Bubble Coalescence Rate Research Articles

Observation of Gap Phenomena and Development Processing Technology for ECDM of Sapphire

The main purpose of this study was to develop observation techniques and processing technology for the electrochemical discharge machining (ECDM) of sapphire wafers. To measure the effect of gas-film thickness, discharge-spark conditions, and droplet sliding frequency on machining quality and efficiency in ECDM, this research utilized high-speed cameras to observe the gas film thickness and formation of the gas film during ECDM. Additionally, this study observed the machining-gap phenomena during ECDM. The formation mechanism and machining characteristics of the gas film were understood through experiments. The machining parameters included the liquid level, working voltage, rotation speed, and duty factor. This study analyzed and discussed the effect of each machining parameter on the gas-film thickness, current, electrode consumption, and droplet sliding frequency. Moreover, this study aimed to obtain optimized machining parameters to overcome the difficulty of machining sapphire. The experimental results indicated that utilizing a high-speed camera to capture the phenomena between electrodes during electrochemical discharge could effectively observe the gas-film thickness and the coverage of the gas film. A higher bubble coalescence rate enhanced the machining capability and reduced the lateral discharge. Therefore, this study could obtain better machining-hole depths through observation and analysis to improve gas-film stability and machining capability. This study demonstrated that a liquid level of 700 µm, a working voltage of 48 V, a duty factor of 50%, and a tool electrode rotational speed of 200 rpm could achieve a hole depth of 86.7 µm and a hole diameter of 129.5 µm.

Investigations on the performance of xanthan gum as a foam stabilizer and assessment of economic and environmental impacts of foam concrete production

The properties of foam concrete are greatly influenced by the quality of the foam used. The stability of the foam is a vital property that needs dire attention when selecting surfactant. This paper evaluates the role of Xanthan gum (XG) as an additive in the performance improvement of foam produced with two different surfactants, viz., Hingot (Natural surfactant) and Nonylphenol ethoxylate (NPE) (Synthetic surfactant). Different characteristics of foam and surfactant, such as initial foam density, foam drainage, foam bubble microstructure, surface tension, and viscosity of surfactants, are experimentally evaluated. Addition of XG results in a 10-fold increment in viscosity for both Hingot and NPE surfactant solutions. The above enhancement in viscosity results in a substantial reduction in bubble size and an increment in lamella thickness. Further, the reduced surface tension of Hingot surfactant also contributes to a significant reduction in the rate of bubble coalescence. Having studied the surfactant and foam behaviour, the next phase comprises studies on the influence of XG on consistency, setting behaviour, compressive strength, and thermal conductivity of foam concrete with two different densities, viz., 1000 kg/m3 and 1500 kg/m3. Results show that the addition of XG results in a reduction in consistency and an increase in demoulding time of foam concrete. Besides improvement in foam performance, a substantial increase in compressive strength and reduction in thermal conductivity of FC is also observed due to XG addition, indicating that foam bubble microstructure has a significant impact. The last phase involves the life cycle assessment (LCA) of the FC produced with the aforementioned surfactants, which is the first of its kind study that considers the impact of surfactants across various environmental categories. Results of the LCA analysis show that the total cumulative energy demand of preparation of 1 kg of Hingot solution is less than that of 1 kg of NPE surfactant solution. The environmental impact analysis also favours Hingot as a sustainable and economical alternative to NPE surfactant. Results of LCA analysis of FC indicate that foam concrete with 1000 kg/m3 performs better than AAC block and conventional clay brick. Furthermore, the studies highlighted that there is more scope for improving the performance of LCA of FC through using various mineral admixtures as replacement for cement.

