Bubble Coalescence Mechanisms Research Articles

Application of CFD to study the flow in a molten salt chlorination furnace

A numerical simulation based on computational fluid dynamics and population balance model has been accomplished to analyze the gas-liquid-solid three phases in molten salt chlorination furnace. A two-way coupled model considering the influence of the bubble size distribution (BSD) on the flow field by transmitting the data of the Sauter mean diameter is firstly set up and validated with the experimental and simulation results. Based on the mechanisms of bubble coalescence and breakup induced by the turbulence and the bubble-particle interaction, some factors like the velocity field, volume fraction, turbulence dissipation rate, BSD, bubble number density and particle distribution are discussed. The results show that 40 m/s is an appropriate operation parameter for the inlet velocity of molten salt chlorination furnace, for the reason that the large bubble in the upper part of the furnace can break up into fine bubbles distributed uniformly in the whole furnace. It is conducive to the increase the interaction opportunity among phases, promote the chemical reactions, and reduce the area of “dead zone”, providing theoretical support and engineering design basis for the production of large-scale molten salt chlorination furnace.

A combined rate theory-population balance model of the evolution of irradiation-induced helium bubbles in metals during annealing

A multiscale model describing the evolution of helium bubbles in the irradiated materials is established based on the rate equations and the population balance equation. The size distribution of the helium bubbles and the evolution law of the statistical average size and number density of the bubbles with time changing predicted by the model are in good agreement with the experimental statistics. The influence of annealing temperature, annealing time, irradiation energy, irradiation flux, and coarsening mechanisms on the evolution of helium bubbles are numerically simulated and discussed, taking FeCrAl alloy as the irradiated material. The results show that the bubble evolution is dominated by the Ostwald ripening mechanism under high temperature annealing conditions (≥ 1073 K), and the time dependence of the average size and density of helium bubbles is consistent with the existing theoretical results. And the number of shrinking helium bubbles caused by Ostwald ripening is effectively reduced by the bubble coalescence effect, leading to further growth of the proportion of large-sized helium bubbles. Consequently, a Gaussian bubble size distribution has been obtained modeling the coupling of the Ostwald ripening mechanism and the bubble coalescence mechanism. The generation rates of helium and vacancy in the material during irradiation, as well as their ratio, are significant factors that can affect the nucleation and evolution of helium bubbles.

Numerical study on the coalescence of multiple argon bubbles at a horizontal plane with different orifice arrangements in liquid steel under laminar flow

ABSTRACT In the current study, the coalescence of multiple argon bubbles at a horizontal plane in liquid steel with different orifice arrangements was numerically investigated using the volume of the fluid model combined with the continuum surface force model. The numerical simulation of pairwise bubbles coalescence was verified with the experimental observation in the air–water system. The arrangements of equilateral triangle, square and rhombic orifices were considered. The effects of bubble size and orifice structure arrangement on the lateral coalescence of multiple argon bubbles were analysed. Argon bubbles coalesce into a bigger one when the initial bubble distance is less than the critical distance of bubble coalescence. The critical distance for bubble coalescence decreases with the increasing bubble size. When the orifice arrangements were arranged in square, rhombic and equilateral triangles, the coalescence of multiple bubbles become easier. In addition, the mechanisms of bubble coalescence and bounce were discussed.

Experimental study of bubble size distribution in bubbly and bubbly-to-slug transition flow

The current work experimentally investigates the evolving characteristics of bubble size distribution (BSD) in bubbly and bubbly-to-slug transition flow in a 25.4 mm inner diameter pipe. Four-sensor conductivity probes used in present research enables accurate acquisition of bubble chord length in the flows with void fraction varying from 0.1 to 0.6. The flow regime transition is implemented by increasing superficial gas velocity while keeping superficial liquid velocity constant. Six right-skewed probability distribution functions (PDFs) commonly used for predicting bubble chord length distribution (CLD) are selected and used to fit the chord length data. The goodness of fit for all PDFs is evaluated by R-square, root-mean-square error (RMSE) and corrected Akaike information criterion. The results show that the PDF which reasonably fits the CLD in bubbly flow fails to fit the CLD in bubbly-to-slug transition flow. The evolving characteristics of CLD in flow regime transition process are qualitatively and quantitatively discussed based on bubble coalescence and break-up mechanisms. It is found that the invalidation of uniform morphology assumption and different mechanisms of bubble coalescence and break-up may result in the failed fitting. To properly predict real bubble size distribution (BSD) in bubbly-to-slug transition flow from CLD, an analytical backward transformation method is proposed to relate the CLD to BSD based on the bubble geometrical characteristics in the transition flow. The predicted BSD can be applied to calculate the interfacial area concentration (IAC) in the transition flow. The calculated IAC shows good agreements with experimental data which demonstrates the validity of predicted BSD in the transition flow.

