An experimental investigation of bubble-induced free convection in a small electrochemical cell

The sodium chlorate production process is run in large electrolysers where electrolyte flows between the electrodes due to the natural convection from hydrogen gas evolution. A brief review is given of electrolytic gas generation at electrode surfaces and of previous studies. A small, enclosed rectangular cell was used to electrolyse both a Na2SO4 and a NaCl/NaClO3 solution, in order to produce hydrogen and oxygen bubbles at one or both of the electrodes. The two-phase flow regimes, bubble sizes, gas fraction and fluid velocities between the electrodes were investigated using microscope enhanced visualisation, laser doppler velocimetry and particle image velocimetry. The practicality of each of the measuring methods is analysed and it is concluded that laser doppler velocimetry is the most robust method for measuring such systems. The experimental results are discussed and conclusions are drawn relating gas evolution to the hydrodynamics of electrolyte flowing through a narrow vertical channel. The major conclusions are that fluid flow in systems with bubble evolution can transform from a laminar to a turbulent behaviour, throughout the length of the cell, and that both turbulence and laminar behaviour can exist across the cell channel at the same horizontal plane.