Modeling local interfacial area concentration for adiabatic and boiling bubbly flows

Bubbly flows appear in various heat and mass transfer systems, and interfacial area concentration (IAC) is a critical geometrical parameter for the flows. The fidelity of accurately computed IACs is essential for reliable three-dimensional computational fluid dynamics analyses of bubbly flows because the IACs directly affect the interfacial force and interfacial heat and mass transfer rates. This study targeted development of a simple and robust model for predicting the local IAC under steady-state bubbly flows with and without phase change. The dependent two-phase flow parameters of the IAC were derived by considering bubble coalescence and breakup rates. The IAC transport equation simplified under steady-state conditions was used for this purpose. A simplified turbulence model was introduced to formulate the energy dissipation rate per unit mass necessary for computing the bubble coalescence and breakup rates. The prediction accuracy of the local IAC model for bubbly flow conditions with area-average void fraction values less than 10 % was estimated to be 20 % based on 803 local data collected for adiabatic air–water bubbly flows in round tubes with diameters ranging from 9.0 to 200 mm. The model was also applied to predict the local IAC data of subcooled boiling flows in heated channels with hydraulic diameters ranging from 18.5 to 22.2 mm. The test conditions covered pressures from 0.1 to 1.46 MPa. The model prediction accuracy for bubbly flow conditions with area-average void fraction values less than 10 % was estimated to be 40 % based on 155 subcooled boiling flow data.

(Invited) Elucidating the Hydrodynamic Behavior of Multi-Species Gas Bubbles in an Electrochemical Solvent Regenerator for Direct Air Capture

Direct Air Capture (DAC) is very significant to bolstering the global drive towards net negative emissions. DAC technologies/plants have emerged in recent years, with the regeneration of the capture solvent used in the process being the major bottleneck of most aqueous technologies. [1] Leveraging electrochemical principles has offered a potential opportunity to simplify the entire solvent regeneration process and potentially reduce its overall capture cost, compared to traditional approaches. The electrochemical approach also offers the benefit of direct integrating clean energy, and also eliminating the high thermal energy requirements of typical methods.[2] Despite these potentials, the complex three-phase (solid-liquid-gas) interaction on the electrode surface pose a significant impediment to this electrochemical approach for DAC. The evolution of gas bubbles in electrochemical cells are well-known to contribute to energy losses in such reactors, from previous studies. [3],[4],[5] Gas bubbles have been shown to influence ohmic overpotentials in electrochemical reactors and studies have shown that the energy demand for water electrolysis can be reduced by 10-25 percent if the formation of gas bubbles is suppressed. [6],[7] However, the evolution of gas bubbles in electrochemical systems remains a complicated issue requiring further investigation. This study advances previous work by investigating the simultaneous evolution of CO2 gas bubbles along with O2/H2 gas bubbles in an electrochemical reactor for DAC. Gas bubbles are infamous for their ability to cover the active area of electrodes, limiting the transport of reactive species to the electrode surface, thus, increasing cell resistance. In this work, we explore changes in the polarization and hydrodynamics behavior of gas bubbles in the electrolyzer used for DAC solvent regeneration owing to the evolution of CO2 bubbles from pH swing, and the implications of the additional CO2 bubble formation to electrode surface coverage. By using a high-speed camera, we observe that bubbles coverage appears to be larger at the edges of the electrodes, and that the orientation of the electrodes influence bubble coalescence and detachment rate. We also employ different cell designs to mitigate the impact of bubble surface coverage towards reducing cell resistance. References Sabatino, A. Grimm, F. Gallucci, M. Van Sint Annaland, G. J. Kramer, M. Gazzani, Joule, 5(8), 2047-2076 (2021).Gao, A. Omosebi, R. Perrone, K. Liu, Journal of The Electrochemical Society (2022).H. Li, Y. J. Chen, Scientific Reports, 11(1), 1-12 (2021).F. Swiegers, R. N. L.Terrett, G. Tsekouras, T. Tsuzuki, R. J. Pace, R. Stranger, Sustainable Energy & Fuels, 5(11), 3004–3004 (2021).Zhao, H. Ren, L. Luo, Langmuir, 35(16), 5392-5408 (2019).Mazloomi, N. B. Sulaiman, H. Moayedi, International Journal of Electrochemical Science, 7(4), 3314-3326 (2012).C. Wang, C. Y. Chen, Electrochimica Acta, 54(15), 3877-3883 (2009).

