Bubble Diameter Research Articles

Numerical and experimental study of wire mesh in the swirl effervescent atomization

Effervescent atomization is widely used in industrial, agricultural, and national defense fields because of its low energy consumption. Quality improvement to spray has been pursued for decades as an important goal without increasing energy consumption. To investigate the relationship between the internal flow and spray characteristic, a swirl effervescent atomizer coupled with a wire mesh was designed in this paper. The effect of wire mesh on spray characteristics and internal flow with different structural parameters was investigated. The experimental results showed that the spray quality (small droplet diameter and stable atomization) was improved by installing a wire mesh in the mixing chamber. The droplet diameter decreased first and then increased with the increase in the effective porosity of the wire mesh, and Sauter mean diameter decreased from 23.1 to 20 μm. Moreover, this study adopts the simulation to explore the influence of bubble size on the spray droplet in the mixing chamber. The simulation results showed that as the effective porosity of the wire mesh increases, the proportion of small-scale bubbles (the bubble diameter Db ≤ 1 mm) increased from 11.3% to 83.37%, resulting in reduced spray droplet diameter. Notably, the number of small-scale bubbles increases first and then decreases, and the mean pressure drops decline.

In-situ TEM bubble to cavity evolution due to annealing post helium and dual ion irradiation in Cu-10Ta and Cu-3Ta

Materials have been essential to every major invention in the history of mankind, e.g. satellites, nuclear reactors, or space shuttles would not exist without advancements in materials withstanding temperature and radiation environments. In this work, we investigate room-temperature implantations of helium and concurrent He and Au via in-situ transmission electron microscopy (TEM) followed by isochronal annealing up to 450 °C to understand the microstructural stability and bubble to cavity evolution in advanced nanocrystalline (NC) Cu-3 at.%Ta and Cu-10 at.%Ta alloys. The room temperature irradiation to high helium concentration and displacements per atom (dpa) levels did not lead to significant coarsening of grains in both Cu10Ta and Cu3Ta. Overall grain sizes remained <150 nm indicating extraordinary microstructural stability in both alloys. Post-irradiation, incubation for a few days resulted in the observation of ~1 nm diameter bubbles homogenously distributed in both the alloys and both irradiation conditions. Further evolution of these bubbles to cavities with annealing to 450 °C indicated Brownian motion migration and coalescence to be the dominant coarsening mechanism involved. A comparison of bubble to cavities evolution in Cu3Ta and Cu10Ta under similar conditions showed smaller bubble size and density in Cu3Ta indicating an overall enhanced resistance of Cu3Ta to swelling under both helium and dual-beam irradiation at high temperatures. This is primarily attributed to the high fraction of essential coherent/incoherent precipitates that act as effective defect sinks in Cu3Ta.

Experimental and numerical analysis on electrolytic bubble group dynamics in the aluminum electrolysis process using the slotted anode

The CuSO4 aqueous solution electrolysis experiment is conducted to investigate the electrical bubble dynamics using the slotted anode to reveal the gas removal mechanism. A three-dimensional transient mathematical model coupled discrete phase model (DPM) − discrete-continuum transition model (DCTM) − volume of fluid (VOF) method is developed to track the dispersed bubble trajectory, bridge the dispersed bubble to continuous gas, and resolve the deformed bubble surface, respectively. The slotted anode decreases the bubble diameter compared with the traditional anode because the slot shortens the bubble motion distance to decrease the bubble residence time. The slot width smaller than the detached bubble diameter can limit the bubble deformation. Increasing current plays a dual role in bubble motion in the experiment: it accelerates the bubble coalescence process but decreases the bubble collision probability. The longitudinal slot is more conducive to reducing the maximum bubble coverage and increasing the bubble release frequency than the transverse slot.

Flow behaviors of multiscale bubbles with porous ceramic membrane distributors: Visualization and modeling

AbstractThe distribution of gas‐liquid two‐phase flow is one of the significant effects on heterogeneous catalytic reactions. Ceramic membrane gas distributors (CMGD) were applied in improving gas‐liquid distribution, and flow behavior of gas as dispersed phase in liquid phase was visualized via high‐speed photograph. The average diameters of multiscale bubbles were measured and modeled ranging from 10−5 to 10−2 m. The coalescence and trajectory of bubbles during rising process were observed, and two typical trajectories straight and spiral types were tracked. To inhibit coalescence of bubbles during rising process, internals manufactured by 3D printing were installed in the channel of ceramic membrane. The average bubble size of CMGD decreases 12% from 392 to 345 μm compared with that of the original CMGD. The CMGD with internals enhances the heterogeneous catalytic reaction performance via providing large quantity of stabile multiscale bubbles which could match the porous structure of catalyst.

