Effect Of Coalescence Research Articles

Investigation of bubble-to-emulsion phase mass transfer at non-isothermal conditions via a coupled CFD-DEM approach

The bubble-to-emulsion mass transfer at non-isothermal conditions is investigated via a coupled computational fluid dynamics-discrete element method (CFD-DEM) approach. The application of CFD-DEM approach in mass exchange prediction is verified. Meanwhile, mass transfer behaviors for an isolated bubble at different injection temperatures are analyzed. It is found that the non-uniform gas concentration is significantly influenced by the uneven temperature inside the bubble besides the impact of flow patterns. Furthermore, the effects of bubble interaction and coalescence on mass transfer are discussed. Two bubbles are injected through the same orifice or two horizontally separated orifices. It is observed that the mass transfer process for two vertically separated bubbles is greatly enhanced in the final period owing to the powerful “suction effect” from the upper bubble. Whereas the mass transfer becomes weak when two bubbles are horizontally injected.

Numerical simulation on bubble wall shape evolution and uniformity in poly(ethylene terephthalate) foaming process

The rheological behaviors of commercial foaming-grade poly(ethylene terephthalate) (PET) were measured and modeled, on the basis of which quantitative simulations on the evolution of the bubble wall shape during PET foaming were conducted. A bubble wall stretching uniformity (BSU) curve was proposed to describe the wall uniformity. A fast dropping BSU curve indicated a nonuniform bubble wall that would lead to the wall rupture. It was shown that the strain hardening and fast relaxation were beneficial for maintaining a uniform bubble wall. Simulations also revealed the initial bubble wall geometry had a strong impact on the bubble wall uniformity. Open-cell foam was produced by aggravating the initial nonuniformity through addition of polytetrafluoroethylene (PTFE) particles. With the analysis on the synergistic effects of bubble rupture and coalescence, the optimized foaming temperature (Topt) for the PET could be determined, which was consistent with the experimental value.

A mechanistic model for nucleate pool boiling including the effect of bubble coalescence on area fractions

A mechanistic model that includes the effect of the bubble coalescence on nucleate pool boiling heat transfer is developed through experimental and theoretical investigations. Transient temperature and heat flux distributions on the boiling surface are obtained using a high-speed infrared imaging technique. According to the temperature and heat flux distributions, the boiling surface is classified into three regions: natural convection, quenching, and evaporation regions. By observing how the heat flux on each region and the corresponding area fraction change as the bubble coalescence occurs, the effect of the bubble coalescence on nucleate pool boiling is investigated. It is found that the bubble coalescence does not affect the heat fluxes on the three regions but affects the area fractions: As the bubble coalescence occurs, the rates of increase in quenching and evaporation area fractions with respect to the wall superheat decrease, while the natural convection area fraction remains the same. By accounting for the effect of the bubble coalescence on quenching and evaporation area fractions, a new correlation of the boiling heat flux is developed and presented. To validate the proposed model, boiling curves for saturated water and FC-72 are reproduced and compared to experimental data. The most distinguishing feature of the model is that it accurately predicts the rate of change of the heat flux with respect to the wall superheat as well as the location of the inflection point at which the slope of the boiling curve begins to decrease. As a result, the error between the reproduced boiling curve and experimental data is reduced by 94% compared to the previous model, which does not include the effect of the bubble coalescence for both water and FC-72.

Characterization of the wake behind a horizontal-axis wind turbine (HAWT) at very high Reynolds numbers

The wake of a three-bladed horizontal axis wind turbine was studied at aerodynamic conditions similar to what is experienced by commercially available turbines. Field relevant Reynolds numbers and tip speed ratios were obtained through the use of a high-pressure wind tunnel, at relatively low velocities. Measurements of the streamwise velocity were acquired through the use of the novel nano-scale thermal anemometry probe (NSTAP), which yields very high spatial and temporal resolution, enabling unattenuated turbulence measurements. Profiles of the mean velocity and turbulent fluctuations are presented, as they demonstrate important features of the wake development, such as wake recovery and tip vortex evolution. One dimensional energy spectra are also presented to provide details on the dominant flow features present in the wake. Reynolds number invariance is shown for mean velocity deficit and streamwise variance profiles for all downstream positions presented. Downstream evolution of streamwise variance profiles provides insight to the dynamic interactions between the tip and root vortex, such as their eventual coalescence. Spectral analysis show that the near wake flow-structures are dominated by the tip vortex, but that other larger structures are present as well, which may be related to the wake meandering phenomenon.

