Effect Of Bubble Coalescence Research Articles

Three-dimensional simulations of liquid/gas flow through radial centrifugal pumps and the effect of bubble coalescence and breakup on the formation of gas accumulations

Three-dimensional liquid/gas flow mixture simulations of a centrifugal pump are performed by a hybrid two-phase solver, which comprises a blending between a Volume-of-Fluid-like interface-sharpening scheme and a classical homogeneous mixture two-fluid model. The hybrid solver is combined with a population balance model and a turbulence-scale adaptive simulation approach. The experimentally measured head drop due to adherent gas pockets at elevated gas loadings is well reproduced. By a thorough validation with optical measurement data, it is shown that the transition from dispersed bubbles to coherent gas accumulations as well as the steady core of gas pockets and their unsteady wake could be captured without any parameter fitting of coalescence and breakup kernels. Regions of high coalescence activity correlate with locations of gas accumulations, stabilizing the gas pockets in the impeller region. Bubble breakup, in contrast, was primarily found in the wake of the accumulation zones where the tip leakage flow of the semi-open impeller re-enters the blade passage. The particular hybrid solver architecture, together with the resolution of turbulence scales and bubble populations, facilitates the reproduction of bubble coalescence and breakup effects on the formation of gas pockets in centrifugal pumps.

Effect of bubble coalescence on two-phase flow boiling heat transfer in raccoon microchannel - A numerical study

A three-dimensional numerical study on two-phase flow boiling is conducted for raccoon and straight microchannels. The effect of flow boiling instability is observed under conjugate conditions with water as coolant. Bubble coalescence in bubble dynamics is carefully studied, and the forces acting are identified. Eotvos and Weber number are calculated to determine the bubble behavior and shape in a raccoon microchannel. Boiling number is also considered to predict the two-phase flow boiling heat transfer. The bubble flow pattern for two-phase boiling in straight and raccoon microchannel are investigated under different parameters such as Waviness (γ∼0 - 0.267) and Mass flux (G ∼ 118 – 590 kg/m2s), keeping saturation temperature constant at Tsat = 310 K corresponding to pressure (p sat = 6500 pa). It is observed that the Eotvos and Boiling numbers are comparatively higher for values of waviness 0 – 0.067 than other raccoon microchannels. This happens due to the presence of long elongated bubbles, causing the flow boiling instability. On the other hand, as the waviness increases, the bubble size changes periodically. Effect of forces on bubble coalescence for different waviness and mass flux is also observed.

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.

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 bubble coalescence on pool boiling heat transfer and critical heat flux – A parametric study based on artificial cavity geometry and surface wettability

Controlling the onset of boiling is highly desirable for enhancing boiling heat transfer. In this study, a systematic set of pool boiling experiments on surfaces with artificial cavities were performed to investigate the effect of surface morphology on bubble coalescence and resultant boiling heat transfer performance. Circular cavities with various diameters and pitch sizes were fabricated on silicon surfaces. The effect of surface wettability on the performance of the structured surfaces were examined with the use of 50 nm thick Teflon film. Using a high speed camera to examine the bubble dynamics, the results reveal that there exists a critical hole pitch size/hole diameter ratio (P/D = 10), below which horizontal bubble coalescence occurs at the lower wall superheats. Furthermore, the visual results indicated that surface wettability alters the critical heat flux (CHF) mechanism. In contrast to the hydrophobic surfaces, hydrodynamic instability is the main reason for CHF occurrence on the hydrophilic surfaces. The results indicate that although increasing the hole diameter enhances the CHF for all the fabricated samples, the effect of pitch size depends on the surface wettability with the CHF increasing with pitch size on the hydrophobic surface and decreasing with pitch size on the hydrophilic surface. A maximum heat transfer coefficient enhancement of 100% was achieved on the hydrophilic structured surface relative to the hydrophobic structured surface. The maximum CHF increase was 100% on the hydrophilic surface and 48% on the hydrophobic surface.

Mixing of spherical bubbles with time-dependent radius in incompressible flows.

The motion of contracting and expanding bubbles in an incompressible chaotic flow is analyzed in terms of the finite-time Lyapunov exponents. The viscous forces acting on the bubble surface depend not only on the relative acceleration but also on the time dependence of the bubble volume, which is modeled by the Rayleigh-Plesset equation. The effect of bubble coalescence on the coherent structures that develop in the flow is studied using a simplified bubble merger model. Contraction and expansion of the bubbles is favored in the vicinity of the coherent structures. Time evolution of coalescence bubbles follows a Lévy distribution with an exponent that depends on the initial distance between bubbles. Mixing patterns were found to depend heavily on merging and on the time-dependent volume of the bubbles.