Impact of electrode-normal magnetic field on oxygen bubbles generated by alkaline water electrolysis at low current densities

The effect of magnetohydrodynamic (MHD) convection on the behavior of oxygen bubbles generated by alkaline water electrolysis is investigated in this paper. In order to avoid mutual obstruction of bubbles and get clear images, a special electrolytic cell with a wire anode is designed to conduct the experiments. The current densities in our experiments are between 0.15 and 0.35 A/cm2, which are relatively low but common values in ordinary industrial electrolyzers. The results show that external magnetic field can reduce the cell voltage slightly and change the distribution of bubble detachment size. The induced MHD convection is found to be able to accelerate the diffusion of gas components in to the bulk electrolyte, resulting in a low comprehensive current efficiency, and further leading to longer growth cycle and larger separation diameter of oxygen bubbles. In addition, different from the “coalesce-and-bounce” detachment pattern, bubbles generated in magnetic field will undergo a sliding motion on the electrode surface during their growth process, which shows a lower coalescence frequency and a smoother detachment. Our work suggests that the driving effect of magnetic field on oxygen bubbles may be positive only at high current densities.

Dynamics of bubbles detached from non-circular orifices: Confinement effect of orifice boundary

In practice, gas usually escapes from non-circular orifices due to machining accuracy, plugging, and other reasons, especially gas leakage in the natural environment. Compared to circular orifices, the bubble behavior and dynamics of non-circular orifices have rarely been investigated. This paper presents three-dimensional imaging system results to obtain comprehensive information on bubble dynamics generated from five elliptical orifices with equal cross-sectional areas and different aspect ratios. Computational fluid dynamics studies using the coupled level set and volume of fluid methods were adopted to assist in the flow field analysis. Experimental data were analyzed statistically to study the effects of orifice wall structure on bubble flow pattern, size, trajectory, and other dynamic parameters. The results show that the equivalent bubble diameter increases with increasing orifice aspect ratio, as does the randomness of the bubble flow field, while the bubble flow pattern complexity increases. A force analysis as the bubble is generated is also conducted. The wall structure of the orifice affects the bubble migration effect to some extent. The work presented in this article provides valuable fundamental information for a range of heater and desalination applications in chemical engineering, wastewater treatment, and immersion leakage identification.

Flow boiling heat transfer enhancement in vertical minichannel heat sink with non-uniform microcavity arrays under electric field

Effects of electrohydrodynamics (EHD) on the bubble behavior and enhanced heat dissipation mechanism for four surface configurations and various heat fluxes in an upward flowing minichannel heat sink were studied experimentally. Three different minichannel configurations with microcavities array were fabricated using direct metal laser sintering. Flow boiling experiments of nanofluid (SiO2-R141b) under different nonuniform electric fields were performed. Homogeneity test of Variance Analysis was introduced to analyze the effect of design and operation parameters on heat transfer. Coupled effect of microcavity and electric field was further found to improve boiling heat transfer efficiency. Visualization shows that the diameter of small bubbles leaving microcavity increased under electric field. The bubbles wiggled between wire electrode and heated wall by action of electric field in bubbly flow district. Furthermore, the mechanism of perturbation of small bubbles flowing along the heating wall under electric field was discussed. It was revealed that the micro-concave cavity array distributed dense downstream and sparse upstream had a large impact on heat transfer coefficient (HTC), especially under electric field. The maximum synthetic enhancement factor of 2.11 was obtained for given flow conditions. Furthermore, a new predictive equation was proposed by a nonlinear regression equation to consider the effects of micro-concave cavity distribution and electric field factors on thermal transfer performance, and this correlation can predict the HTC affected by non-uniform micro-concave arrays well within the ± 10% error band.

