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The behaviour of magnetic fluids under strong nonuniform magnetic field in rotating seal
Journal of Magnetism and Magnetic Materials 65 (1987) 223-226 North-Holland, Amsterdam
BEHAVIOUR OF MAGNETIC MAGNETIC FIELD IN ROTATING
THE
L. Vl?K&
I. POTENCZ,
Polytechnical
Institute of Timipara,
223
FLUIDS UNDER STRONG NONUNIFORM SEAL
D. BICA and R. MINEA 1900 Timipara,
Bd. Mihai Viteazul Nr. I, R.S. Romania
The time dependent local increase of magnetization and effective viscosity are determined for various magnetic fluids subjected to the action of a strong nonuniform magnetic field. Their rate of increase strongly depends on carrier liquid viscosity, particle concentration and dimensional distribution.
1. Introduction Many applications of magnetic fluids, e.g., seals, separators and bearings, utilize the action of strong nonuniform magnetic fields on the liquid. The longer the period r of unperturbed action of the field, the greater the structural changes are due to particle magnetophoresis. The redistribution of particle volume concentration and agglomerate formation produce local modifications of the saturation magnetization M, and effective viscosity verr. A wide variety of methods are currently used to evidence the magnetic field induced structural changes and their influence on the stability of magnetic fluids. In this work we report on the sealing capacity and friction moment changes determined for various magnetic fluids, subjected to the action of strong nonuniform magnetic fields.
2. Experimental
methods
The relative local increase of magnetization, as a function of t, in the case of a magnetic liquid sealing stage may be determined from the following approximative relation [ 11: [Q(t)
- Ap(O)l/Ap(O)J;
= [M,(t)
- M,(O)]/M,(O)J;
= k,
(1)
where Ap( t) is the burst pressure at the moment
t. The value of k corresponds to the initial linear part of the curve Ap = Ap(fi) [l]. The experimental device used [2] is similar to that described in ref. [l]. The pole piece with one or more sealing stages is interchangeable and is installed in the magnetic circuit of an electromagnet. The data presented in ref. [2] and in this paper correspond to 1VH ] = 8 x lo4 Oe/cm at a sealing stage. To evaluate the effective viscosity changes, ae measured the friction moment Mr, corresponding to a thin layer of magnetic fluid between two coaxial cylinders in relative rotating motion (fig. 1). The inner cylinder is a heavy magnetic core, including two annular Almco magnets with = 4.5 MGOe, which levitates on the (BH?,, magnetic liquid layer, while the external one is a light nonmagnetic casing, rigidly connected to the driving and measuring unit. The device has the characteristics of a rotary damper. The magnetic circuit of the core has four annular zones (fig. l), where the field is strongly nonuniform ( 1OH 1 = lo4 Oe/cm), as in the case of a sealing stage. During standstill the field induces structural changes and increases the effective viscosity qerr of the magnetic fluid, consequently it changes the damping characteristics of the device. The longer the period t of the action of the nonuniform magnetic field, the greater the modifications of the friction moment and of the settling time of the oscillatory motion are, which follows a step-like change in the rotation speed of the shaft, coupled
0304-8853/87/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
L. V.4kh.set al. / Magnetic fluid in nonuniform magnetic fields
224
Table 1 The values of coefficients k and K for some magnetic fluids of the same concentration (M, = 400 G)
h the driving and meowring onif mnmaqnetic materlo/ m EA
Magnetic fluid
magnet/? fluid perman en t magnet
FMP* HOl** FMPl*
5
102k
103K
(A)
(s-i’s)
(s-l)
135 120 105
3.37 1.17 1.00
4.64 1.48 0.94
* Prepared at the Polytechnical Institute of Timipara. ** Purchased from Ferrofluidics Corp. (USA) (1979) (Lot. No. PF 3131 L).
