Bubble Size Reduction Research Articles

Effect of NaCl on the flotation response of low-rank coal

ABSTRACTIn this study, the effect of adding NaCl on the flotation enhancement mechanism of low-rank coal was investigated. A gradual reduction in the negativity of the zeta potential of low-rank coal with NaCl addition was conducive to reducing the bubble–particle attachment time and enhancing low-rank coal recovery. In addition, the reduced bubble size and improved foam stability with NaCl addition were moderately beneficial to coal recovery. The flotation rate constant, k, and the maximum combustible recovery, ε∞, increased remarkably as the NaCl concentration increased, indicating that the flotation kinetic of low-rank coal was enhanced by the addition of NaCl.

Study of foam generation and propagation in fully characterized physical-model fracture

Foam greatly reduces gas mobility for gas enhanced-oil-recovery (EOR) projects. It substantially increases both the effective viscosity of gas and gas trapping. Numerous studies have been conducted to understand foam rheology in rock matrix both theoretically and experimentally. The knowledge of foam flow in fractured porous media is incomplete, however. This study aims to contribute to the understating of foam generation and propagation in a fully characterized physical-model fracture. We investigate foam-generation mechanisms and the propagation of pre-generated foam. Gas mobility was greatly reduced as a result of in-situ foam generation. Foam-generation mechanisms similar to those seen in 3D porous media were observed on this model fracture. Foam was generated predominantly by capillary snap-off and lamella division. Lamella division was observed at high gas fractional flow at two different superficial velocities. Fracture wall roughness played an important role in foam generation. In the case of pre-generated foam, two very distinct bubble sizes were injected: fine-textured bubbles much smaller than the roughness scale and coarse-textured foam with bubbles much larger than the roughness scale. The first case did not show any significant change in bubble size as foam propagated through the model fracture, while in the second case, the fracture played a role in reducing bubble size. Inter-bubble diffusion did not regulate bubble size in our apparatus because residence time in the fracture is relatively short. We cannot confirm that foam reached local equilibrium in our experiments, but we believe that local equilibrium lies between the cases of in-situ- and pre-generated foams.

From boom to bust: an operational perspective of demand bubbles

AbstractWhen demand exceeds supply, retailers hedge against shortages by placing multiple orders with multiple suppliers, exceeding customer demand and leading to excess capacity, excess inventory, low capacity utilization and financial losses. This paper provides a comprehensive causal loop diagram and a formal mathematical model of a subset of supplier–retailer relationships. We obtain closed‐form solutions when supplier capacity is fixed, and analyze simulation dynamics when it is variable. Sensitivity analyses provide a deeper understanding of parameters that contribute to demand bubbles and insight on improvement policies. For instance, the ability of the supplier to build capacity quickly can reduce bubble size. The time it takes retailers to perceive and react to supply availability is an important lever in controlling demand bubbles. When retailers learn of shortages with a delay, their reaction is also delayed, which stabilizes the supply chain.© 2018 System Dynamics Society

Effect of hydrostatic pressure on protective bubble characteristic and weld quality in underwater flux-cored wire wet welding

The metal transfer and protective bubble images, real-time electric signal data and weld appearance were obtained in different simulated water depth wet flux-cored arc welding. The electric signal results showed that the high hydrostatic environment deteriorated the stability of the wet welding process. The poor weld appearances with porosity were displayed when it was welded in deep water. The weld dilution rate descended from 51% to 40% with increasing water depth from 0.5 m to 90 m because of arc shrinking and heat input reduction. The reduction of protective bubble size and duration caused by high water pressure was unable to maintain a stable burning arc. With increasing water depth, the increase of pores in welds strongly deteriorated the mechanical property of joints. The process of gas generation and its escape from the droplet during the droplet transfer were observed. The rise of pressure compressed gas pores and prevented the gas from escaping, which resulted in the increase of pores.