Effects of solid particles on bubble breakup and coalescence in slurry bubble columns

Bubble size distribution (BSD) is a crucial parameter in the design and scale-up of slurry bubble columns (SBCs) as it affects the hydrodynamics, heat and mass transfers, and, consequently, the column overall performance. It is necessary to understand the impacts of various parameters on the bubble size to adequately predict the BSD. The population balance model is widely adopted to estimate the BSD by considering the bubble coalescence and breakup rates. Many bubble coalescence and breakup models have been proposed in literature to predict the BSD. However, the available models have been developed for two-phase (gas–liquid) systems. They are, hence, less appropriate for three phase (gas–liquid-solids) systems, as the presence of particles in these systems has considerable impacts on their hydrodynamics. Novel bubble breakup and coalescence models are developed in the present study to predict the effects of solids concentration and particle size on the bubble breakup and coalescence rates. The results showed that the presence of solids can increase both bubble coalescence and breakup rates, while the effect is more pronounced on the coalescence rate. In addition, an increase in the particle size increases the bubble breakup rate and decreases the bubble coalescence rate. An integrated population balance and hydrodynamic model was also developed to estimate the BSD and gas holdup in a slurry bubble column. The developed models in this study were successfully validated by experimental hydrodynamic data from literature and those obtained in our pilot-scale slurry bubble column. A set of parametric studies were accomplished to investigate the effects of solid particles on the bubble breakup, bubble coalescence, and BSD in two common industrial applications of SBCs.

Prediction of sliding bubble coalescence rate during subcooled flow boiling in narrow rectangular channel

Prediction of bubble coalescence rate during subcooled boiling is significant for the simulation of void fraction and heat transfer capacity in the flow channel. In this paper, the boiling visualization experiment based on a four-sides visible test section was carried out to investigate coalescence characteristics of sliding bubbles and predict bubble coalescence rate during subcooled flow boiling in narrow rectangular channel. The experimental phenomena shown that phase interfacial contact velocity at the moment of bubble coalescence increases due to the variation of bubble size in the slide process during the subcooled boiling, which prominently alters the bubble coalescence rate. Besides, bubble collision mainly comes from the relative motion between bubbles due to buoyancy difference and turbulence in this study. Herein, we proposed a correlation of bubble collision frequency by considering the characteristics of rectangular narrow channel. Finally, the bubble coalescence critical velocity model was improved to predict sliding bubble coalescence rate within an error of ±25% during subcooled flow boiling in narrow rectangular channel.

Bubble size distribution for bubbly-to-slug transition flow in narrow rectangular channel

To thoroughly understand the flow structure evolving characteristics in flow regime transition process from bubbly to slug flow in narrow rectangular channel, bubble size distribution (BSD) are experimentally measured by four-sensor conductivity probes. The results show that the T-location distribution fits the measured chord length distribution best in bubbly-to-slug transition flow and the fitting performance becomes worse in slug flow at high void fraction. To solve the problem, two-group bubble fitting for transition flow is proposed and obtains good performance. Function dependence of bubble coalescence and break-up rate is thoroughly analyzed for the transition flow considering its influence on BSD. According to the function dependence and fitting results, empirical correlations are proposed to predict BSD in bubbly and the transition flow. The evolving characteristics of BSD in transition flow are also discussed based on the trends of adjusted parameters in the predicted BSD.

Prediction of Sliding Bubble Coalescence Rate During Subcooled Flow Boiling in Narrow Rectangular Channel

Prediction of Sliding Bubble Coalescence Rate During Subcooled Flow Boiling in Narrow Rectangular Channel

Pore-scale mechanisms and simulations for gas–water two-phase transport processes in natural gas reservoirs

During the development of natural gas reservoirs, predicting the bubble size distribution and transport velocity in the study of gas-water two-phase flow through porous media is extremely important in natural gas recovery. Considering the different forces that act on small and elongated bubbles, a double-bubble phase population balance model was introduced to establish an improved bubble flow model. Accordingly, a two-dimensional visualized model was designed to analyze the bubble diameter distribution and average velocity of bubbles. Dominant bubble coalescence, breakup, and coexistence flows were simulated to evaluate the uniqueness of the diameter distribution of bubbles, the change in the median bubble size at different dimensionless distances, and the average velocity distribution. The peak distribution of the bubble diameter was different in the three flow conditions. Owing to the difference in the bubble breakup and coalescence rates, the peak values of bubbles varied with the increase in the dimensionless distance. Higher bubble breakup or coalescence rates were accompanied by a rapid change in the median bubble size. Furthermore, the average velocity of the transition from small bubbles to elongated bubbles decreased significantly. Thus, the proposed pore-scale gas-water two-phase model can effectively predict the occurrence and transport processes in natural gas reservoirs.

