Effect of insoluble surfactant on turbulent bubbly flows in vertical channels (original) (raw)
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Numerical study of turbulent bubbly downflows in a vertical channel
Physics of Fluids, 2006
Direct numerical simulations are used to study turbulent bubbly downflows in a vertical channel. All flow scales, including the bubbles and the flow around them, are fully resolved using a front-tracking/finite-volume method. The turbulent bubbly channel flow is driven downward by an imposed constant pressure gradient, and the friction Reynolds number of the flow, based on the friction velocity and half-width of the channel, is 127.3, corresponding to a bulk Reynolds number of 3786 for a flow without bubbles. Three cases with several nearly spherical bubbles are examined. The bubble diameter is 31.8 wall units for all cases but the number of bubbles is varied, giving average void fractions of 1.5%, 3%, and 6%. The lift force on the bubbles drives them away from the walls until the mixture in the center of the channel is in hydrostatic equilibrium. Thus, the flow consists of a core region where the average void fraction and the mean vertical velocity are approximately constant and a bubble-free wall layer. The vertical velocity fluctuations in the wall layer decrease as the void fraction increases and the width of the wall layer decreases, but in the bubble-rich core the velocity fluctuations are higher than for a corresponding single-phase turbulent flow.
Polymer drag reduction in surfactant-contaminated turbulent bubbly channel flows
Physical Review Fluids
Polymer additives are commonly utilized to manipulate bubbly flows in various applications. Here we investigate the effects of clean and contaminated bubbles driven upward (upflow) in Newtonian and viscoelastic turbulent channel flows. Interface-resolved direct numerical simulations are performed to examine sole and combined effects of soluble surfactant and viscoelasticity using an efficient three-dimensional finite-difference-fronttracking method. The incompressible flow equations are solved fully coupled with the FENE-P viscoelastic model and the equations governing interfacial and bulk surfactant concentrations. The latter coupling is accomplished by a nonlinear equation of state that relates the surface tension to the surfactant concentration. For Newtonian turbulent bubbly flows, the effects of Triton X-100 and 1-pentanol surfactant are examined. It is observed that the sorption kinetics highly affect the dynamics of bubbly flow. A minute amount of Triton X-100 is found to be sufficient to prevent the formation of bubble clusters restoring the single-phase behavior while even two orders of magnitude more 1-pentanol surfactant is not adequate to prevent the formation of layers. For viscoelastic turbulent flows, it is found that the viscoelasticity promotes formation of the bubble wall-layers and thus the polymer drag reduction is completely lost for the surfactant-free bubbly flows, while the addition of small amount of surfactant (Triton X-100) in this system restores the polymer drag reduction resulting in 25% drag reduction for the Wi = 4 case.
Effect of bubble deformability in turbulent bubbly upflow in a vertical channel
2008
As bubbles rising in a vertical channel with upflow become bigger, it is well known that the void fraction distribution changes in a fundamental way, from a wall peak for small bubbles to a maximum void fraction at the channel center for larger bubbles. Here, we use direct numerical simulations of buoyant bubbles in a turbulent flow to show that it is not the size of the bubbles that matters, but their deformability.
A DNS study of laminar bubbly flows in a vertical channel
International Journal of Multiphase Flow, 2006
Direct numerical simulations are used to examine laminar bubbly flows in vertical channels. For equal size nearly spherical bubbles the results show that at steady state the number density of bubbles in the center of the channel is always such that the fluid mixture there is in hydrostatic equilibrium. For upflow, excess bubbles are pushed to the walls, forming a bubble rich wall-layer, one bubble diameter thick. For downflow, bubbles are drawn into the channel center, leading to a wall-layer devoid of bubbles, of a thickness determined by how much the void fraction in the center of the channel must be increased to reach hydrostatic equilibrium. The void fraction profile can be predicted analytically using a very simple model and the model also gives the velocity profile for the downflow case. For the upflow, however, the velocity increase across the wall-layer must be obtained from the simulations. The slip velocity of the bubbles in the channel core and the velocity fluctuations are predicted reasonably well by results for homogeneous flows.
Studies of Bubbly Channel Flows by Direct Numerical Simulations
Recent DNS studies of buoyant bubbly flows in vertical channels are discussed. Simulations of nearly spherical bubbly flows in vertical channels show that the bubbles move towards the wall for upflow and away from the wall for downflow in such a way that the core is in hydrostatic equilibrium. For downflow the wall layer is free of bubbles but for upflow there is an excess of bubbles in the wall layer. The liquid velocity in the core is uniform. For laminar downflow the velocity in the wall layer can be computed analytically and for turbulent flow the velocity is given (almost) by the law of the wall. For upflow the velocity is strongly influenced by the presence of the bubbles. Results from several simulations, fully resolving the flow around each bubble, are used to discuss the effect of void fraction and bubble size for turbulent downflow.
