The two-phase flow at gas-evolving electrodes: Bubble-driven and Lorentz-force-driven convection
Institute of Fluid Dynamics, Helmholtz-Zentrum Dresden-Rossendorf, Bautzner Landstr. 400, 01328 Dresden, Germany
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Revised: 5 February 2013
Published online: 26 March 2013
We observe electrolysis with gas evolution, a phenomenon occurring in a number of industrial scale electrochemical processes. Here, water electrolysis takes place in a small undivided electrolysis cell consisting of vertical electrodes embedded in a larger glass vessel which contains a dilute NaOH solution. Fluid flow velocities are measured by particle image velocimetry with fluorescent tracers, while size distribution and velocities of the bubbles are determined from bubble shadow images obtained with a high speed camera. Coalescence phenomena are observed in the flow and explain the relatively wide distribution of bubble sizes. Depending on the gap width and the current density, bubbles ascending near the electrodes form two discernible bubble curtains (low average void fraction, wide gaps) or a flow profile more akin to a channel flow (high average void fraction, small gaps). If the flow consists of separate bubble curtains, instabilities develop not unlike to those of a single phase wall jet. Finally, the influence of different wall parallel Lorentz force configurations on the velocity distribution in the cell is investigated. These Lorentz forces are generated by permanent magnets mounted behind the electrodes. Depending on gap width, current density, and magnet configuration, liquid phase velocities can be increased by several times compared to the baseline case.
© EDP Sciences, Springer-Verlag, 2013