Number Of Bubbles Research Articles

Charged Cavitation Multibubbles Dynamics Model: Growth Process

The nonlinear dynamics of charged cavitation bubbles are investigated theoretically and analytically in this study through the Rayleigh–Plesset model in dielectric liquids. The physical and mathematical situations consist of two models: the first one is noninteracting charged cavitation bubbles (like single cavitation bubble) and the second one is interacting charged cavitation bubbles. The proposed models are formulated and solved analytically based on the Plesset–Zwick technique. The study examines the behaviour of charged cavitation bubble growth processes under the influence of the polytropic exponent, the number of bubbles N, and the distance between the bubbles. From our analysis, it is observed that the radius of charged cavitation bubbles increases with increases in the distance between the bubbles, dimensionless phase transition criteria, and thermal diffusivity, and is inversely proportional to the polytropic exponent and the number of bubbles N. Additionally, it is evident that the growth process of charged cavitation bubbles is enhanced significantly when the number of bubbles is reduced. The electric charges and polytropic exponent weakens the growth process of charged bubbles in dielectric liquids. The obtained results are compared with experimental and theoretical previous works to validate the given solutions of the presented models of noninteraction and interparticle interaction of charged cavitation bubbles.

Artificial cavernosa-like tissue based on multibubble Matrigel and a human corpus cavernous fibroblast scaffold.

Ex vivo tissue culture of the human corpus cavernosum (CC) can be used to explore the tissue structural changes and complex signaling networks. At present, artificial CC-like tissues based on acellular or three-dimensional (3D)-printed scaffolds are used to solve the scarcity of primary penis tissue samples. However, inconvenience and high costs limit the wide application of such methods. Here, we describe a simple, fast, and economical method of constructing artificial CC-like tissue. Human CC fibroblasts (FBs), endothelial cells (ECs), and smooth muscle cells (SMCs) were expanded in vitro and mixed with Matrigel in specific proportions. A large number of bubbles were formed in the mixture by vortexing combined with pipette blowing, creating a porous, spongy, and spatial structure. The CC FBs produced a variety of signaling factors, showed multidirectional differentiation potential, and grew in a 3D grid in Matrigel, which is necessary for CC-like tissue to maintain a porous structure as a cell scaffold. Within the CC-like tissue, ECs covered the surface of the lumen, and SMCs were located inside the trabeculae, similar to the structure of the primary CC. Various cell components remained stable for 3 days in vitro , but the EC content decreased on the 7 th day. Wingless/integrated (WNT) signaling activation led to lumen atrophy and increased tissue fibrosis in CC-like tissue, inducing the same changes in characteristics as in the primary CC. This study describes a preparation method for human artificial CC-like tissue that may provide an improved experimental platform for exploring the function and structure of the CC and conducting drug screening for erectile dysfunction therapy.

Predicting shock-induced cavitation using machine learning: implications for blast-injury models.

While cavitation has been suspected as a mechanism of blast-induced traumatic brain injury (bTBI) for a number of years, this phenomenon remains difficult to study due to the current inability to measure cavitation in vivo. Therefore, numerical simulations are often implemented to study cavitation in the brain and surrounding fluids after blast exposure. However, these simulations need to be validated with the results from cavitation experiments. Machine learning algorithms have not generally been applied to study blast injury or biological cavitation models. However, such algorithms have concrete measures for optimization using fewer parameters than those of finite element or fluid dynamics models. Thus, machine learning algorithms are a viable option for predicting cavitation behavior from experiments and numerical simulations. This paper compares the ability of two machine learning algorithms, k-nearest neighbor (kNN) and support vector machine (SVM), to predict shock-induced cavitation behavior. The machine learning models were trained and validated with experimental data from a three-dimensional shock tube model, and it has been shown that the algorithms could predict the number of cavitation bubbles produced at a given temperature with good accuracy. This study demonstrates the potential utility of machine learning in studying shock-induced cavitation for applications in blast injury research.

