Neutron transmission imaging studies on particle-laden liquid metal flow driven by a disk-type rotating permanent magnet induction pump

In metallurgy, gas-stirring ladle treatment of aluminum alloys and steels is applied for the control of non-metallic inclu-sion population. The intense mixing of the molten metal bath by the bubbly flow provides the agglomeration of strongly dispersed solid inclusions and subsequently, the entrapment of agglomerates by gas bubbles floating up into the slag.
This work focuses on flow-induced particle agglomeration in liquid metal. We perform model experiments to study particle-laden liquid metal flow around a circular cylinder. The fluid flow is driven by an electromagnetic induction pump. Neutron transmission radiography is employed for time-resolved visualization of the particle trajectories in the opaque liquid metal.
The experimental setup is designed as a closed liquid metal loop shown in Figure 1a. The flat channel, made of welded stainless steel, has a rectangular inner cross section with 3 mm depth, i.e. parallel to neutron beam direction, and 30 mm width. A 5 mm diameter cylindrical obstacle representing a single rising bubble is placed in the mid-dle of the straight channel section. The disk-type rotating permanent magnet induction pump is locat-ed at the lower U-shaped channel section. By controlling the pump rotational speed in the range of 15…95 min-1, the average flow velocity of 6…40 cm/s results during the measurements.
Low-melting gallium alloyed with 7 wt-% tin (Ga-Sn) is employed as model liquid metal, so that the experimental setup can be operated at room temperature without additional heating. Gadolinium(III) oxide particles (Gd2O3) in a grain size range of 400-600 µm serve as model particles, since they have superior attenuation characteristics for neutron radiation compared to the liquid metal. To introduce the solid Gd2O3 particles into the liquid Ga-Sn, a two-step mixing protocol is developed. It includes the initial preparation of a paste-like suspension with high solid content of wetted particles, followed by a second mixing step inside the Ga-Sn-filled channel by means of the electromagnetic pump. In-creased rotational speed up to 550 min-1 combined with changing rotating direction is applied for in-tense stirring and distributing the particles homogenously in the liquid metal.
The neutron imaging studies are performed at the SINQ beamlines NEUTRA and ICON at the Paul Scherrer Institute, Villigen, Switzerland. By imaging with both high spatial (0.1 mm/px) and temporal (100 fps) resolution, the fast-moving particles are captured as single objects. In terms of en-hanced contrast-to-noise ratio between the Gd2O3 particles and the surrounding liquid Ga-Sn, the currently best neutron image quality is achieved on the ICON beamline due to the higher beam aper-ture (80 mm) yielding the maximum neutron flux of 1.4 x 108 cm-2 s-1 mA-1. The neutron transmission image sequences provide raw data on particle positions and motion in the liquid metal loop, in particular in the shear flow around the cylindrical obstacle. By image processing and data analyzing, we investigate the particle behavior depending on various fluid flow rates.