taniguchi_et_al_2003
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taniguchi_et_al_2003

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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, rotational