A model experiment to study the metal pad roll instability under ambient conditions

Abstract

We present a new model experiment to study the metal pad roll (MPR) instability, which is a limiting factor for the safe operation of aluminum reduction cells. The idea of our experiment is to replace the horizontal electrical currents in the aluminum layer, which are caused by displacements of the cryolite–aluminum interface in industrial reduction cells, with a synthetic current that is supplied through the side walls of the experimental cell. In this way, only one liquid layer of an electrically conducting fluid is required for modeling the MPR instability, allowing the experiment to operate under ambient conditions using the room-temperature liquid alloy GaInSn as the current-bearing fluid. We demonstrate that the experimental model allows self-amplifying MPR waves to be destabilized and maintained in a reproducible way. The setup is equipped with an acoustic measurement technique that facilitates precise submillimeter measurements of liquid metal surface elevations, which makes it possible to determine several key quantities such as MPR growth rates, stability onsets, saturation amplitudes, or viscous and magnetic damping rates. As the MPR destabilizing Lorentz force synthesized in the experiment can be calibrated to the Lorentz forces appearing in real two-layer cells, the proposed model experiment is intended to establish a novel framework for experimental benchmarking.