Abstract
This paper describes the effects of uniform and non-uniform liquid co-flow on the bubbly flow in a rectangular column (with two inlets) deliberately aerated unevenly. The two vertical bubbly streams, comprising uniform bubbles, started interacting downstream of the trailing edge of a splitter plate. This study quantifies the emergence of buoyancy driven flow patterns as a function of the degree of a-symmetric gas sparging and (non-)uniform liquid co-flow by using Bubble Image Velocimetry (BIV) and dual-tip optical fibre probes. Without liquid co-flow, small differences in the gas fraction of the left and right inlet had a large effect on the mixing pattern, whereas a liquid co-flow stabilized a homogeneous flow regime and the flow pattern was less sensitive to gas fraction differences. Void fractions, bubble velocities and chord lengths were measured at two fixed position in the flow channel, whereas BIV provided a global overview of the flow structures. A correlation was developed to predict (a-symmetric) operating conditions for which the gas fraction of the left and right inlet are balanced, such that the bubble motion is governed by advection and no buoyancy driven flow structures arise. The data obtained is highly valuable for CFD validation and development purposes.
Highlights
Besides classic symmetric bubble columns, a-symmetric bubble configurations are widely encountered in the form of air-lift reactors and photobioreactors
While Part I of this paper showed results for a symmetric operation, a-symmetric bubble column configurations were studied for this part, where the superficial gas velocities Usg and/or superficial liquid velocities Usl (L)eft and (R)ight were varied independently
Since we found that Bubble Image Velocimetry can only be applied for low to moderate void fractions, we limit ourselves to show organized flow structures at relatively low Usg, such that the assumption of a 2D flow pattern is plausible
Summary
Besides classic symmetric bubble columns (with or without a liquid co/counter-current flow), a-symmetric bubble configurations are widely encountered in the form of air-lift reactors and photobioreactors. In a similar test facility, De Tournemine and Roig (2010) found stable flow patterns characterized by so-called frontiers between the bubbly streams from the left and right inlets They only observed such frontiers when bubbles were injected on the low liquid velocity side, whereas oscillating boundaries occurred for all cases when bubbles were supplied at the high liquid velocity side. While two parallel bubbly flows (separated by a boundary) develop in vertical direction, the strength of the buoyancy driven flow structures (e.g. liquid entrainment rates into a dense bubble swarm) as a function of the degree of a-symmetry can serve as a very strong benchmark case to calibrate sub-models for interfacial momentum transfer, two-phase turbulence and lateral dispersion of bubbles All these sub-models are strong functions of the (local) void fraction and bubble size (distribution).
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