Modelling of fluidelastic instability in tube bundles under two-phase bubbly flow conditions

One of the major factors affecting the integrity of tube bundles in a steam generator undergoing excessive vibrations is fluidelastic instability. Currently, a comprehensive model for predicting fluidelastic instability in tube bundles under two-phase cross flow has not been developed. Accurate design guidelines on fluidelastic instability in tube bundles is crucial to avoid tube failure due to tube vibrations. Therefore, this study presents a model for the fluidelastic instability (FEI) of a two-phase flow in a normal square tube bundle. The model focuses on the bubbly flow regime of a two-phase flow. This complex flow across the bundle is simplified as a one-dimensional inviscid flow in channels. The liquid is treated as a continuous phase while the gas bubbles are treated as compressible dispersed particles. The motion of each individual bubble is tracked in time domain simulations. Bubble-to-bubble interaction, bubble break-up, and bubble coalescence mechanisms are considered in the current model. The continuity equation is solved using the local instantaneous void fraction. The momentum equation is then solved to calculate the fluid forces on the tube. Using this technique, it was possible to calculate the change in the flow density around the tube. Prediction of the stability threshold showed very promising results when compared with the experimental data.

Evaluating melt foamability of LLDPE/LDPE blends with high LLDPE content by bubble coalescence mechanism

Bubble coalescence mechanism was introduced to evaluate the melt foamability of polymer blends. Linear low-density polyethylene (LLDPE), with excellent mechanical properties but poor foamability, was blended with small amounts of low-density polyethylene (LDPE), which was expected to impart the desired foaming-related properties to LLDPE. The presence of LDPE in LLDPE matrix provided only a small improvement in linear viscoelasticity under small deformations, but a significantly large improvement in nonlinear behavior under large deformations. A critical coalescence radius curve was simulated using a pressure-balanced bubble growth model to evaluate bubble coalescence and foamability of the blends, taking into account the linear and non-linear behaviors. It was found that the steep stress growth caused by LDPE addition can dramatically increase the pressure of large bubbles and then effectively inhibit the coalescence of adjacent bubbles, thus improving the foamability of LLDPE/LDPE blends. The theoretical predictions derived from the bubble coalescence mechanism were also confirmed by experiments.

Bubble coalescence and breakup model evaluation and development for two-phase bubbly flows

