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Rotary Stirring of Liquid Metal without Free Surface Deformation by 
Combination of Rotational and Vertical Traveling Magnetic Fields 
 \u2013 Development of Hybrid Stirrer \u2013 
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 
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. 
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 
 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. 
 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]
Liquid metal 
Coil for 
Vertical Flow 
Coil for 
Coil for 
Coil for 
Vertical Fow 
Fig.3. Schematic of hybrid electromagnetic stirrer.
(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/µe\u3c9\u3c3)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, rotational