taniguchi_et_al_2003

Prévia do material em texto

Rotary Stirring of Liquid Metal without Free Surface Deformation by 
Combination of Rotational and Vertical Traveling Magnetic Fields 
 – Development of Hybrid Stirrer – 
S.Taniguchi1*, K.Maitake2, M.Okubo2, T.Ando2 and K.Ueno2 
1: Graduate School of Environmental Studies 
2: Graduate School of Engineering 
Tohoku University 
Sendai, Japan 
 
Abstract 
An electromagnetic rotary stirrer without free surface deformation is proposed and its performance is 
investigated. This stirrer is composed of two sets of 3-phase coils which impose rotational and vertical 
traveling magnetic fields in a liquid metal simultaneously. Free surface shapes, rotational speeds, and 
velocities of liquid gallium are measured for three cases; rotary stirring, vertical linear stirring, and 
hybrid stirring. It is found that the hybrid stirrer realizes flat free surface at high rotational speed and 
strong turbulence in liquid metal. 
 
Introduction 
Stirring of liquid metal has been applied to realize various metallurgical purposes like 
homogenization of temperature and chemical composition, increase in refining rates, and enhancement 
of inclusion removal. In steelmaking processes, strong stirring is recently noticed again to keep the 
production rate by curtailed equipments [1]. In the field of recycling processes, the strong stirring is 
expected to achieve quick melting of metal scraps and separation of nonmetallics [2]. Furthermore, in 
the field of foundry, semisolid casting under intense stirring is expected to realize fine microstructures 
[3]. Electromagnetic (em) stirring is an exceptional method to impose strong agitation in liquid metal 
without contact. Among various em stirrers, rotary stirrers are well developed and widely applied from 
laboratory-scale to commercial scale equipments [4]. However, the strong centrifugal force results in 
the limitation of the power input because of the free surface deformation and the weak mixing due to 
the suppression of turbulence [5]. 
 The purpose of the present study is to develop 
a new rotary stirrer that enables to enhance 
mixing under the suppression of the free surface 
deformation. 
 
Principle 
 Figure 1 indicates a helical induction stirrer 
proposed by Vives [6]. A cylinder on which 
permanent magnets are arranged helically is 
rotated around a liquid metal container. In this 
case, the metal will receive not only rotational 
forces but also vertical forces. As a result, the 
deformation of the free surface of the metal will 
be suppressed as seen in case (a) of Fig.1. 
Stimulated by this excellent idea, Ueno et al. [7] 
proposed a liquid metal pump composed of 
helical coils which generate rotating twisted 
magnetic field. Figure 2 shows the principle of 
this em pump. 
 
 
* s-tanig@material.tohoku.ac.jp 
Associated Web site: http://www.material.tohoku.ac.jp/~kino/lab%20eng.html 
Proceedings of the Electromagnetic Processing of Materials International Conference 2003 
Fig.1. Helical induction stirrer proposed by Vives.[6]
(a) (b)
 Based on the above works, the performance 
of two types of rotary stirrer is investigated. One 
is the application of the above two principles to 
a rotary stirrer (case-1), and the other is a hybrid 
rotary stirrer composed of two coil systems; one 
is for a rotational traveling field and the other is 
for a vertical traveling field, each of which the 
current is regulated independently (case-2). 
(1) Case-1 
 In the case of em pump, two-pole coil system 
is adopted to impose uniform em forces to liquid 
metal. However, in the case of rotary stirrer, 
downward forces should be localized in the 
outer area where the metal surface is raised. For 
this purpose, 4-pole-coil system is adopted. 
Twelve pieces of oblong coils are arranged 
around the outside of the metal container so as to 
be inclined at an angle of 45 degrees. 
(2) Case-2 
 Figure 3 shows a schematic of the coil arrangement for case2. For axially traveling magnetic field, 
6-pieces of circular coils were arranged to generate a 2-pole magnetic field. Outside these coils, six 
pieces of oblong coil for rotationally traveling magnetic field are arranged along circumference to 
form 2-pole magnetic field. High frequency currents are imposed to the former coils to establish a 
downward flow only in the outer region of the melt by using the skin effect. 
 
Measurements 
 Electric currents were led from a 50-Hz power source to the coil system of case-1 and that for 
rotational flow in case-2. To the coil system for vertical flow in case-2, high frequency currents were 
led through an inverter regulator with variable frequencies. Liquid gallium was contained in a plastic 
crucible with 50-55 mm inside diameter and 100 mm liquid metal height. Coil systems were fixed in 
an annular glass vessel filled with cooling oil. 
 Free surface shapes were measured by an electrical probe method in which a tungsten wire was used 
for the contact probe. The rotational speed of the liquid metal was measured from the rotational speed 
of an impeller which was immersed in the liquid metal. Vives probe was applied to measure the 
velocities of the liquid metal. In the measurements, a low pass filter was adopted to remove the noise 
from the traveling magnetic field generated by the coil system. 
 