The performance of thermal-hydraulics analysis codes heavily depends on the reliable models developed for interfacial transfer terms in the two-fluid model. This requires the accurate modeling of the interfacial area concentration (IAC). The interfacial area transport equation (IATE) developed from the population balance equation is one of the promising ways to be able to dynamically describe the IAC evolutions in the two-phase flows. A lot of efforts have been made to develop the IATE and its bubble coalescence and breakup models in the past two decades. The one-group IATE with its bubble coalescence and breakup models serves as an important and promising model to predict the IAC behaviors in the bubbly flows now. This paper made an extensive survey on the recent theoretical and experimental researches on the IATE and collected the available 8 sets of bubble coalescence and breakup models and the latest experimental databases measured with the up-to-date measuring techniques. The 8 sets of bubble coalescence and breakup models are originally developed to model the bubble evolution in bubbly flows and the small bubble evolution in the flows beyond bubbly flow. The collected data covers the bubbly flows taken the vertical pipes with the diameter ranging from 0.0508 m to 0.200 m and the height-to-diameter ratio ranging from 4 to 113. To confirm the reliability of the 8 sets of bubble coalescence and breakup models, this paper systematically compared and evaluated them with the collected experimental data in the 1D one-group IATE numerical calculations. Although the bubble coalescence and breakup models of Sun et al. (2004, Modeling of bubble coalescence and disintegration in confined upward two-phase flow. Nucl. Eng. Des. 230, 3–26) and Hibiki and Ishii (2000, One-group interfacial area transport of bubbly flows in vertical round tubes. Int. J. Heat Mass Transfer 43, 2711–2726) were found to show the best and second-best predicting performance for the collected latest experimental databases of bubbly flows with data-number-averaging mean absolute relative errors of 33.5% and 61.7% respectively, their relative errors are not satisfactory. Hibiki and Ishii (2000, One-group interfacial area transport of bubbly flows in vertical round tubes. Int. J. Heat Mass Transfer 43, 2711–2726) model only consisting of the contributions of the bubble random collision coalescence and turbulent impact breakup mechanisms can reasonably predict the IAC experimental data in the bubbly flows with high-liquid flow-rate flow conditions and significantly overpredict the IAC experimental data in the bubbly flows with low-liquid flow-rate flow conditions. To further improve the performance of the 1D one-group IATE, this paper has developed a bubble coalescence and breakup model by greatly improving the model of Hibiki and Ishii (2000, One-group interfacial area transport of bubbly flows in vertical round tubes. Int. J. Heat Mass Transfer 43, 2711–2726) with the addition of the contribution of the wake entrainment bubble coalescence mechanism which prevails in low-liquid flow-rate bubbly flows and optimized the adjustable coefficients of the 3 major constitutive models with the collected latest experimental databases of bubbly flows. The data-number-averaging mean absolute relative error of the newly-proposed bubble coalescence and breakup model is accordingly improved to be 17.1% in the predictions of the collected latest experimental databases of bubbly flows.

Three‐dimensional numerical simulation of coalescence and interactions of multiple horizontal bubbles rising in shear‐thinning fluids

The dynamics of multiple horizontal bubbles rising from different orifice arrangements in shear‐thinning fluids was simulated numerically by three‐dimensional Volume of Fluid method. The effects of bubble size, rheological properties of shear‐thinning fluids, and orifice structure arrangements on multiple bubbles interaction and coalescence were analyzed, and the mechanisms of bubble coalescence and breakup were fully discussed and elucidated. The variation of bubble rising velocity during coalescence process and freely rising processes for different orifice arrangements was also deeply investigated. The critical initial horizontal intervals for coalescence of multiple horizontal bubbles with various orifice arrangements were attained by simulation, which could serve as the critical criterion of bubble coalescence or noncoalescence. Furthermore, the critical bubble interval was predicted based on the film drainage model, the prediction accords well with the simulation result and is quite conducive for the design and optimization of perforated gas–liquid contact equipment. © 2015 American Institute of Chemical Engineers AIChE J, 61: 3528–3546, 2015

Measurement of Liquid Film Thickness Between Coalescing Twin Air Bubbles in a Water Pool Using a Modified Laser Extinction Method

The objective of this study is to elucidate the mechanisms of bubble coalescence. The coalescence of horizontally contacting twin bubbles is experimentally investigated using an isothermal air–water system. The liquid film thickness formed between the contacting bubbles is measured using an improved laser extinction method. The variation of liquid film thickness between the bubbles at the rupture location and the distribution of the liquid film thickness are evaluated. The experimental parameters are the airflow rate and the measurement position (transmitted laser position). The bubble approach velocity and the time from the start of collision until coalescence were measured. When bubble coalescence occurs rapidly, the liquid film thickness is thinnest near the center, and this position moves toward the periphery from the center with increasing contact duration. The thinnest film thickness appeared just before coalescence and was approximately 1.0 μm. A ring-shaped thinner area in the liquid film emerged and shifted from the center toward the periphery of the liquid film with the increase in the bubble approach velocity and close contact duration. The thinnest liquid film thickness just before the commencement of rupture in the ring-shaped area was approximately 0.3 μm.

