Large eddy simulation of microbubble transport in a turbulent horizontal channel flow

Liquid-gas multiphase flows occur in many engineering and environmental applications, with the former ranging from the flow of oil and gas in pipelines, of steam and water in nuclear reactors and steam generators, and the evaporation and condensation of refrigerants in refrigeration and air conditioning equipment. In this paper, the dispersion and interaction between microbubbles and turbulence in a horizontal channel flow is investigated using a two-way coupled Eulerian-Lagrangian approach based on large eddy simulation. The microbubbles are considered to be spherical and non-deformable, and are represented by a Lagrangian bubble tracking technique, with the bubbles subject to drag, gravity, buoyancy, shear lift, added mass and pressure gradient forces. Dynamic calibration of a Smagorinsky-type sub-grid scale (SGS) closure is employed to account for the unresolved stresses, whilst a stochastic Markov method is used to describe the effect of the SGS velocity fluctuations on bubble dispersion. Channel flows of water at two shear Reynolds numbers, Reτ=150 and 590, and three different bubble diameters, db=100, 220 and 330µm, are simulated. The results show acceptable agreement with DNS predictions of single- and two-phase flows, with the low density microbubbles migrating towards the upper channel wall with time under the influence of buoyancy, and segregating in the upper half of the channel, with this effect increasing with bubble diameter. The accumulated bubbles near the upper wall modify the liquid velocity field, with the mean velocity profile becoming asymmetric as a consequence and with slight modification of the turbulent stresses. At higher mean velocity and turbulence levels, the buoyancy effect is reduced due to more effective turbulent dispersion of the microbubbles, leading to reduced bubble migration towards the upper channel wall.