CFD-PBM and experimental investigation of a shear thinning fluid in a gas-liquid tank agitated by a helical ribbon impeller

• The hydrodynamics of an agitated gas-liquid reactor has been modelled using CFD-PBM method. • PIV measuring technique has been applied to verify simulation results. • The performance of a dual helical ribbon in non-Newtonian fluid with complex rheology has been investigated. • The influence of impeller speed on bubble size distribution, flow field, and viscosity of shear-thinning fluid have been studied. This study focuses on hydrodynamic characteristics of a shear thinning fluid agitated in a 3D multiphase reactor using a dual helical ribbon impeller. A combination of Computational Fluid Dynamics (CFD) simulation and Population Balance Model (PBM) were employed to study the gas-liquid interactions at various impeller speeds. The standard k - ε model and Eulerian multiphase approach were used to predict better quantities of turbulent flow parameters and its characteristics. Particle Image Velocimetry (PIV) was used to measure the velocity field for the model validation. Simulation results indicated that the bubble breakage and coalescence rate was intensified due to an increase in rotational speed. However, bubble breakage is still the dominant phenomenon since the bubbles would hit the walls and blads due to the turbulent intensity. Further, the helical ribbon impeller significantly reduces the viscosity of the fluid and improves the mixing efficiency that is confirming the authors’ previous experimental results. Furthermore, formation of static vortices adversely affects the efficiency of mixing process proving that an increase in impeller speed does not necessarily enhance the mixing perfiormance. Further, the helical ribbon impeller significantly reduced the viscosity of the fluid and enhanced mixing efficiency, thereby confirming the authors’ previous experimental results.

Characterization of lauryl betaine foam in the Hele-Shaw cell at high foam qualities (80%\u201398%)

Lauryl betaine (LB) as an amphoteric surfactant carries both positive and negative charges and should be able to generate stable foam through electrostatic interaction with nanoparticles and co-surfactants. However, no previous attempts have been made to investigate the influence of nanoparticles and other co-surfactants on the stability and apparent viscosity of LB-stabilized foam. In this study, a thorough investigation on the influence of silicon dioxide (SiO2) nanoparticles, alpha olefin sulfonate (AOS) and sodium dodecyl sulfate (SDS), on foam stability and apparent viscosity was carried out. The experiments were conducted with the 2D Hele-Shaw cell at high foam qualities (80%–98%). Influence of AOS on the interaction between the LB foam and oil was also investigated. Results showed that the SiO2-LB foam apparent viscosity decreased with increasing surfactant concentration from 0.1 wt% to 0.3 wt%. 0.1 wt% SiO2 was the optimum concentration and increased the 0.1 wt% LB foam stability by 108.65% at 96% foam quality. In the presence of co-surfactants, the most stable foam, with the highest apparent viscosity, was generated by AOS/LB solution at a ratio of 9:1. The emulsified crude oil did not imbibe into AOS-LB foam lamellae. Instead, oil was redirected into the plateau borders where the accumulated oil drops delayed the rate of film thinning, bubble coalescence and coarsening.

The role of exothermic reaction in the enhancement of high mass flux spray cooling

Considerable reduction of vapour layer on the hot substrate is the main requirement during the process of ultrafast cooling of metals and this cannot be achieved by implementing the conventional cooling methodologies. Therefore, in the current work, an attempt has been made to reduce the vapour layer effect by using Na2CO3 as the additive in case of high mass flux spray cooling. The heat transfer analysis depicts augmentation and the maximum heat flux of 1.72 MW/m2 is achieved in case of water spray with 50 g Na2CO3. The increment in latent heat extraction period due to the exothermic reaction between sodium carbonate and water, alteration in vapour bubble coalescence rate by the released CO2 after reaction and shifting of thermo-physical properties in the positive direction of heat transfer are the key reasons behind the heat transfer enhancement. In addition to the above, the dropwise evaporative cooling experiments were conducted to reveal and support the heat transfer mechanism more clearly. The variation in Nu, Pr and Bg clearly ensures the augmentation in heat transfer rate. Finally, the effect of Na2CO3 on the surface morphology of the steel plate and the corrosive nature are identified by conducting SEM analysis and corrosion test, respectively.