Analysis of Interaction and Flow Pattern of Multiple Bubbles in Shear-Thinning Viscoelastic Fluids

A numerical study was conducted on the interaction of bubbles with different diameters and arrangements in shear-thinning viscoelastic fluids using OpenFOAM. The Volume of Fluid (VOF) method combined with the surface tension model was used to track the gas–liquid interface, and the rheological properties of the fluid were characterized with the Giesekus model. The numerical results are corresponded with the previous references, verifying the correctness of the simulation method. The influences of the initial bubble diameter, horizontal spacing, and arrangement on the motion state of three parallel bubbles were studied in detail. The flow pattern of the bubble rising was analyzed and compared with the motion state of parallel unequal double bubbles. As the diameter of the bubbles increases, the interaction among three equal size bubbles is changed from coalescence to detachment. Changing the diameter of one of the bubbles will significantly affect the movement of the larger diameter bubble, which is due to the enhancement in kinetic energy. The final state of some arrangement ways is consistent with the phenomenon of unequal double bubbles. The shear thinning effect, the velocity difference between bubbles, and the flow field around bubbles are considered the main reasons that decide the interaction between bubbles.

Novel CFD-DEM approach for analyzing spherical and non-spherical shape particles in spouted fluidized bed

The spouted fluidized bed process is a multiphase heat and mass transfer flow. The process requires effective solid-gas and solid-solid interaction. Various numerical models have been developed to understand and optimize these interactions. However, most of these models have used spherical particle definition, whereas the actual particles are non-spherical. Here we present coupled Computational Fluid Dynamics(CFD) and Discrete Element Method(DEM) based model to analyze the fluidized bed process for non-spherical particles (Faceted cylinder). The model results are validated with experimental results for spherical particles. The model is further used to understand mixing during the process as a function of non-spherical particle geometry. Aspect ratio and corner count were the geometrical input parameters for faceted cylinder-shaped particles. Transient plots for bubble diameter and bed height for spouted fluidized beds were created for all cases. The bubble diameter and bed height values were much lower for faceted cylinder particles than for spherical particles. For non-spherical particles, the increased number of corners was a crucial factor that brought the outcomes closer to those of spherical-shaped particles. The aspect ratio values increased, and the bubble diameter shrank as a result of more resistance to particle movement. The results would be of great use to correctly simulate non- spherical particles based fluidized bed process and optimize various process parameters.

Influence of system pressure on flow boiling in microchannels

The effect of pressure on the flow boiling characteristics of HFE-7200 between 1.0 to 2.0 bar was investigated in a parallel microchannel heat sink (Dh = 0.48 mm, 44 channels). The mass flux and subcooling degree were kept constant at 200 kg/m2 s and 10 K respectively, while heat flux ranged from 26.1 – 160.7 kW/m2. Increasing system pressure decreased the vapour density ratio, thus leading to reduced pressure drop, increased bubble generation frequency and two-phase heat transfer coefficients in the system, though this may not be apparent at low superheat degrees at higher pressures. Smaller bubble diameters were observed at higher pressures and resulted in a delay in flow regime transition to slug flow, which is prone to flow reversal. The experimental work showed promising means to manage flow instabilities and enhance heat transfer performance in two-phase microchannel systems for HFE-7200.

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.

Regulation role of calcium lignosulfonate on the entrainment behavior of serpentine during brucite recycling: A new perspective

The entrainment behavior of fine serpentine seriously deteriorates the recycling efficiency of brucite. In this paper, the effect of calcium lignosulfonate (CLS) on the flotation separation of brucite and serpentine was creatively investigated using a self-made Hallimond apparatus. The experiments revealed that the entrainment rate of fine-grained serpentine decreased from 1.15 to 0.41 with increasing CLS dosage. The pulp viscosity as well as the surface tension analysis, combined with the froth properties investigated using a Foamscan analyzer, confirmed that after CLS addition, the foam generation time increased, while the average bubble diameter gradually decreased, resulting in a decrease in the fluid content of the foam layer as well as the serpentine entrainment rate. Besides, the results were analyzed mechanistically based on Plateau foam structure equilibrium theory. This paper innovatively provides a feasible approach to minimize the content of fine-grained serpentine in brucite concentrate from the perspective of foam entrainment behavior.

Experimental investigation of bubble size evolution released from a fine bubble ejector

AbstractThe effects of gas flow rate, liquid flow rate, and liquid properties on the bubble size evolution at the outlet zone of an ejector have been investigated systemically. The liquid properties, including surface tension and viscosity, were changed by adding glycerol, ethanol, or Carbopol 2020 into tap water. It was found that increasing the liquid flow rate is beneficial for producing smaller bubbles with a narrower size distribution in the ejector, while the gas flow rate investigated here shows less influence on the generated fine bubble diameter. An intense bubble coalescence phenomenon, which has a negative effect on mass transfer, was first observed at the small outlet zone of the ejector by exploring the bubble size evolution along the axial height. It was found that the mean diameter of the fine bubbles could increase by 48% ~ 90% when they rose only 25 cm from the ejector outlet, and the bubble coalescence could be effectively suppressed by adding a small amount of glycerol to water. The bubble diameter could be reduced by 32% ~ 43% by using a 2 wt.% glycerol solution under the experimental conditions. A comparative study with tap water, glycerol solution, Carbopol solution, and ethanol solution was taken, and the underlying mechanism of reducing the generated bubble size by changing the liquid properties was revealed.