Fig.2. E.M. pump proposed by Ueno et al.[2]
Regulator-1
Liquid metal 
Crucible 
Coil for 
Vertical Flow 
Regulator-2
Coil for 
Rotation 
Coil for 
Rotation 
Coil for 
Vertical Fow 
Metal 
Fig.3. Schematic of hybrid electromagnetic stirrer.
Results 
(1) Case-1 
Figure 4 describes the free surface profiles 
measured by the electrical probe method for 
case-1. Two different directions of traveling 
magnetic field are examined, one is upward and 
the other is downward direction. For both 
directions, profiles of free surface are almost 
same, namely, deep depression is formed by the 
centrifugal force due to strong rotational motion. 
From these results, the vertical flow is found to 
be so weak that the free surface deformation is 
not suppressed for case-1. This may be 
attributed to the larger resistance of vertical flow 
than that of rotational flow. It seems difficult to 
regulate the angle of coil inclination to make the 
both flows comparable. From these results, it is 
concluded that the rotary stirrer of case-1 is insufficient for the function to keep flat free surface 
during rotation even though it needs not two power sources like in case-2. 
(2) Case-2 
 The observed free surface profiles during stirring are shown in Fig.5. For the case of rotary stirring 
(Fig.5(a)), free surface depression is clearly seen. On the contrary, for the case of hybrid stirring 
(Fig.5(b)), the depression is reduced remarkably. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 (a) rotary stirring (b) hybrid stirring 
 
 Fig.5. Observed free surface profiles for hybrid stirring of liquid gallium 
with and without vertical flow (Case-2) 
 
 Free surface profiles measured by the electrical probe are indicated in Fig.6. For the case of rotary 
stirring, the difference in the height of free surface between center and rim reaches 10 mm. On the 
other hand, it decreases down to 2 mm for hybrid stirring. The skin depth, (2/µeωσ)1/2, of liquid 
gallium is 36 mm for rotary stirring (50 Hz) and 7.14 mm for vertical stirring (1300 Hz), respectively. 
 Figure 7 shows the relation between rotational speed of liquid gallium and imposed current in the 
coil for rotation. The rotational speed in the hybrid stirring is found to be smaller than that in the 
rotary stirring, nevertheless, the difference is less than 20%. Although these results indicate the 
rotational speed inside the liquid gallium, the rotation of free surface is also confirmed by observation. 
 Measured velocities of liquid gallium with Vives probe are shown in Fig.8 (a)-(c) for the cases of 
vertical stirring, rotationalstirring, and hybrid stirring. In the case of the vertical stirring, Fig.8 (a), 
magnetic field moves downward. There can be seen vigorous fluctuation composed of not only short 
period (less than 1 second) but also long period (several seconds). The short period fluctuation seems 
to be turbulence of liquid flow, and, on the other hand, the long priod fluctuation may be formed by 
the interference between downward and upward flows generated in the liquid gallium. On the 
-5 0 5 10 15 20 25
-3
-2
-1
0
1
2
400Hz, 3.65A
 
 
H
ei
gh
t/m
m
Radius/mm
 Upward
 Downward
Fig.4. Free surface profiles for case-1.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
contrary, there can be seen very small fluctuation 
for the case of simple rotational stirring, Fig.8 (b). 
For the case of rotational stirring, as Spitzer et 
al.[5] pointed out, the rotational speed in the outer 
region is faster than that in the inner region, so that 
turbulent flow is hard to be generated. In such case, 
mixing of liquid metal will be insufficient even if 
a higher power is imposed. Finally, Fig.8 (c) 
indicates the case of hybrid stirring. In this case, 
the level of turbulence is comparable to the 
vertical stirring (Fig.8 (a)) and the long period 
fluctuation seen in the vertical stirring disappears 
completely. Such tendency was also seen in the 
observation of the free surface fluctuation. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Conclusions 
 The present study has been carried out to develop a new rotary stirrer without free surface 
depression that is an essential problem in conventional rotary stirrers if a strong stirring is needed. The 
following results are obtained. 
0 5 10 15 20 25
-10
-8
-6
-4
-2
0
2
4
6
 
 
H
ei
gh
t/m
m
Radius/mm
 Rotatry stirring
 Hybrid stirring
7 8 9 10 11 12
200
250
300
350
400
 