Study on the Method of Coating Defect Elimination and Effect on Mechanical Properties

The casting method in coating preparation has the advantages of short sample preparation period, small raw material loss, low cost, but the paint film often has bubble defects.The defects were characterized by microscope observation and density balance test. The influence of solvent content and ultrasonic shock on coating defects and the influence of defects on coating mechanical properties were investigated. The results show that the density decreases linearly with the increase of the number of bubbles. With the increase of solvent, the viscosity of paint decreases and the content of bubbles decreases obviously. As solvent increased, coating drying time increased, sample preparation period increased.After ultrasonic shock, bubbles in the surface of the paint and the cured coating were significantly reduced, compared with spraying sample preparation, ultrasonic shock sample preparation period was shortened by more than 30%.The samples with defective mechanical properties were broken during bubble emergence, the tensile strength and elongation at break were significantly reduced, and the dispersion was large, which was invalid data.

Coconut oil as an alternative to butter and shortening in bread making.

The characteristics of bread prepared with coconut oil were investigated to determine whether it can be used as an alternative to butter and shortening. Loaf height of the bread increased by adding butter and shortening water content of bread containing oils and fats was lower than that without oils and fats, and baking loss increased with decreasing water content. The addition of oils and fats influenced the baking color of bread and hindered the hardening of bread samples over time. Moreover, the addition and type of oils and fats influenced the crust density of bread samples and dough expansion. Furthermore, numerous fine bubbles were present in bread samples without oils and fats, whereas the size and number of bubbles increased and decreased in bread samples containing oils and fats, respectively. The band concentrations of insoluble proteins at approximately 39, 41, and 48kDa in freeze-dried bread samples without oils and fats were significantly lower than those containing oils and fats. Thirty volatile compounds were detected in all bread samples tested, and the number was high in the following order: bread samples with butter, shortening, and coconut oil, and without oils and fats. However, sensory evaluation showed no significant differences among all bread samples tested. Therefore, it was suggested that bread containing coconut oil had the same characteristics as that containing butter and shortening. PRACTICAL APPLICATION: Butter and shortening are usually used in bread making, although bread prepared with coconut oil can possess the same characteristics as that containing them. Therefore, this study evaluated the characteristics of bread prepared with coconut oil and revealed that use of coconut oil enabled a vegan bread with reduced environmental impact because coconut oil is a vegetable-derived oil that does not require the cutting of tropical rainforests.

Demonstration of Helide formation for fusion structural materials as natural lattice sinks for helium

Fusion power holds promise as an ultimate energy source. However, achieving true sustainability in fusion energy requires addressing the embrittlement of polycrystalline materials in fusion reactors caused by helium, which leads to premature failure, often within a year. Here we experimentally demonstrate that nanodispersoids with constitutional vacancy-like atomic-scale free volume can securely store helium, not only at the matrix-dispersoid interface but also within their bulk lattices, which suggests their effectiveness in delaying critical helium damage of the polycrystalline matrix. The selected model nano-heterophase, fayalite Fe2SiO4, possesses moderately strong lattice sinks for helium while undergoing lattice distortions upon helium absorption. These distortions cause observable changes in peak intensities of X-ray diffraction (XRD) patterns, distinct from changes resulting from other factors like radiation damage. By comparing grazing incidence XRD patterns with ab initio computed patterns, we show that such nano-heterophases can store up to ∼10 at% helium within their bulk lattice, forming a “helide compound.” Incorporating just 1 vol% of Fe2SiO4 reduces helium bubble size and number density by >20 % and >50 %, respectively. These findings suggest that 1–2 vol% of appropriate nano-heterophases can accommodate a few thousand appm of bulk helium, expected to be generated over a 10-year operational period.

Measurement of void fraction in gas-water bubbly flow using a derived multi-eigenvalue sequence from normalized EIT impedance matrix

Void fraction is one of the dominant parameters of gas-water two-phase flow. Its accurate measurement plays an important role in achieving parameter control and reliable operation in industrial processes. This article proposes a more practical method for the measurement of void fraction in gas-water bubbly flow using a derived multi-eigenvalue sequence from a normalized electrical impedance tomography impedance matrix. The relations between eigenvalues and void fraction, bubble radius, number of bubbles are investigated by numerical simulations, which illustrates the superiority of using multi-eigenvalue rather than the largest eigenvalue for void fraction prediction. The nonlinear mapping between the multi-eigenvalue sequence and void fraction is established by applying the XGBoost model with a sliding window of time series. This proposed method is verified by static and dynamic experiments using a self-developed setup in our laboratory, generating stable gas-water bubbly flow with void fraction of less than 0.12. It is shown that the proposed method can predict void fraction with a relative deviation of 10%. Compared with the conventional method based on the largest eigenvalue, the proposed method efficiently improves the measurement accuracy of void fraction in gas-water bubbly flow and applicability in actual measurement of two-phase flow, which can be further extended to other flow regimes.

