Mean Bubble Size Research Articles

Effects of liquid fraction and contact angle on structure and coarsening in two-dimensional foams

Aqueous foams coarsen with time due to gas diffusion through the liquid between the bubbles. The mean bubble size grows, and small bubbles vanish. However, coarsening is little understood for foams with an intermediate liquid content, particularly in the presence of surfactant-induced attractive forces between the bubbles, measured by the interface contact angle where thin films meet the bulk liquid. Rigorous bubble growth laws have yet to be developed, and the evolution of bulk foam properties is unclear. We present a quasistatic numerical model for coarsening in two-dimensional wet foams, focusing on growth laws and related bubble properties. The deformation of bubble interfaces is modelled using a finite-element approach, and the gas flow through both films and Plateau borders is approximated. We give results for disordered two-dimensional wet foams with $256$ to $1024$ bubbles, at liquid fractions from $2\,\%$ to $25\,\%$ , beyond the zero-contact-angle unjamming transition, and with contact angles up to $10^\circ$ . Simple analytical models for the bubble pressures, film lengths and coarsening growth rates are developed to aid interpretation. If the contact angle is non-zero, we find that a prediction of the coarsening rate approaches a non-zero value as the liquid fraction is increased. We also find that an individual bubble's effective number of neighbours determines whether it grows or shrinks to a good approximation.

One‐dimensional kinetic model with novel bubble size equation in molten‐metal bubble column reactors for CH4 pyrolysis

AbstractNon‐oxidative methane pyrolysis produces hydrogen and solid carbon without CO2 in molten‐metal bubble column reactors (MMBCRs). One‐dimensional MMBCR models, which involve bubble hydrodynamics, reactions, heat transfer, and axial dispersion, have been developed for reactor design; however, empirical equations of the mean bubble size derived from bench‐scale experiments have also been used for industrial‐scale MMBCRs. In this study, a novel bubble size equation based on dimensionless numbers was proposed for the bubbling, transient, and slugging flow regimes, which is applicable to both bench‐ and industrial‐scale reactors. Using the MMBCR model coupled with the new bubble size, reaction kinetic parameters were identified for four bench‐scale MMBCRs with Te‐based alloys in the bubbling flow regime. In four industrial‐scale MMBCRs at 980°C, belonging to the transient flow regime for an H2 production of 10,000 Nm3/h, the bubble size and methane conversion were approximately 40 mm and under 35%, respectively.

Effect of gas distributors on hydrodynamics in molten-metal bubble column reactors for fluorinated gas removal using computational fluid dynamics

Fluorinated gases (F-gases) emitted by semiconductor industry have high global warming potential. Repurposing F-gases into valuable metal fluorides using molten-metal bubble column reactors (MMBCRs) is a promising alternative for their removal. Many researchers have reported hydrodynamics of bench-scale MMBCRs using computational fluid dynamics (CFD). However, the effect of the gas distributor type on hydrodynamics in large-scale MMBCRs has not been addressed. This study investigated hydrodynamics of pilot-scale MMBCRs with different gas distributors in a N2-Sn system including 1 % CF4 by using a volume-of-fluid (VOF) CFD model. The VOF-CFD model was validated against the experimental bubble sizes in air-water and Ar-Fe systems. Using the CFD model, the gas holdup (αG), Sauter mean bubble size (d32), and specific interfacial area (as) of three gas distributors (20 spargers, and perforated plates without and with tip) were evaluated in the pilot-scale MMBCR at 650 °C. The bubble size distributions near the gas distributor were significantly different for the three MMBCRs but were similar in the upper zone of the reactor. αG and d32 were approximately 15 % and 22 mm, respectively. as for the three MMBCRs ranged from 55 to 59 m2/m3. The VOF-CFD model predicted the crucial hydrodynamic parameters for reactor design and optimization.

