Average Bubble Research Articles

Modeling and simulation of photocatalytic CO2 reduction into methanol in a bubble slurry photoreactor

One of the attractive but still challenging ways of CO2 utilization is photocatalytic conversion of CO2 into fuels. Since reactor design plays an important role in both the efficiency and quality of the product, moving towards modeling and simulation of the photoreactor is vital. Thus, in this work modeling and simulation of photoreactor has been conducted using appropriate hydrodynamic and kinetic modules. The obtained model has been thoroughly validated by the experimental data which can further be utilized to scale-up the photoreduction process. The important parameters in the reactor design including the number of spargers, inlet gas flux, average bubble diameter, and light intensity have been studied. The methanol concentration after 150 s of the reaction under irradiation with light intensity of 120 W/m2 where 12 spargers delivered carbon dioxide gas into the reactor has been reported and the results have been validated accordingly.

Effect of the Internal Humidity of Concrete on Frost Resistance and Air Void Structure under Different Low Temperature Conditions.

From the perspective of combining macroscopic and microscopic properties, this paper simulates the freeze–thaw cycle process at different freezing low temperatures based on the climate simulation equipment and by setting the curing conditions with different temperatures and relative humidity to produce different moisture conditions in concrete. The frost resistance properties and microscopic air void performance of concrete with different internal water content under different freezing low temperatures in freeze–thaw cycles were systematically studied. The results show that the higher the internal water content of concrete, the more obvious the mass loss rate and dynamic elastic modulus loss of concrete in the freeze–thaw process, and the more serious the deterioration of the air void parameter performance of the air-entraining agent introduced into concrete, which is manifested as the average bubble diameter and bubble spacing factor become larger and the bubble specific surface area decreases. In addition, in the case of the same internal moisture content of concrete, the freezing temperature used in the freeze–thaw cycle also has an important impact on the frost resistance of concrete and air void parameters; the lower the freezing temperature used, the more significant the decline in the frost resistance of concrete, the more obvious the deterioration of air void parameters.

Investigations of bubble size distribution on swirl effervescent atomizer flotation

Atomizer flotation has the advantages of low cost, low energy consumption, adjustable, and long cycle life in industrial applications. The bubble diameter is an important parameter for floatation. The smaller the bubble diameter, the better the flotation effect. However, the bubble diameter produced by the atomizer during flotation separation is large at present. In this paper, a swirl effervescent atomizer is designed and experimental diagnosis through a spray measurement system and submerged spray visualization system. The diameter and size distribution of bubbles in 5 mm from the atomizer outlet to the jet cross-section spray were discussed. The results showed that the swirl effervescent atomizer obtained a fine Sauter mean diameter (SMD is 13–25 μm) compared with the traditional atomizer. Three flow patterns were observed at the submerged spray system: large bubbly flow, discrete bubbly flow, and slug flow. A statistical method of bubbles under high injection pressure was proposed. The effects of the operating conditions of the effervescent atomizer on the bubble diameter were investigated. The average bubble diameter decreased with the increase in injection pressure and gas–liquid mass flow ratios (GLR). However, with the GLR being increased to 0.15, the influence of injection pressure on bubble diameter was reduced. The backpressures (Liquid level heights) have less effect on bubbles compared with injection pressure and GLR. This paper has great significance for the sustainable development of the atomizer in the flotation separation application.

Gas Bubble Dynamics During Methane Hydrate Formation and its Influence on Geophysical Properties of Sediment Using High-Resolution Synchrotron Imaging and Rock Physics Modeling