Droplet Coalescence in Liquid/Liquid Separation

Abstract In this paper, a new correlation is developed to predict liquid/liquid separation dynamics with a focus on a water/oil mixture. The correlation employs a force balance on the droplets to predict the rising velocity of the oil phase. The effect of droplet coalescence on the droplet's rising velocity is investigated, and the new correlation predicts the coalescence rate based on the oil/water volume fraction, as well as the initial droplet diameter. To develop the correlation for droplet coalescence, a series of new numerical simulations of a batch oil/water separation process were conducted. An equivalent experiment was conducted, the results of which agree well with the numerical predictions (relative error of 13.39%). The new correlation can predict the rate of separation with a relative error of 6.35% compared to numerical predictions.

Numerical prediction of dropwise condensation performances on hybrid hydrophobic-hydrophilic surfaces

The use of hybrid surfaces, alternating hydrophobic and hydrophilic (or less-hydrophobic) areas, has been recently proposed as a valuable tool for dropwise condensation performances enhancement. The hydrophilic region, in particular, is designed in order to promote the removal of the droplet from the hydrophobic region, reducing their average size and thus increasing the heat transfer performance. Here, dropwise condensation on heterogeneous surfaces composed by a texture of alternating hydrophobic and hydrophilic (or less-hydrophobic) vertical strips is numerically investigated through a Lagrangian-based phenomenological model, originally developed to study droplet evolution in the framework of in-flight icing phenomenon. The effects of droplet nucleation, growth, coalescence and motion are implemented. The model is extended to the case of curved surface. In order to properly assess the hybrid surface, however, a better knowledge of the interaction between the droplet and the film is required. Thus, additional numerical simulations are carried out using an in-house Eulerian full solver of the liquid film layer, in order to understand the migration mechanism of a drop between two different wettability regions. This will give useful information for a proper implementation of the effect of hydrophilic strips in the phenomenological Lagrangian model. A parametric analysis allows to find the optimal geometric configuration of the hybrid pattern, leading to maximum heat transfer performance, for a given set of wettability properties, nucleation density and condensation rate per unit area. Comparison with experimental data from the available literature ensures the capability of the current Lagrangian model to properly catch the physics behind dropwise condensation process on both planar and curved hybrid surfaces. The uncertainty of the estimate of some model input parameter and their influence on the computed solution are be also discussed.

Effects of bubble coalescence and breakup models on the simulation of bubble columns

This work investigates the effects of coalescence and breakup models on the radial distribution of gas holdup and local bubble size distribution in a laboratory scale bubble column operated at 0.11 m/s, 0.14 m/s, 0.19 m/s and 0.23 m/s. The results quantitatively show the influences of bubble coalescence due to various mechanisms, coalescence efficiency calculated by different models, modification coefficients for coalescence rates, critical energy required for breakup and bubble breakage caused by large scale eddies and viscous shear on numerical simulations. Furthermore, qualitative analysis is performed to analyze the effects of these factors. The performance of various models is evaluated (e.g. Han’s breakup model and Das’s coalescence model). Some optimized combinations of coalescence and breakup models that can be used to accurately predict the local gas holdup and bubble size distribution in laboratory scale bubble columns operated at heterogeneous regime are proposed.

Temperature modulated polymer nanoparticle bonding: A numerical and experimental study

In this research, we investigated the impact of nanoparticle adhesive properties on the size of micro-clusters formed during shear-induced aggregation at different temperatures. To precisely control particle adhesion, we used nanoparticles with a core-shell structure, where the core is composed of polymethyl methacrylate and the shell is composed of a combination of polymethyl methacrylate and polybutylacrylate. Due to significantly different glass transition temperature (Tg) of these polymers, the core act as a hard-sphere, while the presence of polybutylacrylate in the shell, with a glass transition temperature of 50 °C, gives the surface mechanical softness upon increasing temperature. We observed that the size of the aggregates grow significantly when the temperature rises above Tg, indicating an increase of adhesive force between the nanoparticles. Under these conditions, the surface of the nanoparticles exhibits a transition from plastic to viscous behavior that allows core-shell nanoparticles to bond physically upon contact in a controlled coalescence effect. To further investigate the micro-mechanical behavior of the micro-clusters during aggregation, a numerical study of a simple shear flow setup using CFD-DEM with a customized particle interaction model was carried out. This model has the capability to describe non-contact as well as contact forces present in colloidal systems. Depending on the system temperature, the model can simulate either elastic, elastic-plastic or viscoplastic deformation between the interacting nanoparticles. Using this feature, it is demonstrated that it is possible to reproduce the experimentally observed growth in aggregates with temperature rise by simulating an increase in adhesion using primary particle mechanical parameters. Furthermore, these results clearly demonstrate the direct relation between surface properties of the nanoparticles with the macroscopic behavior of the colloidal system.

