Simultaneous measurements of the liquid phase velocity and gas bubble trajectories in a two-phase flow at gas-evolving electrodes

In water electrolysis the efficiency is related to the free area of the electrodes. Therefore a fast transport of the hydrogen bubbles away from the electrodes into the bulk is beneficial. To characterize the flow close to the cathodes surface simultaneous measurements of the fluid velocity and the size and trajectories of hydrogen bubbles were performed. The liquid phase velocity was measured by particle image velocimetry (PIV) as well as particle tracking velocimetry (PTV) using fluorescent tracer particles and laser light illumination. Gas bubble trajectories were determined using particle tracking on bubble shadow images obtained by a second camera. The images were separated by the different wavelength of the illumination/fluorescence. The void fraction in the vicinity of the electrode can become so high that determining both gas bubble and tracer particle velocities becomes quite challenging or even impossible. A comparison between results obtained by PIV and PTV clearly shows that in these regions PTV is better suited for the determination of the liquid phase velocities rather than PIV. A combination of both methods allow for the precise characterization of the evolving wall jet. The use of Lorentz forces, generated by magnets, result in significantly increased wall parallel liquid velocities close to the electrodes [1]. This enhances the transport of the bubbles away from the electrode surface and decreases the fractional bubble coverage. Consequently, the amount of active area for the reduction process is increased and the efficiency of the water electrolysis process can be improved.