Search Results (25)

Splashed droplets in the vacuum chamber play an important role in decarburization and degassing in Ruhrstahl–Heraeus (RH), but the scholars do not pay attention to the behaviors of splashed droplets. Thus, it is necessary to propose a new method to investigate the splashed droplets. A Euler–Euler model and the inter-phase momentum transfer are applied to investigate the interaction between the molten steel and the bubbles, and the gas domain in the vacuum chamber is included in the computational domain in order to describe the movement of the splashed droplets. Numerical results show that the flow field predicted by Euler–Euler model agrees well with the experimental data. There is a higher gas volume fraction near the up-snorkel wall, the “fountain” formed by the upward flow from the up-snorkel exceeds 0.1 m above the free surface, and the center of the vortex between the upward stream and the downward stream is closer to the upward stream in the vacuum chamber. Full article

Supercritical open channel flows contribute substantially to the air–water transfer process in spillways, rivers, and streams. They are characterized by strong turbulent mixing and a substantial amount of air entrainment. The microscopic air–water properties in non-uniform self-aerated flows are investigated experimentally with various chute slopes, including air chord size and air–water transfer frequency. Microscopic air–water structures are primarily affected by chute slope, whereas the approach flow Reynolds number hardly influences them, resulting in self-similarity of the probability distribution of air chord length and air–water transfer frequency distribution in the self-aerated region. The distribution of bubble chord length is more continuous from the small to large scale in the high-air-concentration region for a greater chute slope, and the position of maximum air frequency moves to the higher-aeration zone and gets closely to the free surface. Moreover, empirical relationships are provided to predict the microscopic air–water properties in non-uniform self-aerated flows. Full article

The application of electric fields during pool boiling heat transfer has demonstrated significant potential to enhance thermal performance. However, research on boiling heat transfer enhancement under pulsed electric fields remains insufficient. To further improve pool boiling efficiency, this study systematically investigates the effects of pulsed electric fields, uniform electric fields, and electric field-free conditions on heat transfer performance using the Lattice Boltzmann Method (LBM). The results show that, compared with the uniform electric field and electric field-free condition, the pulsed electric field resulted in the smallest bubble detachment diameter and detachment period, with a higher heat flux density on the wall and the best heat transfer enhancement effect. Under the pulsed electric field, the electric force undergoes abrupt changes at the beginning and end of each pulse peak, exerting greater compression on the bubble base. Simultaneously, this leads to accelerated gas rise inside the bubble, bubble stretching, and contraction of the bottom phase boundary. There exists an optimal pulse frequency that minimizes the bubble detachment period and diameter, resulting in the best wall heat transfer enhancement effect. The effective areas for enhanced boiling heat transfer by pulsed electric fields are the bubble base and the “V”-shaped region connected to the bubble bottom. Full article

Hydrogen (H₂) is considered to be the best carbon-free energy carrier that can replace fossil fuels because of its high energy density and the advantages of not producing greenhouse gases and air pollutants. As a green and sustainable method for hydrogen production, the electrochemical hydrogen evolution reaction (HER) has received widespread attention. Currently, it is a great challenge to prepare economically stable electrocatalysts for the HER using non-precious metals. In this study, a Co/Co₃O₄/Ti₃C₂T_x catalyst was synthesized by supporting Co/Co₃O₄ with Ti₃C₂T_x. The results show that Co/Co₃O₄/Ti₃C₂T_x has excellent HER activity and durability in 1 mol L⁻¹ KOH, and the overpotential and Tafel slope at 10 mA·cm⁻² were 87 mV and 61.90 mV dec⁻¹, respectively. The excellent HER activity and stability of Co/Co₃O₄/Ti₃C₂T_x can be explained as follows: Ti₃C₂T_x provides a stable skeleton and a large number of attachment sites for Co/Co₃O₄, thus exposing more active sites; the unique two-dimensional structure of Ti₃C₂T_x provides an efficient conductive network for rapid electron transfer between the electrolyte and the catalyst during electrocatalysis; Co₃O₄ makes the Co/Co₃O₄/Ti₃C₂T_x catalyst more hydrophilic, which can accelerate the release rate of bubbles; Co/Co₃O₄ can accelerate the adsorption and deionization of H₂O to synthesize H₂. This study provides a new approach for the design and preparation of low-cost and high-performance HER catalysts. Full article