Construction of super hydrophobic paper by modified nano-SiO2 hybrid medium fluoro-epoxy polymer and its properties

Most of the super hydrophobic paper surfaces prepared at present will change the original properties of the paper and can not be used for oil-water separation continuously. There is not yet much systematic research on super hydrophobic modification of paper-based materials. The use of polymers and inorganic fillers for synergistic modification is also relatively rare. In this paper, fluorine-containing epoxy polymer P(GMA-r-DFMA) and KH550 modified nano-SiO2 were mixed to obtain an organic-inorganic hybrid super hydrophobic solution, which was chemically cross-linked to the surface of paper fibers to form a micro- and nano-thin coating to construct a super hydrophobic surface, but the original properties of the paper remain unchanged. The optimal parameters (polymer concentration, soaking time, drying time, drying temperature) for building super hydrophobic surface on paper were explored, and the optimal conditions are the polymer concentration of 7 mg/mL, soaking time of 5 h, drying time of 6 h, drying temperature of 100 °C. Under the optimum conditions, the water contact angle of the modified paper surface is 164 ± 2o and the water rolling angle is 5 ± 2o. The surface of the super hydrophobic modified paper contaminated with toner can be self-cleaned by water droplets. Compared with the unmodified paper, the whiteness, smoothness and glossiness of the surface of the super hydrophobic modified paper have not changed greatly. When the oils were n-hexane, n-octane and trichloromethane, the oil-water separation efficiency of the modified paper could reach 99.9 %, 99.9 % and 98 %, and the effects were stable. Super hydrophobic modified paper in water formed a large number of bubbles on its surface, resulting in silver mirror phenomenon. SEM results showed that the pore structure of the paper was not changed after modification. The surface of the original paper fiber is very smooth, but the surface of the super hydrophobic modified paper fiber has a large number of micro and nano protrusions.

Outgassing behaviour during highly explosive basaltic eruptions

AbstractExplosivity of basaltic eruptions is related to the efficiency in which exsolved gas can separate from the melt during ascent, which is controlled by magma permeability. However, basaltic pyroclasts from eruptions of varying explosivity can show similar permeability, indicating a possible complex relationship between permeability, outgassing and eruptive style. Here, we provide 3D measurements of basaltic pyroclasts using X-ray microtomography. We investigate the role of permeability and outgassing on magma ascent dynamics by using a numerical conduit model. Among the permeable parameters, bubble number density and friction coefficient largely affect explosivity. However, for fast ascending basaltic magmas, gas-melt coupling is maintained independent of magma permeability. In this case, magma storage conditions may determine eruptive style, driving rapid magma ascent, crystallisation and bubble nucleation, producing a highly explosive eruption. Monitoring parameters which reveal pre-eruptive conditions may assist hazard mitigation, particularly for basaltic systems which exhibit a wide range in eruptive style.

Experimental study on bubbles behaviors in pure water flash evaporation process

Experimental observations were conducted on the phenomenon of flash evaporation. The bubbles appear and grow due to the liquid becomes superheated by a sudden decrease in pressure. During experiments the pure water is flashed at an initial temperature range of 55–90 ℃, an initial liquid film height range of 10–60 mm, and a superheat range of 2–22 K. The spatial and temporal distribution of the bubbles are analyzed during the flashing. And the distribution of bubbles under different superheat and the photo of the behaviors of bubbles were studied. The flashing evaporation can be divided into three stages according to the rate of the bubbles number change and the NEF(non-equilibrium fraction) can express the rate of bubble number at different stages. The number of bubbles is affected by the difference of enthalpy under the same volume of the liquid.