Online bubble size analysis in micro flotation

Measuring the size of bubbles is of fundamental interest in process engineering, especially in flotation. The size of a bubble influences its rising velocity, coalescence tendency and interaction with particles and surrounding liquid. This paper presents an online Bubble Size Analyzer (BSA) with automated image analysis software. The importance of isokinetic sampling with short residence time in the sampling system is shown. Key parameters of the image analysis software are pointed out. The BSA was used to characterize a laboratory dissolved air flotation setup. In accordance with literature, bubble size reduction with increasing saturation pressure and recycle flow was measured. It was discovered that bubble generation in the flotation setup happened by classical dissolved air flotation only when it was operated with pure water. With sodium chloride saline and complex protein solution a mixed dispersed/dissolved air flotation was observed. Maximum bubble sizes were found in water, reduced sizes in saline and minimum size in protein solution. Due to its flexible online installation, the BSA is recommended as a suitable tool for bubble size measurements in laboratory and real processes, where inline analysis is not possible.

The effect of gas addition on bubble dynamics in a fluidized bed with flat vertical membranes

This study investigates the hydrodynamics of a fluidized bed with flat vertical membranes for selective species dosing. A custom designed pseudo-2D column, equipped with a multi-chamber area on the back for gas permeation, was used to emulate a membrane assisted fluidized bed (MA-FB). The effect of global and local gas additions on bubble properties, solids hold up, solids circulation patterns and regime transition was investigated using Digital Image Analysis, Particle Image Velocimetry and pressure fluctuation measurements. Gas addition throughout the entire permeable area substantially reduced bubble size and velocity, while increasing the number of bubbles. In addition, a significant reduction in gas channelling through the centre of the column was observed. Concentrating gas addition towards the sides of the column further enhances bubble distribution over the lateral direction and totally removes gas channelling. Gas addition also caused an earlier transition to turbulent fluidization. The estimated bubble-to-emulsion mass transfer rate could be doubled when 30% gas addition was applied in the turbulent regime. These benefits maximise the potential of process intensification for fluidized bed reactor concepts using selective dosing through flat vertical membranes.

Bubbles in noodle dough: Characterization by X-ray microtomography

Bubbles, found in a huge variety of food products, are known to afford desirable quality attributes, especially those related to texture, mouthfeel and taste. However, the presence of bubbles and their effects on wheat flour noodles is an aspect that has been, until now, largely overlooked, despite the positive and negative connotations of bubbly inclusions on Asian noodle quality. X-rays from a synchrotron source (Biomedical Imaging and Therapy facility at the Canadian Light Source) were used to rapidly and non-destructively acquire tomographic images of noodle dough. Appropriate image analysis protocols were used to determine the bubble size distribution, the orientation of bubbles, and their position within the dough sheet. The effect of processing (one or multiple lamination steps) on bubble properties in the dough that was subsequently sheeted (gradual elongation and reduction in thickness) was investigated. Bubble size distributions, well captured by lognormal distribution function, showed that the lamination process induced bubble entrapment and reduction in bubble size. Bubbles were found to be flat, elongated and oriented in the sheeting direction, this effect being less for doughs laminated ten times (90° rotations between lamination steps). Interestingly, a gradient in concentration of bubbles within the dough sheet was found from the noodle core to the sheet edges. Aging effects were also apparent. This first non-destructive study of bubbles in wheat-flour noodle dough provides a more complete knowledge of the dough sheet's internal structure, and how it originates via processing, and this has repercussions on the overall quality of Asian noodles.

Insight into the behavior of colloidal gas aphron (CGA) fluids at elevated pressures: An experimental study

Recently, colloidal gas aphron (CGA) fluids technology has been employed to drill depleted oil and gas reservoirs. Almost all reported experience on CGA fluids were conducted at ambient conditions, and little attention has been paid on the behavior of CGAs at high pressures which is more close to real conditions. In this study, high pressure experiments were conducted by using High Pressure Microscope cell to visualize/monitor the behavior of CGAs at elevated pressures. Single bubble behavior and bubble size distribution (BSD) of CGAs were investigated under different scenarios of pressure change. Results of experiments revealed that BSD of CGAs is controlled by the path of pressure changes, and sudden pressure change affects BSD less than stepwise changes in pressure. The pressure-volume product of bubbles, as a new criterion for analysis of single bubble behavior was introduced. The behavior of pressure-volume product for a single bubble shows significant loss of air via diffusion through tri-layer film into the bulk phase during compression as well as a hysteretic behavior during compression and decompression processes. Monitoring single bubble behavior at elevated pressures revealed that CGAs meet three distinct regions with time: first, they exhibited a sharp reduction in bubble size (region I), then became stable with minimum change in bubble diameter (region II), and finally, the size of bubbles decreased suddenly and after a short time they were collapsed (region III). Observations which showed aphrons were survived at pressures as high as 2000psig may support the idea for field applications of CGAs as drilling fluids. Results of this work may provide thorough insight into behavior of CGA fluids at elevated pressures.