Numerical study of the geometric characteristics and kinetic behavior of single bubble rise processes in different liquids

The characterization of single bubble in gas–liquid two phase flow is a critical yet unresolved issue in both science and industry. In this study, the volume-of-fluid (VOF) method is used to numerically simulate and experimentally investigate the effect of initial bubble diameter, liquid viscosity, and surface tension on bubble deformation and the internal flow field of the bubble in a pool of stationary liquid. The findings indicate that as liquid viscosity increases, the bubble's rising speed decreases, and the bubble tends to oscillate. The variation in bubble deformation ratio and the degree of fluctuation increase with the bubble's initial diameter and decrease with the viscosity of the liquid phase. Additionally, as the surface tension of the liquid decreases, the bubble becomes more prone to rupture, and the number of ruptures increases. The flow field inside the bubble can be classified into three categories: “double main vortex type,” “double main vortex type with separated vortex,” and “double main vortex type with scattered vortex.” The velocity reaches its maximum at the center of each vortex type, and the velocity at the interface varies as the bubble interface shape changes. This work lays the foundation for the study of the flow field inside the bubble and improves the predictability of gas–liquid equipment design.

Air Pocket Removal during Siphon Priming

Siphon frequently is employed in pump stations as a pump outlet with a convenient flow cutoff mode for pump shutdown, and it usually works with axial-flow pumps with a large discharge and low head. After the pump starts, the initial air pocket in the siphon should be removed quickly from the siphon to generate a primed siphon flow, a process called priming. In this study, experiments were conducted to investigate the characteristics of air pocket removal during priming, in which two stages can be defined: air-compression and air-entrainment stages. During the air-compression stage, the air is compressed by the pumped water, and the increased air pressure can break the downstream water seal and expel some air out of the siphon. During the air-entrainment stage, the air is removed mainly in the form of bubbly flow. The air-entrainment stage can be divided further into two states: at a small water discharge, the priming process stays in State 1, in which the air pocket always remained in the siphon hump; and at a large discharge, the residual air pocket can be entirely swept out of the siphon in State 2, finally generating the primed siphon flow. The air-compression stage accounts for less than 5% of the priming time but discharges 20%–60% of the initial air. For the bubbly flow during the air-entrainment stage, the bubble diameters follow a log-normal distribution with the median of about 2.3 mm. The critical flow rate for generating primed siphon flow was predicted based on the bubble clearing velocity and force and on the motion analysis of the bubble, which can be a rough value for the calculation of the critical flow rate. The prediction indicates that the critical flow rate should increase with the descending angle and cross-section area of siphon.

Numerical study on the interfacial heat and mass transfer characteristics of bubble direct contact condensation with the effect of non-condensable gas

Bubble condensation is one of the fundamental issues in the direct contact condensation, which is also affected significantly by the existence of non-condensable gas. In this study, a further investigation is carried out on air-steam bubble condensation with the Proportion Integration Differentiation (PID) control model which adjusts the mass transfer coefficient of Lee model in each time step by comparing the parameters obtained in the simulation with the ones calculated based on the empirical correlations originated from the experiments. Furthermore, species transport equations are employed to obtain the local mass fractions of steam and air. The proposed simulation schema is verified by the existing experimental data.Results show that the air-steam bubble condensation can be divided into initial and stable stages. The mass transfer coefficient used in Lee model can be regarded as linear growth over time at each stage, but with different slopes. The non-condensable gas can weaken the heat and mass transfer of condensation, but stabilize the bubble deformation. The present study also reveals the influences of subcooling and initial bubble diameter on the air-steam bubble condensation, which is expected to be instructive for better understanding the effect of non-condensable gas on the interfacial characteristics of bubble condensation in subcooled water.