A model of coupled oscillation of bubble cluster in liquid cavity wrapped by viscoelastic medium

Considering the interactions between bubbles in a multi-bubble system in a liquid micro-cavity, a spherical bubble cluster in a liquid cavity is modeled in order to describe the dynamical effect of the viscoelastic medium outside the liquid cavity on the oscillation of bubbles, and the coupled equations of bubbles are obtained. Subsequently, the acoustic response characteristics of bubbles are investigated by analyzing the radial oscillation, the stability of the non-spherical shape of bubbles and the threshold of inertial cavitation. The results show that the confinement of the cavity and the bubble cluster facilitates the suppression of bubble oscillation, however, it might enhance the nonlinear properties of bubbles to a certain extent. From the acoustic response curve at 1 MHz, it is found that the main resonance peaks shift leftward with the increase of the bubble number, which means a minor resonant radius can be obtained. The nonlinear stability of bubbles in a confined environment is mainly determined by acoustic pressure amplitude and frequency, the initial bubble radius, and bubble number density, while the effect of the cavity radius is enhanced with the increase of the driving pressure. There is a minimum unstable driving acoustic pressure threshold, depending on the initial bubble radius, and the unstable regions are mainly located in a range of less than 4 μm. With the increase in bubble number density, the strip-type stable region scattered of the unstable region in the map is gradually transformed into a random patch-like distribution, which indicates that the bubble oscillation under high acoustic pressure is more sensitive to the parameters, and it is very susceptible to interference, produces unstable oscillation and then collapses. When the bubble equilibrium radius is in a range greater than 4 μm, the influences of frequency and bubble number density on the inertial thresholds are particularly significant.

Structural stability analysis of spherical bubble clusters in acoustic cavitation fields

The upwelling growth and evolution of spherical bubble clusters appearing at one-quarter wavelength from the water surface in ultrasonic cavitation fields at frequencies of 28 kHz and 40 kHz are studied by high-speed photography. Due to the interactions among bubbles, the stable bubble aggregation occurs throughout the rise of the bubble cluster, whose vertical pressure difference leads to a more significant spreading in the upper part of the cluster in the standing-wave field. At 28 kHz, the rising speed is about 0.6 m/s, controlled by the primary acoustic field. After a violent collapse of the bubble clusters, the aggregating structure begins to hover near the water surface. The size and stability of the structure are affected by the frequency and pressure of the primary acoustic field. If two clusters are close to each other, the clusters deviate from the spherical shape, even trailing off, and eventually merge into a single bubble cluster. By considering the influence of water-air boundary, based on the mirror principle, a spherical bubble cluster model is developed to explore the structure stability of the clusters, and the modified dynamics equations are obtained. The effects of driving acoustic pressure amplitude, bubble number density, water depth, and bubble equilibrium radius on the optimal stable radius of the spherical bubble cluster are numerically analyzed by using the equivalent potentials at 28 kHz and 40 kHz. The results show that the optimal stabilizing radius of spherical bubble cluster is in a range of 1–2 mm, and it tends to decrease slightly with the increase of the driving acoustic pressure and bubble number density. It is worth noting that the nonlinearity is enhanced by increasing acoustic pressure, which may promote the stability of the cluster structure. The smaller the unstable equilibrium radius, the easier it is to grow, and the stable size at 40 kHz is slightly smaller than that at 28 kHz. Generally, spherical clusters first appear in a high-pressure region and then move to a low-pressure region. If the acoustic pressure drops below a certain critical value, bubble clusters disappear. The theoretical analysis is in good agreement with the experimental observation. The analysis of the growth and structural stability of spherical bubble cluster is helpful in understanding the behavioral modulation of bubbles.

Computational analysis on fluid flow characteristics in a modified annular dump combustor of a ship

The improvement of systems of aeronautical and marine propulsion able to meet the forthcoming drastic certification requirements on the reduction of the level of pollutants emission heavily relies on the development of new concepts of combustion chamber technologies. In view of this, a novel numerical investigation has been performed on the basis of characteristics of the flow of a fluid passing over a simple annular-shaped dump combustor and a modified dump combustor with certain central restrictions like rectangular and spherical shapes. The restrictions are located at a fixed distance of 0.14 m from the inlet. Flow characteristics and patterns are presented sequentially in the form of streamline contours, velocity contour, pressure contour, axial velocity profiles, and centerline velocity profiles. The above-mentioned flow characteristics are studied at several locations in the flow field for various Reynolds numbers (Re), ranging from 103 to 106 for all considered models. The flow regulating mass and momentum conservation partial differential governing equations along the x-axis and y-axis are discretised with the help of the control volume-based finite volume approach. The SIMPLE algorithm and second-order upwind scheme are used to solve iteratively the discretised equations for the case of a fixed aspect ratio of 3 and for a central abstraction of 120 %. The analysis corroborates and concludes that more numbers of bubbles are formed in a geometrically modified dump combustor than in a plain dump combustor. Additionally, it has been found that the number of recirculating bubbles increases along with the value of the Reynolds number. Consequently, the core recirculation zone is improved, which ensures the desirable mixing of fluids in the ship combustor and helps significantly in the reduction of NOx emission and combustion residuals. Furthermore, as the Reynolds number (Re) increases, the variation in axial velocity also raises for all considered Reynolds numbers, indicating an improved chance of better mixing. At a Re of 106, the axial velocity profile reaches its peak for both the modified combustors, although at different locations. Specifically, the magnitude of the axial velocity profile is 1.05 at the position of 0.14 m for the rectangular restriction and 0.10 m for the spherical restriction. Consequently, a higher Reynolds number may be recommended for optimal mixing.

