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This study aims to reveal the development process of artificial void waves generated in a fully developed turbulent channel flow. Void waves generated with periodically fluctuated air flow rate were optically visualized by a high-speed video camera at three locations along the channel. The measured spatio-temporal fluctuations of void waves maintained the initial frequency given at the injection during the downstream propagation. The diffusion coefficients of void distributions were calculated to evaluate the influence of diffusion in a two-phase turbulent flow. The coefficients denoted that diffusion effect was larger than that of turbulent eddy viscosity on void wave deformation. [ABSTRACT FROM AUTHOR]

Removing inclusions from the melt is an important task in metallurgy with critical impact on the quality of the final alloy. Processes employed with this purpose, such as flotation, crucially depend on the particle size. For small inclusions, the aggregation kinetics constitute the bottleneck and, hence, determine the efficiency of the entire process. If particles smaller than all flow scales are considered, the flow can locally be replaced by a plane shear flow. In this contribution, particle interactions in plane shear flow are investigated, computing the fully resolved hydrodynamics at finite Reynolds numbers, using a lattice Boltzmann method with an immersed boundary method. Investigations with various initial conditions, several shear values and several inclusion sizes are conducted to determine collision efficiencies. It is observed that although finite Reynolds hydrodynamics play a significant role in particle collision, statistical collision efficiency barely depends on the Reynolds number. Indeed, the particle size ratio is found to be the prevalent parameter. In a second step, modeled collision dynamics are applied to particles tracked in a fully resolved bubbly flow, and collision frequencies at larger flow scale are derived.

• The bubble shape variation in the shear-thinning fluid with bubble swarms is investigated using high-speed imaging. • Influence of superficial gas velocity and solution concentration over the bubble shape is systemically evaluated. • The relationship between the bubble aspect ratio and equivalent diameter is revealed. • The novel aspect ratio prediction correlations based on dimensionless group Re · Eo are proposed for oblate and prolate spherical bubbles, respectively. Gas/shear-thinning liquid two-phase flow is often involved in the petroleum, chemical, and food industries, with the bubble shape playing a vital role in the theoretical analysis and numerical simulation of the flow. This study investigates the bubble shape variation in a shear-thinning liquid (carboxymethyl cellulose sodium solution) under the bubble swarm condition using high-speed imaging and a bubble recognition algorithm. The results show that trailing bubbles accelerate as they rise along the flow path of leading bubbles. During acceleration, the bubbles are subjected to lateral forces that cause them to deform from an oblate spheroid to a prolate spheroid. The effect of the shear-thinning property is enhanced owing to the increased superficial gas velocity and solution concentration, leading to an increase in the number of prolate spherical bubbles. Considering the significant difference in energy transfer in the flow fields around oblate and prolate particles, new aspect ratio correlations were developed for the two shapes based on the correlations of Re · Eo derived from the Newtonian fluid case. Compared to the previous correlation developed by assuming that the two shapes have the same aspect ratio, the proposed method significantly improves the prediction accuracy of the bubble aspect ratio for a shear-thinning fluid case. [ABSTRACT FROM AUTHOR]

The local mass transfer of a bubble under swarm turbulence influence is the key to the performance of gas-liquid contactors. For measurements, optical accessibility is reduced due to an increased void fraction. The performance of a flexible grid of rigid particles to mimic the hydrodynamic conditions of bubble-induced swarm turbulence is assessed using spectral energy density distribution and an approach to connect the turbulent intensity to the according length scale of the flow. Despite the significant physical property differences to real bubbly flow the flexible grid is found to realistically mimic the swarm turbulence of a cocurrent bubble column. Hence, a single bubble approaching the proposed flexible grid experiences hydrodynamic conditions comparable to real bubble-induced turbulence. [ABSTRACT FROM AUTHOR]

The study of bubble swarm distributions in a direct-contact heat exchanger was addressed both theoretically and experimentally. But recently developed approach based on uniform design for measuring bubbles uniformity and mixing efficiency within rectangular or square region cannot be used to characterize the mixture homogeneity and mixing time within circular region. This paper discussed the difference between rectangular or square region and circular region. A quantitative metric of mixing uniformity is used to characterize the bubble patterns within circular region. Simulation studies are also used to illustrate the proposed methodologies while the experimental case considered demonstrate that the proposed approach evaluated the experimental data rather well. The space-time features of the mixing transient have been successfully derived when the bubble swarms or other objects distribute in a circular region. This work brings new insights to evaluate mixing quality of different systems practically, which has a high degree of accuracy (the percentage of enhancing achieved is about 40%). [ABSTRACT FROM AUTHOR]