Gas bubble in aquatic sediments has a significant effect on its geophysical and geomechanical properties. Recent studies have shown that methane gas and hydrate can coexist in gas hydrate–bearing sediments. Accurate calibration and understanding of the fundamental processes regarding such coexisting gas bubble dynamics is essential for geophysical characterization and hazard mitigation. We conducted high-resolution synchrotron imaging of methane hydrate formation from methane gas in water-saturated sand. While previous hydrate synchrotron imaging has focused on hydrate evolution, here we focus on the gas bubble dynamics. We used a novel semantic segmentation technique based on convolutional neural networks to observe bubble dynamics before and during hydrate formation. Our results show that bubbles change shape and size even before hydrate formation. Hydrate forms on the outer surface of the bubbles, leading to reduction in bubble size, connectivity of bubbles, and the development of nano-to micro-sized bubbles. Interestingly, methane gas bubble size does not monotonously decrease with hydrate formation; rather, we observe some bubbles being completely used up during hydrate formation, while bubbles originate from hydrates in other parts. This indicates the dynamic nature of gas and hydrate formation. We also used an effective medium model including gas bubble resonance effects to study how these bubble sizes affect the geophysical properties. Gas bubble resonance modeling for field or experimental data generally considers an average or equivalent bubble size. We use synchrotron imaging data to extract individual gas bubble volumes and equivalent spherical radii from the segmented images and implement this into the rock physics model. Our modeling results show that using actual bubble size distribution has a different effect on the geophysical properties compared to the using mean and median bubble size distributions. Our imaging and modeling studies show that the existence of these small gas bubbles of a specific size range, compared to a bigger bubble of equivalent volume, may give rise to significant uncertainties in the geophysical inversion of gas quantification.

Bubble dynamics in a binary Gas-Solid fluidization system of Geldart B and Geldart D particles

In order to gain a more fundamental understanding of binary particle systems, a two-dimensional (2D) fluidized bed was used to study the bubble dynamics and dense phase composition. Digital image analysis (DIA) was employed to measure bubble diameter, bubble rise velocity, bed expansion, and the distribution of the two types of particles in the bed. Magnetite and sand particles as the single components and binary mixtures were used as the bed materials. The bubble diameter and bubble rise velocity both increased with the distance above the gas distributor and the excess gas velocity. The bubble rise velocity also increased while bubbles ascending in the bed due to an increase in bubble size and buoyancy. Both mono particle systems had higher average bubble sizes than that of the system with nearly identical particle composition, which had the smallest bubble size and the lowest bubble rise velocity. The bubble coalescence and splitting processes were also imaged and the total bubble volume was found to increase after coalescence and decrease after splitting due to some uncaptured smaller bubbles. In addition, a preliminary experiment was carried out to trace dense the phase composition and the overall bed density was estimated based on the dense phase composition and bed expansion.

Lattice Boltzmann method for simulation of solid–liquid conjugate boiling heat transfer surface with mixed wettability structures

Based on the one-component multiphase lattice Boltzmann method, a novel solid–liquid conjugate boiling heat transfer pseudo-potential lattice Boltzmann (LB) model is tentatively proposed in this paper. By respectively introducing the physical property parameters of solids and liquids into the relaxation time τT of the temperature distribution equation, different energy transfer rates in solid, liquid, and vapor regions can be successfully predicted. After verifying the accuracy, stability, and reasonability of this model, the bubble detaching behavior and boiling heat transfer performance on the rectangular cavity structure are analyzed through setting different contact angles of the cavity surface and plane heating surface. The results show that the hydrophobic cavity surface can initialize bubble nucleation earlier and obviously increase the bubble detaching frequency because of its gas-bounding character, while the hydrophilic plane heating surface can restrict the expansion of bubbles and delay the appearance of film boiling. Moreover, for uniform wettability surfaces, the bubble detaching period varies in the quadratic equation with the surface contact angle due to the interaction of surface tension and buoyancy, and there is a minimum detaching period. While for the mixed wettability surfaces, the bubble detaching period also has a minimum value with the decrease in the contact angle the cavity surface, but the average bubble detaching diameter basically does not change with the cavity surface contact angle; moreover, the cavity surface contact angle corresponding to the minimum detaching period also increases with the increase in the plane heating surface contact angle. In addition, for the boiling heat transfer surface with cavity structure, the maximum heat flux and temperature gradient occur on the cavity surface, and the local heat flux of the hydrophobic cavity surface is higher than that of the hydrophilic cavity surface. This work will provide useful help for the further development of the conjugate boiling heat transfer LB model and clarify the mechanism of enhanced boiling heat transfer on a mixed wettability surface.

