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Non-intrusive assessment of transport phenomena at gas-evolving electrodes

Tuesday, 29 June, 2010 - 10:00
Campus: Brussels Humanities, Sciences & Engineering campus
Flora Tomasoni
phd defence

Gas-evolving electrodes are systems where gas bubbles are produced after a
heterogeneous reaction. This thesis provides new insights into the multidisciplinary
and multi-scale nature of the involved phenomena. A combined theoretical and
experimental approach allows to successfully interpret and model the bubble
nucleation, growth and detachment from the electrode. The importance of the
different forces acting on moving bubbles is examined and a model is developed for
describing the effect of bubble evolution on mass transfer by solving the unsteady
diffusion-advection equation. The experimental techniques for measuring bubble size
are reviewed and backlighting is selected as the most suited one. In order to overcome
the difficulties arising with out-focus bubbles and bubbles overlapping, a model for
bubble shadow and a module for bubble ``erosion'' are developed and implemented
(FROG). This brings an increase of four times of the void fraction limit of the
technique itself. The effect of the contact angle on the bubble diameter is investigated
by varying the electrode potential difference.

Experimental results are presented and analysed for bubble evolution in A.C.
graining, in a turbulent bubbly channel flow and in a rotating bubble plume. In order
to better understand the bubble layer development, a model for micro bubble motion
is developed and successfully validated with the measurements. The methodology
developed proved to be effective.