Bubble Size Distribution Research Articles

Research on bubble generation method based on flow control mechanism

The method of bubble generation is the key to the application of bubbles, of which the flow control method is widely used because of its low energy consumption and high efficiency, and the flow mechanism involved can be subdivided into turbulence, shock, and cavitation. All three effects can be achieved by venturi tubes. This paper systematically introduces the basic physical principles of the three effects, the bubble size distribution, and the applicable void fraction. Among them, turbulence and cavitation can generate bubbles in the millimeter and micro-nano range respectively, respectively, and the shock wave is in between.

A modelling approach to investigate the performance of slurry bubble column reactors implementing Fischer–Tropsch synthesis

AbstractA reliable estimation of the reactor performance is a crucial aspect of slurry bubble column reactor design. Hydrodynamics, mass transfer, and reaction rate are the main parameters influencing the overall performance. A new hydrodynamic model was adopted in this study to properly predict the effect of solids loading, particle size, pressure, and temperature on the gas holdup, bubble size distribution, and mass transfer coefficient. The reactor was divided into small cells with individual hydrodynamic parameters. The results were compared with the experimental data and showed that the model can acceptably predict the hydrodynamic and mass transfer parameters in various process situations. A parametric study was accomplished to understand the effect of catalyst loading, superficial gas velocity, H2/CO ratio, L/D ratio, pressure, temperature, and catalyst attrition on the conversion rate, catalyst productivity, and space–time yield. A cobalt/silica catalyst was adopted in this study. Based on the obtained results, the syngas conversion increases by catalyst loading, L/D, and temperature, while it decreases by Ug and pressure. The H2/CO ratio results in a maximum conversion somewhere between 2 and 2.5. Three different scenarios were determined to study the effect of catalyst attrition on the reactor performance. The results show that the attrition decreases the syngas conversion due to the decrease in the catalyst size and the catalyst loss. Also, the performance remains constant if a sufficient amount of fresh catalyst is added to the system continuously.

The entrainment and evolution of gas bubbles in bread dough—A review

AbstractBackground and ObjectiveFor control of bread quality to achieve high loaf volume and uniform crumb structure, gas bubble dynamics in dough needs to be better understood throughout different breadmaking processes. The objective of this review was to establish a solid theoretical basis on how flour type, water and salt content, and mixing conditions affected the incorporation, evolution, and stabilization of gas bubbles in a dough.FindingsBubble dynamics including entrainment, disentrainment, break‐up, disproportionation, growth, and coalescence were outlined and their effects on the gas phase of the dough were assessed. The application, advantages, and disadvantages of microscopy, magnetic resonance imaging (MRI) and X‐ray microtomography techniques for qualitatively or quantitatively characterizing the void fraction and bubble size distribution (BSD) in the dough at various stages of the breadmaking process have been discussed.ConclusionsSince the BSD evolution in bread dough is associated with the quality of the resultant products, to devise strategies for improving the product quality, dough formulation, and mixing conditions need to be considered.Significance and NoveltyDue to the obvious challenges of monitoring the fast evolution of BSD in yeasted dough, future research needs to focus on the effects of yeast activity on dough's BSD.

Are nucleation bubbles in a liquid all independent?

The spontaneous formation of tiny bubbles in a liquid is at the root of the nucleation mechanism during the liquid-to-vapor transition of a metastable liquid. The smaller the bubbles the larger their probability to appear, and even for moderately metastable liquid, it is frequent to observe several tiny bubbles close to each other, suggesting that they are not all independent. It is shown that these spatially correlated bubbles should be seen as belonging to one single density depression of the liquid due to fluctuations (called LDR for Low Density Region) and should be counted as one event instead of several. This has a major impact on the characterization of the bubble density in a liquid, with consequences (i) for understanding liquid-to-vapor transitions which proceed through growing and merging of these correlated bubbles, and (ii) for free energy profile and barrier calculations with molecular simulation techniques which require to convert the calculated size distribution of the largest bubble into the size distribution of any bubble. Remarkably, the average number of LDRs in a given volume simply relates to the probability of not having bubbles in the liquid.

