Abstract
Mass transfer is sometimes rate-determining step in a high temperature process. For enhancement of production efficiency, concentration boundary layer formed in the vicinity of interface between two phases should be controlled in these processes. Ultrasound is one of the candidates for this purpose because it can excite acoustic streaming, micro-jet and accompanying flow in a high temperature liquid from its outside. Thus a lot of researches on the effect of the ultrasound on the mass transfer have been done. However, researches on dynamic behavior of concentration boundary layer under the irradiation of the ultrasound have not yet been done until now. In this study, effect of the ultrasound irradiation on the boundary layer formed in an aqueous solution by anodic reaction of a copper electrode has been investigated. Because concentration of bivalent copper ions is related to brightness of the solution by Lambert-Beer law, the concentration boundary layer was evaluated using recorded data of a high speed camera. A constant voltage was imposed on the solution for excitation of the anodic reaction. The observed boundary layer thickness was roughly agreed with the theoretically calculated boundary layer thickness using Kármán-Pohlhausen methods when the ultrasound was not irradiated. As soon as the ultrasound was irradiated, brightness increased within 0.05 seconds in the region away from the anodic electrode more than 20 μm. This was caused by the micro-jet and/or flow excited by it. On the contrary, the current gradually increased and its relaxation time was 0.5seconds.