Monitoring Polymer-Enhanced Foam Displacements Through Heterogeneous Porous Media: A Pore-Scale Study

AbstractIn this work, fundamental understanding of phase displacements involved in polymer-enhanced air foam is investigated which was not well discussed in the available literature. To do this, a series of foam injection experiments were performed on heterogeneous rock-look-alike micromodels in the presence and absence of a single fracture. The models were initially saturated with crude oil and experienced post polymer-enhanced foam injection process. We observed for the first time the mechanism of synergetic upstream snap-off and lamella division in the vicinity of the area where the permeability contrast was obvious. Observations showed two opposite effects of oil emulsioning and bubble coalescence when gas bubbles came in contact with oil in pore bodies. Fractal dimension analysis of front polymer-enhanced foam illustrates a noticeable improvement in oil displacement. Primary enhanced foam injection to oil saturated micromodel causes bubble coarsening which leads to less efficient oil displacement process. The lower the polymer concentration, the less stable the foam; consequently, the less efficient oil displacement is observed. Lower viscosity oil results in lower recovery efficiency as the stability of foam decreases. To shed light on the dynamic behavior of polymer–surfactant interface, some dynamic surface tension tests were conducted. Results showed that repellency between surfactant and polymer molecules causes surfactant molecules to be present on the surface making the initial dynamic interfacial tension (IFT) decrease. Results of this work help to better understand how polymer could enhance the efficiency of foam floods in heterogeneous systems.

Free-Rising Bubbles Bounce More Strongly from Mobile than from Immobile Water-Air Interfaces.

Recently it was reported that the interface mobility of bubbles and emulsion droplets can have a dramatic effect not only on the characteristic coalescence times but also on the way that bubbles and droplets bounce back after collision (Vakarelski, I. U.; Yang, F.; Tian, Y. S.; Li, E. Q.; Chan D. Y. C.; Thoroddsen, S. T. Sci. Adv.2019, 5, eaaw4292). Experiments with free-rising bubbles in a pure perfluorocarbon liquid showed that collisions involving mobile interfaces result in a stronger series of rebounds before the eventual rapid coalescence. Here we examine this effect for the case of pure water. We compare the bounce of millimeter-sized free-rising bubbles from a pure water–air interface with the bounce from a water–air interface on which a Langmuir monolayer of arachidic acid molecules has been deposited. The Langmuir monolayer surface concentration is kept low enough not to affect the water surface tension but high enough to fully immobilize the interface due to Marangoni stress effects. Bubbles were found to bounce much stronger (up to a factor of 1.8 increase in the rebounding distance) from the clean water interface compared to the water interface with the Langmuir monolayer. These experiments confirm that mobile surfaces enhance bouncing and at the same time demonstrate that the pure water–air interfaces behave as mobile fluid interfaces in our system. A complementary finding in our study is that the ethanol–air interface behaves as a robust mobile liquid interface. The experimental findings are supported by numerical simulations of the bubble bouncing from both mobile and immobile fluid interfaces.

Microfibers formation in two-phase fluid flowing in a channel with the abrupt constriction: Numerical modeling

Numerical modeling was used to study the patterns of droplet deformation in two-phase Newtonian fluids flowing through a three-dimensional rectangular microchannel with a sharp narrowing. The elongation of single droplets of different viscosities was investigated in different channel zones. Calculations were carried out for different confinement parameter—the ratio of droplet diameter to the gap thickness. The increase in this parameter was shown to lead to the substantial increase in the droplet relative elongation. The effect of coalescence to microfiber formation in flowing emulsion was considered.

Effects of bubble coalescence and breakup on CO2 absorption performance in nanoabsorbents

The complexity and dynamics of bubble coalescence and breakup have fascinated scientists for decades. In this study, we performed experiments and simulations involving successively rising bubbles in a rectangular bubble column for carbon dioxide absorption in nanoabsorbents (methanol with various concentrations of alumina nanoparticles). The variations of the bubble behavior were captured by a high-speed camera in experiments and simultaneously visualized via simulations. Firstly, we distinguished the orifice region and the bulk liquid region. The bubble diameter was determined by only the gas flow rate and orifice diameter in the orifice region; however, it changed significantly in the bulk liquid region, experiencing approximately four stages. Wake entrainment and eddy capture dominated the bubble coalescence in the bulk liquid region; however, the bubble breakup was mainly caused by eddy collision and the coalescence of two bubbles with a small surface tension force. Both coalescence and breakup were more likely to occur in liquids with a higher concentration of nanoparticles. Additionally, we considered the mass transfer coefficient in the liquid phase and found that it can be enhanced by the coalescence and breakup, through the generation of polydisperse bubble swarms. A correlated parameter was deduced that can be substituted in the Hughmark equation to predict the mass transfer coefficient of the CO2–methanol–nanoparticle system.

