Measurements of rising velocity of a small bubble in a stagnant fluid in one- and two-component systems

摘要

Despite its apparent simplicity, the problem of accurate determination of the shape and velocity of a bubble in a still fluid is far from being solved. Most of the models available in the literature make use of three dimensionless parameters to correlate the data, but, according to experimental evidence, the number of independent variables ruling the phenomenon is continuously increasing. The purpose of the present paper is to assess the degree of accuracy of relatively simple and general correlations available in literature for the terminal rising velocity and aspect ratio of a bubble over a wide experimental data set, obtained by injecting bubbles in two different fluids (water and FC-72, a fluoroinert trade mark of 3M) with different injection devices (nozzles and orifices). Both two-components (gas in a different liquid) and one-component (vapor in its liquid) systems were considered. To this aim, a simple experimental apparatus was built and the bubble velocity and shape were determined by analyzing the images taken with a high-speed camera. Measured shapes (aspect ratio) and rising velocities of single bubbles were compared with available correlations. The comparison showed that the aspect ratio was well correlated by the Tadaki number or, to a little lesser extent, by the Weber number. Among the correlations tested, the ones proposed by Taylor & Acrivos and Vakhrushev & Efremov gave the better results in the observed range. Concerning the terminal rising velocity, several correlations were selected and their predictions exhibited generally an error up to 50% throughout the observed range, the best accuracy being given by the recent model of Tomiyama et al. (c) 2006 Elsevier Inc. All rights reserved.