stirring, 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. 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 \u3b8-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  The Division of High-Temperature Processes, ISIJ, Innovative Propositions to Achieve High Efficiency in Refining and Solidification Processes, ISIJ, 2002.  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.  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