Effects of diversity and coalescence of species assemblages on ecosystem function at the margins of an environmentalshift.

Sea level rise is mixing formerly isolated freshwater communities with saltwater communities. The structure of these new aquatic communities is jointly controlled by pre- and post-colonization processes. Similarly, since salinity is a strong abiotic determinant of post-colonization survival in coastal systems, changes in salinity will likely impact community composition. In this study, we examine how a strong abiotic gradient affects the diversity and structure of bacterial and zooplankton communities and associated ecosystem functions (decomposition and carbon mineralization). We ran a six week dispersal experiment using mesocosm ponds with four distinct salinity profiles (0, 5, 9, and 13 psu). We find that salinity is the primary driver of both bacterial and zooplankton community composition. We find evidence that as bacterial richness increases so does the amount of decomposition. A phenomenological model suggests carbon mineralization may decrease at mid-salinities; this warrants future work into possible mechanisms for this apparent loss of function. Understanding how salinization changes community structure and ecosystem function may be paramount for managing and conserving coastal plain ecosystems where salinity is increasing due to sea level rise, saltwater intrusion, storm surges, and drought.

Biocompatible nanoparticles production by pulsed laser ablation in liquids

The preparation of spherical metallic nanoparticles (NPs), in the range of 10–100 nm, using pulsed laser ablation in water has particular interest in many scientific fields. The fast released laser energy to the solid metal in water produces plasma at the solid-liquid interface generating NPs in solution. The size distribution and the solution concentration depend by many parameters concerning the laser source (wavelength, pulse energy, pulse duration), the irradiation conditions (target depth in water, focal spot, repetition rate, irradiation time), and the medium where the ablation occurs (water, solution concentration, presence of surfactants). Optimal conditions can be found to control the average particle size, the size distribution, and the coalescence effect. A study of the NPs dependence from the ns pulsed Nd:YAG laser, irradiating metals in water is presented, discussing also the physical characterization of the produced NPs employing microscopy and optical analyses.

Numerical simulation research of subcooled flow boiling based on MUSIG model under low pressure

Subcooled boiling heat transfer is widely used in modern high-power and high-load internal combustion engines (ICEs) because of its efficient heat transfer characteristics. In the present work, a numerical simulation of subcooled boiling under low-pressure conditions is performed on CFX with a user-defined program. Lee’s subcooled boiling experiment results are used for calculations. The bubble departure diameter is simulated by Du’s model. In order to apply the simulation method to engine cooling system with complex channel, the Favre Averaged Drag Model and Tomiyama Lift Force Model are used which makes up for shortcomings of the experience factor needed to be adjusted. The Multiple Size Group (MUSIG) model considering the effects of vapor bubbles coalescence and breakup is used to describe the different sizes of bubbles in the liquid phase. A qualitatively good agreement with the experimental data is achieved without changing model parameters, which lays a good foundation for the application of the simulation method in an engine cooling system.

Description of spreading dynamics by microscopic network models and macroscopic branching processes can differ due to coalescence

Spreading processes are conventionally monitored on a macroscopic level by counting the number of incidences over time. The spreading process can then be modeled either on the microscopic level, assuming an underlying interaction network, or directly on the macroscopic level, assuming that microscopic contributions are negligible. The macroscopic characteristics of both descriptions are commonly assumed to be identical. In this work we show that these characteristics of microscopic and macroscopic descriptions can be different due to coalescence, i.e., a node being activated at the same time by multiple sources. In particular, we consider a (microscopic) branching network (probabilistic cellular automaton) with annealed connectivity disorder, record the macroscopic activity, and then approximate this activity by a (macroscopic) branching process. In this framework we analytically calculate the effect of coalescence on the collective dynamics. We show that coalescence leads to a universal nonlinear scaling function for the conditional expectation value of successive network activity. This allows us to quantify the difference between the microscopic model parameter and established estimates of the macroscopic branching parameter. To overcome this difference, we propose a nonlinear estimator that correctly infers the microscopic model parameter for all system sizes.