Evaluation and dynamic breakup of bubble size distribution of liquified natural gas release underwater

With the demand for lower carbon emissions and the increasing use of clean energy instead of traditional fossil fuels, offshore production of liquefied natural gas (LNG) has become a major mode of energy transportation and storage. Due to its cryogenic characteristics, LNG produces special physical phenomena once it leaks from cargo ships or offshore pipelines. To investigate the mechanical behavior and thermal interaction between the jet-released LNG and surrounding ambient water, a set of laboratory experiments with various measurements was designed and developed to perform controllable cryogenic liquid horizontal release under water. The hydrodynamic phenomenon of flow behavior, effect of release dynamics, and instability mechanism of the breakup process caused by the formation of vapor bubbles were observed and quantified under different orifice sizes and release pressures. In addition, a four-stage physical model, established by Raj, that described the breakup dynamics transition from jet liquid to diffusion plume was validated and recorded using a visualized high-speed video camera. Two commonly used mathematical methods, the Rosin-Rammler distribution and lognormal distribution, were used to quantify and evaluate the average bubble sizes in 1.95 ± 0.06 mm, 2.18 ± 0.06 mm, and 2.2 ± 0.1 mm from 1, 3–5 mm, using image processing techniques. The bubble breakup mechanisms and empirical equations were developed and discussed with a complete set of literature data based on a non-dimensional analysis of various release conditions, which was validated with droplet size data obtained from laboratory studies in both gas and liquid co-flowing jets. The −0.38 power-law scaling relationship between the dimensionless length scale of d/lm and the mixture Weber number was established to predict the breakup of the vapor bubble size from subsea release fluids.

Role of Air Bubble Inclusion on Polyurethane Reaction Kinetics.

In this study, we investigated the influence of mixing conditions on the foaming process of water blown polyurethane (PU) foams obtained at different mixing speeds (50, 500, 1000 and 2000 rpm). In particular, the morphological evolution during the foaming process, in terms of the bubble size and bubble density, was studied via optical observations, while the effects on the reaction kinetics were monitored using in situ FTIR spectroscopy. At the slow mixing speed (50 rpm), no air bubbles were included and the early foaming process was characterized by the formation of new bubbles (CO2 nucleation), provided by the blowing reaction. Later on, it was observed that the coalescence affected the overall foaming process, caused by the gelling reaction, which was inhibited by the indigent mixing conditions and could not withstand the bubbles expansion. As a result, a PU foam with a coarse cellular structure and an average bubble size of 173 µm was obtained. In this case, the bubbles degeneration rate, dN/dt, was −3095 bubble·cm−3·s−1. On the contrary, at 500 rpm, air bubbles were included into the PU reaction system (aeration) and no formation of new bubbles was observed during the foaming process. After this, the air bubbles underwent growth caused by diffusion of the CO2 provided by the blowing reaction. As the gelling reaction was not strongly depleted as in the case at 50 rpm, the coalescence less affected the bubble growth (dN/dt = −2654 bubble·cm−3·s−1), leading to a PU foam with an average bubble size of 94 µm. For the foams obtained at 1000 and 2000 rpm, the bubble degeneration was first affected by coalescence and then by Ostwald ripening, and a finer cellular structure was observed (with average bubble sizes of 62 µm and 63 µm for 1000 rpm and 2000 rpm, respectively). During the first foaming stage, the coalescence was less predominant in the bubble growth (with dN/dt values of −1838 bubble·cm−3·s−1 and −1601 bubble·cm−3·s−1, respectively) compared to 50 rpm and 500 rpm. This occurrence was ascribed to the more balanced process between the bubble expansion and the PU polymerization caused by the more suitable mixing conditions. During the late foaming stage, the Ostwald ripening was only responsible for the further bubble degeneration (with dN/dt values of −89 bubble·cm−3·s−1 and −69 bubble·cm−3·s−1, respectively).

Numerical Modeling of Transient Two-Phase Flow and the Coalescence and Breakup of Bubbles in a Continuous Casting Mold.

