Search Results (145)

This investigation employs focused ion beam (FIB) and transmission electron microscopy (TEM) techniques to systematically analyze the distribution characteristics of fission products in medium-burnup (40.6 GWd/tU) UO₂ fuel and their impact on fuel cracking behavior. The findings indicate that grain boundary embrittlement is predominantly attributed to the accumulation of spherical particles of solid fission products, including Mo, Ru, Rh, and Pd, which preferentially segregate around impurity particles, leading to localized stress concentration. Intragranular cracks are associated with the strip-like segregation of fission elements and the amorphization process. It also reveals that the size and number density of intragranular Xe bubbles are ~6.24 ± 0.24 nm and 5.2 × 10²² m⁻³, respectively, while Xe did not, under the analyzed conditions, significantly influence crack nucleation. This research elucidates the correlation mechanism between fission product distribution and fuel cracking behavior at medium burn up, offering experimental evidence to enhance the reliability and safety of nuclear fuel assemblies. Full article

Hydrodynamic cavitation is one of the most promising technologies for sustainable process intensification in the food, nutraceutical, and environmental sectors, due to its ability to generate highly localized and intense implosions. Venturi-type devices, known for their simplicity and efficiency, are widely used for non-thermal extraction, microbial inactivation, and cellular disruption. However, the effectiveness of cavitation critically depends on internal geometry—particularly the perimeter-to-area ratio (P/A), which influences both pressure gradient distribution and the density of nucleation sites. In this context, an innovative configuration based on the Reuleaux triangle is proposed, allowing for a significant increase in the P/A ratio compared to conventional circular-section devices. This theoretical study extends the Navier–Stokes and Rayleigh–Plesset models to describe bubble dynamics and assess the influence of geometric and rotational variants (VRAt) on the localization and intensity of cavitation collapse. The results suggest that optimized internal geometries can reduce treatment times, increase selectivity, and improve the overall energy efficiency of cavitation processes, offering strong potential for advanced and sustainable industrial applications. This work is entirely theoretical and is intended to support the future design and experimental validation of next-generation cavitating devices. Full article

This study provides a novel experimental setup and methodology for the quantitative investigation of evaporation-induced foaming behaviors in a polymer/small-molecule solution system (PSMS). In traditional dynamic test methods, it is difficult to precisely describe the evaporation-induced foaming process of a multicomponent solution because the concentration of light components in solution continuously decreases during ebullition, causing undesired changes in foaming behavior. In this study, a precisely controlled condensation reflux module was introduced into the setup to maintain pressure, temperature, and concentration of the PSMS at constant levels during the entire ebullition process, allowing dynamic test methods to quantify the evaporation-induced foamability. With this newly proposed device, experimental data of typical PSMS, polyolefin elastomer (POE)/n-hexane solution system, were obtained and modeled to illustrate the foam growth profile, thereby characterizing the dynamic foaming process based on a logistic growth function. The corresponding dimensionless number Σevap was calculated to evaluate evaporation-induced foam stability by analyzing the foam growth profile under varying pressure, concentration, and energy input levels. Furthermore, given that the PSMS represents a highly non-ideal system, the bubble nucleation rate J was modified in this work by introducing a correction coefficient δ to account for the non-ideal effects of macromolecules present in solutions. Additionally, another correction coefficient λ was incorporated into the Gibbs free energy term to adjust for supersaturation of liquid during nucleation. The experiment’s data align well with the modified bubble nucleation rate mechanism proposed herein. Full article

The microstructure evolution and hardness variations of as-fabricated and recrystallized 12Cr oxide dispersion strengthened (ODS) steel after dual-ion (6.4 MeV Fe³⁺ and energy-degraded 1 MeV He⁺) irradiation at 973 K up to 10.6 displacements per atom (dpa) at peak damage and 8900 appm He are investigated. Results show that the oxide particles slightly shrink in the as-fabricated specimen, while they are stable in the recrystallized specimen. Furthermore, larger helium bubbles are trapped at the grain boundaries in the as-fabricated specimen, and the size of helium bubbles in the grains is almost the same for both as-fabricated and recrystallized specimens, indicating that reduction of grain boundaries would reduce the potential nucleation sites and suppress the helium segregation. Moreover, no obvious hardening occurs in the as-fabricated specimen, whereas the hardness increases a little in the recrystallized specimen. Based on the barrier model, the barrier strength factor of helium bubbles is calculated. The value is 0.077, which is much smaller and suggests that helium bubbles seem not to significantly induce irradiation hardening. Full article