End-to-End Bubble Size Distribution Detection Technique in Dense Bubbly Flows Based on You Only Look Once Architecture.

Accurate measurements of the bubble size distribution (BSD) are crucial for investigating gas-liquid mass transfer mechanisms and describing the characteristics of chemical production. However, measuring the BSD in high-density bubbly flows remains challenging due to limited image algorithms and high data densities. Therefore, an end-to-end BSD detection method in dense bubbly flows based on deep learning is proposed in this paper. The bubble detector locates the positions of dense bubbles utilizing objection detection networks and simultaneously performs ellipse parameter fitting to measure the size of the bubbles. Different You Only Look Once (YOLO) architectures are compared, and YOLOv7 is selected as the backbone network. The complete intersection over union calculation method is modified by the circumferential horizontal rectangle of bubbles, and the loss function is optimized by adding L2 constraints of ellipse size parameters. The experimental results show that the proposed technique surpasses existing methods in terms of precision, recall, and mean square error, achieving values of 0.9871, 0.8725, and 3.8299, respectively. The proposed technique demonstrates high efficiency and accuracy when measuring BSDs in high-density bubbly flows and has the potential for practical applications.

Simultaneous monitoring of flow patterns, and bubble, and plastics micro-particle characteristics in Dissolved Air Flotation (DAF)

Dissolved air flotation (DAF) is an important water treatment process for removing suspended solids. Understanding the characteristic size and spatial distribution of both bubbles and suspended solids, including their hydrodynamics, within flotation tanks is key to optimising flotation performance. In this study, a continuously operating lab scale DAF tank was constructed to simultaneously monitor size distributions and motions of microbubbles (Ø = 67–86 ± 13–19 µm) and spherical polyethylene microplastic particles (Ø = 69.8 ± 5.3 µm) in unbuffered water at circumneutral pH without salt addition and with 0.5 g L−1 sodium chloride (NaCl) added. Microplastic particles were used for the first time in such a setup. Particle tracking showed different flow regimes in the separation zone, with short-circuiting flow paths varying at different flow rates. Within the range of flowrates employed, the bubble bed that formed was not prominent enough to promote stratified flow. The addition of NaCl greatly reduced microplastic particle counts within the separation zone during flotation. This new multi-factorial approach provides critical insights into the variations in DAF performance.

Study on the Strengthening Mechanism of a MIBC-PEG Mixed Surfactant on Foam Stability.

In the flotation process, the frother, which is typically a surfactant, can be added to the pulp to reduce the surface tension and create stable foam. Currently, the nonionic mixed surfactant is widely employed as the frother for fine coal flotation. In this study, we focused on examining the foam properties of a mixed surfactant comprising short-chain methyl isobutyl carbinol (MIBC) and long-chain polyethylene glycol-1000 (PEG). Analytical techniques such as surface tension measurement, dynamic foam stability measurement, bubble morphology observation, and foam film drainage measurement were used to investigate the foam properties in single and mixed surfactant solution from a macroscopic scale to a microscopic scale. The surface tension results indicated that PEG exhibited higher surface activity than MIBC, and the addition of PEG to MIBC resulted in a significant reduction in solution surface tension. The dynamic foam stability analysis revealed that the incorporation of a small amount of PEG into MIBC solution notably improved foam stability. Furthermore, the addition of PEG to the MIBC solution led to a shift in the bubble size distribution curve from a "double peak" to a "single peak" shape. This shift indicated a substantial reduction in bubble size, indicating an enhanced inhibition of bubble coalescence. Additionally, the liquid film drainage rate was significantly slowed down, and the stability of the liquid film was improved upon the addition of PEG to MIBC. This improvement can be attributed to the synergistic effect of MIBC and PEG molecules adsorbed at the gas-liquid interface. The synergistic effect of mixed MIBC-PEG was due to the additional surface tension gradient created by the difference in surface activity between PEG and MIBC. This surface tension gradient enhances the Marangoni flow of surfactant molecules, thereby improving the self-healing ability of the liquid film and increasing its stability.

