Abstract
A mechanism behind the force pushing bubbles towards the tube center in vertical bubbly flow was proposed in this study, which related to a transition of sliding-core bubbly flows. Preliminary tests with a vertical tube, stagnant water and constant bubble generation, confirmed that the bubble concentrated at the center of tube at some axial distance from the bubble inlet while after the start of the wall-side-bubble generation or as the bubble-passing-number increased, and that the bubble concentration was generated by self-induced bubble motion caused by three-dimensional wake.