Bubble size distribution and turbulence characterization in a bubbly flow in the presence of surfactant

This study aimed to quantify the less explored complex multiphase hydrodynamics of a bubble swarm in the presence of surfactant which is the key to flotation process widely used in the resources and environmental engineering applications.Experiments were conducted in a rectangular column (cross-section: 100 mm × 100 mm) in batch mode by varying the gas flux (0.02 to and 0.08 cm/s) in the presence of an anionic surfactant sodium dodecyl sulphate (5 to 15 ppm). High-speed imaging was then used to visualise the unsteady bubble plume dispersion behaviour and estimate the bubble size distribution (BSD) which showed a reduction in the mean bubble size with the increasing gas flux and surfactant concentration.Next, particle image velocimetry (PIV) was utilised to measure the instantaneous velocity field which was used to determine the turbulence characteristics. It was shown that the bubble plume contributes to significant anisotropy in the flow field which increased in the higher surfactant concentration and gas flux cases. The energy containing turbulence length scale was characterized by the integral length scale, which was observed to increase linearly with both the gas flux and surfactant concentration. Also, the local turbulence energy dissipation rate exhibited a strong linear correlation with the bubble surface area flux parameter. In the presence of surfactant, the turbulence energy spectrum of the system exhibited a less steep slope in the inertial subrange regime compared to the Kolmogorov −5/3 slope. The spectrum also showed a leftward shift indicating energy addition to the larger turbulence length scales which was reflected in the formation of large recirculation zones around the bubble plume.

Artificial neural network (ANN) modelling to estimate bubble size from macroscopic image and object features

Bubble size measurements in aerated systems such as froth flotation cells are critical for controlling gas dispersion. Commonly, bubbles are measured by obtaining representative photographs, which are then analyzed using segmentation and identification software tools. Recent developments have focused on enhancing these segmentation tools. However, the main challenges around complex bubble cluster segmentation remain unresolved, while the tools to tackle these challenges have become increasingly complex and computationally expensive. In this work, we propose an alternative solution, circumventing the need for image segmentation and bubble identification. An Artificial Neural Network (ANN) was trained to estimate the Sauter mean bubble size (D<sub>32</sub>) based on macroscopic image features obtained with simple and inexpensive image analysis. The results showed excellent prediction accuracy, with a correlation coefficient, R, over 0.998 in the testing stage, and without bias in its error distribution. This machine learning tool paves the way for robust and fast estimation of bubble size under complex bubble images, without the need of image segmentation.

The size distribution of the agitated saline microbubbles for contrast transcranial Doppler generated using standard manual methods.

Agitated saline microbubbles (MBs) are a common contrast agent for determining right-to-left shunt (RLS) by the contrast transcranial Doppler (c-TCD). The size of the generated bubbles is not standardized in clinical practice. MBs were generated using the recommended manual method by reciprocating motion through two syringes. The bubble size distributions (BSD) were measured using the microscopic shadow imaging technique. The results show that the diameter of MBs is mainly distributed between 10 and 100 μm, the mean bubble size is between 21 and 34 μm, the Sauter mean diameter (D32) is primarily between 50 and 300 μm, and the standard deviation (SD) is between 6 and 17 μm in 80 experiments. It provides a more accurate basis for the recommended manual method instability. The high variance values of the BSD indicate that the manual method has low stability and repeatability. The results of this study can be useful for further improvement of the reliability of c-TCD in detecting RLS. RESEARCH HIGHLIGHTS: This study provided the first detailed descriptions of the MBs size distribution in a flowing contrast agent by the microscopic shadow imaging technique. It reveals significant differences in the bubble size of manual foaming during repeated manipulations for each individual and between individuals.

Hydrodynamics of CH4-Sn molten-metal bubble columns under electromagnetic field using computational fluid dynamics

The use of molten metal bubble column reactors (MMBCRs) for CO2-free H2 production via non-oxidative CH4 pyrolysis has gained attention due to little catalyst deactivation and simple downstream processing. Maintaining a small size of bubbles is crucial to enhance CH4 decomposition rates in MMBCRs. In this study, the effect of electromagnetic field on the hydrodynamics of MMBCRs was investigated using volume-of-fluid computational fluid dynamics (VOF-CFD) coupled with a large-eddy simulation (LES) model for turbulence and a magnetohydrodynamic (MHD) model for the electromagnetic field. Hydrodynamic parameters such as the gas holdup (αG), mean bubble size (d32), and gas–liquid interfacial area (as) were obtained from the validated VOF-CFD model in the bench-scale CH4-Sn system at 900 °C. A horizontal electromagnetic field with alternating current (AC) at an electromagnetic field strength of 1.0 T promoted bubble breakup and increased as from 8.6 to 18.2 m2/m3 compared to the MMBCR without electromagnetic field.

