Suppression of temperature fluctuations by rotating magnetic field in a large scale Rayleigh-Benard cell

Temperature fluctuations are observed experimentally in a cylinder of liquid mercury heated from below and cooled from above under action of a rotating magnetic field (RMF). The Grashof number (Gr) exceeds that of earlier studies by a factor of 10 at least. The setup models a proposed modification of the Czochralski crystal growth process where crucible rotation is replaced by RMF. It is known that the RMF characterized by the magnetic Taylor number Ta=15Gr^0.8 suppresses buoyant temperature fluctuations in a cylinder cooled from above [1]. To achieve a comparable effect by a mechanical rotation of the crucible, a considerably higher rotation rate is required and the difference grows with Gr. The effect has been demonstrated up to Gr~5x10⁷ that corresponds to a medium size gallium arsenide liquid encapsulated Czochralski process. Does the “0.8” scaling keep as Gr is increased? This is the central question of the study. One of the possible explanations of the phenomenon is a hypothetical stabilizing action of the core meridional flow. It is known that the angular velocity scaling Omega~Ta^2/3 experiences a change at Ta~2x10⁷ (that is able to stabilize the Gr~5x10⁷ configuration). Besides, it is known that the product of mean radial velocity Vr and mean angular velocity Omega equals the magnetic force (Ta~V, Omega). Thus, if the meridional flow indeed plays a major role, then there should be a change of the Tacr(Gr) scaling above Gr~5x10⁷. The experiment will allow Gr in the range [5x10⁷ : 5x10⁸]. It is prepared and scheduled for April 2010