Poly-disperse simulation of condensing steam-water flow inside a large vertical pipe

The condensation of saturated steam bubbles in sub-cooled water inside a vertical pipe was studied by poly-disperse CFD simulations. Six test cases with varied pressure, liquid sub-cooling and diameter of the gas injection orifices were simulated. Baseline closures presented for non-drag forces in previous work were found to be reliable also in non-isothermal cases. The effect of bubble coalescence and breakup is over-weighting in the region close to steam injection in case of small orifice diameter. With the increase of orifice diameter, breakup becomes dominant in determining bubble size change. The effect of interphase heat transfer coefficient correlations was investigated. The widespread Ranz–Marshall correlation was found to under-estimate the condensation rate, especially at high pressure levels. In contrast, satisfying agreement with the experimental data was obtained by the Tomiyama correlation.

Effects of crystallization and bubble nucleation on the seismic properties of magmas

AbstractSeismic tomography of potentially hazardous volcanoes is a prime tool to assess the location and dimensions of magmatic reservoirs. Seismic velocities are strongly affected by processes occurring within the conduit or in the magma chamber, such as crystallization and bubble exsolution. However, the limited number of constrained measurements does not allow yet to link seismic tomography and the textural state of a particular volcanic system. In this study, we investigated a chemically simplified melt in the system CaO‐Na2O‐Al2O3‐SiO2‐H2O‐CO2, which undergoes plagioclase crystallization and bubble exsolution. A Paterson‐type internally heated gas pressure apparatus was employed to measure ultrasonic velocities at a constant pressure of 250 MPa and at temperature from 850 to 700°C. Magmatic processes such as crystallization, bubble nucleation, and coalescence have been recognized throughout the measurements of seismic velocities in the laboratory. Compression and shear wave velocities increase nonlinearly during crystallization. At a crystal fraction exceeding 0.45, the formation of a crystal network favors the propagation of seismic waves through magmatic liquids. However, bubble nucleation induced by crystallization leads to an increase of magma compressibility resulting in a lowering of the wave propagation velocities. These two processes occur simultaneously and have a competing influence on the seismic properties of magmas. In addition, as already observed by previous authors, when the bubble fraction is less than 0.10, the decrease in seismic velocities is more pronounced than for higher bubble fractions. The effect of bubble coalescence on elastic properties is thus lower than the effect of bubble nucleation.

Numerical investigation on boiling flow of liquid nitrogen in a vertical tube using bubble number density approach

An average bubble number density (ABND) model was formulated and numerically resolved for the subcooled flow boiling of liquid nitrogen. The effects of bubble coalescence and breakup were taken into account. Some new closure correlations describing bubble nucleation and departure on the heating surface were selected as well. For the purpose of comparison, flow boiling of liquid nitrogen was also numerically simulated using a modified two-fluid model. The results show that the simulations performed by using the ABND model achieve encouraging improvement in accuracy in predicting heat flux and wall temperature of a vertical tube. Moreover, the influence of the bubble coalescence and breakup is shown to be great on predicting overall pressure beyond the transition point.

Surface force at the nano-scale: observation of non-monotonic surface tension and disjoining pressure.

Nano bubbles and films are important in theory and various applications, such as the specific ion effect of bubble coalescence, flotation and porous medium seepage; these rely greatly on the fundamental aspects of extended-DLVO surface forces. However, the origin and validation of the non-DLVO forces are still obscure, especially at the nano scale (1-5 nm). Herein, we report the first determination of the disjoining pressures of aqueous electrolyte nano-films using molecular dynamics (MD) simulations. Our results showed that adding salt does not lead to a decrease in the disjoining pressure. On the contrary, higher concentrations results in greater disjoining pressures. In addition, the temperature was found to significantly change the pattern of the disjoining pressure isotherm. These results aid the understanding of a number of underlying mechanisms, involving nano solid-liquid-gas surfaces.

Quantification of vesicle characteristics in some diatreme-filling deposits, and the explosivity levels of magma–water interactions within diatremes

Vesicles within juvenile fragments in mafic pyroclastic deposits contain important information about the state of the magma at the time of fragmentation. There have been few vesicle studies of juvenile pyroclasts from mafic phreatomagmatic deposits, however, and none we can find from maar–diatreme volcanoes. In this paper we document the vesicularity and vesicle-population characteristics of juvenile fragments sampled from non-bedded lithified deposits of the Coombs Hills diatreme complex, part of the Ferrar large igneous province, Antarctica. The diatreme-filling pyroclastic deposits, dominated by lapilli tuffs and tuff breccias, contain typically abundant lithic clasts derived mostly from the enclosing sedimentary sequence, and several types of juvenile clasts ranging from blocky to fluidal or “raggy”.In the samples measured, 77–80% of the juvenile pyroclasts ranging in size from 0.5mm to fine lapilli is in the ‘non-vesicular’ to ‘incipiently vesicular’ range (<20% vesicles). Such low vesicularities are expected for pyroclasts from maar–diatreme volcanoes where fragmentation takes place at depth in the diatreme or root zone due to magma–water interaction. A few juvenile clasts, however, are more vesicular, and seven of these were chosen and sectioned for 2D analysis of vesicle shapes and orientation, vesicle number densities (Nv), and vesicle volume distributions. The shapes of the vesicles in the studied sections are mostly elliptical (sometimes polylobate), with mean aspect ratios ranging between 0.67 and 0.72. Circular statistics are used to test for trends in the vesicle long-axis orientation data; non-uniformity of orientations is found in most cases, but the trends are weak. Vesicle volume distributions are often bimodal due to variable coalescence. Total Nv values range from 1.0×102 to 5.7×103mm−3; taking the effects of bubble coalescence into account, these values are similar to those found in pyroclasts from other phreatomagmatic volcanoes, although they also overlap partly with those seen in fire fountain deposits and some basaltic Plinian eruptions.Fluidal- or rag-shaped juvenile clasts, some circular vesicles, and the lack of microlites all suggest that the Coombs Hills magma had a relatively low viscosity prior to fragmentation, despite the basaltic andesite composition. This low viscosity allowed parts of the magma to be fragmented in a non-brittle fashion during phreatomagmatic explosions and to form fluidal clasts. Phreatomagmatic explosions in diatremes can therefore produce diverse types of juvenile clasts simultaneously, and the proportions of each will depend on the explosivity of the magma–water (slurry) interaction and other factors. Recycling of fragments is also thought to be an important factor in generating mixtures of different types of juvenile fragments in diatremes.