This paper presents a numerical study of the gravity-driven motion of single bubbles and bubble swarms through a vertical channel, using High-Performance Computing (HPC) and Direct Numerical Simulation (DNS) of the Navier-Stokes equations. A systematic study of the wall effect on the motion of single deformable bubbles is carried out for confinement ratios CR = {2,4,6} in both circular and square channels, for a broad range of flow conditions. Then, the rising motion of a swarm of deformable bubbles in a vertical channel is researched, for void fractions α = {8.3%, 10.4%, 12.5%} and CR = {4, 6}. These simulations are carried out in the framework of a novel multiple marker interface capturing approach, where a conservative level-set function is used to represent each bubble. This method avoids the numerical and potentially unphysical coalescence of the bubbles, allowing for the collision of the fluid particles as well as long time simulations of bubbly flows. Present simulations are performed in a periodic vertical domain discretized by 2 × 10 6 control volumes (CVs) up to 16.6 × 10 6 CVs, distributed in 128 up to 2048 processors. The collective and individual behavior of the bubbles are analyzed in detail. [ABSTRACT FROM AUTHOR]

Electrochemical, acoustic and imaging techniques are used to characterise surface cleaning with particular emphasis on the understanding of the key phenomena relevant to surface cleaning. A range of novel techniques designed to enhance and monitor the effective cleaning of a solid/liquid interface is presented. Among the techniques presented, mass transfer of material to a sensor embedded in a surface is demonstrated to be useful in the further exploration of ultrasonic cleaning of high aspect ratio micropores. In addition the effect of micropore size on the cleaning efficacy is demonstrated. The design and performance of a new cleaning system reliant on the activation of bubbles within a free flowing stream is presented. This device utilised acoustic activation of bubbles within the stream and at a variety of substrates. Finally, a controlled bubble swarm is generated in the stream using electrolysis, and its effect on both acoustic output and cleaning performance are compared to the case when no bubbles are added. This will demonstrate the active role that the electrochemically generated bubble swarm can have in extending the spatial zone over which cleaning is achieved. [ABSTRACT FROM AUTHOR]

The bubble swarm behavior in a system comparable to a flat sheet membrane module was investigated. The parameters channel depth, superficial gas velocity, superimposed liquid velocity and viscosity of the continuous phase were varied. Besides water as continuous phase, a non-Newtonian liquid was used with respect to the rheology of real waste water sludge. The gas hold-up was determined for 144 parameter combinations with high speed imaging and the shear stress – important for the fouling mitigation – was determined for 72 parameter combinations with the electrodiffusion method. To the authors׳ knowledge, this is the first application of the electrodiffusion method in a bubbly flow with a non-Newtonian liquid. For the two properties gas hold-up and the shear stress describing correlations were found taking all parameters into account. In both cases, the strongest influences were found for the gas velocity and the rheology of the continuous phase. Regarding the shear stresses, the median values were up to 3.2 Pa while the maximum values were up to 9.1 Pa. Depending on the parameter combination, a significant influence of the rheology was found disagreeing with the statement of the transferability of the results gained in water to real waste water systems mentioned in some papers. [ABSTRACT FROM AUTHOR]

Based on the experimental finding that microbubble swarms dramatically promote heat transfer from a vertical heated wall, despite their potentially adiabatic nature, tests of microbubble fluid mechanics in the isothermal state are performed to clarify the unique motion characteristics of microbubble swarms. At constant bubble flow rate, the microbubble swarm shows a significant pulsatory rise along a vertical flat wall, particularly for small bubbles. Particle tracking velocimetry applied to the microbubbles shows that a two-way interaction between the microbubbles and the liquid flow self-excites the pulsation during their co-current rise. The sequence consists of the following processes: (i) increase in the bubble number density close to the wall as a result of the liquid velocity gradient driven by the microbubbles themselves; (ii) wave generation inside the microbubble swarm to induce the pulsatory rise of the swarm; and (iii) amplification of the waves, which results in void-bursting motion in the final stage. [ABSTRACT FROM AUTHOR]

Abstract: This paper proposes a novel method to quantify the synergy of bubble swarm patterns and heat transfer performance in a direct-contact heat exchanger (DCHE) by using computational homology. Betti numbers are used to estimate the number of bubbles aggregating in flow patterns and to obtain the pseudo-homogeneous time. A simple linear model of a bubble swarm and the heat transfer performance of a DCHE is constructed on the basis of experimental analysis, in which a new index ( ) is defined by the Betti number average as well as the pseudo-homogeneous time. A good fitting curve between and the volumetric heat transfer coefficient average is obtained with a correlation coefficient of 0.95. A paradigm is established on the basis of this novel method for the study of flow patterns and heat transfer performance, and it offers an alternative route to explore the relationship of flow patterns and heat transfer in other heat transfer processes such as convection, multiphase flow, and turbulent flow. [Copyright &y& Elsevier]

