Investigation of hydrodynamics and heat transfer at liquid metal downflow in a rectangular duct in a coplanar magnetic field

Abstract

Hydrodynamics and heat transfer in a liquid metal downflow in a rectangular duct with an aspect ratio of approximately 3/1 in a coplanar magnetic field (MF) are studied upon inhomogeneous (one-sided) heating of the duct. The flow in the heat-transfer duct of the cooling system of a liquid-metal blanket module of the tokamak-type thermonuclear reactor is modeled. Experiments were carried out at the mercury magnetohydrodynamic (MHD) test facility, which is a part of the MHD-complex of Moscow Power Engineering Institute–Joint Institute for High Temperatures of the Russian Academy of Sciences. A probe technique is used for measurements in the flow. The studies are performed within the following ranges of regime parameters: Reynolds numbers Re = 10000–55000, Hartmann numbers Ha = 0–800, and Grashof numbers Grq = 0–6 × 108. Averaged profiles of velocity, temperature, temperature fluctuations of the flow, and duct wall temperature are presented for two typical flow regimes. Detailed measurements are performed in the duct cross-section distant from the heating beginning in the region of homogeneous MF. MF leads to the turbulent transport suppression, owing to which the temperature on the heated wall increases. A considerable influence of the heat-gravitational counter-convection, the interaction of which with the external MF leads in some regimes to the appearance and development of instabilities in the laminarized flow, is revealed under the downflow conditions. Generation of large-scale secondary vortices with the axis parallel to the MF induction causes temperature fluctuations of the abnormal intensity that considerably exceeds the level of turbulent fluctuations. Such temperature fluctuations easily penetrate into the duct wall and can lead to the fatigue breakdown of thermonuclear reactor cooling paths. Ranges of unallowable regime parameters are determined and the boundary in coordinates Gr-Re is presented, where this effect is revealed or vanishes. The numerical simulation of studied parameters is performed under the conditions corresponding to the experiment. Computational results coincide satisfactorily with the experimental data in the flow regimes with large Reynolds numbers, where the effect associated with the development of instabilities caused by the heat-gravitational convection is absent or is relatively small.