This study systematically investigates the influence of mix proportion on and the early-age properties and CO₂ uptake of CO₂-mixed cement paste, focusing on variations in the water-to-binder (w/b) ratio, slag content, and air-entraining agent (AEA) dosage. Mineralogical characteristics were analyzed using X-ray diffraction (XRD) and thermogravimetric analysis (TGA), while pore structures were assessed via nitrogen adsorption. CO₂ uptake was quantified immediately after mixing. Results indicate that a low w/b ratio limits CO₂ dissolution and transport, favors hydration over carbonation, and leads to a coarser pore structure. At moderate w/b ratios, excess free water facilitates concurrent carbonation and hydration; however, thinner water films ultimately hinder CaCO₃ precipitation and C-S-H nucleation. Slag contents up to 30% slightly suppress early carbonation and hydration, while higher dosages significantly delay both reactions and increase capillary porosity. An increasing AEA dosage stabilizes CO₂ bubbles, suppressing immediate CO₂ dissolution and reducing the early formation of carbonation and hydration products; excessive AEAs promotes bubble coalescence and results in an interconnected pore network. An optimized mix design, moderate water content, slag below 30%, and limited AEA dosage enhance the synergy between carbonation and hydration, improving early pore refinement and reaction kinetics. Full article

This paper investigates and assesses the potential applicability of global mass transfer coefficients derived from large-scale experiments to the bubble growth of a single bubble in a super-saturated flow $(\sigma =9)$. Therefore, it presents, for a specific flow velocity $(u=1{\displaystyle \frac{\mathrm{m}}{\mathrm{s}}}, Re=\mathrm{10,678})$, a comparison between correlation-based modelling and 3D Large Eddy Simulation–Volume of Fluid (LES-VOF) Computational Fluid Dynamics (CFD) simulations (minimum cell size of 10 µm, Δt = 10 µs). After the verification of the CFD with pool nucleation bubbles, two cases are regarded: (1) the bubble flowing in the bulk and (2) a bubble on a wall with a crossflow. The correlation-based modelling results in a nearly linear relationship between bubble radius and time; meanwhile, theoretically, the self-similarity rule offers $r~{Bt}^{0.5}$. The Avdeev correlation gives the best agreement with the CFD simulation for a bubble in the flow bulk (case 1), while the laminar approach for calculation of the exposure time of the penetration theory shows good agreement with the CFD simulation for the bubble growth at the wall (case 2). This preliminary study provides the first quantitative validation of global mass transfer coefficient correlations at the single-bubble scale, suggesting that computationally intensive CFD simulations may be omitted for rapid estimations. Future work will extend the analysis to a wider range of flow velocities and bubble diameters to further validate these findings. Full article

This work addresses enhancing flow boiling heat transfer via the use of engineered surfaces possessing specific novel geometries created via femtosecond laser texturing. Surface functionalization can result in improved, more controlled, and denser nucleation as well as controlled surface rewetting, leading to reduced incipient superheats, higher heat transfer coefficients, reduced flow instabilities, and increased critical heat fluxes with respect to a non-modified reference surface. Specifically, this study investigates how bubble dynamics and heat transfer performance are affected by three different surface textures fabricated on 200 µm thick 316L stainless steel foils using a femtosecond (fs) laser. The examined textures consist of inclined (=45°) microgrooves, inclined (=45°) conical microholes, and laser-induced periodic surface structures (LIPSSs). Each textured surface’s degree of heat transfer enhancement is assessed with respect to a plain reference surface in identical operating conditions. The working fluid is PP1, a replacement of 3M™ FC-72 in heat transfer applications. Among the tested surfaces, submicron-scale LIPSSs contribute to the rewetting of the surface but only show a slight improvement when not combined with bigger microscale structures. The inclined grooves result in the most gradual onset, showing almost no incipient overshoot. The inclined conical microholes achieve superior results, improving heat transfer coefficients up to 70% and reducing the incipient temperature up to 13.5 °C over a plain reference surface. Full article