Turbulence modulation and microbubble dynamics in vertical channel flow
International Journal of Multiphase Flow, 2012
In this paper we examine the mutual interactions between microbubbles and turbulence in vertical channel flow. An Eulerian-Lagrangian approach based on pseudo-spectral direct numerical simulation is used: bubbles are momentum coupled with the fluid and are treated as pointwise spheres subject to gravity, drag, added mass, pressure gradient, Basset and lift forces. Two different flow configurations (upward and downward channel flow of water at shear Reynolds number Re s = 150) and four different bubble diameters are considered, assuming that bubbles are non-deformable (i.e. small Eotvos number) and contaminated by surfactants (i.e. no-slip condition applies at bubble surface). Confirming previous knowledge, we find macroscopically different bubble distribution in the two flow configurations, with lift segregating bubbles at the wall in upflow and preventing bubbles from reaching the near-wall region in downflow. Due to local momentum exchange with the carrier fluid and to the differences in bubble distribution, we also observe significant increase (resp. decrease) of both wall shear and liquid flowrate in upflow (resp. downflow). We propose a novel force scaling to examine results in vertical turbulent bubbly flows, which can help to judge differences in the turbulence features due to bubble presence. By examining two-phase flow energy spectra, we show that bubbles determine an enhancement (resp. attenuation) of energy at small (resp. large) flow scales, a feature already observed in homogeneous isotropic turbulence. Bubble-induced flow field modifications, in turn, alter significantly the dynamics of the bubbles and lead to different trends in preferential concentration and wall deposition. In this picture, a crucial role is played by the lift force, which is a delicate issue when accurate models of shear flows with bubbles are sought. We analyze and discuss all the observed trends emphasizing the impact that the lift force model has on the simulations.
Impact of surfactant on terminal velocity bubbles bellow 0.7 mm (suspended bubbles)
Using a circulating flow that balances buoyancy and drag, small bubbles (b 1 mm) are held in a column that classifies them according to their terminal velocities. Without frother, the terminal velocities fall between the values predicted by Hadamard-Rybczynski for fluid spheres, and those predicted by Stokes for hard spheres. Although it is commonly believed that industrial surfactants have little to no impact on such small bubbles, this study demonstrates a trend comparable to that of larger bubbles, namely that the addition of frother can retard the bubbles even beyond the predictions of the hard sphere model. Hence the motion of small bubbles appears to be impeded by mechanisms similar to those acting on larger bubbles. The frothers studied were MIBC and Dowfroth 250.
Inertial and buoyancy effects on the flow of elongated bubbles in horizontal channels
International Journal of Multiphase Flow, 2021
When a long gas bubble travels in a horizontal liquid-filled channel of circular crosssection, a liquid film is formed between the bubble and the channel wall. At low Reynolds and Bond numbers, inertial and buoyancy effects are negligible, and the liquid film thickness is a function of the capillary number only. However, as the tube diameter is increased to the millimetre scale, both buoyancy and inertial forces may become significant. We present the results of a systematic analysis of the bubble shape, inclination, and liquid film thickness for a wide range of capillary, Bond, and Reynolds numbers, namely 0.024 ≤ Ca l ≤ 0.051, 0.11 ≤ Bo ≤ 3.5, and 1 ≤ Re l ≤ 750. Three-dimensional numerical simulations of the flow are performed by employing the Volume-Of-Fluid method implemented in OpenFOAM. In agreement with previous studies, we observe that buoyancy lifts the bubble above the channel axis, making the top liquid film thinner, and thickening the bottom film. As the Bond number approaches unity, the cross-sectional shape of the bubble deviates significantly from a circular shape, due to flattening of the bottom meniscus. The simulations demonstrate the existence of a cross-stream film flow that drains liquid out of the top film and drives it towards the bottom film region. This drainage flow causes inclination of the bubble, with a larger inclination angle along the bottom plane of the bubble than the top. As buoyancy becomes even more significant, draining flows become less effective and the bubble inclination reduces. A theoretical model for the liquid film thickness and bubble speed is proposed embedding dependencies on both capillary and Bond numbers, which shows good agreement with the reported numerical results. Inertial forces tend to shrink the bubble cross-section and further lift the bubble above the channel centreline, so that the bottom film thickness increases significantly with the Reynolds number, whereas the top film thickness is less sensitive to it.
Physics of Fluids, 2011
The behavior of small spherical bubbles immersed in a homogeneous isotropic turbulent carrier flow of a heavier fluid has been experimentally studied. Air bubbles with diameters between 10 and 900 lm were injected in the test section of a horizontal water channel and allowed to interact with the turbulence induced by a grid located at the entrance to the test section. Point measurements of the bubble diameter and convective and rise velocities were taken from light interferometry data, together with flow visualizations that showed the instantaneous concentration field of bubbles in the carrier flow. The effect of the turbulence on the bubbles was found to alter the concentration field of bubbles leading to preferential accumulation at small scales, a phenomenon referred to as clustering, and to a decrease in the rise velocity of bubbles in the flow below the value measured and predicted for bubbles in a stationary fluid. These results are interpreted in terms of the different forces acting on the bubble in an inhomogeneous flow and in particular as the effect of pressure fluctuations that drive the bubbles preferentially to the core of vortices.
Dispersion of bubbles in fully developed channel flow
Journal of Physics: Conference Series, 2011
Dispersion and preferential concentration of small, low Stokes number bubbles in horizontal turbulent channel flow is studied by DNS and experiments. A DNS of turbulent channel flow at Reτ = 360 with Lagrangian tracking of one-way coupled bubbles (d < η, St = 1.3 × 10 −3) shows that equilibrium bubble concentration profiles can be described by a gradient diffusion hypothesis in analogy to flows with suspended sediment as studied by Rouse (1937). The conditionally averaged flow around the bubbles is measured by simultaneous PIV and bubble shadowgraphy and confirms the finding of the DNS that bubbles are preferentially concentrated in large-scale downward flowing fluid regions, which compensates for the rise velocity of the bubbles. This clustering is not an inertia effect, but results from the combination of a concentration gradient and turbulent mixing.