Bubble breakup criteria for the population balance model of gas–liquid flow simulations

This work presented a critical review of various literature models with focus on the bubble breakup criteria. A general formulation of critical energy for breakup criteria was derived. Based on the framework of Luo and Svendsen (1996), effects of breakup criteria and effective eddy scale on model outputs were examined. Results showed that a joint criterion consisting of surface energy criterion and force criterion (Cf + Wed', Wec = 2) exhibited both reasonable breakup frequency and daughter size distribution (M-shape), and an optimized kernel function was proposed. A correlation between fragmentation ratio and eddy scale was introduced, and a new breakup model with a simplified kernel function was obtained based on the model of Qi et al. (2022). Both kernel functions were implemented in CFD-PBM simulations, and the results agreed with bubble size distributions. The confusions about literature bubble breakup criteria were cleared and two bubble breakup models were suggested.

Bubble dynamics properties of B-particles in a quasi-2D gas-solid fluidized bed: Computational particle fluid dynamics numerical simulation and post-processed by digital image analysis technique

Bubble dynamics properties play a crucial and significant role in the design and optimization of gas-solid fluidized beds. In this study, the bubble dynamics properties of four B-particles were investigated in a quasi-two-dimensional (quasi-2D) fluidized bed, including bubble equivalent diameter, bubble size distribution, average bubble density, bubble aspect ratio, bubble hold-up, bed expansion ratio, bubble radial position, and bubble velocity. The studies were performed by computational particle fluid dynamics (CPFD) numerical simulation and post-processed with digital image analysis (DIA) technique, at superficial gas velocities ranging from 2umf to 7umf. The simulated results shown that the CPFD simulation combining with DIA technique post-processing could be used as a reliable method for simulating bubble dynamics properties in quasi-2D gas-solid fluidized beds. However, it seemed not desirable for the simulation of bubble motion near the air distributor at higher superficial gas velocity from the simulated average bubble density distribution. The superficial gas velocity significantly affected the bubble equivalent diameter and evolution, while it had little influence on bubble size distribution and bubble aspect ratio distribution for the same particles. Both time-averaged bubble hold-up and bed expansion ratio increased with the increase of superficial gas velocity. Two core-annular flow structures could be found in the fluidized bed for all cases. The average bubble rising velocity increased with the increasing bubble equivalent diameter. For bubble lateral movement, the smaller bubbles might be more susceptible, and superficial gas velocity had a little influence on the absolute lateral velocity of bubbles. The simulated results presented a valuable and novel approach for studying bubble dynamics properties. The comprehensive understanding of bubble dynamics behaviors in quasi-2D gas-solid fluidized beds would provide support in the design, operation, and optimization of gas-solid fluidized bed reactors.

Double‐sensor conductivity probe applied in a flotation system

AbstractThis study proposed a new approach for measuring bubble size distribution, bubble mean diameter, Sauter mean bubble diameter, and gas holdup using a double‐sensor conductivity probe in an air/water two‐phase system bubble column. The results for the two‐phase system were compared and calibrated using analyses from bubble images taken by a digital camera from the side of the column wall. Good agreement was observed between the two techniques. The same double‐sensor conductivity was used in an air/water/solids three‐phase system. The conductivity probe captured the change in bubble dynamic behaviour inside the pulp phase; however, the presence of the solids made it more challenging to measure. As a result, the VisioFroth commercial package, using images taken from the top of the froth layer, could be used in conjunction with the double‐sensor conductivity probe to show the dynamic evolution of mineralized bubbles from the pulp zone to the froth zone in a flotation process.

