Sea Water MHD : Electrolysis and Gas Production in Flow

Abstract

The work presented is principally experimental, it concerns mainly the coupling between sea water electrolysis and hydrodynamics (in both ways). The first series of measurements presented serve as a reference background to our study. It is realised in the conventional working conditions of an electrochemical cell containing an agitated solution of NaCl 35g/l. The working electrode is a rotating disc of Platinum. Using polarography and chronovoltametry in the range of tension and current studied, we observe two anodic reactions are in competition : production of chlorine or di-oxygen. The second series of measurements is much more relevant to sea water MHD propulsion. A real flow( maximum 10 m/s) of a solution of NaCl 35g/l, is imposed in a semi-transparency test section (4 cm × 4 cm cross section, lm length). The working electrodes are rectangular plates of Platinum coated Titanium (Pt/Ti) placed as the two horizontal walls. The electrochemical measurements using the same method as in the first series, demonstrate that the wall region hydrodynamics of a TBL (Turbulent Boundary Layer) is strongly promoting the mass transfer and thus completely change the anodic limit current. The hydrodynamic measurements are based on the combined use of a PDPA (Phase Doppler Particle Analyser) and flow visualisation. They demonstrate that the electrolysis micro-bubbles does perturb the TBL and that they can be considered as passive tracers of the flow. Most of the experimental results are explained using basic theoretical concept as for instance : the hydrodynamic boundary layer and the diffusion layer, combined to balance principle as for instance : Faraday law (for the total amount of electrolysis gas produced).