Bubble swarm velocity in a column

Abstract

In an air-water-only system under batch operation, a bubble interface was created in a laboratory column by introducing a step change in gas flow, and the rise velocity of this interface, u in, was measured using a novel method based on electrical conductivity with a custom-designed fast-response conductivity meter. The u in was plotted against J g2 J g1 , where J g1 and J g2 are the superficial gas velocities before and after the step change, respectively. The velocity of the undisturbed system was estimated by interpolation at J g2 J g1 = 1 . This velocity was referred to as the hindered velocity, u h . The local average gas velocity, u g, loc (= J g loc ε g, loc ) [corrected from the average gas velocity ( = J g ε g ) to account for the local pressure], was different from u h . It is argued that u h is the real bubble swarm velocity and u g, loc is the bubble swarm velocity for the ideal case of uniform bubble size. Unlike u g, loc , the u h was constant along the column, a result supported by theoretical momentum analysis. The u h is proposed as the key characteristic swarm velocity of the system.