Air-bubble entrainment by translating turbulent jets in stagnant water

This paper presents an experimental study on air-bubble entrainment in quiescent water by translating circular turbulent jets. The jet diameters, plunging heights, impact velocities, and translating velocities were varied during the experiments to investigate their relative effect on the bubble characteristics. The experimental observations reveal that the jet translation affects the air bubble entrainment mechanism and the distribution of bubble size. A rotating cavity forms around the plunging jet due to the translation of the jet. Depending on the translating velocity, the air bubble emanates from the cusp of the cavity and the downstream water surface meniscus with the jet. The bubble swarm produced by the translating jet exhibits vortex shedding with Strouhal numbers between 0.22 and 0.27, comparable to a circular cylinder in cross-flow. The peak of the bubble size distribution varies between 0.5 and 1.5 mm, while the Sauter mean diameter varies between 1.8 and 2.8 mm. The maximum penetration depth of bubbles is found to be a function of the jet impact to translating velocity ratio, and the Capillary number of the air–water interface. The spatial distribution of bubbles along the plume cross section exhibits Gaussian distributions. Finally, the terminal rising velocity of the bubbles shows no obvious effect of the jet translation.

Harmful algal bloom aerosols and human health.

Harmful algal blooms (HABs) are increasing across many locations globally. Toxins from HABs can be incorporated into aerosols and transported inland, where subsequent exposure and inhalation can induce adverse health effects. However, the relationship between HAB aerosols and health outcomes remains unclear despite the potential for population-level exposures. In this review, we synthesized the current state of knowledge and identified evidence gaps in the relationship between HAB aerosols and human health. Aerosols from Karenia brevis, Ostreopsis sp., and cyanobacteria were linked with respiratory outcomes. However, most works did not directly measure aerosol or toxin concentrations and instead relied on proxy metrics of exposure, such as cell concentrations in nearby waterbodies. Furthermore, the number of studies with epidemiological designs was limited. Significant uncertainties remain regarding the health effects of other HAB species; threshold dose and the dose-response relationship; effects of concurrent exposures to mixtures of toxins and other aerosol sources, such as microplastics and metals; the impact of long-term exposures; and disparities in exposures and associated health effects across potentially vulnerable subpopulations. Additional studies employing multifaceted exposure assessment methods and leveraging large health databases could address such gaps and improve our understanding of the public health burden of HABs.

CFD Modeling of Phase Change during the Flashing-Induced Instability in a Natural Circulation Circuit

Flashing-induced instability (FII) has a significant impact on the safe operation of a natural circulation circuit, a phenomenon frequently encountered in the cooling systems of advanced light water reactors. While one-dimensional system codes are commonly used for the engineering design and safety analysis of FII, there is a strong academic interest in understanding the underlying physical mechanisms. To address this, high-resolution computational fluid dynamics (CFD) simulations serve as a valuable tool. However, the current state of CFD modeling for two-phase flows with phase change, which are particularly highly transient fluctuating flashing flows, is still in its early stages of development. In this study, we establish a CFD model that focuses on interphase heat transfer to analyze the phase change during FII. By incorporating experimental data from the literature, we investigate the transient flow field and thermodynamic behavior in the riser of the GENEVA test facility. The study provides valuable insights into the non-equilibrium and interfacial transfer phenomena during the phase change as well as the effect of high-frequency fluctuation. Additionally, we discuss in detail the challenges associated with FII modeling and the limitations of the current model. We also provide suggestions for potential improvements in future numerical studies. The results show that the thermal phase change and heat transfer coefficient model adopted for the simulation reasonably captures the evaporation and condensation process. However, it tends to under-predict the evaporation rate, which results in a larger pressure drop through the riser. The observation that the void fraction close to the wall is higher than that in the riser center evidences that the reliable modeling of bubble size distribution as well as the inclusion of non-drag forces are important for predicting the transverse void distribution. Furthermore, it reveals that both the temperature and pressure change in an FII, and their effects on phase change should be taken into account simultaneously.

