Two-phase Mixing Through a Narrow Gap

Mixing of a two-phase flow through a gap connecting adjacent channels was investigated for a variety of gap heights and flow rates for both balanced and unbalanced flows. Flow could be unbalanced based on liquid flow alone, gas flow alone, or by both being unbalanced. The water inlet Reynolds number was varied from 40k to 100k, and volumetric gas quality from 0 to 0.35. 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. 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. Net gas transfer, the void fraction distributions, bubble size distributions, and gas interface phase velocities were measured using dual-plane wire mesh conductivity sensors at both inlets and outlets. Additionally, imaging of the fluorescent tracer dye and air bubbles was used to further examine the mixing mechanisms. For the balanced flows large coherent structures dominated the mixing. However, 5% volumetric quality gas injection could significantly suppress these structures reducing mixing by up to 80%. For unbalanced flows, the mixing could be simply driven by the lateral pressure gradient, often leading to a large net mass transfer across the gap.