluid seal structure. The results show that the actual pressure value of the interlaced divergent FS is consistent with its theoretical pressure value well. Compared to the common ferrofluid seal, the interlaced divergent FS has better PR.

Experimental study on self-repairing performance of the interlaced ferrofluid seal

To improve the self-repairing performance of common ferrofluid seal (FS) and meet sealing requirements of long life under vacuum conditions, the interlaced FS device was developed. Effects of the number of axial pole teeth (PT), the number of radial PT, the width of the axial sealing gap (SG), and the height of the radial SG on their self-repairing performance were studied through experimental methods and compared with the self-repairing performance of the common FS. The results reveal that the self-repairing performance of interlaced FS is significantly better than that of the common FS with an increase in the number of FS ring ruptures. The ability to self-repairing of the interlaced FS diminishes or remains unchanged. As the quantity of axial PT increases, the self-repairing performance of interlaced FS is enhanced and then weakened. As the number of radial PT increases, the self-repairing performance of interlaced FS is weakened and then enhanced and then weakened. When the axial SG width exceeds 0.1 mm, the self-repairing performance of interlaced FS is weakened with the increase of SG. When the height of the radial SG exceeds 0.1 mm, the self-repairing performance of interlaced FS is weakened with the increase of SG.

Design and analysis of combined standard ferrofluid and centrifugal seals

PurposeFerrofluid seals are known for their low friction torque and high tightness. However, they have some limitation due to the allowable rotational speed. The work presented here analyzes the performance of newly designed seals which are a combination of a ferrofluid and a centrifugal seal. The new seals can operate at high speeds. The purpose of this study is to theoretically predict the performance of combined seals.Design/methodology/approachThree seals were designed and selected for analysis. A version of the seals with a nonmagnetic insert is also considered, the purpose of which is to facilitate the installation and return of ferrofluid during low rotational speeds. The analyses were based on combining the results of numerical simulation of magnetic field distribution with mathematical models.FindingsA combination of ferrofluid sealing and centrifugal sealing is possible. Analyses showed that the combined seal could hold a minimum pressure of 190 kPa in the velocity range of 0–100 m/s. The problem with this type of seal is the temperature.Originality/valueNew seal designs are presented. Key parameters that affect the seal operation are discussed. A methodology that can be used in the design of such seals is presented.Peer reviewThe peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-07-2023-0221/.

Sealing mechanism investigation of convergent ferrofluid seals with staggered pole teeth

To reveal the pressure transfer and the failure mechanism of the convergent ferrofluid seal (CFFS) with staggered pole teeth (SPT), an experiment observation apparatus has been designed to direct monitoring of morphological transformations in the ferrofluid film. The pressure transfer and failure processes of CFFS-SPT during the pressurization process was investigated experimentally. The effects of eccentricity distance of axial pole teeth and magnetic isolation materials on pressure resistance of the seal were investigated. The results indicate that the pressure transfer in the radial chamber of CFFS-SPT is faster than that in the axial chamber. When seal fails, CFFS-SPT experiences micro-leakage and complete leakage stages. Moreover, the change in eccentricity distance of axial pole teeth does not affect the sealing mechanism.

Modeling shape deformation of ferrofluid ring section and secondary flow in ferrofluid seals with rectangular polar teeth using BEM and FVM

The focus of this study is on modeling the shape deformation of ferrofluid ring sections and secondary flows in ferrofluid seals with a rectangular polar tooth. These factors determine the sealing capability and homogenization of particles in ferrofluids. The model consists of governing equations for ferrofluid flows, the balance equation of normal stresses on ferrofluid free surfaces, the magnetic field equations and the ferrofluid volume equation. To solve the mathematical model, a method utilizing a combination of boundary element method and finite volume method is proposed. Discretizing the azimuthal velocity equation with boundary element method enables efficient determination of ferrofluid-ring section shape deformation. Furthermore, a discretely fitting method is proposed to solve the issue of lack of analytic expressions for the magnetic field distribution in sealing structures with rectangular polar teeth. Numerical tests of the magnetic field and azimuthal velocity distribution verify the validity and accuracy of the proposed solution method. Numerical experiments conducted on an actual ferrofluid sealing structure show that increasing the rotational speed of the shaft, in conjunction with effect of the centrifugal force, leads to deformation of the ferrofluid-ring section shape and reduces the contact area between the ferrofluid ring and shaft. This results in a decrease in pressure resistance. Typically, at a speed of 10 m/s, the pressure resistance falls by around 20%. The maximum value of secondary-flow velocity is significantly affected by the rotational speed of the shaft, although it does not exceed 0.8% of the linear velocity on the shaft surface in our study.