Critical consideration of gas bubbles to replace fat globules in acidified milk matrices

The influence of low gas volume fraction and bubble deformability on rheological properties and microstructure of acidified milk matrices was investigated; functionality was compared with that of fat globules. With increasing fat volume fraction, stiffness and yield stress of the acidified milk models increased, but decreased with increasing gas volume fraction. This was caused by the large size of the bubbles compared with the fat globules and different processing of the samples containing gas and fat, respectively. A mixture of 1% fat together with gas bubbles resulted in a smaller mean bubble size and thus stiffer bubbles. Simultaneously, an increase in stiffness and yield stress of the bubble-containing matrices was observed, although not to the same extent as for unfoamed matrices with increasing fat content. Beyond a critical gas volume fraction of 0.27, gas bubbles were no longer separated and interacted with other matrix components and themselves, weakening the structure.

Optical measurement of the shear stress and velocity distribution in an idealized deglutition process

The rheological properties of food have an impact on physical interaction with various haptic sensors in the mouth during the chewing and swallowing process. Therefore, the investigations on food rheology are dedicated to improving the texture or mouthfeel of a product. It is advantageous to perform such rheological measurements in geometries and under conditions that reflect the flow conditions during the food distortion in the mouth cavity. Such investigations have been limited primarily to viscous or lumpy foodstuffs. Our study is dedicated to the compression of liquid foams in an idealized deglutition process. To reflect the flow conditions in such a process, we developed an experimental setup consisting of a quasi-two-dimensional replica of the palate and a movable tongue. During the idealized deglutition process, the optical measurements of bubble displacement and deformation were performed. The deglutition process was modelled by varying the compression speed of the tongue towards the static palate. The variation in mean bubble size and liquid fraction of foam for two tongue geometries with different roughness are available. It is observed that the velocity distribution of bubble motion during the deglutition corresponds to a rigid body flow, notwithstanding the surface roughness. The resulting wall-shear stress, however, was higher for the rough tongue plate for all foam bubble sizes and liquid fraction. The resulting velocity and wall-shear stress distributions can provide information about the haptic perception in the mouth during the swallowing process.

A phenomenological kinetic flotation model: Distinct Time-Variant floatability distributions for the pulp and froth materials

A simple and easy-to-use phenomenological kinetic flotation model, strongly connected with the physics of the process, is proposed in this paper. The model explicitly contains the cell volume, aeration rate, volumetric hold-up, mean bubble size, and particle density as input variables. It can be employed to characterize the floatability distributions of the particles in the pulp and the froth separately any time during the flotation process. Two new time-dependent kinetic parameters, the bubble loading factor φ(t) and the maximum cell mass transfer capacity Ṁmax(t) also appear in the model expression. φ(t) is a measure of the degree of crowding of the bubble surfaces and accounts for the deviations from the first-order rate equation. Ṁmax(t) describes the maximum amount of mass that can be transported to the froth phase by the bubble population in the cell. Screen fractionation of each froth product collected at different time intervals during a single kinetic flotation test is sufficient to generate the data required by the model for analysis. Application of the model to this data yields directly time-dependent functions for the floatability of the particles reporting to froth Kf(t) or remaining in the cell Kp(t) for each size fraction separately, without the need for any empirical parameters. The test of the model was carried out using published kinetic flotation data from the literature.

Formation and stabilisation of microbubbles in acidified milk products

In recent years, several studies have attempted to use gas as a fat replacer. However, large bubbles affect the creaminess perception negatively. Therefore, this study examined whether microbubbles behave oppositely and are able to enhance creaminess. The first part of this study addressed the formation and stabilisation of microbubbles in an acidified milk model matrix. After optimising the gas injection process and the matrix formulation with different hydrocolloids, it was possible to generate microbubbles with a mean bubble size of 34 μm. The second part of this study investigated the influence of microbubbles on textural parameters like stiffness, yield stress and viscosity compared with large bubbles. It could be shown that in the model matrix containing microbubbles an increased stiffness arose with increasing gas volume fraction while the opposite trend occurred for large bubbles. However, such an effect did not occur for the yield stress and viscosity.