Fig. 1. Sketch of the friction moment measuring device (all dimensions in mm).
to the casing. The rotation speed step was chosen small enough, O-45 rpm, in order to cause only a minimal perturbation of the magnetic fhtid layer during measurements. Further details may be found in ref. [2]. The first part of the curves M, = M,(t) will be approximated with straight lines and then, supposing that Mr - 9, the friction moment increase may be characterized by the following coefficient:
-4@w4@) x t g heff0) - seffwl/%ffw~ = K-
the values of k and K are, as a consequence of the higher migration velocity of the large particles (see table 1). k and K have much greater values for the less well purified FMP magnetic fluid, with an important fraction of large particles, than for the carefully prepared HO1 and FMPl fluids. Several magnetic fluids with different qualities were examined using the above described meth-
!6
W,(r)
(2)
3. Results and discussions As expected, the mean particle diameter 5 has a significant effect on the migration process and agglomerate formation, that is, on the rate of increase of Ap and M,, during the period t of unperturbed action of the nonuniform magnetic field on the liquid. All the ferrofluids of the type FMP and FMUTR30 were stabilized with oleic acid. The Ap = Ap(fi) and iI4r = M,(t) curves [2], obtained for three petroleum based magnetic fluids of the same concentration, but with different dimensional distributions of particles [3], show that the greater the diameter of particles, the greater
8
6
IO
20
30
10
40
60
80
f/min)
Fig. 2. The behaviour of a petroleum based magnetic fluid of reduced stability.
225
L. V&is et al. / Magnetic fluid in nonuniform magnetic fields
ods. It is especially useful to compare the behaviour of magnetic fluids on the same basis and with the same concentration of particles. The Ap = Ap(fi) and M, = M,(t) curves offer the possibility to detect the deficiencies of their behaviour in nonuniform magnetic fields. This is well illustrated in fig. 2 in the case of a less well prepared petroleum based magnetic fluid. The resulting great values for k and ‘K, as compared to those obtained for HO1 and FMPl, indicate its low degree. of stability. The modifications of nerr induced during the time period t by a nonuniform magnetic field, include not only the effect of particle redistribution, but also that of the formation of particle structures of increasing complexity. Particle concentration and carrier liquid viscosity have a significant influence on the above processes (fig. 3). Increasing the concentration, the value of K reduces, denoting the effect of particle interactions
20
40
60
80
100
on the migration process. In the case of an approximatively one order of magnitude higher viscosity, transform oil based magnetic fluid, the particle migration is strongly inhibited. As the M, = M,(I) curves show (fig. 3), the HO1 and FMPl type magnetic fhtids have different behaviour. The reduction of K with increasing concentration is considerably greater for the HO1 magnetic fluid. This is probably due to some quantity of excess surfactant which increases its viscosity in comparison with FMPl, especially at higher particle concentrations. (The viscosity at 20°C of HO1 (700) is 32.3 CP versus only 13.4 CP of FMPl (700)). The measurements of M,, which are more sensitive to particle structure formation than those on Ap, evidenced also some kind of ageing effect (fig. 4). While the M,(t) curve for FMPl(400) is well reproduced after a period of more than one year, the more concentrated FMPl(700) fluid changed
no
140
f80
200 C(min)
Fig. 3. The effect of particle concentration and carrier liquid viscosity on the rate of increase of friction moment.
L. V&is
226 ff fP
et al. / Magnetic fluid in nonuniform magnetic field
ml
Fig. 4. The influence of particle concentration on the long-term behaviour of magnetic fluids.
its behaviour. The observed, at about 2.5 times, reduction of K is due, probably, to spontaneous particle structure formation, which is not observed at lower concentrations.
The rate of increase of the sealing capacity and that of the friction moment may be used to characterize and compare the behaviour of magnetic fluids in nonuniform magnetic fields.
4. Conclusions
References
Particle migration and structure formation in magnetic fluids subjected to the action of strong nonuniform magnetic fields is a time dependent process, strongly influenced by carrier liquid viscosity, particle concentration and dimensional distribution.
PI
S. Taketomi, Japan. I. Appl. Phys. 19 (1980) 1920.
PI I. Anton, L. Vekfts, I. Potencz and D. Bica, Magnitnaia Ghidrodinamika 3 (1985) 13, in Russian; see also St. cert. Met. Apl. 43 (1984) 451, in Romanian. [31 M. Trqculescu and M. Lifti, Rev. Roum. Sci.’ Techn.-M&. Appl. 30 (1985) 317.
