UC Berkeley

Mixing of a two-phase flow through a gap connecting adjacent channels was investigated for a variety flow rates and two gap heights. The mixing was characterized by the amount of gross liquid and net gas mass transfer from one channel to the other. The liquid mixing through the gap was calculated based on the measured mass flow rates and dye tracer concentrations at the inlet and outlet of each channel. The gas net transfer, phase fraction distributions, bubble size distributions, and gas interface velocity were measured using dual-plane wire mesh conductivity sensors at both inlets and outlets. Additionally, imaging of the fluorescent tracer dye and air bubbles were used together with Spectral Proper Orthogonal Decomposition (SPOD) to further characterize the mixing mechanisms. The water inlet Reynolds number was varied from 4 to 10×104 and the volumetric gas quality from 0 to 0.35 which resulted in gas volume fractions up to 15%. All conditions were in the bubbly flow regime. Air was introduced by a needle array producing nominally monodispersed bubbles at lower gas flow rates with broader bubble size distributions at higher gas flow rates. The Sauter mean diameter varied from 3 to 8 mm, depending on flow rates. For these balanced flows large coherent structures dominated the mixing. However, even mere O(5%) volumetric quality gas injection could significantly suppress the coherent structures reducing mixing by over 50%.