The escalating thermal demands of high-power electronic devices and energy systems necessitate advanced thermal management solutions. Flow boiling in small/micro channels has emerged as a promising approach, yet its practical implementation is hindered by flow instabilities and heat transfer deterioration under high-heat fluxes. This study presents a systematic numerical investigation of subcooled boiling flow and heat transfer in a rectangular small channel under high-heat-flux conditions, employing the VOF method coupled with the Lee phase change model. The increasing heat flux accelerates bubble nucleation and coalescence while reduced mass flux promotes early local slug formation, shifting flow transitions upstream and degrading thermal performance. A local vapor volume fraction threshold of α_ν = 0.2 is identified for the bubbly-to-sweeping flow transition and α_ν = 0.4 for the sweeping-to-churn transition. Furthermore, a novel dimensionless parameter β is proposed to classify dominant flow regimes, with critical β ranges of 12–16 and 24–32 corresponding to the two transitions, respectively. These findings provide new quantitative tools for identifying flow regimes and improve the understanding and design of compact boiling-based thermal management systems under extreme heat- flux conditions. Full article

N-butylnitroxyethylnitramine (BuNENA) is a high-energy plasticizer with high plasticizing ability, low sensitivity, and high energy. It has broad application prospects in HTPE propellants. Nevertheless, as an energetic plasticizer, it requires treatment to reduce its sensitivity. To this end, the passivation process for BuNENA was simulated using a mixing model analogous to nucleate boiling. This method involves tracking the formation and movement of bubbles using a Lagrange frame, and the bubbles themselves are modeled as rigid spheres subject to buoyancy and viscous forces. A variational multiscale (VMS)-based Euler framework was employed to simulate the fluid surrounding the bubble. The movement process of the bubbles was analyzed, and it was found that the amount of bubbles and the movement speed were higher at high temperatures and in a high vacuum, and the passivation effect on BuNENA was better. At a pressure of 40 mbar and a temperature of 50 °C, BuNENA demonstrated an 89% water removal rate. A comparison of the experimental results with the simulation results revealed slight discrepancies between them. A meticulous analysis of the passivation process for BuNENA is rendered possible by integrating experimental and simulation methodologies, a feat that has immense implications for the realm of composite solid propellant passivation. Full article

Green hydrogen, produced via electrolysis using renewable energy, is a zero-emission fuel essential for the global transition to sustainable energy systems. Optimizing hydrogen production requires a detailed understanding of bubble dynamics at the cathode, which involves three key stages: nucleation, growth, and detachment. In this study, hydrogen bubble growth was investigated in a custom-built electrolysis cell with microelectrodes, combining high-speed imaging and electrochemical measurements with a potentiostat. The results reveal distinct growth regimes governed by a potential-dependent time exponent, captured through a power law. Within the evaluated range of potentials, three regions with different bubble departure behaviors were identified: (i) at low potentials (2.0–2.6 V), bubbles depart without coalescing, (ii) in the transitional region (2.6–3.2 V), bubbles coalesce to varying degrees before detachment, and (iii) at high potentials (≥3.2 V), large, coalesced bubbles dominate. These findings highlight the significant impact of coalescence on bubble growth and departure behavior, affecting electrode coverage with gas and, consequently, electrolysis efficiency. Understanding these interactions is crucial for improving hydrogen evolution efficiency by mitigating bubble-induced mass transport limitations. The findings contribute to advancing electrolysis performance, offering insights into optimizing operating conditions for enhanced hydrogen production. Full article

Boiling heat transfer plays a crucial role in various engineering applications, requiring accurate numerical modeling to capture phase-change dynamics. This study employs the pseudopotential lattice Boltzmann method (LBM) to simulate boiling heat transfer at different reduced temperatures, aiming to provide deeper insights into bubble dynamics and heat transfer mechanisms. The LBM framework incorporates a multi-relaxation-time approach and the Peng–Robinson equation of state to enhance numerical stability and thermodynamic consistency. Simulations were performed to analyze bubble nucleation, growth, and detachment across varying reduced temperatures, considering the influence of surface wettability, surface tension and gravitational acceleration. The results indicate a strong dependence of bubble behavior on the reduced temperature, affecting both heat flux and boiling regimes. The numerical findings show reasonable agreement with theoretical predictions and experimental trends, validating the effectiveness of the LBM approach for phase-change simulations. Additionally, this study highlights the role of contact angle variation in modifying boiling characteristics, emphasizing the necessity of accurate surface interaction modeling. The outcomes of this work contribute to advancing computational methodologies for boiling heat transfer, supporting improved thermal management in industrial applications. Full article