Multi-parameter optimization of ferrofluid seal pole teeth based on magnetic-flow coupling and taboo genetic algorithm

Ferrofluid seals are a high-performance seal technology. Rational pole tooth structure and size can enhance the performance of ferrofluid seals. However, most current methods for optimizing pole tooth structure and size ignore the influence of the flow field. Therefore, this paper presents a pole tooth optimization method based on magnetic-flow coupling and considers the shaft rotational speeds. Combining this method with the taboo genetic algorithm establishes the mathematical model and simulation model of magnetic pole tooth size optimization. The optimal pole tooth size at different rotational speeds is obtained through optimization. Additionally, the effects of rotational speed and pole tooth parameters on pressure capacity are studied. The results show that both the rotational speeds and the parameters of the pole teeth significantly affect pressure capacity. The rotational speeds also influence the parameters of the pole teeth.

Preparation of diester-based ferrofluid and applied research in sealing

To improve the sealing performance of ferrofluid, a Fe3O4 magnetic nanoparticles with different coating times was synthesized by the chemical co-precipitation method, and the performance of diester-based Fe3O4 ferrofluid was investigated and characterized. A convergent ferrofluid seal (CFFS) device was designed and the prepared ferrofluid was applied to it. The influence of important factors such as seal clearance and pole teeth of CFFS on the pressure capability of the CFFS are investigated using simulations and experiments. The results show that the prepared magnetization intensity of the ferrofluid is 29.5 kA/m. The simulation and experimental results of the CFFS agree well, and its performance is 3.3 times that of the ordinary ferrofluid seal.

Influence of Magnetic and Rheological Properties on a Leakage Channel in a Fluid Ring in Ferrofluid Seal

A ferrofluid ring can be compared to a liquid O-ring seal. This liquid ring is used in various applications, such as seals, valves, compressors, dampers, or thrust bearings. Its main advantage is that it maintains tightness and is held in a given place by a magnetic field. One of the main problems is the occurrence of leakage due to pressure differences. This affects the volume of liquid ejected outside the ferrofluid ring region and the value of the leakage itself. The research area of this publication is related to the recognition of how magnetic and rheological properties influence the mechanism of leakage channel formation in a single fluid ring. The purpose was to research basic parameters that characterize the leakage channel, such as the pressure jump value, the cross-sectional area, airflow, and leakage value. The results allow us to understand this problem better and provide insight into the behavior of magnetic particles in a colloid. Studies have shown that an increase in the number of magnetic nanoparticles increases the cross-sectional area of the leakage channel. The opposite effect is observed when a viscosity increase affects fluid resistance to deformation. The study also shows that the share of large particles does not significantly influence the leakage channel. However, the magnetic particles viscous drag affects the ability of particles to react to change.

Start-up test of ferrofluid seal based on rheometer

Sealing is one of the most successful applications of ferrofluid. However, the seal often has a sudden change in speed during the start-up process, which causes unstable vibration of the system and may result in accidental failure of the seal. In this work, we designed a ferrofluid seal test equipment based on rheometer to study the start-up characteristics of ferrofluid seal. We scan the rotational speed of the ferrofluid seal to determine the torque range of the fixed torque start, and found that the magnetic field strength and the number of pole teeth will affect the torque value and the slope of the curve. We experimentally analyzed three start-up modes of ferrofluid seal. During the start-up process at a fixed rotating speed, the torque fluctuates periodically, and the fluctuation amplitude increases with the increase of magnetic field strength or the number of pole teeth. During the start-up process of fixed torque, the rotating speed fluctuates periodically, and the fluctuation amplitude declines with the increase of torque. In the process of variable torque start-up, as the number of torque changes increases, the rotation speed increases more slowly and the fluctuation range is smaller, which leads to a more stable start-up of the ferrofluid seal. Ultimately, we believe that variable torque starting will be used to promote the application of ferrofluid seals in high-end precision applications.