Effects of DC electric fields on meso-scale structures in electrostatic gas-solid fluidized beds

Electric fields have great potential to control meso-scale structures, typically bubbles and agglomerates in gas-solid fluidized beds. However, previous research only considered the effects of polarization forces and ignored the electrostatic effects. This work, by means of cold model experiments, for the first time investigates the effects of DC electric fields on meso-scale structures in a 3-D fluidized bed with electrostatic effects. Results show that particles will also experience the Coulomb force under the effects of electrostatics besides of polarization forces, and the effects of these two electric field forces are different. The Coulomb forces make particles migrate at lower field strengths, then increases the bubble size and breaks up agglomerates. While at higher field strengths, the polarization forces result in formation of agglomerates and reduction of bubble size. Therefore, the effects mechanism of electric fields is the competition effects between the Coulomb force and the polarization forces. The critical transition field strength, at which the predominant position of Coulomb and polarization forces on bubbles and agglomerates changes, is significant in the control of meso-scale structures by DC electric fields.

Experimental studies of condensing vapor bubbles in subcooled pool water using visual and acoustic analysis methods

In this study, smooth bubble condensation that occurs in subcooled pool water was examined to understand condensation phenomena at the beginning of boiling. The behaviors of condensable and non-condensable bubbles were investigated with respect to various temperatures and bubble sizes. The experimental data were analyzed using visual and acoustic methods including Phase Interface Binarization (PIB), Tridimensional Reconstructing Assumption (TRA), and acoustic data conversion. For visual analyses, (1) the PIB method determined bubble departure frequency, condensation time, and rising distance, and (2) the TRA method determined departure bubble size and volume reduction rate. The bubble detached with smaller volume and occurred more frequently with a smaller nozzle and a higher subcooling degree. Because the condensation always occurred during the growth, necking, and detachment of a bubble, the bubble was detached before it grew sufficiently. Lower condensation time and rising distance were associated with higher subcooling degrees and smaller injected bubbles. With respect to the acoustic analysis, sound signals were measured using a hydrophone, and the obtained analog data were converted to sound pressure units. The results revealed that higher volume reduction rate resulted in stronger sound pressure. As a result, the condensation phenomena in the smooth bubble regime were visually observed, and the possibilities of acoustic monitoring for earlier boiling were investigated.

The effect of mesh-type bubble breaker on two-phase vertical co-flow

The influence of a mesh-type bubble breaker in a two-phase vertical co-flow is experimentally investigated using high-speed imaging. A range of liquid and gas flow rates were considered that correspond to the gas-to-liquid flow rates ratios (GLRs) from 0.05 to 39.6 covering bubbly, slug and churn flow regimes. The mesh-type bubble breaker was found to be effective in reducing the size of nozzle-generated bubbles in two-phase co-current vertical flow. The bubble size reduction by 50%-70% is observed in the presence of the mesh-type bubble breaker compared to that without a bubble breaker. The results also show that at a given GLR, the mesh-type bubble breaker delayed the flow regime transition compared to that without a bubble breaker. A Froude number correlation was proposed to predict the mean bubble size downstream of the bubble breaker for the bubbly flow regime, which accurately predicted the bubble size over a range of bubble breaker parameters. It is observed that the geometry of the bubble breaker influence the two-phase flow regime transition. At low liquid flow rates, small pore size of the breaker was found to be effective. This effect diminishes at higher liquid flow rate due to the elongation of the bubbles by the liquid inertia. The results also show that a longer bubble breaker located closer to the bubble generating nozzle is the most effective in producing bubbly flow over a longer range of gas-liquid flowrates ratios.