CFD modelling of polydispersed bubbly two-phase flow around an obstacle

A population balance model (the Inhomogeneous MUSIG model) has recently been developed in close cooperation between ANSYS-CFX and Forschungszentrum Dresden-Rossendorf and implemented into the CFD-Code CFX [Krepper, E., Lucas, D., Prasser, H.-M, 2005. On the modelling of bubbly flow in vertical pipes. Nucl. Eng. Des. 235, 597–611; Frank, T., Zwart, P.J., Shi, J.-M., Krepper, E., Rohde, U., 2005. Inhomogeneous MUSIG Model—a population balance approach for polydispersed bubbly flows, International Conference “Nuclear Energy for New Europe 2005”, Bled, Slovenia, September 5–8, 2005; Krepper, E., Beyer, M., Frank, Th., Lucas, D., Prasser, H.-M., 2007. Application of a population balance approach for polydispersed bubbly flows, 6th Int. Conf. on Multiphase Flow Leipzig 2007, (paper 378)]. The current paper presents a brief description of the model principles. The capabilities of this model are discussed via the example of a bubbly flow around a half-moon shaped obstacle arranged in a 200 mm pipe. In applying the Inhomogeneous MUSIG approach, a deeper understanding of the flow structures is possible and the model allows effects of polydispersion to be investigated. For the complex flow around the obstacle, the general structure of the flow was well reproduced in the simulations. This test case demonstrates the complicated interplay between size dependent bubble migration and the effects of bubble coalescence and breakup on real flows. The closure models that characterize the bubble forces responsible for the simulation of bubble migration show agreement with the experimental observations. However, clear deviations occur for bubble coalescence and fragmentation. The models applied here, which describe bubble fragmentation and coalescence could be proved as a weakness in the validity of numerous CFD analyses of vertical upward two-phase pipe flow. Further work on this topic is under way.

Effect of bubble coalescence on entrainment in gas fluidized beds

An entrainment flux model for freely bubbling gas-solid fluidized beds was described. The model was based on particle trajectories in the freeboard wit

The use of the chemical method for the determination of interfacial areas in gas—liquid contactors

The interfacial area a chem in a gas—liquid contactor as determined by the chemical method deviates from the true geometrical interfacial area a geo, because the overall conversion of the gas phase reactant represents an incorrect average if bubble sizes and residence times are not uniform. The deviations of a chem from a geo become larger the broader the distribution τ b/ d b and the higher the overall conversion Ω A of the reactant in the gas phase. Model calculations, which take into account both the effect of gas phase backmixing as well as the effect of bubble coalescence on the deviation of a chem from a geo, are performed for a mechanically agitated gas—liquid reactor and a bubble column at practical micro- and macromixing conditions. For a gas—liquid model reaction, which is first-order in the gas phase reactant, it is found that: (1) for a mechanically agitated reactor the error in a chem will always be smaller than 10% if Ω A is lower than 0.99, and (2) for a bubble column the error in a chem will be smaller than 20% for most practical applications if Ω A is lower than 0.99. Gas—liquid model reaction systems with absorption of CO 2 in alkanolamine solutions are recommended for the determination of interfacial area in gas—liquid contactors.

Bubble eruption model for entrainment phenomena below the TDH for gas-solid fluidized beds.

A bubble eruption model in which the effect of bubble coalescence near the bed surface on particle ejection from a gas-solid fluidized bed is proposed. It is a statistical model based on detailed measurement of the relationship between bubble coalescence at the bed surface and particle swarm ejection caused by bubble eruption. By this model, the passing frequency of particle swarms in the freeboard, the maximum rise height of the particle swarms, and the entrainment rate below the TDH can be simulated, In particular, the effect of the superficial gas velocity on the passing frequency or the entrainment rate, which has not been evaluated by model analysis, can be predicted properly.