The factors, which are insignificant at small velocities, become rather considerable with increasing the shaft surface velocity as far as the magnetic fluid sealing complex (MFSC) is concerned They impact both on the pressure drop restrained by the sealer and on MFS operational resource.

Primarily, MFSC hydrodynamics is complicated because centrifugal forces are added to magnetic forces to restrain magnetic fluid (MF) in the working gap, and, moreover, radial pressure gradient ~*ν*^{2}/ R due to fluid motion is added to axial pressure gradient resulted from magnetic volume force *μ*_{I}M_{S}Î.

While the rotation frequency and the sealed shaft diameter are increasing the linear velocity of the shaft surface and the value of centrifugal forces are also increasing. These forces throw the fluid off the shaft surface which causes a namber of negative consequences, including the complete ejection of magnetic fluid from the working gap.

The problem of flow stability between the coaxial cylinders (Taylor Couette flow) is considered classical for hydrodynamical and magnetohydrodynamical stability. The specific feature of the MFS gap in comparison with the gap in the hydrodynamics of conventional fluid is the fact that it is of rather complicated geometrical form (it is necessary for focusing the magnetic flux and the fluid volume is limited by not only the solid but also by the deformed free boundaries. The problem is also complicated by the following factors: the probable non-uniformity of the working gap, the non-linear characteristics of magnetization and magnet reological properties of the magnetic fluid itself.

Moreover, these rotational flows are also specific because the secondary laminar regime immediately occurs when the primary laminar flow loses its stability but not turbulent flow, the system of period laminar flows straps on the primary laminar flow along the shaft axis of meridian vortex, so called Taylor vortexes [1].

Before they emerge, the fluid’s laminar flow is damaged because the axial wave process vortexes emerge, the velocity of strapped waves being equal to 1/3 of the velocity on the shaft surface and the third harmonic of the vibrational process emerging (mode *m* = 2) [2]. Azimuth component of the velocity vector will be divided into a series of odd harmonics and around the gap there will be the integre number of waves, the process being discrete: screw lines of the current are closed and the flow is quasilaminar.

The paper [1] shows that secondary laminar flow is unstable even in the weak magnetic field. It is in a good accord with the data referred to in [3], where they indicate that the increase in shaft rotation can take place at the modes *m* = 0 and *m* = 1 in weak magnetic fields. It should be kept in mind that it is practically impossible to determine which mode is the most unstable because the mode depends on the value of magnetic field, MF properties etc.

The experience in bench testing [4] and the author’s survey for MFS service in “Ferrohydrodynamics” shows that the problems of MF ejection from the gap are due to the excess in the linear velocity in the gap of 15 m/sec. It was noticed visually that fine disperse MF drops are thrown off to the azimuthal direction, which testifies to the fact that the development of instable free surface causes the ejection. When the linear velocity is increasing up to 35 m/sec, it is impossible to restrain magnetic fluid in the MFSC working zone of any classical design. Primarily, it depends on the fact that during the execution of concentrators of the magnetic flux, the action of centrifugal forces on magnetic drives of the sealer coincides with the direction of magnetic induction gradient.

It is known that the issues of friction in high-velocity magnetic fluid sealing complexes have been described in detail. The friction in MFSCs is insignificant, but some researches [6, 7] etc., note that it should not be neglected because viscous friction leads to MF heating, its subsequent evaporation and desorption of the molecules of surface active substances from the particles surface. The both factors put magnetic fluid out of operation in the sealer gap and the magnetic fluid sealer itself. At the same time the analysis of the operation of more than 4000 magnetic fluid seal complexes under various operational conditions conducted by “Ferrohydrodynamica” shows that the failures, due to magnetic fluid temperature heating, have not been registered. The contradiction is explained in [8]: for high-velocity MFSCs it is important to take into account thermo-magnetic convection which results in the intensive circulational flow in the meridional plane in the narrow working gap above the pole, thus reducing the fluid temperature in this region. Generally speaking, it is the positive factor, therefore special measures for additional flows should not be taken.

To increase the operational capability of high-velocity magnetic fluid seals, it is necessary to take a number of measures for restraining magnetic fluid in the working gap, excluding the impact of centrifugal forces by means of MFSC design changes.

The execution of shaft labyrinths is the most effective and widely used method in sealing technology (hydrodynamical, impeller, labyrinth, slot seals, etc.) for reducing the influence of centrifugal forces [9]. Tooth concentrators of the magnetic flux can similarly be made on the shaft but not on the magnetic circuits. In this case centrifugal forces throw magnetic fluid to the tooth top into the region of maximal magnetic field blocking the decrease in the critical pressure drop and magnetic fluid throw-off from the gap.

In [3] the author attracts the attention to the fact that there has not been obtained the criterion for classifying rotation as stable or instable for Couette flow of complex geometry of the working gap. The nature of instable flow of magnetic fluid in the MFSC is determined for each particular case because they are determined by magnetic fluid properties.

Therefore it is only direct numerical calculation that can define the flow character. The aim of the research is to analyse the interaction of magnetic and centrifugal forces in the seal working gap and consequently, to design the magnetic fluid seal complex for wider applications.

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