Stability of various particle-stabilised aluminium alloys foams made by gas injection

Aluminium alloy foams are created by injecting air into liquid alloys containing non-metallic particles. In addition to an alloy containing the usual SiC particles, other types of metal/particle composites are studied, which are created by in situ reactions in the melts: two fluoride salts react and form TiB2 particles, and Ca addition or addition of CuO and SiO2 gives rise to the formation of various oxides and spinel particles. Injecting air into the molten composites through two different steel cannulas leads to the formation of first bubbles and then foam. The entire process is monitored in situ by X-ray radioscopy. The goal is not only to understand how and what kind of particles stabilise gas injected foams better, but also to reduce the fraction of added particles, which could improve mechanical properties, solve recycling issues and reduce production costs. All the observed composites are shown to have the potential to be processed to metallic foam. Melts containing TiB2 particles are found to perform as well as those containing SiC even at lower volume fractions. Oxidation of alloy melts promoted by Ca addition gives rise to melts that exhibit good foamability. Melts oxidised by CuO and SiO2 addition show partial stability. Mg is found to be a required alloying element to create stable foams. Smaller bubbles can be produced using smaller injector needle openings. By reducing bubble size and using new variants of in situ generated particles, more stable foams can be achieved with a lower number density of stabilising particles.

CFD modeling the hydrodynamics of binary particle mixture in pseudo-2D bubbling fluidized bed: Effect of model parameters

The hydrodynamics of binary coal-sand mixture in a pseudo-2D rectangular bubbling fluidized bed (0.385m×0.005m×0.128m) was simulated using the multi-fluid Eulerian model incorporating the kinetic theory of granular flow. Parametric studies of the boundary wall condition, particle-particle restitution coefficient, friction packing limit, as well as transport equation for granular temperature were performed to investigate their influences on the predicted mixing/segregation behavior. The CFD simulation results demonstrated that the predicted mixing behavior was closely related to the expression for granular temperature transport equation and specularity coefficient. When the full transport equation for granular temperature was adopted, the predicted mixing degree decreased with the increase of specularity coefficient. And the best agreement between simulation results and experimental data was achieved when specularity coefficient was equal to 1.0. Nevertheless, when the algebraic transport equation for granular temperature was adopted, the system was always predicted in well-mixing rather than segregation state. Under the full transport equation for granular temperature and the no slip boundary wall condition, the predicted mixing degree decreased with the increase of the particle-particle restitution coefficient and frictional packing limit. The supplementary simulations indicated that for the considered gas-solid system there exist a critical bed thickness larger than which the system was in well-mixing state and the simulation results were independent from the investigated parameters. The hydrodynamic analysis indicated that the reduction of bubble size and the solid axial movement could be the mechanism responsible for the occurrence of axial segregation.

Bubble Formation at a Submerged Orifice in High-Speed Horizontal Oscillation

Reducing the cell size of aluminum foams is always a hot and difficult topic in the fabrication of aluminum foams by gas injection route. There lacks theoretical guidance for the bubble size reduction when foaming by the dynamic gas injection method. For the convenience of observation, the aqueous bubbles from small-sized orifice in the high-speed horizontal oscillation were investigated in this paper. A bubble formation and detachment model in the high-speed horizontal oscillation system was proposed. The high-speed system with horizontal simple harmonic oscillation could reduce the average bubble size of aqueous foam effectively. The regularity of bubble formation and the influence of experimental parameters on average bubble size can be predicted by the theoretical model, and the experimental results agree well with the theoretical calculation. The results have shown that bubbles generally detach from the orifice at deceleration periods of the simple harmonic oscillation, and there exist several fixed sizes of bubbles with the fixed experimental parameters due to the effects of periodic forces. The average bubble size decreases with the increase of oscillation frequency and amplitude, and it roughly increases with the increase of gas flow rate. Using the high-speed horizontal oscillation method to prepare aluminum foams, the cell size can be reduced to about 1 mm. Moreover, the cell sizes of aluminum foam can be well predicted by this theoretical model.

