The combined action of a magnetic field and an electric field on a sea water flow is a well known mean of direct sea water propulsion. The seawater can be considered as a poorly conducting high velocity flow. Consequently the MHD coupling is weak (Rm << 1 and N << 1) and the flow, in an MHD thruster, is almost an ordinary hydrodynamic flow submitted to a non uniform JxB field. The current density, applied to the flow, is very moderate in order to minimise the Joule dissipation. The fact that sea water is an electrolyte makes the real specificity of the problem. The interface flow / electrode is the place for an electrolysis which produces electrical looses and gaseous emission (micro-bubbles). The typical current density and the mean flow velocity involved are completely different from that of industrial seawater electrolysis cells which makes the problem quite new.
The work presented, mainly experimental, concerns the coupling between sea water electrolysis and hydrodynamics (in both ways). 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).
The first series of measurements, briefly presented, 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 voltametry in the range of tension and current studied, two anodique 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 (4cm x 4cm cross section, 1m 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 hydrodynamical 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 not perturb the TBL and that they can be considered as passive tracers of the flow.