Mechanisms of bubble coalescence in silicic magmas

Bubble coalescence is an important process that strongly affects magmatic degassing. Without coalescence, bubbles remain isolated from one another in the melt, severely limiting gas release. Despite this fact, very little has been done to identify coalescence mechanisms from textures of magmatic rocks or to quantify the dynamics of bubble coalescence in melts. In this paper, we present a systematic study of bubble-coalescence mechanisms and dynamics in natural and experimentally produced bubbly rhyolite magma. We have used a combination of natural observations aided by high-resolution X-ray computed tomography, petrological experiments, and physical models to identify different types of bubble–bubble interaction that lead to coalescence on the timescales of magma ascent and eruption. Our observations and calculations suggest that bubbles most efficiently coalesce when inter-bubble melt walls thin by stretching rather than by melt drainage from between converging bubble walls. Orders of magnitude are more rapid than melt drainage, bubble wall stretching produces walls thin enough that inter-bubble pressure gradients may cause the melt wall to dimple, further enhancing coalescence. To put these results into volcanogical context, we have identified magma ascent conditions where each coalescence mechanism should act, and discuss the physical conditions for preserving coalescence structures in natural pumice. The timescales we propose could improve volcanic eruption models, which currently do not account for bubble coalescence. Although we do not address the effect of shear strain on bubble coalescence, the processes discussed here may operate in several different eruption regimes, including vesiculation of lava domes, post-fragmentation frothing of vulcanian bombs, and bubbling of pyroclasts in conduits.

Coalescence efficiency of bubbles in bubble columns

AbstractBubble size distribution was modelled by employing the population balance equation (PBE). All three bubble coalescence mechanisms (turbulence, buoyancy and laminar shear) and the main bubble breakup mechanism (breakup due to turbulent eddies) were considered in the model. Local bubble size distributions at the top and bottom of the column were obtained by solving this PBE. The results were compared with the experimental data for seven independent multiphase systems (water/air, isomax diesel/air, kerosene/air and four other liquid mixture/air) at two diverse gas velocities. The experimental adjustable constant in the coalescence efficiency function was determined by fitting the population balance to the experimental bubble size distributions. An empirical correlation was proposed for the coalescence efficiency by the dimensional analysis, which includes Reynolds and Weber numbers. © 2011 Canadian Society for Chemical Engineering

多流体モデルによる気泡塔内多分散気泡流の数値計算(流体工学,流体機械)

Bubble coalescence and breakup models are implemented into the (N+2)-field model, which is a hybrid combination of a multi-fluid model and an interface tracking method to examine the applicability of available bubble coalescence and breakup models to poly-dispersed bubbly flows in bubble columns. The implemented models are a breakup model proposed by Luo & Svendsen and coalescence models proposed by Prince & Blanch and Wang et al. The validation of these models is carried out using experimental data for a coalescence-dominating flow, a breakup-dominating flow, a flow in-between, and a flow with low coalescence-breakup frequencies. Good predictions of void and bubble size distributions for the four kinds of poly-dispersed bubbly flows in bubble columns are obtained. It is also demonstrated that all the possible bubble coalescence mechanisms, i.e. bubble coalescence caused by bubble collisions due to turbulent eddies, the difference in bubble velocities and shear slip, and bubble coalescence due to wake entrainment, should be taken into account for obtaining good predictions.

Bubble Coalescence in Gas–Liquid Flow at Microgravity Conditions

Bubbly flow at microgravity conditions is investigated experimentally. The evolution of bubble size has been measured in tubes of various diameters (6 to 40 mm). It is shown that the bubble size significantly increases along the tube due to coalescence. To predict the bubble size distribution along the tube a mechanistic model based on the transport equations of the moment of the bubble-diameter distribution has been developed. In these equations the coalescence rate is modeled in taking into account the two mechanisms of bubble coalescence encountered in microgravity: the turbulence-induced coalescence and the shear-induced coalescence. The results of the model are then compared to the experimental data.

The Mechanisms of Bubble Coalescence in a Non‐Newtonian Fluid

AbstractThe coalescing mechanism of two in‐line oblate‐cusped bubbles rising in a non‐Newtonian fluid has been qualitatively and quantitatively studied by using particle image analyzer. Through effects of the shear thinning and viscoelasticity on the flow structure, the acceleration of the trailing bubble to the leading bubble is evident. While two bubbles are getting closer, the drained liquid is circulated to the back of the trailing bubble. To encounter these dynamic flow structures, the bubble shape evolution for the leading bubble in each stage is from oblate‐cusped to elongated oblate‐cusped, to invert pentagonal, and to oblate. Suppressed by the trailing bubble and the flow structure, the shape of leading bubble changes from oblate‐cusped to oblate, to flattened oblate, and to flattened oblate with concave surface.

