Experimental study of the suppression of Rayleigh-Bénard instability in a cylinder by combined rotating and static magnetic fields

Abstract

We consider experimentally transitions in a liquid metal cylinder heated from below and subject to superimposed rotating and static magnetic fields. The applied static magnetic fields are too weak to influence the characteristic velocity of the rotating field driven basic flow. Being itself turbulent, a strong enough magnetically driven flow suppresses considerably the temperature fluctuations due to the thermogravitational convection. The remaining background fluctuations are caused by unsteady Taylor vortices generated near the sidewall. Our experiment shows that the superimposed static "cusp" magnetic field reduces the amplitude of these remaining temperature fluctuations by a factor of 4, compared to the case with a superimposed uniform axial field. The observed behavior agrees well with the static field effect on the amplitude of the additional unstable Taylor vortex-type solutions. These solutions bifurcate subcritically and represent the governing structures in the background turbulence. Thus, the observations are consistent with the description of the background turbulence as an irregular phase trajectory around the skeleton of the subcritical flow states. If this "skeleton" is compressed by an external influence ͑the cusp static field in our case͒, then also the amplitude of turbulent fluctuations decreases by the same factor. Another effect of the cusp field is to sharpen the transition between buoyancy and magnetic forcing dominated regimes. This allowed us to obtain an empirical expression for the conditions of this transition. We conclude that the rotating magnetic-field-driven flow suppresses the buoyant flow at a much lower angular velocity than a rigid-body mechanical rotation.

The suppression of temperature fluctuations by a rotating magnetic field in a high aspect ratio Czochralski configuration

“…4. The dashed line of slope OðGr 0:8 Þ is the extrapolation of our previous experimental results in a smaller cell [5]. …”

“…A higher initial melt level, in turn, enables the growth of larger crystals in the same crucible. The stabilizing effect of an RMF has been studied so far in relatively small cylinders only [3][4][5][6]. Can this tendency be extrapolated to higher parameters characteristic for medium size Czochralski techniques?…”

“…So, what may then be the advantage of an RMF? A recent investigation showed that the necessary angular rotation rate is several times lower in case of an RMF [5] as compared to a mechanical rotation, and this difference increases with the Grashof number. Thus, the RMF may have a potential to increase the initial melt level, particularly in a larger crucible.…”

“…This limits the initial melt depth since the thermo-gravitational instability depends mainly on the vertical scale [2]. Being itself turbulent, a sufficiently strong rotating magnetic field (RMF) driven flow is able to suppress significantly the amplitude of the buoyancy driven temperature fluctuations [3][4][5]. The mechanical rotation usually applied in Czochralski growth is also suppressing the thermo-gravitational instability [2].…”

Travelling magnetic fields applied to bulk crystal growth from the melt: The step from basic research to industrial scale

“…In this paper, the capacity even of TMF for industrial applications will be discussed. Further, a higher versatility is expected from combined fields like SMF with RMF [37,38], SMF with AMF [39], RMF with TMF [40] or AMF and TMF with SMF [41]. Of course, in the end economical constrains decide over the applicability of such combinations in production.…”