The bubble size distribution and O2 mass transfer in an oscillating, meso-scale tube

Abstract

This work aims at characterizing bubble size distribution and its control in a small-scale, constricted tube in the presence of an oscillating flow. From previous works, it became clear that the full modelling of the dispersed gas -liquid flow in the constricted tube requires a deeper clarification of the impact of tube geometry and the hydrodynamic parameters on the observed values of the oxygen mass transfer coefficient (kLa). In order to do so, data on the size and shape of the generated bubbles have been obtained with a CCD camera during the steady, continuous operation of a 350 mm constricted tube in the presence of flowing liquid and gas phases, at different combinations of oscillation frequency (f) and amplitude (x0). These data were used to generate bubble size distribution curves. The results demonstrated that the operation at smooth oscillatory flow conditions (f ≤ 10 Hz) leads to a decrease of the bubble size, while for higher values of f (15-20 Hz) the increased oscillatory velocities cause an increase in bubble size. The simultaneous increase of gas hold-up for the same oscillatory flow conditions (f = 15-20 Hz) suggests that the gas hold-up correlates with the bubbles’ diameter for this particular geometry. A correlation between the bubbles’ diameter and the mass transfer coefficient in the liquid, kL, with the oscillatory flow mixing conditions is also presented.