Interfacial Area Transport of Bubbly Flow in a Small Diameter Pipe

In relation to the development of the interfacial area transport equation, this study focused on modeling of the interfacial area transport mechanism of vertical adiabatic air-water bubbly flows in a relatively small diameter pipe where the bubble size-to-pipe diameter ratio was relatively high and the radial motion of bubbles was restricted by the presence of the pipe wall. The sink term of the interfacial area concentration was modeled by considering wake en-trainment as a possible bubble coalescence mechanism, whereas the source term was neglected by assuming negligibly small bubble breakup for low liquid velocity conditions based on visual observation. One-dimensional interfacial area transport equation with the derived sink term was evaluated by using five datasets of vertical adiabatic air-water bubbly flows measured in a 9.0mm-diameter pipe (superficial gas velocity: 0.013–0.052 m/s, superficial liquid velocity: 0.58–1.0m/s). The modeled interfacial area transport equation could reproduce the proper trend of the axial interfacial area transport and predict the measured interfacial area concentrations within an average relative deviation of 1±11.1%. It was recognized that the present model would be promising for predicting the interfacial area transport of the examined bubbly flows.

Interfacial Area Transport of Bubbly Flow in a Small Diameter Pipe.

In relation to the development of the interfacial area transport equation, this study focused on modeling of the interfacial area transport mechanism of vertical adiabatic air-water bubbly flows in a relatively small diameter pipe where the bubble size-to-pipe diameter ratio was relatively high and the radial motion of bubbles was restricted by the presence of the pipe wall. The sink term of the interfacial area concentration was modeled by considering wake en-trainment as a possible bubble coalescence mechanism, whereas the source term was neglected by assuming negligibly small bubble breakup for low liquid velocity conditions based on visual observation. One-dimensional interfacial area transport equation with the derived sink term was evaluated by using five datasets of vertical adiabatic air-water bubbly flows measured in a 9.0mm-diameter pipe (superficial gas velocity: 0.013–0.052 m/s, superficial liquid velocity: 0.58–1.0m/s). The modeled interfacial area transport equation could reproduce the proper trend of the axial interfacial area transport and predict the measured interfacial area concentrations within an average relative deviation of 1±11.1%. It was recognized that the present model would be promising for predicting the interfacial area transport of the examined bubbly flows.

Two-group interfacial area transport equations at bubbly-to-slug flow transition

In relation to the development of the interfacial area transport equation, the basic concept of two-group interfacial area transport equations was demonstrated. This study focused on (1) the formulation of the two-group interfacial area transport equations, (2) the classification of bubble interactions between spherical/distorted bubbles and cap/slug bubbles, (3) the categorization of basic mechanisms of bubble coalescence and breakup, (4) the preliminary modeling of sink and source terms in the interfacial area transport equation, (5) the construction of several data sets at bubbly-to-slug flow transition by assuming the shape of a cap bubble, and (6) the validation of the two-group interfacial area transport equations by means of the data sets at the bubbly-to-slug flow transition. Excellent agreement was obtained between predicted and measured interfacial area concentrations within the average relative deviation of ±3.61% for seven data sets. It was expected that this preliminary work would serve to show the capability of the two-group interfacial area transport equations as well as to understand the basic behavior of the interfacial area transport at the transition from bubbly to slug flow.

Regular-irregular bubble departure patterns of film boiling on a horizontal heating wire

The transition between regular and irregular bubble departure pattern of film boiling on a horizontal heating wire is investigated experimentally. A range of transition heat flux can be identified. The transition is gradual in nature and is proposed as caused by bubble coalescence mechanism. A simple estimate for the transition heat flux under saturated film boiling is proposed and compared with experiments.

Summary paper on fluidization and transport phenomena

The papers presented in the sessions on fluidization and transport phenomena cover a wide range of subjects pertaining to fluid—particle systems. Examples of these subjects include bubble coalescence mechanisms, entrainment, pressure effects and chaos in bubbling and turbulent fluidized beds, wave phenomena and cluster structure in circulating fluidized beds, and spectral and fractal characteristics in gas—liquid—solid fluidized beds. In this summary paper, an overview based on these papers is given and salient results are highlighted.