BEHAVIOUR OF MAGNETIC MAGNETIC FIELD IN ROTATING
THE
L. Vl?K&
I. POTENCZ,
Polytechnical
Institute of Timipara,
223
FLUIDS UNDER STRONG NONUNIFORM SEAL
D. BICA and R. MINEA 1900 Timipara,
Bd. Mihai Viteazul Nr. I, R.S. Romania
The time dependent local increase of magnetization and effective viscosity are determined for various magnetic fluids subjected to the action of a strong nonuniform magnetic field. Their rate of increase strongly depends on carrier liquid viscosity, particle concentration and dimensional distribution.
1. Introduction Many applications of magnetic fluids, e.g., seals, separators and bearings, utilize the action of strong nonuniform magnetic fields on the liquid. The longer the period r of unperturbed action of the field, the greater the structural changes are due to particle magnetophoresis. The redistribution of particle volume concentration and agglomerate formation produce local modifications of the saturation magnetization M, and effective viscosity verr. A wide variety of methods are currently used to evidence the magnetic field induced structural changes and their influence on the stability of magnetic fluids. In this work we report on the sealing capacity and friction moment changes determined for various magnetic fluids, subjected to the action of strong nonuniform magnetic fields.
2. Experimental
methods
The relative local increase of magnetization, as a function of t, in the case of a magnetic liquid sealing stage may be determined from the following approximative relation [ 11: [Q(t)
- Ap(O)l/Ap(O)J;
= [M,(t)
- M,(O)]/M,(O)J;
= k,
(1)
where Ap( t) is the burst pressure at the moment
t. The value of k corresponds to the initial linear part of the curve Ap = Ap(fi) [l]. The experimental device used [2] is similar to that described in ref. [l]. The pole piece with one or more sealing stages is interchangeable and is installed in the magnetic circuit of an electromagnet. The data presented in ref. [2] and in this paper correspond to 1VH ] = 8 x lo4 Oe/cm at a sealing stage. To evaluate the effective viscosity changes, ae measured the friction moment Mr, corresponding to a thin layer of magnetic fluid between two coaxial cylinders in relative rotating motion (fig. 1). The inner cylinder is a heavy magnetic core, including two annular Almco magnets with = 4.5 MGOe, which levitates on the (BH?,, magnetic liquid layer, while the external one is a light nonmagnetic casing, rigidly connected to the driving and measuring unit. The device has the characteristics of a rotary damper. The magnetic circuit of the core has four annular zones (fig. l), where the field is strongly nonuniform ( 1OH 1 = lo4 Oe/cm), as in the case of a sealing stage. During standstill the field induces structural changes and increases the effective viscosity qerr of the magnetic fluid, consequently it changes the damping characteristics of the device. The longer the period t of the action of the nonuniform magnetic field, the greater the modifications of the friction moment and of the settling time of the oscillatory motion are, which follows a step-like change in the rotation speed of the shaft, coupled
0304-8853/87/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
L. V.4kh.set al. / Magnetic fluid in nonuniform magnetic fields
224
Table 1 The values of coefficients k and K for some magnetic fluids of the same concentration (M, = 400 G)
h the driving and meowring onif mnmaqnetic materlo/ m EA
Magnetic fluid
magnet/? fluid perman en t magnet
FMP* HOl** FMPl*
5
102k
103K
(A)
(s-i’s)
(s-l)
135 120 105
3.37 1.17 1.00
4.64 1.48 0.94
* Prepared at the Polytechnical Institute of Timipara. ** Purchased from Ferrofluidics Corp. (USA) (1979) (Lot. No. PF 3131 L).