Controlling bubble coalescence in metallic foams: A simple phase field-based approach

The phase-field method is used as a basis to develop a strictly mass conserving, yet simple, model for simulation of two-phase flow. The model is aimed to be applied for the study of structure evolution in metallic foams. In this regard, the critical issue is to control the rate of bubble coalescence compared to concurrent processes such as their rearrangement due to fluid motion. In the present model, this is achieved by tuning the interface energy as a free parameter. The model is validated by a number of benchmark tests. First, stability of a two dimensional bubble is investigated by the Young-Laplace law for different values of the interface energy. Then, the coalescence of two bubbles is simulated until the system reaches equilibrium with a circular shape. To address the major capability of the present model for the formation of foam structure, the bubble coalescence is simulated for various values of interface energy in order to slow down the merging process. These simulations are repeated in the presence of a rotational flow to highlight the fact that the model allows to suppress the coalescence process compared to the motion of bubbles relative to each other. Moreover, since density is treated as an auxiliary variable “slaved” to the volume occupied by a given phase, the present model allows realization of arbitrarily large liquid-gas density ratios. This property is demonstrated by simulation of a system with ρl/ρg=10,000.

On the link between Earth tides and volcanic degassing

Abstract. Long-term measurements of volcanic gas emissions conducted during the last decade suggest that under certain conditions the magnitude or chemical composition of volcanic emissions exhibits periodic variations with a period of about 2 weeks. A possible cause of such a periodicity can be attributed to the Earth tidal potential. The phenomenology of such a link has been debated for long, but no quantitative model has yet been proposed. The aim of this paper is to elucidate whether a causal link between tidal forcing and variations in volcanic degassing can be traced analytically. We model the response of a simplified magmatic system to the local tidal gravity variations and derive a periodical vertical magma displacement in the conduit with an amplitude of 0.1–1 m, depending on the geometry and physical state of the magmatic system. We find that while the tide-induced vertical magma displacement presumably has no significant direct effect on the volatile solubility, the differential magma flow across the radial conduit profile may result in a significant increase in the bubble coalescence rate at a depth of several kilometres by up to several multiples of 10 %. Because bubble coalescence facilitates separation of gas from magma and thus enhances volatile degassing, we argue that the derived tidal variation may propagate to a manifestation of varying volcanic degassing behaviour. The presented model provides a first basic framework which establishes an analytical understanding of the link between the Earth tides and volcanic degassing.

Non-Newtonian Behavior Effect on Gas-liquid Mass Transfer using an Anchor Impeller for CSTR Bioreactors: A CFD Approach

Objectives: kL a mass transfer coefficient was predicted using CFD (computational fluid dynamics) for analyzing non-newtonian effects on gas liquid mass transfer in a 10 L bioreactor stirred with an Anchor Impeller. Methods/Statistical Analysis: The set up bioreactor configurations were defined by typical culturing conditions used for fungi organism. Bubble breakage frequency and coalescence rate were simulated using Luo - Colaloglou and Tavlarides models and PBM approaches, respectively. Simulated results from different shear rates due to non-newtonian behaviour are compared by analyzing its influences in bubble size and power input. Findings: A clear relationship between high levels of shear rates and small bubble sizes is found in this work. The later is also associated with the high values of kL a simulated (270 h-1) and compared to levels found at low shear rates (62 h-1). Application/Improvements: Impressed by these findings new design optimizations for non-newtonian bioprocessing applications would be improved using CFD. Keywords: Anchor, Bubble Coalescence, Non-Newtonian, Sauter Diameter, CFD

CFD model simulation of bubble surface area flux in flotation column reactor in presence of minerals

