Heat Transfer in a “Tube-in-Channel” Combined System with an Upward Flow of Liquid Metal in a Transverse Magnetic Field

Investigations were performed of heat transfer to a forced upward flow of mercury in a tube inserted into a heated channel with a rectangular cross-section under the effect of a transverse magnetic field. The outer channel is filled with mercury and connected to a natural circulation loop. Liquid metal heat transfer is simulated in a cell of the cooling system of the channel-type liquid metal blanket for a Tokamak fusion reactor. Experimental data on temperature fields and heat-transfer performance in the inner tube and the outer channel were obtained in the mercury magnetohydrodynamic test rig using microthermocouple probes. Three different cases of natural circulation loop operation are examined: (I) the loop is off, convective flow can occur only in the space between the tube and the channel wall; (II) the loop is open and operates under adiabatic conditions; (III) the loop is open, water cooling is on. The results of measurement in the inner tube demonstrate that heat transfer in the tube-in-channel system is enhanced compared to the heat transfer in a separate tube both with and without a magnetic field. Under the experimental conditions, natural convection is induced by the buoyancy and electromagnetic forces in the gap between the tube and the channel wall. The configuration and structure of the flow in the gap change drastically in a transverse magnetic field, and the heat-transfer rate depends on the operating conditions in the natural circulation loop. Convection reduces temperature nonuniformities in the gap, and the heat transfer in the investigated “tube-in-channel” enhances greater when the natural circulation loop is activated and, especially, when it is additionally cooled. Low-frequency high-amplitude fluctuations induced by the instability of the natural convection and magnetohydrodynamic flows are observed in the gap.