Experimental Study of Bubble-driven Liquid Metal Flow in a DC Magnetic Field

摘要

Bubble-driven flows have found wide applications in industrial technologies. Gas bubbles are injected into a bulk liquid metal to drive the liquid into motion, to homogenise the physical and chemical properties of the melt or to refine the melt. For such gas-liquid metal two-phase flows, external magnetic fields provide a possibility to control the bubble motion in a contactless way. Our interest is devoted to the motion of gas bubbles in stagnant liquid metals under the influence of a DC magnetic field. Previous experimental work showed the effect of transverse and longitudinal magnetic fields, respectively, on the slip ratio and the bubble dispersion in a turbulent channel flow. Because the gas bubble is electrically non-conducting, it does not experience the effect of the electromagnetic force directly. However, the bubble behaviour is, of course, influenced by the magnetically induced modifications in the liquid flow structure around the bubble. The possibility to influence the bubble wake by an additional body force may also contribute to a better general understanding of the interaction between bubble path and wake. Our experiments were performed within an open, cylindrical container made from Perspex with a diameter of D = 100mm. The cylinder was filled until a height of H = 220mm with the ternary alloy GalnSn as working fluid. The set-up is positioned concentrically inside a Helmholtz configuration of two water-cooled copper coils. The magnetic field direction was chosen to be parallel or perpendicular to the mean bubble path, respectively. Several nozzles made from stainless steel with inner diameters between 0.3 and 5mm were used to inject argon bubbles into the liquid. The nozzle outlet was positioned in the midpoint of the cylindrical cross section 10mm above the cylinder bottom. The gas flow rate was controlled using a mass flow controller. The DOP2000 velocimeter (Signal Processing SA) with a standard 4 MHz transducer (TR0405LS) was used to carry out the velocity measurements.