Bubble surface area flux (Sb) is one of the main design parameter in flotation column that typically employed to describe the gas dispersion properties, and it has a strong correlation with the flotation rate constant. There is a limited information available in the literature regarding the effect of particle type, density, wettability and concentration on Sb. In this paper, computational fluid dynamics (CFD) simulations are performed to study the gas–liquid–solid three-phase flow dynamics in flotation column by employing the Eulerian–Eulerian formulation with k-ε turbulence model. The model is developed by writing Fortran subroutine and incorporating then into the commercial CFD code AVL FIRE, v.2014. This paper studies the effects of superficial gas velocities and particle type, density, wettability and concentration on Sb and bubble concentration in the flotation column. The model has been validated against published experimental data. It was found that the CFD model was able to predict, where the response variable as indicated by R-Square value of 0.98. These results suggest that the developed CFD model is reasonable to describe the flotation column reactor. From the CFD results, it is also found that Sb decreased with increasing solid concentration and hydrophobicity, but increased with increasing superficial gas velocity. For example, approximately 28% reduction in the surface area flux is observed when coal concentration is increased from 0 to 10%, by volume. While for the same solid concentration and gas flow rate, the bubble surface area flux is approximately increased by 7% in the presences of sphalerite. A possible explanation for this might be that increasing solid concentration and hydrophobicity promotes the bubble coalescence rate leading to the increase in bubble size. Also, it was found that the bubble concentration would decrease with addition of hydrophobic particle (i.e., coal). For instance, under the same operating conditions, approximately 23% reduction in the bubble concentration is predicted when the system was working with hydrophobic particles. The results presented are useful for understanding flow dynamics of three-phase system and provide a basis for further development of CFD model for flotation column.

Laboratory study of the Non-Newtonian behavior of supercritical CO2 foam flow in a straight tube

Rheological properties of supercritical CO2 (ScCO2) foam fluids are essential to understand foam displacement mechanisms in the research & development of efficient CO2-EOR and CO2 geological storage technologies. In this paper, a systematic laboratory study concerning non-Newtonian ScCO2 foam flow characteristics in a straight tube was carried out under different gas-liquid ratios. Through modifying experimental pressure and temperature conditions, comparative studies on liquid phase CO2 and gaseous CO2 foam rheology were also carried out. The experimental results show obvious shear-thickening behavior for the scCO2 foam (45 °C and 8.0 MPa) with flow behavior index increasing from 1.03 to 1.76 with increasing foam quality. And the apparent viscosity of scCO2 foam could reach up to 140 times of the single-phase water. The liquid phase CO2 foam (25 °C and 8.0 MPa) also exhibits clearly the shear-thickening characteristics with flow behavior index ranging from 1.56 to 1.88. The gaseous CO2 foam, on the other hand, behaves in a shear-thinning or pseudo-plastic manner with flow behavior index of 0.70 < n < 0.76. Due to the decreased interfacial tension and less bubble coalescence rate, much finer foam textures are observed for ScCO2 and liquid CO2 bulk foam, which mainly contributes to the shear thickening behavior for ScCO2 and liquid CO2 foam fluid.

Prediction of bubble size and axial position distribution through hybridization of population balance modelling and dynamic gas disengagement

Bubble Column Reactors (BCRs) are suitable for multiphase fluid systems due to their low energy consumption, ease of fabrication and relatively high degree of mixing. When predicting the properties of the bubble population, it was shown in our previous study that the global Bubble Number Density Distribution (BNDD) could be determined through the coupling of Electrical Resistance Tomography (ERT) to the Dynamic Gas Disengagement (DGD) process. However, due to the low spatial resolution associated with the ERT, the local BNDD, bubble coalescence and breakage rates of the fluid dispersion, and the BCR hydrodynamic parameters could not be predicted.In the present paper, these limitations are addressed by hybridizing the steady-state Population Balance Model (PBM) with the DGD results. In the first stage of the hybridization, the PBM having the source terms of bubble coalescence and breakup rates was solved to simulate the bubble evolution process in a BCR of height 1.56m and diameter 0.29m designed with a porous tubular gas distributor. The PBM solution was utilized as an initial condition for a modelling of the unsteady state DGD process as an advection transport of mean bubble sizes under the influence of bubble expansion rates. The unsteady state PBM that models the DGD process was solved by the method of characteristics to obtain an analytical solution of the local transient disengaging gas volume. Accordingly, the inlet mean and standard deviation of bubble sizes parameters, the coalescence and breakage rates, and the local flux gradient parameter during DGD were adjusted to ensure the modelled local static and dynamic gas holdups during DGD process agree with the corresponding experimental measurements.Validation was achieved through comparison of the macro-properties that can be determined from the resolved BNDD. Such properties include the axial distributions of Sauter Mean Bubble Diameter (SMBD), gas holdups, and the volumetric gas-liquid mass transfer rates of the steady-state bubble population swarming. The comparison of predicted hydrodynamic parameters with two literature results yielded R-squares of 0.88 and 0.82, Chi-squares of 0.86 and 1.08 and f-statistics of 35.96 and 27.24 indicating good agreement. It is therefore proposed that hybridization of the PBM and DGD process yielded a high spatial prediction of the Size and Axial Distribution (SAD) of bubble dispersion and the bubble coalescence and breakage rates even though ERT offers a relatively low spatial resolution.

