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

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VKI PHDT 2010-04, Flora Tomasoni, Vrije Universiteit brussel, Belgium, June 2010, ISBN 978-2-87516-010-2

Non-intrusive assessment of transport phenomena at gas-evolving

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

by Flora Tomasoni

VKI PHDT 2010-04, Vrije Universiteit Brussel, Belgium, June 2010, ISBN 978-2-87516-010-2

Gas-evolving electrodes, which are systems in which gas bubbles are produced by a heterogeneous reaction, represents a special case of multiphase flow. Their intrinsic difficulty arises from the multidisciplinary and multi-scale nature of the phenomena involved. Not only a dispersed phase, i.e. the bubbles, is generated at the electrode by means of an electrochemical reaction, but also mass transfer at the electrode is modified by the evolution of the bubbles themselves (e.g. mass transfer enhancement induced by micro/macro convection). Additionally in many industrial applications, the electrolyte is moving with respect to the electrode, which again affects the flux of material reacting (kinetic control versus transport control). It is the aim of this thesis to gain more physical insight on the modelling of mass transfer at gas-evolving electrodes, by combined theoretical and experimental approaches.

The assessment of the two-phase flow properties is essential to understand the complex phenomena occurring during multi-phase electrochemical reactions: hence, chapter 2 is devoted to a theoretical discussion of the transport phenomena taking place at gas evolving electrodes. The transport problem is first described and the role of bubble evolution is emphasised. The mechanisms involved in bubble nucleation, growth and detachment are then examined and the bubble-electrolyte interaction is discussed. This approach allows to en-light the importance of the different forces acting on moving bubbles; especially it is concluded that bubbles in high vortical flow are accelerating more, causing an increase in the mixture velocity gradient, with important consequences on heat and mass transfer.

A new model is proposed to describe the effect of bubble evolution on mass transfer, by solving the unsteady diffusion-advection equation, with a constant vertical velocity. According to the solution proposed, the current transient becomes steeper as the velocity is more negative (from the bulk toward the electrode), which means that the concentration variation occurs faster as fresh solution is brought to the electrode, i.e. the mass transfer rate is enhanced.

The techniques for measuring bubble size are reviewed in chapter 3. It is found that backlighting is the most suited to investigate gas evolution in industrial applications. In order to overcome the backlighting weak point, i.e. that out-focus bubbles are also measured, causing a large measurement error, the image formation is analysed and a model is proposed for the bubble shadow. By implementing this model, a new powerful software, named FROG, Focused- Recognition- Overlapping-Globule, is developed, which allows the discrimination between in and out of focus objects, leading to the possibility of assessing void fraction. An “erosion” module is also implemented, which allows the discrimination of overlapping bubbles: the void fraction limit of backlighting technique is in this was increased of four times with respect to the standard software capability.

The experimental results are presented and analysed in x4.2, for bubble evolution in A.C. graining, in x4.3, for a turbulent bubbly channel flow and in x4.4, for a rotating bubble plume. A uniquely strong correlation is measured during A.C. graining between the anodic potential peaks evolution and bubble size, which resulted fundamental in clarifying the mechanisms involved in bubble evolution. The bubble break-off diameter is found proportional to the pit size, because of the surface tension, and to the hydrogen flux, because of the gas momentum flux, in agreement with the bubble break-off model developed in chapter 2. The experiments in the turbulent bubbly flow (x4.3) suggested the existence of a back-coupling between the electrolyte and the bubbles. The flow influenced both the bubble trajectory and the bubble break-off diameter, which decreased as the Reynolds number was increased.

On the other hand, the flow field itself experienced the bubble presence, particularly in the logarithmic and sub-layer region, where the velocity gradient resulted higher compared to the single-phase flow condition. The results of the model proposed in x2.4, about micro bubble motion, were in good agreement with the measurements, and the model has been used to better understand the bubble layer development. Finally, by changing the electrode potential, it was possible to experience the effect of the contact angle variation on the bubble diameter: as the electrode potential was increased, the bubble diameter decreased. Concerning the bubble influence on mass transfer, it was concluded that the effect of bubbles was negligible at low working electrode potential difference, while their effect could be well modelled as a constant vertical velocity for higher voltage, in agreement with the model proposed in chapter 2. The measurements performed in the rotating bubble plume (x4.4) allowed to characterise bubble size and motion as function of the voltage and of the rotational speed. However, since no steady-state condition was achieved in the current response to a potential step, even introducing a disk rotation, no comparison could be done with the theoretical model presented in x2.5.3.1. Simultaneous flow and bubble measurements will be helpful to characterise the bubble-flow interaction and to measure the bubble-plume induced flow.

 

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