Vorticity dynamics using PIV: Quantitative analysis of water perturbation by bubble rise

Abstract

Bubble rise behavior and its disturbance of surrounding water are widespread in nuclear power plants and have a significant impact on the safety and efficiency of the installations. In this work, the dynamic parameters of six different sizes of bubbles and the perturbation of water are investigated, with the help of particle image velocimetry (PIV) and numerical image post-processing techniques. The morphology, trajectory, instantaneous velocity, velocity vector fields, and vorticity fields as well as their interactions are analyzed, by extracting data from the images. The results show that the shape of the rising bubbles is elliptical with small eccentricity most of the time, but not a standard round. In the initial stage of bubble generation, the eccentricity is large and the Weber number (We) of the bubble is small due to the surface tension. In addition, a standard annular flow field and two symmetric vorticities are found around the bubble in the initial stage, by calculating the velocity vector field and vorticity field of water. However, the equilibrium between the two vorticities can only be maintained for tens of milliseconds, and once the vorticity completely occupies the tail of the bubble, the bubble turns in the direction of the vorticity. This phenomenon occurs four to five times in the observed region and manifests itself macroscopically as a swing of the bubble trajectory. Further, the trajectory swing frequency and the bubble average velocity are found to decrease with the size of the bubble, which indicates that larger bubbles are more stable. However, the average velocity of the water increases with the increase of the bubble size, indicating that the water perturbation is more sensitive to the size of the bubble compared to the trajectory and morphology. Finally, a correlation is proposed for calculating the water velocity based on Reynolds number (Re), Eötvös number (Eo), and bubble velocity, which shows a good adaptation after comparing with the experimental data.