Effects of particle size and concentration on bubble coalescence and froth formation in a slurry bubble column

A new approach for simulating the formation of a froth layer in a slurry bubble column is proposed. Froth is considered a separate phase, comprised of a mixture of gas, liquid, and solid. The simulation was carried out using commercial flow simulation software (FIRE v2014) for particle sizes of 60–150μm at solid concentrations of 0–40vol%, and superficial gas velocities of 0.02–0.034m/s in a slurry bubble column with a hydraulic diameter of 0.2m and height of 1.2m. Modelling calculations were conducted using a Eulerian–Eulerian multiphase approach with k–ε turbulence. The population balance equations for bubble breakup, bubble coalescence rate, and the interfacial exchange of mass and momentum were included in the computational fluid dynamics code by writing subroutines in Fortran to track the number density of different bubble sizes. Flow structure, radial gas holdup, and Sauter mean bubble diameter distributions at different column heights were predicted in the pulp zone, while froth volume fraction and density were predicted in the froth zone. The model was validated using available experimental data, and the predicted and experimental results showed reasonable agreement. To demonstrate the effect of increasing solid concentration on the coalescence rate, a solid-effect multiplier in the coalescence efficiency equation was used. The solid-effect multiplier decreased with increasing slurry concentration, causing an increase in bubble coalescence efficiency. A slight decrease in the coalescence efficiency was also observed owing to increasing particle size, which led to a decrease in Sauter mean bubble diameter. The froth volume fraction increased with solid concentration. These results provide an improved understanding of the dynamics of slurry bubble reactors in the presence of hydrophilic particles.

Hydrodynamic Study of a Gas–liquid–solid Bubble Column Employing CFD–BPBM Method

AbstractThe computational fluid dynamics – bubble population balance model (CFD–BPBM) was employed to predict the hydrodynamic characteristics of a gas–liquid–solid bubble column. A 3D time dependent numerical study was performed and the bubble size distributions at the conditions of different superficial gas velocity (0.089 m/s–0.22 m/s), solid volume fraction (0.03–0.30) and particle density (2500 kg/m3–4800 kg/m3) in the three–phase system were investigated, and the simulation results were compared with the experimental results. The bubble diameters ranging from 1 mm to 64 mm were divided into ten classes. The predicted pressure changing with the bed height had a good agreemeet with the experimental result. The bubble number density predicted decreased when the bubble size increased at each superficial gas velocity, and the bubble coalescence rate became greater than the breakup rate when Ugshifted from 0.089 m/s to 0.16 m/s. The bubble interaction was similar at 0.16 m/s and 0.22 m/s both at particle size dp= 75 μm and 150 μm. The bubble size corresponding to the maximum of the bubble volume fraction increased as Ugincreased. The particles can make the bubble break up and coalesce simultaneously when the solid volume fraction was larger than 0.20, and therefore the particles had a contribution to both of the bubble coalescence and breakup in the bubble coalescence regime (Ug= 0.16 m/s). The effect of the particle density was similar with that of the solid volume fraction. Increasing the particle density can enhance the breakup rate of the large bubbles.