Morphology Of Gas Bubbles Research Articles

Integrated simulation of U-10Mo monolithic fuel swelling behavior

A separate computational branch has been implemented within the DART (Dispersion Analysis Research Tool) computational code to simulate the swelling behavior of U-10Mo monolithic fuel under the operating conditions of high-power research and test reactors (RTRs). The monolithic branch of the DART code implements a mechanistic rate-theory-based fission-gas-behavior model for the calculation of fission gas swelling, as well as a suite of thermal, physical, and mechanical models to consider various processes occurring in RTR fuels during irradiation. To accurately simulate and eventually predict U-10Mo monolithic fuel irradiation behavior, the code uses materials properties calculated with lower length-scale computational methods, such as gas atom diffusivity and U-Mo surface energy from atomic simulations and grain-morphology-specific recrystallization kinetics (recrystallized fuel volume fractions versus fission density) predicted using the phase-field method. The remainder of the fission gas behavior parameters used in the model were calibrated with measured intergranular bubble size distributions. With this integrated simulation approach, the swelling behavior of U-10Mo monolithic fuel was simulated for various initial grain sizes at different operating conditions and compared with measured data. Furthermore, because limited experimental data exist for parameter calibration, detailed sensitivity studies for the important parameters used in the fission gas behavior model were performed to examine their impact on both intergranular gas bubble morphology at low fission density, and on total porosity at high fission density.

A level set approach to Ostwald ripening of trapped gas bubbles in porous media

Ostwald ripening of gas bubbles is a thermodynamic process for mass transfer, which is important for both foam enhanced oil recovery and geological CO2 storage. We present a methodology for simulating Ostwald ripening of gas ganglia surrounded by liquid in arbitrary pore geometries. The method couples a conservative level set model for capillary-controlled displacement and a ghost-bubble technique that calculates mass transfer based on difference in chemical potentials. The methodology is implemented in a software framework for parallel computations. As a validation of the model, we show that simulations of bubble ripening in a pore throat connecting two pore bodies are consistent with previously reported trends in similar geometries. Then we investigate the impact of gas type, compressibility factor, and local capillary pressure on gas-bubble ripening in various water-wet pore geometries. The results confirm that gas solubility and compressibility factor are proportional to the rate of mass transfer. Our simulations suggest that Ostwald ripening has largest impact in heterogeneous or fractured porous structures where differences in gas-bubble potentials are high. However, if the liquid separating the gas bubbles is also a disconnected phase, which can happen in intermediate-wet porous media, the resulting local capillary pressure can limit the coarsening and stabilise smaller bubbles. Finally, we simulated Ostwald ripening on a 3-D pore-space image of sandstone containing a residual gas/water configuration after imbibition. Characterization of gas-bubble morphology during the coarsening shows that large ganglia get more ramified at the expense of small spherical ganglia that cease to exist.

Gas-Graft Coverage After DMEK: A Clinically Validated Numeric Study.

PurposeWe investigate the influence of positioning, gas fill, and anterior chamber size on bubble configuration and graft coverage after Descemet's membrane endothelial keratoplasty (DMEK).MethodsWe use a mathematical model to study the bubble shape and graft coverage in eyes of varying anterior chamber depths (ACD). The governing equations are solved numericly using the open source software OpenFOAM. Numeric results are validated clinically so that clinical gas fill measures can be correlated with numeric results providing gas-graft coverage information otherwise clinically inaccessible.ResultsIn a phakic eye (ACD = 2.65 mm) with a gas fill of 35%, graft contact ranged from 35% to 38% depending on positioning and increased to 85% to 92% with a 70% fill. In contrast, positioning of a pseudophakic eye (ACD = 4.35) with a gas fill of 35% results in graft contact ranging from 8% to 52%, increasing to 63% to 94% with a 70% fill. The mathematical model demonstrates negligible differences between air and SF6 results and interestingly, a very thin central patch of aqueous humor within the gas bubble is found in some cases.ConclusionsGraft coverage in phakic eyes (ACD ≤ 3 mm) is dominated by the gas fill and less sensitive to patient positioning. In pseudophakic eyes with larger values of ACD, the graft coverage depends on gas fill and patient positioning with positioning even more important as ACD increases.Translational RelevanceAnterior chamber depth markedly influences the role of patient positioning in gas-filled eyes after DMEK due to the interplay between anterior chamber anatomy and gas bubble morphology.

A three-dimensional numerical study on the dynamics and deformation of a bubble rising in a hybrid Carreau and FENE-CR modeled polymeric liquid

New results are reported on the simulation of the rise of a deforming gas bubble surrounded by a liquid exhibiting a blend of shear-thinning and viscoelastic properties. A new hybrid model has been developed which combines the Carreau model for emulating the shear-thinning property of the liquid with the Chilcott–Rallison (FENE-CR) model for treating the viscoelastic property of the liquid. The new hybrid model allows one to independently prescribe the shear-thinning and viscoelastic properties (parameters) to be used in a simulation. Computations are implemented using the coupled level-set and volume-of-fluid (CLSVOF) method for tracking the deforming gas-liquid interface. The interfacial jump conditions are enforced with the sharp interface approach. In this study, four parameters, the Eötvös number, Carreau model parameter for slope of decreasing viscosity, Deborah number, and viscoelastic polymer dumbbell length parameter, are varied in order to gain understanding for gas bubble morphology in liquids exhibiting both shear-thinning and viscoelastic properties. It is found that the extent of bubble deformation and the liquid stress field are strongly dependent on these 4 parameters with minimal correlation between the shear-thinning and viscoelastic parameters. The ability to vary shear-thinning and viscoelastic parameters independently of one another in the new hybrid Carreau and FENE-CR model enables one to conveniently predict gas bubble behavior in complex liquid polymers by parameterizing the new model from basic measurements of the polymers.