Modeling coalescence effects within bubble collectives is a challenge with grid-based methods because of the extensive remeshing. Mesh-free simulation methods, such as the smoothed particle hydrodynamics (SPH) approach offer an alternative for systems with highly deformable free surfaces and complex geometries. This contribution investigates the suitability of the SPH method for the simulation of bubble collectives. The dynamics of a single bubble coalescing with a planar surface is studied. Moreover, the ability of the SPH approach to model bubble collectives is highlighted considering the coalescence of two bubbles of different sizes. [ABSTRACT FROM AUTHOR]

Abstract: A systematic computational study of the dynamics of gas bubbles rising in a viscous liquid is presented. Two-dimensional simulations are carried out. Both the dynamics of single bubbles and small groups of bubbles (bubble swarms) are considered. This is a continuation of our previous studies on the two-bubble coalescence and vortex shedding [A. Smolianski, H. Haario, P. Luukka, Vortex shedding behind a rising bubble and two-bubble coalescence: a numerical approach, Appl. Math. Model. 29 (2005) 615–632]. The proposed numerical method allows us to simulate a wide range of flow regimes, accurately capturing the shape of the deforming interface of the bubble and the surface tension effect, while maintaining the mass conservation. The computed time-evolution of bubble’s position and rise velocity shows a good agreement with the available experimental data. At the same time, the results on the dynamics of bubble interface area, which are, up to our knowledge, presented for the first time, show how much the overall mass transfer would be affected by the interface deformation in the case of the bubble dissolution. Another set of experiments that are of interest for chemical engineers modelling bubbly flows concerns the bubble swarms and their behavior in different bubble-shape regimes. The ellipsoidal and spherical shape regimes are considered to represent, respectively, the coalescing and non-coalescing bubble swarms. The average rise velocities of the bubble swarms are computed and analyzed for both regimes. [Copyright &y& Elsevier]

• The bubble aspect ratio tends to be a constant value with increasing the bubble diameter. • None of the available correlations can accurately correlate the bubble aspect ratio with We or Ta number. • Eo and Eo - Re based correlations proposed by Besagni and Inzoli (2016) and Besagni and Deen (2020) show a good performance. • Predicted values by correlations developed in this work are close to the experiments. The bubble shape affects the gas–liquid interface momentum, heat and mass transfer, as well as the flow field around the bubble. Correctly predicting the bubble shape is challenging but indispensable under bubble swarm conditions. In this work, the bubble aspect ratio, which is adopted for the characterization of the bubble shape, is obtained through high-speed photography combined with an image processing algorithm from several experiments with an oscillating bubble plume. The results show that the bubble aspect ratio tends to be a constant value with increasing the bubble diameter, rather than decreasing as predicted by various empirical correlations developed in single bubble experiments. None of the available empirical correlations can accurately correlate the bubble aspect ratio with Weber number or Tadaki number. Among the available correlations in literature, Eo based correlation proposed by Besagni and Inzoli (2016) and Eo-Re based correlation proposed by Besagni and Deen (2020) show a better performance. The predicted values of Eo-Re based correlation developed in this work are the closest to the experimental data compared with others. [ABSTRACT FROM AUTHOR]

Removing inclusions from the melt is an important task in metallurgy with critical impact on the quality of the final alloy. Processes employed with this purpose, such as flotation, crucially depend on the particle size. For small inclusions, the aggregation kinetics constitute the bottleneck and, hence, determine the efficiency of the entire process. If particles smaller than all flow scales are considered, the flow can locally be replaced by a plane shear flow. In this contribution, particle interactions in plane shear flow are investigated, computing the fully resolved hydrodynamics at finite Reynolds numbers, using a lattice Boltzmann method with an immersed boundary method. Investigations with various initial conditions, several shear values and several inclusion sizes are conducted to determine collision efficiencies. It is observed that although finite Reynolds hydrodynamics play a significant role in particle collision, statistical collision efficiency barely depends on the Reynolds number. Indeed, the particle size ratio is found to be the prevalent parameter. In a second step, modeled collision dynamics are applied to particles tracked in a fully resolved bubbly flow, and collision frequencies at larger flow scale are derived. [ABSTRACT FROM AUTHOR]

The bubble swarm trajectory in the jet bubbling reactor is measured through the bubble image velocimetry (BIV) technique. The result shows that the bubble swarm rises straightly when the jet Reynolds number is lower than 7,000. However, when the jet Reynolds number exceeds 14,000, the bubble swarm exhibits vortex‐like motion, and the bubble vortices oscillate periodically. The oscillating frequency of bubble vortices under the gas bubbling condition is lower than the flapping frequency of pure liquid jet. Moreover, the moving region and oscillating frequency of bubble vortices increase with the jet Reynolds number. The superficial gas velocity has little effect on the bubble swarm trajectory and the oscillating frequency. An empirical correlation between the oscillating frequency of bubble vortices and the jet Reynolds number is built based on the simple harmonic vibration theory. [ABSTRACT FROM AUTHOR]