Effect of Shock-Wave-Mediated Collapse on Nanobubble Characteristics.

To enhance the comprehension of the cavitation mechanism and explore its practical use in industrial production, this study developed models involving oxygen, varying bubble radii, and bubble quantities. This study uses molecular dynamics simulations coupled with the momentum mirror method to examine the collapse characteristics of bubbles during ultrasonic cavitation. The investigation uncovers patterns in the fluctuation of the maximum local density of water molecules, released pressure, and temperature. The findings demonstrate that, when oxygen-containing bubbles collapse at identical radii, the local density is notably higher and diminishes more rapidly. Moreover, the changes in the shape exhibit greater regularity. During the bubble collapse, a depression forms on the bubble's surface, coinciding with a notable surge in local density around the depression. As bubble radii and quantities increase, so does the local density along with a concurrent rise in the maximum pressure. Intriguingly, the model demonstrates the lowest pressure at Z = 35 Å accompanied by the emergence of a small crescent-shaped region with a reduced density. Throughout the pressure ascension phase, the rate of the maximum pressure change escalates with an increase in the number of bubbles. Conversely, during the pressure descent phase, the rate of the maximum pressure change diminishes with a growing number of bubbles. However, it is important to note that the maximum pressure does not exhibit a direct correlation with the number of bubbles. Ultimately, this study provides valuable technical guidance and a theoretical foundation for the integration of ultrasonic cavitation in industrial production processes.

Dynamic modeling of cavitation bubble clusters: Effects of evaporation, condensation, and bubble–bubble interaction

We present a dynamic model of cavitation bubbles in a cluster, in which the effects of evaporation, condensation, and bubble–bubble interactions are taken into consideration. Under different ultrasound conditions, we examine how the dynamics of cavitation bubbles are affected by several factors, such as the locations of the bubbles, the ambient radius, and the number of bubbles. Herein the variations of bubble radius, energy, temperature, pressure, and the quantity of vapor molecules are analyzed. Our findings reveal that bubble–bubble interactions can restrict the expansion of bubbles, reduce the exchange of energy among vapor molecules, and diminish the maximum internal temperature and pressure when bursting. The ambient radius of bubbles can influence the intensities of their oscillations, with clusters comprised of smaller bubbles creating optimal conditions for generating high-temperature and high-pressure regions. Moreover, an increase in the number of bubbles can further inhibit cavitation activities. The frequency, pressure and waveform of the driving wave can also exert a significant influence on cavitation activities, with rectangular waves enhancing and triangular waves weakening the cavitation of bubbles in the cluster. These results provide a theoretical basis for understanding the dynamics of cavitation bubbles in a bubble cluster, and the factors that affect their behaviors.

Attempts to Electrochemically Synthesize Palladium Superhydrides By High Pressure Method – Combination of Electrolytic Hydrogen Charging and Electroplating of Protective Coatings –

