Macroscopic thermomagnetic convection: a more generic case and optimization

A. Cramer - X. Zhang - G. Gerbeth

Forschungszentrum Dresden--Rossendorf, Institute of Safety Research, P.O. Box~510119, 01314~Dresden, Germany

Abstract
It was demonstrated in a previous paper that the interaction between a thermoelectric current \bvec j and an imposed static magnetic field \bvec B produced thermoelectromagnetic convection (TEMC). The prominent feature was utilisation of a wall material exhibiting a big difference in thermoelectric power S in comparison to the fluid. Although high flow velocities were accomplished, the vigour was potentially mitigated by two design issues. Firstly, the side walls parallel to the temperature gradient \Grad T were made of the same electrically high conducting material as the isothermal walls. This, in parts, shorts the thermoelectric current. Moreover, these walls might be regarded as lowering the degree of the experiment of being generic. Secondly, the field created by a permanent magnet covered only a small fraction of the fluid volume. Again, albeit being mirror symmetric, the three-dimensional distribution of \bvec B lowers the degree of being generic. The present paper reports on an experimental study on TEMC in a square box, wherein the necessary \Grad T is accomplished by heating and cooling two opposing side walls, respectively, whereas the other two side walls are electrically non-conducting. An almost two-dimensional distribution of \bvec B is applied to a relatively large area of the interface between the fluid and the bottom of the container. The pole shoes of the magnet are specifically designed so as to have a high value of the curl of the Lorentz force \Rot \bvec F_L = \bvec ∇×(\bvec j ×\bvec B), the non-vanishing of which is another pre-requisite for TEMC. Two containers with different bottom materials are build. The ferromagnetic nickel with negative S used in [1] is replaced by isotan in one variant, offering probably the highest absolute value of S among metals. To consider also the counterpart, nichrome with a high positive S is used in the construction of the second container. Ultrasonic Doppler velocimetry is employed to quantify the TEMC flow field. The results of all three configurations are compared and discussed. In addition, first results on more developed turbulent regimes are presented, which could not be obtained in the previous setup [1] because of a more limited ∆S ·∆T. Figs 5, Refs 3.