Numerical and experimental investigation of a large gap convergent ferrofluid seal with the stepped pole piece

To solve the problem of low pressure resistance of the conventional ferrofluid seal (FFS) under the condition of single magnetic source with large gap, which is difficult to meet the demand of vacuum sealing, the convergent FFS with stepped pole piece structure was designed. The designed structure is also compared with the conventional FFS structure. The effects of radial and axial sealing gaps (SGs), number of steps, step height on the pressure resistance and self-repairing ability of the seal were experimentally investigated. The results show that the new FFS structure can withstand 1 atm when the number of steps is 4 and the radial SG is 0.4 mm, which can provide a reference for vacuum sealing. The innovative FFS has a maximum pressure resistance 23 times higher than the conventional FFS for the same size. The self-repairing ability of the seal is negatively correlated with the radial and axial SG, the number of steps, and the height of the steps. The results of this study are important for the design of the FFS structure with single magnetic source that meets the requirements of a vacuum environment.

Performance of the ferrofluid-spiral labyrinth combined seal for sealing liquid

When the ferrofluid is used to seal liquid, the relative movement between the ferrofluid and the sealed liquid causes the stability problem of their interface, and the sealing performance is poor. However, the spiral and the spiral sleeve of the spiral labyrinth seal interact with the sealed liquid to make it reverse flow to prevent leakage when the shaft rotates. In order to improve the performance of rotary ferrofluid seal, the ferrofluid-spiral combined seal structure and the ferrofluid-spiral labyrinth combined seal structure are designed. A combined seal test-bench is built. The pressure resistance experiments and seal life experiments of ferrofluid seal and two type of combined seals are carried out. The theoretical and experimental results show that the sealing performance of the ferrofluid-spiral labyrinth combined seal is better than that of the ferrofluid-spiral combined seal ferrofluid seal. Ferrofluid-spiral labyrinth combined seal can not only solve the failure problem of ferrofluid seal at high speed, but also solve the leakage problem of spiral seal at shutdown and low speed, so as to achieve a more stable sealing effect at different speeds. The higher the speed is, the better its sealing performance is.

Development of a Model and Experimental Study of Thermal Processes in a Ferrofluid Sealer

The aim of the work is to create an interconnected numerical model of the magnetic, hydrodynamic and temperature fields of a ferrofluid sealer and to analyze thermal processes occurring in highspeed seals. This goal is achieved by selecting the necessary equations, boundary conditions, assumptions and physical properties of the magnetic fluid when building the numerical model of the sealer’s working gap, verification of the developed model by the results of the physical experiment. The important results of the work are the obtained and analyzed data on the influence both of physical properties and the geometry of the working gap of the ferrofluid sealer on the heating of the ferrofluid. With a shaft radius of 140 mm and a linear velocity at the shaft surface of 25 m/s due to viscous heating the ferrofluid temperature exceeding the ambient temperature can reach values up to 80 degrees and higher, it has been shown. The use of the equation proposed by V.E. Fertman to determine the thermal conductivity of ferrofluid and the mixing rule to determine its heat capacity allows us to describe with sufficient accuracy for engineering calculations the thermophysical properties of concentrated ferrofluids, it was shown. The significance of the results consists in the possibility of using the developed numerical model in the study of interrelated physical processes in the working gap of the ferrofluid sealer of rotating shafts. The physical and concentration parameters of the synthetic oil-based magnetic fluid given in the paper and the results of its test operation as part of a ferrofluid seal can be used to verify the results of newly developed models of ferrofluid devices.