IntroductionIncited by the possibility of room-temperature superconductors in superhydrides, numerous them have been synthesized using a diamond anvil cell under high pressures up to several hundred GPa. Although palladium has widely researched for a long time as a prototype hydrogen-absorbing metal, the palladium superhydrides has not yet been obtained under such high pressures. A recent theorical calculation has predicted that the palladium superhydrides (e.g., PdH10) may be synthesized by combing electrolysis and high pressure1), but it is not certain whether it can be achieved. In this study, we attempted to synthesize the palladium superhydrides by using electrolytic hydrogen charging under high pressures, and subsequently applied protective coatings to suppress the hydrogen desorption from the palladium hydrides at ambient pressure.ExperimentalFigure shows a schematic illustration of the high-pressure apparatus. The high-pressure apparatus can apply the pressure of up to 400 MPa to the electrolyte in the electrolytic cell by pumping water into a sealed high-pressure vessel using a plunger pump and pressure multipliers. A cold-rolled palladium foil of 30 ~ 40 μm in thickness was used as a cathode, and a zinc plate was utilized as a soluble anode. Hydrogen was electrochemically loaded into the palladium foil at 0 V vs. Zn in an electrolyte consisting of 0.1 mol dm-3 sulfuric acid and 4.6 mmol dm-3 zinc sulfate under various pressures for 6 hours. Subsequently, the palladium foil was coated with the zinc film at constant current density 25 mA cm-2 for 120 seconds under the high pressure. The specimen was removed from the electrolytic cell in the high-pressure vessel at ambient pressure, and the hydrogen concentration of the palladium hydride PdH x was determined by thermal desorption spectroscopy. Structural analysis was conducted using X-ray diffraction and scanning electron microscopy.Results and discussionA number of bubbles were observed on the palladium foil surface during the electrolytic charging at ambient pressure. On the other hand, no bubbles were observed at 300 MPa. The zinc film with approximately 1.4 μm thick was coated on the PdH x surface after the electrolytic hydrogen charging. The hydrogen concentration of the PdH x was approximately x = 0.7 immediately after synthesis, which decreased to x = 0.2 without the zinc coating. On the other hand, the hydrogen concentration of the PdH x was maintained at least 1 week with the zinc coating. This result indicates that the zinc film acts as a barrier to hydrogen desorption from PdH x , and suggests that if the palladium superhydrides could be synthesized by electrolytic hydrogen charging under high pressure, it could be taken out to ambient pressure while maintaining its concentration.Reference1) W. Guan, R. J. Hemley, and V. Viswanathan, PNAS, 118, e2110470118 (2021) Figure 1

Some problems of modeling the liquid cavitation degassing. I. Acoustic cavitation.

In recent decades, cavitation methods of liquid degassing have become widely used, which today have practically replaced the traditional time-consuming mechanical and chemical methods of degassing in industry. The application of cavitation methods is based on the fact that a part of the neutral gases present in the liquid is not in a dissolved state, but in a so-called "free" state in the composition of a large number of vapor-gas bubbles, the size of which is measured on the scale of micro- and nanometers. The nature of the stable, long-term existence of such micro-bubbles has not yet found a reasonable explanation and is the subject of debate among researchers. Cavitation methods of degassing, both hydrodynamic and acoustic, are aimed precisely at the rapid removal of these bubbles from the liquid together with the free gas present in them. The advantage of using acoustic cavitation methods is the ability to precisely control the frequency and intensity of ultrasound, as well as the duration of sounding. Acoustic degassing methods are based on two mechanisms: the passage of dissolved gas inside pulsating bubbles due to the effect of "directed diffusion" and the convergence and subsequent coalescence of neighboring bubbles under the influence of force Bjerknes As a result, the growing bubbles quickly float up and leave the liquid together with the free gas. In recent years, a large number of articles on the comprehensive study of acoustic degassing processes have been published. According to the authors of these publications, the mechanism of degassing at the microscopic level and all the diversity of bubble dynamics, depending on the frequency and intensity of the sound, remain unclear. This article examines the main problems of modeling acoustic degassing processes, which confirm the absence of generally accepted clear ideas about the physical nature and mechanisms of cavitation phenomena and a general approach to the analysis of the obtained results. In order to develop research in this direction, the article also presents the results of a computational experiment on the coalescence of pulsating bubbles, conducted by the authors on the basis of a model of the dynamics of a single bubble previously created by them. As a result of the theoretical study, new, previously unknown information about the force interaction of pulsating bubbles of different sizes was obtained, which can be considered as a certain contribution to the understanding of the mechanisms of acoustic degassing.