The multiphase flow and spatial distribution of bubbles inside a continuous casting (CC) mold is a popular research issue due to its direct impact on the quality of the CC slab. The behavior of bubbles in the mold, and how they coalesce and break apart, have an important influence on the flow pattern and entrapment of bubbles. However, due to the limitations of experiments and measurement methods, it is impossible to directly observe the multiphase flow and bubble distribution during the CC process. Thus, a three-dimensional mathematical model which combined the large eddy simulation (LES) turbulent model, VOF multiphase model, and discrete phase model (DPM) was developed to study the transient two-phase flow and spatial distribution of bubbles in a continuous casting mold. The interaction between the liquid and bubbles and the coalescence, bounce, and breakup of bubbles were considered. The measured meniscus speed and bubble diameter were in good agreement with the measured results. The meniscus speed increased first and then decreased from the nozzle to the narrow face, with a maximum value of 0.07 m/s, and appeared at 1/4 the width of the mold. The current mathematical model successfully predicted the transient asymmetric two-phase flow and completely reproduced the coalescence, bounce, and breakup of bubbles in the mold. The breakup mainly occurred near the bottom of the submerged entry nozzle (SEN) due to the strong turbulent motion of the molten steel after hitting the bottom of the SEN. The average bubble diameter was about 0.6 mm near the nozzle and gradually decreased to 0.05 mm from the nozzle to the narrow face. The larger bubbles floated up near the SEN due to the effect of their greater buoyancy, while the small bubbles were distributed discretely in the entire mold with the action of the molten steel jet. Overall, the bubbles were distributed in a fan shape. The largest concentration of bubbles was in the lower part of the SEN and the upper edge of the SEN outlet.

The effect of feed solids concentration on flotation performance using the Reflux Flotation Cell

This study examines the separation performance achieved using the Reflux Flotation Cell (RFC) when treating low economic value coal waste streams over a range of feed solids concentrations. Feed solids concentrations of 1.3, 2.4 and 5.7 wt% were examined using a fine coal suspension of nominally 50 wt% head ash, covering the typical range emerging from the overflow of a desliming hydrocyclone. This work was also extended to include much higher levels of ultrafine gangue through the addition of significant levels of silica, resulting in an extreme feed ash of 97 wt% head ash, the aim being to assess whether these levels of gangue minerals affect hydrophobic particle capture. For the actual plant feed, as the feed solids concentration was increased the flux of hydrophobic particles fed to the RFC increased in a fixed ratio to the flux of hydrophilic particles. These results showed a greater gas flux was required to maintain combustible recovery as the feed flux of hydrophobic particles increased. The relationship between bubble surface coverage and hydrophobic particle recovery was also investigated. Estimates of the average bubble surface coverages, ranging 6–18%, showed a decreasing trend between recovery and surface coverage. By establishing a strong positive liquid bias flux, ∼0.7 cm/s in magnitude, the product ash was maintained within a narrow band of 7.0–11.9 wt% ash across the series of experiments, demonstrating consistent and strong product desliming over the up to 16-fold increase in feed solids concentration.

Effect of gravity conditions on heat transfer and flow fluctuations of liquid hydrogen in a non-isothermal horizontal circular tube

Liquid hydrogen phase transition is a common phenomenon in space missions for space vehicles using low temperature liquid hydrogen as propellant. In this study, a numerical model with coupled RANS solver and VOF/Level-set method was used to simulate the liquid hydrogen phase transition in a non-isothermal horizontal circular tube under different gravity conditions (1g-10−4 g). The gas phase hydrogen produced by evaporation of liquid hydrogen was calculated by Lee model. The statistics of the overall volume, heat flux, mass flow rate, mean velocity of gas phase hydrogen was carried out. The data results shown that the flow fluctuations were strongest under the gravity acceleration of 10−1 g relative to other gravity conditions. The average bubble volume at 10−1 g was the smallest, which was 11.58% smaller than that at 10−3 g condition. The intermittent contact with the tube wall, which leaded to intermittent long bubble and flow resistance, was the main reason.

