Gerd Mutschke

Control of Flow Separation Using Electromagnetic Forces

If a fluid is electrically conductive, its flow may be controlled using electromagnetic forces. Meanwhile, this technique is a recognized tool even on an industrial scale for handling highly conductive materials like liquid metals. However, also fluids of low electrical conductivity as considered in the present study, like sea-water and other electrolytes, permit electromagnetic flow control. Experimental results on the prevention of flow separation by means of a streamwise, wall parallel Lorentz force acting on the suction side of inclined flat plates and hydrofoils will be presented.

Clarifying the mechanism of reverse structuring during electrodeposition in magnetic gradient fields.

Deviating from the common expectation, magnetoelectrochemical structuring during deposition of diamagnetic ions was demonstrated, very recently. To achieve this, electrochemically inert paramagnetic ions have to be added to the electrolyte and the deposition has to be performed in a magnetic gradient field. A reverse structuring occurs, yielding thinner deposits near high gradient regions. In this paper we aim to clarify the mechanism of this reverse structuring. Potentiodynamic and potentiostatic investigations were performed, including measurements of the deposited mass with an electrochemical quartz crystal microbalance (EQCM). The convection of the electrolyte was studied in situ by astigmatism particle tracking velocimetry (APTV). It was revealed that during the reverse structuring a convection is induced in the electrolyte, which is directed away from the working electrode in regions of high magnetic gradients. Due to this additional convection, the overall deposition rate is increased, whereby it is locally reduced in regions of high magnetic gradients. The mechanism for reverse structuring is discussed in detail. Also, the influence of all relevant magnetic forces is addressed.

Numerical simulation of the onset of mass transfer and convection in copper electrolysis subjected to a magnetic field

Numerical investigations have been performed in order to simulate the transient behaviour of copper electrolysis in a rectangular cell with vertical electrodes in a galvanostatic regime where the kinetics of the electrodes is controlled by charge-transfer. The transient behaviour observed for a binary electrolyte reproduces the temporal evolution of the concentration distribution measured in recent experimental work. Horizontal magnetic fields that vary linearly between the electrodes create Lorentz forces that either enhance or attenuate natural convection. The different time scales of natural convection and convection driven by the Lorentz force lead to interesting transient effects. The simulations performed for the case of an attenuating Lorentz force explain the dynamics of vertical inhomogeneities of the concentration boundary layer during the initial stages of electrolysis that were previously observed experimentally.