Effects of Y2O3 nanoparticles / Ni matrix interface on the mechanical properties and helium swelling resistance in NiMoCr-Y2O3 alloys

The use of oxide dispersion-strengthened (ODS) nickel-based alloys has emerged as a promising choice for structural materials in molten salt reactors. In this study, a novel Y2O3 nanoparticles dispersion-strengthened nickel-base alloy was prepared by powder metallurgy method. The NiMoCr-Y2O3 alloy exhibits superior yield strengths and ultimate tensile strengths, surpassing those of the Hastelloy N (HN) alloy, both at room temperature and high temperatures. This enhancement can be attributed to the dispersion-strengthening effect imparted by Y2O3 nanoparticles. Additionally, the NiMoCr-Y2O3 alloy displays commendable plasticity. Nevertheless, it is worth noting that the stress concentration at the Y2O3 nanoparticles/Ni matrix interfaces makes it susceptible to stress-induced breakage, resulting in lower plasticity compared to the HN alloy. Transmission electron microscopy (TEM) observations reveal that the Y2O3 nanoparticles /Ni matrix interfaces effectively trap helium (He) bubbles, leading to a reduction in helium bubble size. Furthermore, X-ray absorption fine structure (XAFS) analysis indicates a higher vacancy density in NiMoCr-Y2O3 alloy compared to HN alloys during irradiation, contributing to an increased number of helium bubbles in the NiMoCr-Y2O3 alloy. This effect is primarily due to the preferential absorption of a significant quantity of interstitial particles by the Y2O3 nanoparticles/Ni matrix interfaces. Consequently, the volume fraction of helium bubbles in the NiMoCr-Y2O3 alloy (∼0.12 %) is notably lower than that observed in the HN alloy (∼0.52 %), showing that NiMoCr-Y2O3 alloys exhibit excellent He swelling resistance.

Modeling the distribution characteristics of vapor bubbles in cavitating flows

Dispersed vapor bubbles are the dominant rheology in cloud cavitation, and their size distribution is directly associated with cavitation noise and erosion. However, the numerical resolution of large numbers of dispersed bubbles remains a challenge. In this work, we establish a new cavitation model based on the population balance equation (PBE) that can predict the size distribution and spatiotemporal evolution of bubbles within cloud cavitation under different cavitation numbers. An expression for the phase transition source term without empirical parameters is derived based on the bubble size distribution (BSD) function, enabling the coupling of mass transfer in the governing equations with the PBE cavitation model. The cavitation model is solved alongside the Eulerian homogeneous mixture flow. The mass transfer between water and vapor, and the bubble coalescence and breakup under turbulent flows, are modeled to determine the BSD. The numerical model is carefully validated through comparisons with experimental results for cavitation flows on a wedge-shaped flat plate, and good agreement is achieved with respect to the pressure distribution, void fraction, and BSD. This confirms that our proposed cavitation model can accurately predict the void fraction and BSD within the cloud cavitation region.

Effects of heterogeneous nucleation model on computational fluid dynamics simulation of flow boiling heat transfer in the mini-channel

Bubble nucleation is the initial stage of flow boiling and plays an important role in boiling heat transfer. However, bubble nucleation occurs at a microscopic scale, rendering it challenging for the macroscopic computational fluid dynamics method to realistically simulate this intricate process. In this paper, based on the coupled volume-of-fluid and level set method, a heterogeneous nucleation model is improved and conducted to simulate the subcooled flow boiling in a rectangular mini-channel, considering these conditions both in the presence and absence of a microlayer. The coefficient of the original heterogeneous nucleation model is adjusted across a range from 0.1 to 10.0 times its previous value to establish multiple new nucleation models for illustrating their effects on flow patterns and heat transfer characteristics. For flow boiling without a microlayer, when the coefficient of the original heterogeneous nucleation model is halved, the nucleate boiling intensity upstream of the channel diminishes, resulting in a reduction in the heat transfer coefficient. Nevertheless, this alteration mitigates the formation of slug flow and the appearance of dry patches near the channel outlet, consequently averting a sharp increase in outlet wall superheat. Quantitatively, relative differences of 23.83% and 90.48% in average and local maximum wall superheat are observed, respectively. In contrast, the presence of a very thin microlayer beneath the growing and slipping bubble in flow boiling with a microlayer is notable. This microlayer quickly evaporates, dissipating more than 77% of the input heat flux and substantially expanding the bubble volume. Consequently, under identical wall superheat conditions, the influence of variations in the number of activated bubbles induced by different heterogeneous nucleation models on heat transfer and flow patterns in flow boiling is significantly attenuated. Specifically, when the difference in nucleus site density remains within a tenfold range, the differences in the average and maximum wall superheat are limited to just 16.78% and 33.86%, respectively. Concerning flow boiling in a mini-channel featuring a microlayer, the simulation results verify that large deviations in the activated bubble number have few effects on the flow pattern and wall superheat, greatly reducing heterogeneous nucleation model requirement and promoting the numerical study of flow boiling.