Bubble Size Effects on Dispersed Phase Motion in Vertical Bubbly Pipe Flow

Abstract

Three-dimensional measurements of bubble size, and velocity are presented along with the associated statistics for dilute bubbly flow in a vertical pipe. These measurements were obtained through a combination of precise index-of-refraction matching and a stereoscopic high-speed imaging system. An image-processing algorithm has been developed and used to extract instantaneous bubble size, shape, velocity and trajectory information and statistics corrected for magnification errors (maximum of 2%). The experiments have been conducted at a Reynolds of 14,600 in vertical pipe-flow and a volumetric gas flow ratio of 0.29%. Bubble sizes ranged from sub millimeter to several millimeters. The results indicate that bubbles less than approximately 20 wall units with very low ratios of rise velocity-to-liquid-average-velocity occupy the full breadth of the test section, while bubbles in the range between 20 and 60 wall units with ratios of rise-velocity-to-liquid-average-velocity greater than 0.5 are efficiently trapped by the wall and are almost exclusively found in the inner wall region (y+ < 30). Bubbles larger than 60 wall units with ratios of rise-velocity-to-liquid-average-velocity less than 0.5 are found throughout the pipe cross-section although with a strong preference to the wall neighborhood. This bubble behavior is put in perspective considering four mechanisms of bubble migration: (a) turbulent dispersion due to near-wall large scale structure, (b) inviscid transverse forces (Magnus), (c) viscous transverse forces (Saffman), and (d) unsteady transverse forces because of vortex shedding and bubble shape deformation.