Effect of Clay Minerals on the Flotation Performance of Coal and Mean Bubble Size

Flotasyon, ince boyutlu kömürlerin zenginleştirilmesinde kullanılan en etkili yöntemlerdendir. Bununla birlikte, kömürün yan kayaç olarak kil minerallerini bulundurması flotasyon işlemini olumsuz etkilemektedir. Bu çalışmada kil minerallerinin (kaolin ve montmorillonit) ve flotasyon reaktiflerinin Tunçbilek linyit kömürünün flotasyon performansına ve ortalama kabarcık boyutuna etkisi araştırılmıştır. Deneysel çalışmalarda; bastırıcı olarak sodyum silikat, toplayıcı olarak gaz yağı kullanılırken, köpürtücü olarak ise metil izobütil karbinol (MIBC) ve Dowfroth 250 kullanılmıştır. Kil türü ve miktarı, bastırıcı miktarı ve köpürtücü türü ve miktarı çalışılan deneysel parametrelerdir. Bastırıcı miktarı ve kil içeriğinin etkisinin belirlendiği deneysel çalışmalarda, kaolin içerikli numunelerde %40-55, montmorillonit içerikli numunelerde ise %30-47 aralığında yanabilir verim değerleri elde edilmiştir. Köpürtücü olarak Dowfroth 250 ile daha iyi sonuçlar elde edilmiş ve montmorillonit içeren kömür numunesi, kaolin içeren kömür numunesine göre daha büyük kabarcıklar oluşmasına neden olmuştur. Bu çalışmadan elde edilen sonuçlar yan kayaç olarak kil bulunduran kömürlerin flotasyon davranışlarının anlaşılabilmesi ve çözüm önerileri sunulabilmesi için temel bir altyapı oluşturacaktır.

Effects of SDS surface-active agents on hydrodynamics and oxygen mass transfer in a square bubble column reactor: Experimental and CFD modeling study

This work examined experimentally and numerically the effects of sodium dodecyl sulfate (SDS) surfactant agent on mass transfer coefficients kLaL and kL in a high-aspect square bubble column reactor. To this end, SDS aqueous solutions at concentrations between 5 ppm and 30 ppm were prepared. Superficial gas velocity was varied from 0.74 cm.s−1 to 9.44 cm.s−1 covering all flow regimes to analyze the interaction between different phases and the effect of liquid surface tension on gas-liquid interfaces. Experimental data indicated that the presence of the SDS in the liquid phase augmented the overall gas holdup up to 25% at a constant gas velocity (Ug = 7.3 cm.s−1) in the transition regime, which is due to the role of SDS in bubble interactions. The impact of SDS addition on the mass transfer is twofold. Firstly, it reduced kL because the SDS molecules migrate towards the bubble-liquid interface thanks to the lower O2 diffusivity in water, thus hindering the O2 transfer from one phase to the other. Secondly, it increased aL owing to the inhibition of bubbles coalescence and, therefore, the lower mean bubble size. Among these two effects, the latter is prevailing, which led to an increase in kLaL in contaminated systems. In the second part of this work, a population balance model (PBM) was coupled with a computational fluid dynamics (CFD) model based on large-eddy simulation (LES) to analyze the effect of both tap water and SDS contaminated systems on the breakage phenomena, local flow pattern, and the resulting hydrodynamics and mass transfer features in the reactor. Ultimately, experiments and numerical simulations agreed well in terms of global gas holdup and kLaL coefficient. In particular, the transition points in the case of the SDS media (except for foam formation) provided the model with adequate PBM definitions for the dispersed phase.