 
R
ot
at
io
n 
sp
ee
d 
/rp
m
Current(rotation) /A
 Rotary stirring
 Hybrid stirring
Fig.6. Free surface profiles for case-2. 
 50Hz, 7.5A for rotary stirring, 
1300Hz, 12A for vertical stirring. 
Fig.7. Rotational speed as a function of 
 imposed current for rotary and hybrid 
stirring.
Fig.8. Velocities of liquid gallium measured by Vives probe for (a) vertical stirring, (b) rotational 
stirring, and (c) hybrid stirring. 
0 5 10 15
-100
-50
0
50
(a) Velocity in z-direction
 Average:-25.7 cm/s
 Downward (1300Hz,12A)
 
 
Ax
ia
l v
el
oc
ity
/c
m
 s
-1
Time/s
0 5 10 15
-50
0
50
100
(b) Velocity in θ-direction
 Average:29.1cm/s
 Rotation (50Hz,7.5A)
 
 
C
irc
um
fe
re
nt
ia
l v
el
oc
ity
 /c
m
 s
-1
Time/s
0 5 10 15
-100
-50
0
50
(c) Velcity in z-direction
 Average: -12.7 cm/s
 Hybrid: downward(1300Hz,12A)
 rotation(50Hz,7.5A)
 
 
Ax
ia
l v
el
oc
ity
 /c
m
 s
-1
Time/s
(1) A stirrer with helical coils of which the angle of inclination is 45 degrees is examined of its 
performance by using liquid gallium. Free surface profile measured by an electrical probe is not 
improved at all. It is thought that the flow resistance in the circumference direction is much 
smaller than that in the vertical direction, and as a result, the vertical flow which is expected to 
lower the raised surface in the outer region does not work well. 
(2) A hybrid stirrer composed of two coil systems generating vertical and rotational flow, is proposed 
and its performance is examined. The profile of the free surface of liquid gallium measured by an 
electrical probe is found to be almost flat if the downward flow is generated in the outer area of 
the liquid metal. 
(3) Rotational speed of liquid gallium measured by an impeller immersed in a liquid metal is slightly 
smaller in the case of hybrid stirring compared with the case of rotary stirring. 
(4) Liquid velocities are measured by Vives probe. For the case of vertical stirring without rotation, 
there can be seen a strong turbulence accompanied by long period fluctuations. This long period 
fluctuation also appears as the vigorous free surface oscillation. For the case of simple rotary 
stirring, the level of turbulence is quite small because of the suppressing effect of turbulence in a 
rotating flow filed. For the case of hybrid stirring, strong turbulence is observed but the long 
period fluctuation is not observed. This means that the hybrid stirring is able to generate intense 
turbulence without free surface deformation or oscillation. 
 
Acknowledgment 
 This study was partly supported by the Ministry of Education, Science, Sports and Culture, Grant- 
in-Aid for Scientific Research on Priority Areas (1998-2001). 
 
References 
[1] The Division of High-Temperature Processes, ISIJ, Innovative Propositions to Achieve High 
Efficiency in Refining and Solidification Processes, ISIJ, 2002. 
[2] T.Ozono, Research and Development of Advanced Technology on Recycling of Aluminum 
Materials, The Journal of Japan Institute of Light Metals, 50 (9) (2000) 468-474. 
[3] O.J.Ilegbusi, J.Szekely, Mathematical Representation of the Velocity, Temperature and Solid 
Fraction in an Electromagnetically Stirred Solidifying Melt, ISIJ International, 30 (5) (1990) 372-
380. 
[4] Y.Miki, H.Shibata, N.Bessho, Y.Kishimoto, K.Sorimachi and T.Hirota, Cleaning Molten Steel with the 
Centrifugal Flow Tundish, Tetsu-to-Hagane, 86 (4) (2000) 239-246. 
[5] K.H.Spitzer, M.Dubke and K.Schwerdtfeger, Rotational Electromagnetic Stirring in Continuous 
Casting of Round Strands, Metall. Trans.,B, 17B (March) (1986) 119-131. 
[6] C.Vives, Mew Electromagnetic Manufacturing Processes for Semisolid Alloys and Metal Matrix 
Composites Synthesis, Proc. of Int. Symp. on EPM, EPM’94, Nagoya, (1994), 223. 
[7] T.Ando, K.Ueno, S.Taniguchi and T.Takagi: Induction Pump for High-Temperature Molten 
Metals Using Rotating Twisted Magnetic Field, IEEE Transactions of Magnetics, 38 (4) (2002) 
1789-1796.