Microchannel heat exchangers, with their large specific surface area, exhibit high heat/mass transfer efficiency and have a wide range of applications in chemical engineering and energy. To enhance microchannel flow boiling heat transfer, a top-connected microchannel heat exchanger with a Ni/Ag micro/nano composite surface was designed. Using anhydrous ethanol as the working fluid, comparative flow boiling heat transfer experiments were conducted on regular parallel microchannels (RMC), top-connected microchannels (TCMC), and TCMC with a Ni/Ag micro/nano composite surface (TCMC-Ni/Ag). Results show that the TCMC-Ni/Ag’s maximum local heat transfer coefficient reaches 179.84 kW/m²·K, which is 4.1 times that of RMC. Visualization reveals that its strongly hydrophilic micro/nano composite surface increases bubble nucleation density and nucleation frequency. Under medium-low heat flux, the vapor phase converges in the top-connected region while bubbles form on the microchannel surface; under high heat flux, its capillary liquid absorption triggers a thin-liquid-film convective evaporation mode, which is the key mechanism for improved heat transfer performance. Full article

A lot of novel surface treatment technologies have appeared over the last few decades, offering great possibilities for practical use. Modified surfaces have confirmed their successful application in thermal engineering for boiling heat transfer enhancement and single-phase convection. Several classification approaches for boiling surfaces exist in the literature; however, a full, physically based, and commonly accepted universal system is still missing. This paper proposes such a classification system, based on considerations of physical mechanisms underlying the nucleation process and enhancement mechanism during different stages of vapor bubble growth. It also presents an overview of recent advances in the development of enhanced boiling surfaces. Full article

The microcellular injection molding (MuCell^®) process, which uses supercritical fluid (SCF) as a foaming agent, is considered an important green molding solution to reduce product weight, molding energy, and cycle time and to improve the foam quality. However, maximizing the foaming density while keeping size uniformity in the foaming cell requires further attention. In this study, H₂O and the SCF N₂ were employed as cofoaming agents in the MuCell^® process of polypropylene (PP). Owing to the different critical points of N₂ and H₂O, bubble nucleation was expected to occur in interactive ways. Various process parameters were investigated, including the SCF N₂ content, the moisture content adsorbed within the resin under targeted PP weight reductions of 30% and 40%, the melt and mold temperature conditions, and the gas counter pressure. The resulting foaming morphology was examined to evaluate the foam quality in terms of the foaming density and bubble size distribution. The bubble coalescence, particularly in the skin layer, was examined, and the associated gas permeability flow rate was measured. The results indicated that H₂O-assisted foaming led to bubble coalescence and allowed for gas penetration in the direction of the part thickness direction, resulting in an overall increase in foaming density, particularly in the skin layer. Under high SCF N₂ and H₂O contents, the solid skin layer disappeared, regulating the gas permeability from one surface side to the other. Under the optimized process parameters, the gas permeability flow rate in the filter-like foaming PP material reached 300–450 mL/min. The application of gas counter pressure also helped increase the foam density and bubble coalescence, enhancing the gas permeability in the PP material to about 500 mL/min. These results demonstrate the potential application of microcellular injection molding using water as a cofoaming agent in moisture-release devices. Full article

Despite the numerous benefits of high-temperature superconducting (HTS) power transformers, they are highly sensitive and vulnerable from a thermal perspective, particularly under fault current conditions due to their fault current tolerance properties. Ensuring the proper operation of the cooling system can enhance the transformer’s performance during fault and overload conditions. To improve the thermal management of this transformer in both convective heat transfer and nucleate boiling conditions, utilizing liquid nitrogen (LN₂) nanofluid instead of conventional LN₂ is a promising solution. In this study, a two-phase Eulerian model using ANSYS Fluent software is employed to analyze the impact of different volume fractions (VFs) of Al₂O₃ nanoparticles with a 40 nm diameter on the cooling performance of a power HTS transformer. The numerical simulations are conducted using the Ranz–Marshal method for heat transfer and the finite element method for solving the governing equations. Nanoparticle concentrations ranging from 0 to 1% are evaluated under various fault conditions. Additionally, the influence of nanoparticles on bubble behavior is examined, partially mitigating the blockage of cooler microchannels. The simulation reveals that adding nanoparticles to the fluid reduces the temperature of the hotspot by 29% in steady state and by 34–52% under different fault currents as a result of 0–46% enhancement of nucleate boiling heat transfer, thereby improving the cooling efficiency of the transformer. Full article