Enhanced production of hydrogen from alkaline electrolysis by microbubbles removal on bionic electrode

In this paper, a plane electrode reactor with gas electro-generation in alkaline water electrolysis was developed. In such electrochemical reactors, the efficiency is closely linked to the hydrodynamics of the electrogenerated bubbles acting as movable electrical insulators. The electric and flow fields of the electrodes were studied by numerical simulation methods and the data revealed even electric potential distributions of the novel bionic type when compared to conventional grid type. The Murray leaf-like total pressure drop was the lowest and the flow field was mostly uniform, resulting in synergetic effect of electric field and flow field with higher hydrogen concentration reaching about 50% the traditional one. The experimental data indicated lower overpotential of Murray leaf-like structure by 12% than those obtained by traditional grid electrode plates. Furthermore, the visual experiments showed that the mean bubble size of the bionic leaf-like electrodes was smaller than that of the grid type by 45%, suggesting possible optimization of generated and detachment of hydrogen bubbles on the bionic electrode surface. In sum, the combination of uniform current distribution and efficient removal of hydrogen bubbles by optimizing the electrodes through bionic design could promote the efficiency of the alkaline water electrolysis for hydrogen production.

Foam coarsening under a steady shear: interplay between bubble rearrangement and film thinning dynamics.

Aqueous foams are unstable and age by drainage and coarsening. Today, these effects are well described, as also their impact on foam properties. In that respect, the foam viscoelastic properties evolve in time as a consequence of coarsening which tends to increase the mean bubble size. Here, we investigate the reverse coupling, and study if and how the continuous flow of a foam can impact its dynamics of coarsening. We introduce a new protocol where brief oscillatory measurements are inserted during a constant steady shear, allowing us to monitor the relative variation of the bubble size with time (obtained from the one of the elastic modulus G') as a function of the applied shear rate. It turns out that the coarsening rate is strongly impacted by the applied shear: this rate is continuously reduced above a critical shear rate, which itself decreases with the bubble size. This coarsening-rate reduction is interpreted as the result of out-of-equilibrium and shear-dependent film thicknesses, being higher than at rest. The critical shear rate, above which films are dynamically sustained at higher thickness than at equilibrium, emerges from the competition between the rate of rearrangements and the time required to drain the thick film created during the rearrangement. We thus report here a first experimental proof and measurements of out-of-equilibrium film thicknesses within a sheared foam, and of the impact this has on coarsening.

Evaluation of the statistical reliability of micro-flotation experiments using a hallimond flotation cell

ABSTRACT This paper presents the evaluation of the statistical reliability of micro-flotation experiments using a Hallimond flotation cell. The study was conducted by three experimental analysts using different micro-flotation conditions and samples. Statistical analysis of 20 micro-flotation experiments was performed. Using the Kolmogorov–Smirnov test, the data from all experiments were found to come from a normal distribution. The coefficient of variation ranged from 0.5% to 8.2%. The high variability was mainly justified by the poor reproducibility of the bubble size distribution; this was corroborated by the high deviation of the estimated mean bubble size in the experiments, which was 742 ± 249 µm. A frequency histogram showed that although extreme values up to 21.3% could appear, 70% of the experiments had coefficient of variation less than 4%. The Asymptotic Test and the Modified Signed-likelihood Ratio Test showed that there were not statistically significant differences between the variability of the 20 micro-flotation experiments. The overall confidence interval for the coefficient of variation from 2.2% to 4.5% was established. The coefficient of variation tended to decrease when the repetitions of the micro-flotation experiments increased. The main source of variability of the micro-flotation experiments conducted in the Hallimond cell is the repeatability of the method.

Effect of Operating Parameters on the Coalescence and Breakup of Bubbles in a Multiphase Pump Based on a CFD-PBM Coupled Model

When the multiphase pump is running, the internal medium often exists as bubble flow. In order to investigate the bubble occurrence characteristics in the pressurization unit of the multiphase pump more accurately, this paper couples computational fluid dynamics (CFD) with a population balance model (PBM) to investigate the bubble size distribution law of the multiphase pump under different operating conditions, taking into account the bubble coalescence and breakup. The research shows that the mean bubble size in the impeller domain gradually decreases from 1.7013 mm at the inlet to 0.6179 mm at the outlet along the axis direction; the average bubble diameter in the diffuser domain fluctuates around 0.60 mm. The bubbles in the impeller region gradually change from the trend of coalescence to the trend of breakup along the axial and radial directions, and the bubbles in the diffuser tend to be broken by the vortex entrainment. The bubble size development law is influenced by the inlet gas volume fraction (IGVF) and the rotational speed, showing a more obvious rule, where the gas phase aggregation phenomenon enhanced by the increase in IGVF promotes the trend of bubble coalescence and makes the bubble size gradually increase. The increased blade shearing effect with the increase in rotational speed promotes the trend of bubble breakup, which gradually reduces the size of the bubbles. In addition, increasing the bubble coalescence probability is a key factor leading to changes in bubble size; the bubble size development law is not very sensitive to changes in flow, and the bubble size is at its maximum under design conditions. The research results can accurately predict the performance change of the multiphase pump and provide technical guidance for its safe operation and optimal design.