Fig. 1. Sketch of the friction moment measuring device (all dimensions in mm).
to the casing. The rotation speed step was chosen small enough, O-45 rpm, in order to cause only a minimal perturbation of the magnetic fhtid layer during measurements. Further details may be found in ref. [2]. The first part of the curves M, = M,(t) will be approximated with straight lines and then, supposing that Mr - 9, the friction moment increase may be characterized by the following coefficient:
-4@w4@) x t g heff0) - seffwl/%ffw~ = K-
the values of k and K are, as a consequence of the higher migration velocity of the large particles (see table 1). k and K have much greater values for the less well purified FMP magnetic fluid, with an important fraction of large particles, than for the carefully prepared HO1 and FMPl fluids. Several magnetic fluids with different qualities were examined using the above described meth-
!6
W,(r)
(2)
3. Results and discussions As expected, the mean particle diameter 5 has a significant effect on the migration process and agglomerate formation, that is, on the rate of increase of Ap and M,, during the period t of unperturbed action of the nonuniform magnetic field on the liquid. All the ferrofluids of the type FMP and FMUTR30 were stabilized with oleic acid. The Ap = Ap(fi) and iI4r = M,(t) curves [2], obtained for three petroleum based magnetic fluids of the same concentration, but with different dimensional distributions of particles [3], show that the greater the diameter of particles, the greater
8
6
IO
20
30
10
40
60
80
f/min)
Fig. 2. The behaviour of a petroleum based magnetic fluid of reduced stability.
225
L. V&is et al. / Magnetic fluid in nonuniform magnetic fields
ods. It is especially useful to compare the behaviour of magnetic fluids on the same basis and with the same concentration of particles. The Ap = Ap(fi) and M, = M,(t) curves offer the possibility to detect the deficiencies of their behaviour in nonuniform magnetic fields. This is well illustrated in fig. 2 in the case of a less well prepared petroleum based magnetic fluid. The resulting great values for k and ‘K, as compared to those obtained for HO1 and FMPl, indicate its low degree. of stability. The modifications of nerr induced during the time period t by a nonuniform magnetic field, include not only the effect of particle redistribution, but also that of the formation of particle structures of increasing complexity. Particle concentration and carrier liquid viscosity have a significant influence on the above processes (fig. 3). Increasing the concentration, the value of K reduces, denoting the effect of particle interactions
20
40
60
80
100
on the migration process. In the case of an approximatively one order of magnitude higher viscosity, transform oil based magnetic fluid, the particle migration is strongly inhibited. As the M, = M,(I) curves show (fig. 3), the HO1 and FMPl type magnetic fhtids have different behaviour. The reduction of K with increasing concentration is considerably greater for the HO1 magnetic fluid. This is probably due to some quantity of excess surfactant which increases its viscosity in comparison with FMPl, especially at higher particle concentrations. (The viscosity at 20°C of HO1 (700) is 32.3 CP versus only 13.4 CP of FMPl (700)). The measurements of M,, which are more sensitive to particle structure formation than those on Ap, evidenced also some kind of ageing effect (fig. 4). While the M,(t) curve for FMPl(400) is well reproduced after a period of more than one year, the more concentrated FMPl(700) fluid changed
no
140
f80
200 C(min)
Fig. 3. The effect of particle concentration and carrier liquid viscosity on the rate of increase of friction moment.
L. V&is
226 ff fP
et al. / Magnetic fluid in nonuniform magnetic field
ml
Fig. 4. The influence of particle concentration on the long-term behaviour of magnetic fluids.
its behaviour. The observed, at about 2.5 times, reduction of K is due, probably, to spontaneous particle structure formation, which is not observed at lower concentrations.
The rate of increase of the sealing capacity and that of the friction moment may be used to characterize and compare the behaviour of magnetic fluids in nonuniform magnetic fields.
4. Conclusions
References
Particle migration and structure formation in magnetic fluids subjected to the action of strong nonuniform magnetic fields is a time dependent process, strongly influenced by carrier liquid viscosity, particle concentration and dimensional distribution.
PI
S. Taketomi, Japan. I. Appl. Phys. 19 (1980) 1920.
PI I. Anton, L. Vekfts, I. Potencz and D. Bica, Magnitnaia Ghidrodinamika 3 (1985) 13, in Russian; see also St. cert. Met. Apl. 43 (1984) 451, in Romanian. [31 M. Trqculescu and M. Lifti, Rev. Roum. Sci.’ Techn.-M&. Appl. 30 (1985) 317.