CFD-modelling of boiling in a heated pipe including flow pattern transition

Flow boiling occurs when a subcooled liquid enters a heating pipe and its temperature near the heating wall exceeds the boiling onset temperature. Bubbles are generated on the heating wall and the more downstream the larger the average bubble size due to progressing evaporation and coalescence. Further downstream, the two-phase flow morphologies may change from bubbly to slug, plug, and annular flow. Since these flow patterns have a great impact on the heat and mass transfer rates, an accurate prediction of them becomes critical. In this work, the recently developed GENeralized-TwO Phase concept (GENTOP) was used for flow patterns transition modelling and their effects on the wall heat transfer during the upward subcooled flow boiling inside a vertical heating pipe. Furthermore, a previously developed mechanistic bubble dynamics model was implemented in the GENTOP framework as a sub-model. This model is based on the force balance on a single growing bubble considering evaporation of the microlayer underneath the bubble, thermal diffusion and condensation around the bubble as well as the dynamic inclination and contact angles. It does not require a recalibration of parameters to predict the bubble dynamics. For implementing this model in the Euler-Euler (E-E) framework an extension of the current nucleation site density and heat partitioning model was required. Eventually, for a generic test case, flow boiling regimes of water in a vertical heating pipe were simulated using ANSYS CFX 18.2.

Microfluidic Investigation of Foam Coarsening Dynamics in Porous Media at High-Pressure and High-Temperature Conditions

Coarsening or Oswald ripening, induced by interbubble gas diffusion, is considered to dominate foam structure evolution in porous media. We present the first study of trapped foam coarsening dynamics under realistic deep reservoir conditions (up to 3200 psi/22 MPa of pore pressure and 100 °C of temperature) in a high-pressure and high-temperature microfluidic system. The findings are expected to help predict foam structure evolution in applications such as enhanced oil recovery and CO2 geological sequestration. It is shown that, in porous media, larger bubbles grow at the expense of smaller bubbles. The growth rate of the average bubble area (⟨a⟩) over time shows a long-term linear increase when ⟨a⟩ is between 1/5 and 1/2 of the average pore size. The foam coarsening kinetics are determined by the liquid film permeability, gas-liquid interfacial tension, and the molar volume of the dispersed phase. In summary, foams prepared with less water-soluble gases (e.g., N2 and air) and lower foam quality show slower coarsening kinetics due to a lower film permeability. Foam coarsening is more sensitive to surfactant concentration (than surfactant type), as it determines the interfacial tension that controls the mass transfer driving force (capillary pressure difference). The transport properties of the dispersed phase depend strongly on its density, which increases with increasing pore pressure and decreasing temperature. At the same experimental conditions, gas CO2 foam shows a 10-fold faster coarsening rate than N2 foam. However, dense (i.e., liquid and supercritical) CO2 foams show a remarkable 20-500-fold reduction in coarsening kinetics compared with gas N2 and CO2 foams due to the significantly reduced mass transfer driving forces. In a sense, trapped CO2 foam can be stronger than N2 foam at high-pressure and high-temperature conditions.

Exploring the Impacts of Conditioning on Proton Exchange Membrane Electrolyzers by In Situ Visualization and Electrochemistry Characterization.

For a proton exchange membrane electrolyzer cell (PEMEC), conditioning is an essential process to enhance its performance, reproducibility, and economic efficiency. To get more insights into conditioning, a PEMEC with Ir-coated gas diffusion electrode (IrGDE) was investigated by electrochemistry and in situ visualization characterization techniques. The changes of polarization curves, electrochemical impedance spectra (EIS), and bubble dynamics before and after conditioning are analyzed. The polarization curves show that the cell efficiency increased by 9.15% at 0.4 A/cm2, and the EIS and Tafel slope results indicate that both the ohmic and activation overpotential losses decrease after conditioning. The visualization of bubble formation unveils that the number of bubble sites increased greatly from 14 to 29 per pore after conditioning, at the same voltage of 1.6 V. Under the same current density of 0.2 A/cm2; the average bubble detachment size decreased obviously from 35 to 25 μm. The electrochemistry and visualization characterization results jointly unveiled the increase of reaction sites and the surface oxidation on the IrGDE during conditioning, which provides more insights into the conditioning and benefits for the future GDE design and optimization.

