The measuring method of the averaged liquid velocity fields around a large bubble rising in stagnant liquid in a vertical round pipe using Ultrasonic Velocity Profile monitor (UVP), which the authors have developed in the previous report, was applied to make clear the effect of pipe inner diameter, D, on the velocity field in this report. Two ultrasonic transducers inclined +20 degrees and -20 degrees from the horizontal line, respectively, were used for the measurement to get two dimensional velocity vectors. Three pipes of D = 32, 42 and 54mm were used in the measurement. According to the measured results, the velocity fields in the liquid phase above the large bubble, in the liquid film around the large bubble, and in the wake were presented and discussed, focusing especially on the effect of pipe inner diameter D.
Water electrolysis has been recently recognized as a very important technology for hydrogen production. It has been well-known that electrolytic bubbles cause an increase in effective resistivity of the electrolyte, and there have been many electrochemical studies of the bubble dispersion effects on electrolysis characteristics. There are, however, very few studies that measure the growing phenomenon of bubbles from generation to departure on an electrode surface, the bubble rising velocity distribution and the void fraction in the interelectrode gap; moreover, their effects on the electric resistance characteristics have not been discussed extensively. We conducted an experimental study focusing on a small-scale electrolytic cell for hydrogen production in order to resolve the these issues, and discussed the fluid forces (surface tension, buoyancy, drag) acting on hydrogen bubbles from a viewpoint of gas-liquid two-phase flow. We used a PIV technique with a high-speed video camera and a shallow focal image system. As a result, we were able to successfully capture the images of hydrogen bubbles during the growth, and found that there was a linear relationship between log(We) and log(Re), where We is the Weber number and Re is the Reynolds number for a circulation type of the electrolytic cell. We also found an optimal gap between two electrodes when the flow pattern changes from bubbly flow to plug flow.