Numerical investigation of the role of free-stream turbulence in boundary-layer separation

The aerodynamic performance of lifting surfaces operating at low Reynolds number conditions is impaired by laminar separation. In most cases, transition to turbulence occurs in the separated shear layer as a result of a series of strong hydrodynamic instability mechanisms. Although the understanding of these mechanisms has been significantly advanced over the past decades, key questions remain unanswered about the influence of external factors such as free-stream turbulence (FST) and others on transition and separation. The present study is driven by the need for more accurate predictions of separation and transition phenomena in ‘real world’ applications, where elevated levels of FST can play a significant role (e.g. turbomachinery). Numerical investigations have become an integral part in the effort to enhance our understanding of the intricate interactions between separation and transition. Due to the development of advanced numerical methods and the increase in the performance of supercomputers with parallel architecture, it has become feasible for low Reynolds number application ($O(10^{5})$) to carry out direct numerical simulations (DNS) such that all relevant spatial and temporal scales are resolved without the use of turbulence modelling. Because the employed high-order accurate DNS are characterized by very low levels of background noise, they lend themselves to transition research where the amplification of small disturbances, sometimes even growing from numerical round-off, can be examined in great detail. When comparing results from DNS and experiment, however, it is beneficial, if not necessary, to increase the background disturbance levels in the DNS to levels that are typical for the experiment. For the current work, a numerical model that emulates a realistic free-stream turbulent environment was adapted and implemented into an existing Navier–Stokes code based on a vorticity–velocity formulation. The role FST plays in the transition process was then investigated for a laminar separation bubble forming on a flat plate. FST was shown to cause the formation of the well-known Klebanoff mode that is represented by streamwise-elongated streaks inside the boundary layer. Increasing the FST levels led to accelerated transition, a reduction in bubble size and better agreement with the experiments. Moreover, the stage of linear disturbance growth due to the inviscid shear-layer instability was found to not be ‘bypassed’.

Patients With Intravitreal Gas Bubbles at Risk of High Intraocular Pressure Without Exceeding Elevation of Surgery: Theoretical Analysis

The purpose of this study was to show the mechanism responsible for high peak IOP in patients with intravitreal gas bubbles resulting from a descent to low elevation and a return ascent, without exceeding the surgical elevation. A computational model reconstructed four clinical cases, using published elevations, ascent rates, and initial bubble sizes. In each case, patients first underwent surgery (790 m), then went home (790 m, 790 m, 325 m, 240 m). When returning for follow-up visits, patients descended to a low elevation (20 m, 0 m, 25 m, -310 m), then ascended to surgical elevation (790 m). The computational model output bubble size, aqueous humor volume, and IOP during the patients' travels. A parametric study was conducted to investigate the role of each modeling parameter. All four simulated cases showed increased peak IOP (34-50 mm Hg). Intraocular pressure returned to a normal value (15 mm Hg) after prolonged exposure to the surgical elevation. Over the course of the entire path, the gas bubble volume changed approximately 5%, decreasing in size during descent and then increasing during ascent. In our simulations the change of bubble size outpaced the change of aqueous humor volume resulting in a 2-fold risk to patients. First, the bubble size reduction at the low elevation may increase the risk of ocular hypotony and postsurgical retinal detachment. Second, the combined increasing bubble size and accumulated aqueous humor puts patients at risk of high peak IOP after ascent even without exceeding the surgical elevation. The risks are primarily dependent on rates of elevation change and duration spent at the different elevations.

Bubble swarm characteristics in a bubble column under high gas holdup conditions

Local bubble characteristics were measured using a novel monofibre optical probe manufactured for elevated pressures and high gas holdups. The objective of this study is to test the performance of the optical probe, as well as to investigate the impact of operating conditions on bubble characteristics in a bubble column when operating under high gas holdup conditions. Experiments were conducted in a 101.6mm diameter column operating at pressures up to 6.5MPa. A 0.5wt% aqueous ethanol solution was used in this study to simulate high gas holdups observed in many industrial reactors containing liquid mixtures with surface-active compounds. The probe struggled to detect all the bubbles due to significant bubble size reduction and non-rectilinear bubble rise. However, successfully detected bubbles were deemed representative of the mean bubble rise velocity as indicated by the dynamic gas disengagement technique. Global gas holdup profiles in conjunction with the drift flux analysis showed that pressure does not have a significant impact at elevated liquid flow rates as bubbles tend to follow the convective current of the liquid. Relatively narrow chord length distributions were observed, where 90% of the bubbles were 1.0mm or less. The energy dissipated through the gas–liquid distributor plate was shown to have a significant impact on the initial bubble size generated and high gas holdups were also achieved at atmospheric pressure by varying the open-surface area of the distributor.

