Experimental Investigation of Bubble Lateral Motion in Shear Flow
thesisposted on 03.01.2019, 19:30 by Ke Tang
In two-phase flow, the void fraction and its distribution are two major factors describing the characteristic of flow patterns. Better understanding of void fraction distribution in two-phase flow would help improve safety and efficiency in the nuclear industry as the heat transfer process is significantly affected by the void distribution in nuclear reactor fuel bundles. Lift force is proposed to explain the lateral migration of bubbles in the shear flow (Feng & Bolotnov, 2017, Lucas & Tomiyama, 2011, Akio Tomiyama, Tamai, Zun, & Hosokawa, 2002). However, the mechanism of lift force is unclear and the research on lift force is limited.
An experimental investigation is performed on the lift force of single bubble in weak linear shear flow field in water. In addition, characteristics of bubble motion including bubble terminal velocity, aspect ratio and oscillation amplitude are studied and comparisons are made with existing models.
It was found that the model proposed by Tomiyama et al. (A. Tomiyama, Celata, Hosokawa, & Yoshida, 2002) has the best prediction of bubble terminal velocity with introduction of a tuning factor in consideration of the asymmetric deformation of bubble. Bubble aspect ratio is found to significantly affect its terminal velocity, and a new model is proposed to best fit the experiment data. It is also observed that the shear rate magnitude has no influence on bubble aspect ratio in this study. Oscillation was observed for all the bubbles in this experiment. Oscillation amplitude scattered widely and it was difficult to correlate it only with the bubble equivalent diameter. In terms of lift force, positive lift coefficient was observed for small size bubbles and transits to negative value with growing size. Due to the high Reynolds number of flow and low viscosity of water, widely scattered data is found in the results. Although the accurate prediction of lift coefficient is difficult to obtain in the experiment, the lift coefficient transition trend is given and agrees with many other research. In addition, this research provides a database for further lift coefficient investigation.