Bubble‐scale observations of polyurethane foam expansion

AbstractWe describe experiments designed to inform computational models of the dynamic filling process of chemically blown, polyurethane foams, especially subgrid models to predict bubble size affecting foam properties. Three experimental methods are used to observe the evolution of bubble sizes during blowing. Magnified views of bubbles at a transparent wall of a channel are recorded during the foaming. The bubble sizes in the final frame after the expansion has stopped are compared to scanning electron microscope images of the interior of the cured samples to determine wall effects. In addition, diffusing wave spectroscopy is used to determine the average bubble sizes across the width of a similar channel during foam expansion. We conclude that the bubble size distribution is dependent on the formulation of foam being tested, temperature, the height in the foam bar, the proximity to a wall, and the degree of overpacking.

Rheological and morphological characteristics of foam fluid using hydroxypropyl guar and surfactant

In order to systematically study the rheological and morphological characteristics of foam fracturing fluid, hydroxylpropyl guar (HPG) was selected as the foam stabilizer, sodium dodecyl sulfate (SDS) and cetyl trimethyl ammonium bromide (CTAB) were selected as the foaming agents, and foam fracturing fluid used for coalbed methane (CBM) stimulation was prepared. The viscosity, quality and half-life of foam fracturing fluid under different additive concentration levels were tested, and the bubble morphology and its relationship with half-life were observed and quantitatively analyzed with microscopy. In addition, the influence of pulverized coal on foam stability was also studied. The results show that the influence of pulverized coal on the basic parameters of the foam fracturing fluid correlated with the particle size and concentration of the pulverized coal. The small particle size of pulverized coal increased the liquid phase viscosity and half-life of the foam, but reduced the foam quality. With increasing pulverized coal concentration, the effects were enhanced. The average bubble diameter decreased with increasing HPG concentration, and the changing trend of the bubble diameter conformed to the gamma distribution. The foam half-life had a positive and negative correlation with the bubble morphology parameters α and β, respectively. The results can deepen the understanding of the influence of foaming agents, foam stabilizers, and coal particles on the properties of foam fracturing fluids, and provide a reference for the optimization of foam fracturing fluid formulations.

Review of foam stability in porous media: The effect of coarsening

Foam stability in porous media is crucial to industrial applications such as enhanced oil recovery (EOR) and CO 2 geological sequestration. Though many studies of foam stability in porous media have been carried out, the results from these studies often interpreted the combined effects of drainage, coalescence, and coarsening. This convoluted interpretation does not add to the understanding and contribution of the isolated foam instability mechanisms. Coarsening or Ostwald ripening dominates the evolution of foam structure in porous media due to the scale effect. To bridge the knowledge gap, a comprehensive review of the mass transfer fundamentals and the experimental methods for observing the foam coarsening process in porous media was conducted. Unlike an open system where bubbles can grow freely, the restriction from the geometric confinement in the rock (or rock-like) matrix decreases the foam coarsening rate. The confinement can even reverse the mass transport direction to go from larger bubbles to smaller bubbles (anti-coarsening). Various foam coarsening models based on Fick's diffusion law from the bubble-scale to the bulk-scale are reviewed to better understand the foam coarsening mechanisms. From the reviewed literature, foam coarsening dynamics are dominated by the average bubble size, the liquid film thickness between bubbles, and the gas-liquid interfacial properties. The proper selection of the foaming agents (surfactants and nanoparticles) and the gas phase (N 2 and CO 2 ) is critical to control the foam coarsening kinetics. The review of the experimental methods shows that both optical and Atomic Force Microscopy (AFM) are commonly used in studying the foam coarsening process at the bubble scale. X-ray microtomography (μCT) provides a robust tool to investigate the temporal evolution of trapped gases at the pore scale in real rocks. Microfluidics allow seeing the foam kinetics in 2D and 2.5D feature representations of the porous media. The fabrication of 3D micromodels with controlled wettability remains a challenge. Additionally, the challenge of capturing the dynamics of the foam coarsening process with moving bubbles remains understudied and ill-resolved. • Compared with drainage and coalescence, coarsening is the most significant process impacting foam stability in porous media. • Fick's diffusion law-based coarsening models are introduced from bubble-scale to bulk-scale. • The selection of the foaming agents and the gas phase (N 2 and CO 2 , etc.) is critical to the coarsening kinetics of foams. • A micromodel is a robust visualization tool of the foam structure evolutionin porous media.