Experimental study on the critical pressure of converging ferrofluid seal with alternating pole teeth

For the improvement of pressure capability in ferrofluid seal with a small gap, a structure of converging ferrofluid seal (CFS) with alternating pole teeth is proposed in this study. Distribution of magnetic field in this structure is analyzed by using finite element method, and then theoretical values of withstanding gas pressure are calculated. The influence of ferrofluid volume, sizes of radial and axial sealing gap, numbers of radial and axial pole teeth on the critical pressure of CFS with alternating pole teeth is studied on the test bench. Then experimental results are compared and analyzed with the experimental pressure resistance of ordinary stepped ferrofluid seal. It is demonstrated that the calculated value of pressure resistance of the CFS with alternating pole teeth fits well with the experimental data. The volume of the injected ferrofluid can reach saturation. The sealing performance of CFS with alternating pole teeth is related not only to the clearance between pole teeth and stepped shaft, but also to the number of radial and axial pole teeth. The sealing performance of CFS with alternating pole teeth is better than that of ordinary stepped ferrofluid seal.

КОНЕЧНО-ЭЛЕМЕНТНОЕ МОДЕЛИРОВАНИЕ МАГНИТНЫХ ПОЛЕЙ МАГНИТОЖИДКОСТНЫХ УПЛОТНЕНИЙ

The paper shows the process of constructing a model of a magnetic system of a ferrofluidic seal by means of femm 4.2. The results of a finite element simulation of a magnetic field are presented. It is found out that finite element modeling of the magnetic field gives the opportunity to assess qualitatively the efficiency of the magnetic system of ferrofluidic seals.

Experimental Study and Simulation on the Seal Pressure of a Ferrofluid Seal Using Radial-Charged Magnets

As one of the most mature applications of ferrofluid, the pole shoes of a ferrofluid seal are easily damaged under the situation of the large radial beating of the main shaft. Meanwhile, due to the small size, the pole teeth commonly used in pole shoes harbor problems such as poor processability and low seal pressure under a large gap. In order to solve the aforementioned problems, we proposed a new structure of the ferrofluid seal that uses a radial-charged ring magnet as the magnetic source. Then, considering that the ring magnets have problems of uneven magnetization and installation difficulties, we used split magnets to take the place of ring magnets and proposed the second new structure. According to the study on the distribution of magnetic field in seal gap axially and circumferentially, we obtained the simulation results of the two new structures’ seal pressure. The results revealed that the new structure of the ferrofluid seal with rectangular magnets can solve the aforementioned problems and exhibited a certain amount of sealing pressure.

Pressure capability analysis of magnetic powder seals and pole tooth design by multiparameter optimization

Ferrofluid seals have been an increasingly popular sealing technique with unique advantages like zero leakage, long lifetime and low friction torque. But ferrofluid seals are incapable of specific working conditions, due to intolerance of extreme temperatures and limited pressure capability per stage. Recently, a novel sealing method using magnetic powders has been put forward to solve the problems. However, studies on the seal performance and theory of design are still blank. Here for the first time, we give a theoretical analysis of pressure capability of magnetic powder seals. The formula of pressure capability is derived, and a determinant coefficient is put forward. For sealing structure design, four derivative-free optimization algorithms are used to optimize geometric parameters of pole teeth. Finally, a sealing test bench was established, and the experimental data were compared with the simulation results. This research provides a basic theoretical and experimental guidance for magnetic powder seals.

Sealing Behavior Research on the Radial Ferrofluid Seal Structure with Oblique Teeth

Considering various unstable factors when the large-diameter spindle operates at a high speed, such as eccentricity, centrifugal force, etc., the ordinary ferrofluid seal structures will show poor sealing performance. This paper proposes an axial-radial bidirectional ferrofluid seal structure with radial oblique teeth to improve the sealing performance. The pressure resistance of the radial ferrofluid seal structure in the magnetic circuit is theoretically analyzed. The magnetic flux distribution characteristic in the gap of the oblique teeth is studied by magnetic field simulation. According to the analysis results of the magnetic induction intensity, to obtain the larger theoretical pressure resistance, the optimal angle of oblique teeth is 77.87°, 64.28° and 62.81° under the radial seal structure with different gaps of 0.1 mm, 0.15 mm and 0.2 mm, respectively. In addition, simulation analysis is carried out to obtain the fluid pressure and velocity distribution of the radial ferrofluid seal structure with different oblique teeth angles. When the oblique teeth angle is small, the pressure drops and gas flow speeds in the ferrofluid area are all lower, and the pressure resistance is higher.