Bioinspired Topological Surface for Directional Oil Lubrication.

Directional droplet transport widely exists in biological surfaces that greatly inspire the development of a great deal of engineered devices allowing for directional liquid transport in diverse energy and water applications. Despite extensive advances in this area, it remains a challenge to guide the directional spreading of lubricating oils by virtue of the bioinspired design of surface topography in the lubricity field. In this paper, we develop a bioinspired topological surface textured with simple V-shaped posts elegantly distributed in the parallel channels, which allows for an efficient and unidirectional transport of various lubricating oils. We also clarify the propagation of a precursor film and the coalescence effect between the original droplet and the precursor film in the preferential direction, as well as the pinning effect in the reverse direction, which integrate seamlessly to create a long-range directional oil transportation. The directional oil transportation promises a potential application of directional lubrication, creating a functional surface consisting of two zones with different lubrication properties as evidenced by different friction coefficients.

The origin of the elements and other implications of gravitational wave detection for nuclear physics

The neutron-star collision revealed by the event GW170817 gave us a first glimpse of a possible birthplace of most of our heavy elements. The multi-messenger nature of this historical event combined gravitational waves, a gamma-ray burst and optical astronomy of a “kilonova”, bringing the first observations of rapid neutron capture (r process) nucleosynthesis after 60 years of speculation. Modeling the r process requires a prodigious amount of nuclear-physics ingredients: practically all the quantum state and interaction properties of virtually all neutron-rich nuclides, many of which may never be produced in the laboratory! Another essential contribution of nuclear physics to neutron stars (and their eventual coalescence) is the equation of state (EoS) that defines their structure and composition. The EoS, combined with the knowledge of nuclear binding energies, determines the elemental profile of the outer crust of a neutron star and the relationship between its radius and mass. In addition, the EoS determines the form of the gravitational wave signal. This article combines a tutorial presentation and bibliography with recent results that link nuclear mass spectrometry to gravitational waves via neutron stars.

Control of crystal morphology in energetic drop-on-demand inkjet method

Nano-energetic materials have attracted the worldwide attention since they play an important role in fabricating insensitive high-energy explosives, micro-energetic devices, and explosive detectors. Here, we report a simple, but an effective strategy to control droplet coalescence during inkjet printing, as a major variable, to tailor the nanoscale morphology of energetic materials produced upon evaporation of all-liquid inks. We expect to achieve energetic materials with nanoscale particles by integrating the deposition and the nanocrystallisation of energetic droplets in one step. A proprietary uniform energetic micro-droplet printing equipment is utilised to reveal the influence of temperature, frequency on the particle size of energetic materials. Uniform line, with the particle size between nanometre and microns, is successfully obtained, showing the feasibility of the proposed method for preparing the microscale charge of nano-energetic materials. We also discussed the effect of droplet coalescence on the crystal morphology.

Hydrocylone for oil-water separations with high oil content: Comparison between CFD simulations and experimental data

Production separators for gas-oil-water separation are huge, require long residence times, and present several internals to improve oil-water separation. As an alternative to production separators, specially for subsea facilities, this work presents a hydrocyclone which separates a 40% oil-in-water mixture with 93% ± 2% total efficiency and flow ratio of 7%. Such high oil concentration is typical of mature wells and hydrocyclones have not been designed for such purpose before. In order to design this hydrocyclone, fractional factorial techniques have been used to consider the influence of seven geometrical variables. CFD has been used to assess the performance of each hydrocyclone of the factorial design. The selected hydrocyclone has been tested experimentally in order to validate its performance. PIV measurements have also been carried out in order to assess tangential and axial mean velocity profiles for only water flowing in the hydrocyclone. FBRM has been used to measure droplet size distribution at inlet, overflow and underflow streams. As results, computational reduced total efficiency has been experimentally confirmed. Numerical simulations captured tangential velocity near the centerline of the hydrocyclone, but failed in predicting axial velocity profiles and tangential velocity peaks and near wall regions. It has been discussed though that these are probably not responsible for oil droplets separation in the hydrocyclone when water is the continuous phase. A methodology to analyze the effect of droplet breakup and coalescence, using measured inlet, overflow and underflow cumulative size distribution, has been presented. It has been shown that for the inlet cumulative undersize distribution used, breakup was negligible and coalescence occurred only for droplet diameters higher than those separated with 100% grade efficiency. In this case, no breakup and coalescence models are thus needed in the mathematical model and this explains why reduced total efficiency has matched in both numerical simulations and experiments.