Characteristics and dynamics of electrolyzed bubbles in magnetohydrodynamic bubbly jet

BackgroundThe magnetohydrodynamics (MHD) are highly relevant to practical applications, including marine propulsion and hydrogen production etc. For better implementation to applications, typical characteristics of the MHD bubbly jet, such as the size distribution and motion of oxygen and hydrogen bubbles, need to be well realized. MethodsThe MHD bubbly jet generated by high current density associated with strong permanent magnet is experimentally investigated. Two fields of view are taken to analyze the details of micro-sized bubbles and the whole pattern of bubbly jet. To couple the magnetoelectric effects and bubble dynamics, the Reynolds number based on the Lorentz force ReL is applied in the parametric analysis. FindingsDependence of the mean size of bubble on the current density is generally verified. Two regimes regarding the bubble sizes are distinguished. The proposed mechanisms of these two regimes are supported by the pattern of bubbly jet. For ReL≤2000, size of bubble monotonically increases with current density, so that referred to as the electrochemistry regime. Nevertheless, the size of bubble remains nearly unchanged for ReL>2000, referred to as the flow regime in which the turbulent breakup of the bubbly jet core starts to evolve.

Application of CFD to study the flow in a molten salt chlorination furnace

A numerical simulation based on computational fluid dynamics and population balance model has been accomplished to analyze the gas-liquid-solid three phases in molten salt chlorination furnace. A two-way coupled model considering the influence of the bubble size distribution (BSD) on the flow field by transmitting the data of the Sauter mean diameter is firstly set up and validated with the experimental and simulation results. Based on the mechanisms of bubble coalescence and breakup induced by the turbulence and the bubble-particle interaction, some factors like the velocity field, volume fraction, turbulence dissipation rate, BSD, bubble number density and particle distribution are discussed. The results show that 40 m/s is an appropriate operation parameter for the inlet velocity of molten salt chlorination furnace, for the reason that the large bubble in the upper part of the furnace can break up into fine bubbles distributed uniformly in the whole furnace. It is conducive to the increase the interaction opportunity among phases, promote the chemical reactions, and reduce the area of “dead zone”, providing theoretical support and engineering design basis for the production of large-scale molten salt chlorination furnace.

Numerical study of flashing pipe flow using a TFM-PBM coupled method: Effect of interfacial heat transfer and bubble coalescence and breakup

In the present work, the two-fluid model (TFM) is coupled with the population balance model (PBM) to trace the spatial and temporal change of bubble size and interfacial area concentration (IAC) in flashing flows. The model is first validated for bubble growing in stagnant superheated liquid, and satisfactory predictions of the bubble size under low and moderate superheat are obtained. It is then applied to flashing pipe flows, which are characterized by low superheat and high turbulence intensities. The results show that in these cases, coalescence and breakup are important phenomena changing the bubble size distribution in addition to growth. The neglect of their contribution leads to a significant under-prediction of the bubble size and consequently over-prediction of IAC. In addition, choosing an appropriate closure for interfacial heat transfer coefficient (HTC) is another key point in flashing simulation. In high-Reynolds cases (e.g. Re > 106), the enhancement due to turbulence is non-negligible.

The modification of bubble characteristics and chalcopyrite surface hydrophobicity in water containing NaCl