Pulse Efficiency and Gap Status of Rotary Ultrasonic Assisted Electrical Discharge Machining and EDM Milling

This study proposed an ultrasonic assisted (UA-EDM) and rotary ultrasonic assisted EDM (RU-EDM) spindle with ultrasonic transducer for die sinking EDM and EDM milling processes. The research includes investigation of the ultrasonic amplitude effects, spindle revolution speed, peak current, feed-rate control of EDM on the removal rate and tool wear rate for machining of SKD 61 steel. The significant factors that affect the removal efficiency and the tool wear are further verified. Especially, the fundamental studies of the gap status including gas bubble morphology and movement, debris expelling efficiency and waveform of voltage and current pulses are also investigated through experiments with a transparent observation window. Experiment results reveal that RU-EDM is superior to conventional EDM in removal rate in both sinking and milling EDM, but with the cost of a larger tool wear rate. Explanation from gap status and characteristics of the EDM pulse trains with and without rotary ultrasonic waves are compared.

First-principles study of the surface properties of U-Mo system

U-Mo alloys are promising fuels for future high-performance research reactors with low enriched uranium. Surface properties, such as surface energy, are important inputs for mesoscale simulations (e.g., phase field method) of fission gas bubble behaviors in irradiated nuclear fuels. The lack of surface energies of U-Mo alloys prevents an accurate modeling of the morphology of gas bubbles and gas bubble-induced fuel swelling. To this end, we study the surface properties of U-Mo system, including bcc Mo, α-U, γ-U, and γ U-Mo alloys. All surfaces up to a maximum Miller index of three and two are calculated for cubic Mo and γ-U and non-cubic α-U, respectively. The equilibrium crystal shapes of bcc Mo, α-U and γ-U are constructed using the calculated surface energies. The dominant surface orientations and the area fraction of each facet are determined from the constructed equilibrium crystal shape. The disordered γ U-Mo alloys are simulated using the Special Quasirandom Structure method. The (1 1 0) and (1 0 0) surface energies of γ U-7Mo and U-10Mo alloys are predicted to lie between those of γ-U and bcc Mo, following a linear combination of the two constituents’ surface energies. To better compare with future measurements of surface energies, the area fraction weighted surface energies of α-U, γ-U and γ U-7Mo and U-10Mo alloys are also predicted.

3D Tomography Analysis of the Inner Structure of Pebbles and Pebble Beds

AbstractAn analytical tool to monitor the arrangement of pebbles in a pebble bed as well as the morphology of gas bubbles in as fabricated and neutron irradiated beryllium pebbles is presented. The context of this study is the Helium Cooled Pebble Bed (HPCB) blanket design for the forthcoming generation of fusion reactors. The thermal‐mechanical behavior of pebble beds is a basic issue for the HPCB. It depends strongly on the configuration of the pebbles in the bed, and in particular on the number of contacts between pebbles, and between pebbles and the blanket walls. The related contact surfaces play also a major role. The knowledge on the inner structure of the pebbles is required since during the life cycle of a power reactor helium and tritium bubbles are produced inside the beryllium pebbles and the tritium build‐up can be in excess of several kilograms, being thereby a key safety issue. All the non‐destructive analyses are based on 3D computer aided microtomography using a very powerful synchrotron radiation x‐ray source with high spatial resolution. The data analysis relies on a topological operator called filtered medial line applied to the entire data volumes and the related graph representation. By this technique the number of contacts between the pebbles in pebble packs and their angular distribution are obtained, as well as the corresponding contact surfaces. The evaluation of bubble sizes and densities in single pebbles, the assessment of the pore channel network topology, the 3D reconstruction of the fraction of interconnected bubble porosity, and the open‐to‐closed‐porosity ratio are among the most interesting findings.

Oil chemistry and its impact on heavy oil solution gas drive

A series of laboratory depletion experiments are conducted at reservoir temperatures with three different live crude oils, as well as mineral oils. These tests examine the role of oil composition on the heavy-oil solution gas drive process. The morphology of gas bubbles, pore pressure, critical gas saturation and oil recovery is compared. Core-level depletion indicates that oil composition plays a role in determining metastability of dispersed gas bubbles in foamy oil. Generally, systems that exhibit effluent gas bubbles on the order of the size of pores tend to recover more oil, all other factors being equal. The concentration of organic acid and base groups as well as asphaltene content of crude oil is measured to characterize each crude-oil sample and develop an understanding of the influence of oil chemistry. Results suggest that significant asphaltene content as well as substantial acid number (the amount of potassium hydroxide in milligrams that is needed to neutralize the acid groups in one gram of crude oil) and base number (the amount of potassium hydroxide in milligrams needed to neutralize the acid titrant for one gram of crude oil) are indicators of whether oil is foamy. The partitioning of acid and base groups between the asphaltene fraction and deasphalted oil is also studied. Organic acid and base groups are clearly present in the asphaltene fraction. The role of such functional groups on oil foam stability is investigated by measuring the lifetime of single foam films formed from crude oil and asphaltene solutions. Transparent micromodels etched with a sandstone pore network and containing gas dispersed within the oil are also used to investigate the correlation of acid number, base number, and asphaltene content on gas-bubble coalescence. The results show that a high concentration of asphaltene that exhibits acid and base functional groups tends to increase foamability and film lifetime of gas/crude-oil dispersions. The deasphalted fraction is not foamy despite possessing significant acid and base number. We conclude that acid and base groups within asphaltene, and their interaction at the gas–oil interface, are a source of interfacial stability.