Transient rising of oxygen bubbles in a fast varying gravity

We report experiments on the transient motion of an oxygen bubble rising in liquid oxygen in a fast varying magneto-gravitational acceleration. In the set-up, a strong magnetic field gradient is applied, which creates in bulk oxygen a body force opposed to its weight. When this gravity compensation is suddenly suppressed, the resulting apparent gravity increases within a fraction of a second from 0 g0 to 0.4 g0 (where g0 is the terrestrial gravitational acceleration). Oxygen bubbles of diameters ranging from 6 mm to 18 mm rise rapidly in the liquid by buoyancy, for values of Galilei and Bond numbers rarely reached with common fluids in terrestrial gravity. The shape of the bubble interface is initially ellipsoidal and evolves toward more complicated shapes during the motion. The time-dependent rising velocity of the bubble is measured during the fast variation of gravity. A comparison of the transient bubble dynamics with classical results obtained in a constant gravity environment enables the time scale of the gravity variation to be precisely quantified.

Simulation of Spatial Distribution of Multi-Size Bubbles in a Slab Continuous-Casting Mold Water Model.

In this paper, a fully coupled large eddy simulation model, including the volume of fluid model, the discrete phase model, the bubble-collision model, and the bubble-breakup model was used to simulate the spatial distribution of multi-size bubbles and its impact on the instantaneous two-phase flow in a slab continuous-casting mold. The influence of the bubble-interaction model on the bubbles' three-dimensional spatial distribution and size distribution, as well as on two-phase flow was discussed. By comparison with the velocity on the meniscus and the average bubble diameter inside a continuous-casting slab water model, the appropriate numerical model was recommended to accurately simulate the two-phase flow and characteristics of discrete bubbles. The submerged entry nozzle and the area around it saw bubble coalescence and breakage more frequently than other areas. The key interaction between the bubbles was their bouncing in the deep region of the mold. In the mold, the average bubble diameter was 0.741 mm, and 44.5% of the total number of bubbles had an approximately average diameter.

CFD-VOF-DPM numerical simulation of enhanced boiling heat transfer characteristics of microencapsulated phase change material slurry

Two-dimensional (2D) computational fluid dynamics (CFD) simulations of water-based microencapsulated phase change material suspension (MPCMS) on the flow boiling in a gas–liquid–solid flow system was performed with volume of fluid (VOF) method and discrete particle model (DPM). The Lagrangian particles were linked to the Eulerian phases through the interchange terms such as the drag force in the respective momentum equations. Influences of particle properties including mass fraction and core phase transition temperature, fluid properties including liquid surface tension force and viscosity, detachment time and rise velocity of gas bubbles and particle entrainment in the gas–liquid–solid flow under ambient conditions were numerically investigated. The effects of particles on bubble nucleation, growth and rupture during boiling were studied by visualization. The results show that MPCM can enhance the boiling heat transfer ability of the base liquid. The maximum heat transfer enhancement rate of MPCMS (28 °C) is 4.8%, the maximum heat transfer enhancement rate of MPCMS (90 °C) is 5.1%, the maximum heat transfer enhancement rate of MPCMS (110 °C) can reach 6.7%. MPCM can promote the formation of new bubbles and the rupture of large bubbles, reduce the departure diameter of bubbles, and enhance the boiling heat transfer capacity of the base liquid. The MPCM with core phase change temperature higher than the boiling temperature of base fluid has the best enhancement effect. Through the combination of numerical simulation methods such as VOF and DPM, the complex phase transition heat transfer process of gas-liquid-solid particle coupling of latent thermal functional thermal fluid can be accurately simulated. The work lays a foundation for further explorations on the gas–liquid–solid flows and possible industry applications.

Effect of particle size and shear parameter on the characteristic of particle-bubble cluster in high-intensity conditioning of coal

ABSTRACT Recent studies have reported that particle-bubble clusters formed and are deduced to play a potentially important role in high-intensity conditioning (HIC). In this study, the effect of particle size and shear parameter on the characteristic of particle-bubble cluster in HIC of coal was investigated in details. Results showed that, for coal of +75 μm fraction, the number of bubbles and the mean bubble diameter generated by pusher blade and sawtooth-disk blade were close. By contrast, the effect generated by four-pitched blade was inferior. The number of bubbles is fairly related with particle size. Under the same conditions, compared to that of +75 μm fraction, the number of bubbles attached to the coal surface of 45 ~ 75 μm is 1 to 2 orders of magnitude less. The mean diameter of the bubbles also tends to decrease in the same way. Particle-bubble cluster was also found in the fraction of −45 μm coal. Compared to the other two blades, the number of bubbles generated by pusher blade was the largest. In summary, pusher blade produced the best performance of cluster formation for each particle size fraction. This study is expected to provide guidance for the optimization of shear parameters for cluster formation during HIC.