Recycled water has been utilized in various industries as a response to global water scarcity, despite the accumulation of salts in process water and the subsequent alteration of water structures and interfacial properties caused by salts. This study investigated the effect of NaCl, a dominant salt in process water in mineral flotation plants, on chalcopyrite flotation, a major source of copper. For the first time, this study decoupled the effects of surface hydrophobicity and bubble characteristics modified by NaCl on chalcopyrite flotation and also considered the effect of NaCl on the true floatability by removing the effect of collectors and the recovery caused by mechanical entrainment. Cyclic voltammetry (CV) and Cryogenic x-ray photoelectron spectroscopy (Cryo-XPS) were combined to understand chalcopyrite surface oxidation and the formation of hydrophobic products, while bubble size distributions were measured to understand bubble characteristics. It was demonstrated that the addition of NaCl improved the true flotation of chalcopyrite slightly from de-ionised water to 0.1 M NaCl solution, but significantly from 0.1 M NaCl solution to 0.7 M and 1.2 M NaCl solutions. This trend was consistent with the extent of oxidation on the chalcopyrite surface, but inconsistent with the atomic concentration of hydrophobic oxidation products formed. It was identified that the atomic concentration of hydrophobic oxidation products on the chalcopyrite surface was optimised in 0.1 M NaCl solution, suggesting that the surface hydrophobization by NaCl was not a primary factor in chalcopyrite flotation. In contrast to the reduced amount of hydrophobic oxidation products on the chalcopyrite surface in 0.7 M and 1.2 M NaCl solutions, the bubble size was reduced significantly, suggesting that bubble size played a significant role in chalcopyrite flotation in saltwater containing NaCl.

Bubble size distribution at early stage of hydrodynamic cloud cavitation

The bubble size distribution (BSD) in hydrodynamic cloud cavitation is poorly understood, in spite of its importance in cavitation erosion and noise. Challenges arise owing to the heterogeneous turbulent flows and high void fraction in the cavitation regime. The use of a fiber optical probe enables us to obtain the BSD in a cavitation cloud. Two distinct power law scalings at the early stage of cloud cavitation are identified. The first generation of bubbles is produced by the fission to the shedding cavitation pocket by large-scale turbulence, whose isotropic part leads to the basic scaling −10/3, while the anisotropic part due to the effect of hydrofoil wall contributes to the deviation. The successive fragmentation of bubbles accompanied with turbulent energy cascade results in the fairly uniform scaling −4/3. The results indicate that turbulence plays a dominant role in bubble breakup at the early stage of cloud cavitation.

Numerical study of the cavitating flow over backward facing step with a polydisperse two-phase flow model

We study the cavitating flow over a backward facing step with an incompressible polydisperse cavitation model. The model can predict experimental observations for this flow reasonably well, including the shedding cloud characterized by the condensation front, cavity length, void fraction, and shedding frequency. All model variations produced shedding cavities, but the turbulence model and grid resolution are essential for better predictions, with delayed detached eddy simulation (DDES) performing better than Reynolds-averaged Navier–Stokes. Quantities, such as pressures at key points, maximum void fraction location, and shedding frequency, are mildly sensitive to those factors. Finer DDES grid resolution, crucial to resolve small vortices where cavitation occurs in their low pressure cores, improves predictions. Since a fully incompressible model produces a condensation front that follows well the experimental trends, it is concluded that compressibility is not a necessary condition for the formation of a condensation front. Consequently, the speed of sound in the mixture does not appear to play an important role in the front formation and evolution. The polydisperse nature of the model allows prediction of the bubble size distribution. Small bubbles concentrate on the downstream section of the cavity, where cavity collapse is strongest and bubble fission is most intense, while larger bubbles reside near the step where the flow is milder. The condensation front is a moving source of vorticity for the liquid phase where the “compressibility,” in the sense of mixture density changes due to void fraction changes, and baroclinic effects are significant, but the buoyancy effect is negligible.

Comparison of bubbles interaction mechanisms of two-group Interfacial Area Transport Equation model

This work deals with 3D simulations of complex bubbly, cap-bubbly and churn regimes exhibiting bubbles of different shapes and with broad bubble size distribution. The first contribution of this work is to investigate and compare several bubble interaction mechanisms of coalescence and fragmentation for the 2-Group Interfacial Area Transport Equation (IATE) model. For two of these models, this is the first time their performances are assessed within a CFD code. The second contribution is to propose and assess a novel model of fragmentation and coalescence. Finally, a validation versus experimental data on three different configurations and three different regimes is performed. In particular, the interaction mechanisms are analyzed for one specific regime. The implementation of the two-group IATE model has been systematically performed in the 3D NEPTUNE CFD code.