De-mixing characteristics of fine coal in an air dense medium fluidized bed

An air dense medium (magnetite) fluidized bed (ADMFB) provides an alternative method for dry coal beneficiation. The de-mixing (segregation) characteristics can significantly affect the quality of the final products. In this study, the segregation characteristics of fine coal (1–3mm) in an ADMFB were compared to those in an ordinary fluidized bed (FB). Specifically, the pressure drop, density-segregation behaviors, segregation degree, motions of the coal/magnetite mixture, and bubble size were determined with various gas velocities. Density-segregation occurred at a low gas velocity in the ADMFB. However, the particles started to mix when the gas velocity was increased. There was an optimum gas velocity to produce the maximum amount of segregation. The comparative analysis (ash segregation degree and bubble size characteristics) between ADMFB and FB suggested that the bubble-drive flotsam-jetsam mechanism can explain the particle segregation. Furthermore, adding dense medium to a fluidized bed led to a higher amount of segregation because of the reduction in bubble size.

Post-Formation Shrinkage and Stabilization of Microfluidic Bubbles in Lipid Solution.

Medical ultrasound imaging often employs ultrasound contrast agents (UCAs), injectable microbubbles stabilized by shells or membranes. In tissue, the compressible gas cores can strongly scatter acoustic signals, resonate, and emit harmonics. However, bubbles generated by conventional methods have nonuniform sizes, reducing the fraction that resonates with a given transducer. Microfluidic flow-focusing is an alternative production method which generates highly monodisperse bubbles with uniform constituents, enabling more-efficient contrast enhancement than current UCAs. Production size is tunable by adjusting gas pressure and solution flow rate, but solution effects on downstream stable size and lifetime have not been closely examined. This study therefore investigated several solution parameters, including the DSPC/DSPE-PEG2000 lipid ratio, concentration, viscosity, and preparation temperature to determine their effects on stabilization. It was found that bubble lifetime roughly correlated with stable size, which in turn was strongly influenced by primary-lipid-to-emulsifier ratio, analogous to its effects on conventional bubble yield and Langmuir-trough compressibility in existing studies. Raising DSPE-PEG2000 fraction in solution reduced bubble surface area in proportion to its reduction of lipid packing density at low compression in literature. In addition, the surface area was found to increase proportionately with lipid concentration above 2.1 mM. However, viscosities above or below 2.3-3.3 mPa·s seemed to reduce bubble size. Finally, lipid preparation at room temperature led to smaller bubbles compared to preparation near or above the primary lipid's phase transition point. Understanding these effects will further improve on postformation control over microfluidic bubble production, and facilitate size-tuning for optimal contrast enhancement.

Hydrodynamics of gas–solids flow in a bubbling fluidized bed with immersed vertical U-tube banks

We apply a two-fluid model (TFM) from the open-source code Multiphase Flow with Interphase eXchanges (MFIX) to investigate hydrodynamics in a gas–solids fluidized bed with immersed vertical tubes. The cut-cell method implemented in MFIX is used to fully resolve the flow around vertical U-tube banks. Simulations are performed in a bed diameter of 0.145m with square and triangular tube arrangements, for inlet gas velocities of U0/Umf=2.3, 4.5 and 6.8. Simulation results are compared with experimental results from the literature and show very good agreement for the bubble size. The efficiency of vertical tubes in reducing bubble size depends upon inlet gas velocity and tube arrangement. Reduction in bubble size is due to the vertical tubes preventing bubble coalescence and promoting bubble splitting. In-bed vertical tubes result in uniform distribution of bubbles within the bed with increase in bubble frequency. The bubble frequency is higher within the bed for square tube arrangements. For a bed with vertical tubes, the bubble shape is generally elongated, which results in high bubble rise velocity. Axial solid velocity and solids circulation patterns are significantly affected by the vertical tubes, where triangular tube arrangements rarely show any solids circulating zone.