Hydrodynamics of molten-metal bubble columns in the near-bubbling field using volume of fluid computational fluid dynamics

Using molten-metal bubble columns (MMBCs), non-oxidative CH4 pyrolysis producing H2 and solid carbon has emerged as an efficient low-carbon H2 production compared to steam-methane reforming (SMR). Because hydrodynamic parameters such as gas holdup and bubble size have significant impacts on heat and mass transfer, understanding the hydrodynamics of MMBCs is crucial for determining the optimal operating conditions and reactor design. A volume of fluid computational fluid dynamics (VOF-CFD) model coupled with a large eddy simulation (LES) turbulence equation was developed and validated for MMBCs. A novel bubble detection algorithm was also proposed for the CFD simulation. Using the validated CFD model, the hydrodynamics of a CH4-Sn system were investigated in a near-bubbling field at 900 °C and ambient pressure. The gas holdup increased from 1.1 to 1.9 % as the superficial gas velocity rose from 2.6 to 5.2 mm/s. The mean bubble size also increased from 4.7 to 5.4 mm for the gas velocities under the bubbling flow regime. The interfacial surface area between the gas and liquid phases was approximately 8.6 to 15.1 m2/m3 in the bubbling flow regime. The VOF-CFD simulations near the nozzle may thus provide the bubble size generated from the gas distributor according to operational and geometrical modifications.

Towards more efficient hydroformylation of long‐chain alkenes in aqueous biphasic system using microbubbles

AbstractMicrobubbles were applied for the first time to enhance the aqueous biphasic hydroformylation of long‐chain alkenes. The gas–liquid–liquid multiphase hydroformylation was carried out using 1‐dodecene as model higher olefin and dihexadecyl dimethyl ammonium bromide (DDAB) as surfactant. The increasing liquid temperature resulted in the increased mean bubble size and total bubble number, but this trend became inapparent at high temperatures due to the reduced liquid viscosity. The increasing gas flow rate was favorable to increase the multiphase interfacial area, thus leading to the increasing mass transfer coefficient KLa. The conversion of 1‐dodecene reached maximum 95% in the three‐phase microemulsion system, and at a gas flow rate of 40 ml min−1. Further increasing the gas flow rate to 80 ml min−1 decreased the conversion, which was attributed to the destroyed flow field, as observed in micro‐PIV. Owing to the increased multiphase mass transfer efficiency, the application of microbubbles allowed to intensify the hydroformylation reaction with reduced reaction pressure, higher chemoselectivity, and increased turnover frequency (TOF).

Effect of particle characteristics and foaming parameters on resulting foam quality and stability

In this study, the effects of ten different food-grade particles on bubble quality and stabilization of particle-stabilized food foams in batch and continuous foaming with and without polyglycerol ester (PGE) as an emulsifier were investigated. Particle properties, such as contact angle and porosity, and varying process parameters, such as shear rate and gas fraction, were assessed with respect to their impact on bubble size x50,0, bubble size distribution width and drainage.The smallest bubble size x50,0 in foams without PGE could be achieved with banana powder (88 μm), calcium carbonate (89 μm) and microcrystalline cellulose (79 μm) particles. In comparison, the smallest size in the reference without particles were 105 μm. Combining the use of particles with PGE further reduced bubble size by up to 57% and drainage by up to 100%. Increasing the shear rate from 4922 s−1 (35 μm) to 9844 s−1 (14 μm) resulted in smaller mean bubble sizes and significantly narrower bubble size distributions whereas no distinct correlation between gas fraction and resulting bubble size was found.This study shows that using suitable particles in combination with an optimized foaming process promotes both bubble quality and the stability of foams.