Hydrodynamics of an Airlift Column for Aeration in Molten Sulfur

The airlift column is a promising technology for the removal of volatile gas from high-viscosity molten sulfur. However, a detailed analysis is lacking on the hydrodynamic properties inside the column, due to the difficulty in flow behavior detection in the opaque molten sulfur. In this work, we adopted the computational fluid dynamics simulation to understand the hydrodynamic behaviors in an airlift column for molten sulfur aeration. In addition, we analyzed the impacts of the superficial gas velocity (UGr) and column height on the hydrodynamic characteristics, such as gas holdup, average bubble diameter, and liquid circulation velocity (ULr) in the column. The simulation shows that at a constant column height of 15 m, an increase on gas holdup can be obtained with the increase of the superficial gas velocity, while the bubble diameter remains almost constant. Once the superficial gas velocity exceeded 0.333 m/s, the liquid circulation velocity increased slowly. With a variation on the column height from 5 to 25 m, a negligible change on gas holdup, but an obvious increase on liquid circulation velocity and bubble diameter is observed at the given superficial gas velocity of 0.0389 m/s. Furthermore, the simulation shows a similar trend, but with considerably more detailed information, on the relationship between the gas holdup and liquid circulation velocity when compared to the predictions from the Chisti correlation (1988) and an optimized correlation proposed in this work.

Flow aeration by an offset aerator: Air entrainment, bubbly flow turbulence, and adaptive-window cross-correlation processing

Chute aerators are used to induce flow aeration to prevent cavitation damages to flood discharge structures. The aerated chute flows are traditionally studied in terms of the air concentration distributions and air entrainment rate, whereas further detailed characterization of the bubbly flow associated with the turbulence modification has been missing. Herein the air–water flow turbulence was studied experimentally for a simple offset aerator model in a sloping chute. Advanced interfacial turbulence analyses were applied to the air–water flow measurement results using a dual-tip phase-detection probe. The new adaptive-window cross-correlation approach was first applied to the turbulence intensity estimate in both regions upstream and downstream the impact point. The overall spatial distributions of basic air–water flow properties agreed favorably with literature, and highlighted different patterns of air–water flow development in different sections of the flow. The turbulence level of the bubbly flow, and the bubble-turbulence interactions, were quantitatively described with the interfacial turbulence intensity, correlation turbulent length scale, and some measures of the bubble clustering behaviors. The interfacial turbulence intensity was obtained in the same order of magnitude as the common water-phase turbulence intensity observations, except for the impact zone. The results revealed the shear flow development and turbulence dissipation in the bottom layer of the aerated chute flow, which affected the local average bubble counts, the relative motions of neighboring bubbles, the diffusive transport of bubbly eddy structures, and consequently the near-wall bubble distributions critical for the protection of chute surface from cavitation erosion.

Implementation of a Multi-cell Approach in the Multi-Physics Calculations of an Aqueous Homogeneous Reactor

Nowadays, the Aqueous Homogeneous Reactor (AHR) technology is under study for its use in the production of medical isotopes. This technology has proven to be potentially advantageous for this purpose due to its low cost, small critical mass, inherent passive safety, and simplified fuel handling, processing, and purification characteristics. Among the studies being carried out is computational modeling and simulation, which represents a key technology in the pursuit for improvements in efficiency, safety, and reliability of these systems. This paper aims to expand upon a previous AHR computational model through the implementation of a multi-cell approach for the improvement of the calculations using a methodology for the multi-physics and multi-scale coupling of the neutronic and thermal-hydraulic codes. It was found that these additions to the original model cause a small change to the overall reactor behavior. Thermal-hydraulic parameters such as, average fuel solution temperature and velocity, gas volume fraction and average radiolytic gas bubbles velocity undergo a variation of 0.161 °C, 0.0009 m/s, 0.015% and 0.0003 m/s. In contrast, significant local differences were obtained mainly for the fuel solution temperature and radiolytic gas bubbles volume fraction. It was verified that a simplified AHR computational model consisting of a 20° section of the fuel solution is able to adequately reproduce the results of the full AHR computational model.