Lotus-type porous metals, characterized by low densities, large surface areas, and directional properties, are contemporarily utilized as lightweight, catalytic, and energy-damping materials; heat sinks; etc. In this study, the effects of dimensionless working parameters on the morphology of lotus-type pores in metals during unidirectional solidification were extensively investigated via general algebraic expressions. The independent dimensionless parameters include metallurgical, transport, and geometrical parameters such as Sieverts’ law constant, a partition coefficient, the solidification rate, a mass transfer coefficient, the imposed mole fraction of a solute gas, the total pressure at the top free surface, hydrostatic pressure, a solute transport parameter, inter-pore spacing, and initial contact angle. This model accounts for transient gas pressure in the pore, affected by the solute transfer, gas, capillary, and hydrostatic pressures, and Sieverts’ laws at the bubble cap and top free surface. Solute transport across the cap accounts for solute convection at the cap and the amount of solute rejected by the solidification front into the pore. The shape of lotus-type pores can be described using a proposed fifth-degree polynomial approximation, which captures the major portions between the initial contact angle and the maximum radius at a contact angle of 90 degrees, obtained by conserving the total solute content in the system. The proposed polynomial approximation, along with its working parameters, offers profound insights into the formation and shape of lotus-type pores in metals. It systematically provides deep insights into mechanisms that may not be easily revealed with experimental studies. The prediction of a lotus-type pore shape is thus algebraically achieved in good agreement with the available experimental data and previous analytical results. Full article

Extracorporeal gas exchange therapies evolved considerably within the first three–four decades of their appearance, and have since reached a mature stage, where minor alterations and discrete fine-tuning might offer some incremental improvement. A different approach is introduced here, making use of modern, purely diffusive membrane materials, and taking advantage of the elevated concentration gradient ensuing from gas pressure buildup in the gas chamber of the oxygenator. An assortment of silicone membrane gas exchangers were tested in vitro as per a modified protocol in pursuance of assessing their gas exchange efficiency under both regular and high-pressure aeration conditions. The findings point to a stark performance gain when pressurization of the gas compartment is involved; a 40% rise above atmospheric pressure elevates oxygen transfer rate (OTR) by nearly 30%. Carbon dioxide transfer rate (CTR) does not benefit as much from this principle, yet it retains a competitive edge when higher gas flow/blood flow ratios are employed. Moreover, implementation of purely diffusive membranes warrants a bubble-free circulation. Further optimization of the introduced method ought to pave the way for in vivo animal trials, which in turn may potentially unveil new realms of gas exchange performance for therapies associated with extracorporeal circulation. Full article

With the aim of understanding cavitation damage of stainless steel under the effect of cavitation collapse in an aqueous environment, morphology, phase, chemical state, electrochemical, and other properties after cavitation and erosion are methodically examined. The obtained results indicate that the addition of metal ions in pure water strengthens the dynamic effect of cavitation collapse. When the cavitation collapses, it is capable of releasing a high temperature and transferring it to the surface of the stainless steel in a short time. Then, through the rapid cooling of the pure water environment, the martensitic structure is generated, but the presence of a massive amount of metal ions leads to a decrease in the speed of heat conduction and absorption of some heat. After the collapse of the bubble, the passivation film on the surface of the stainless steel incorporates into the creation of initial micropores. A concentration difference and a growth of the current density at the bottom of the hole, as well as a reduction in the pH value are detectable. Compared with pure water, 3.5% NaCl solution contains more free ions and its local current density is higher, so its corrosion resistance is worse. Full article

A novel approach for the treatment of volatile organic compounds from gaseous streams was developed. In order to accomplish this, a semi-batch bubble reactor was used, aiming to assess the toluene (selected as model compound) degradation from gaseous streams via heterogeneous Fenton oxidation. Activated carbon-based catalysts—metal-free or iron-impregnated—with different textural and chemical surface properties were used for the first time as catalysts, in order to degrade gaseous toluene using such technology. Complementary characterization techniques, such as nitrogen adsorption at –196 °C, elemental analysis, pH at the point of zero charge (pH_PZC), inductively coupled plasma optical emission spectrometry (ICP-OES) and transmission electron microscopy (TEM), were used. The materials’ chemical surface properties, particularly the presence of N-surface groups, were herein found to play an important role in toluene adsorption and catalytic performance. The maximum amount of toluene transferred, 6.39 × 10⁻³ mol, was achieved using melamine-doped activated carbon (N-doped material) that was impregnated with iron (sample herein called ACM-Fe). This iron-based catalyst was found to be quite stable during three reutilization cycles. Full article

This study develops robust numerical algorithms for the simulation of three-phase (solid-liquid-gas) boiling and bubble formation problems in rectangular channels. The numerical algorithms are based on the Finite Volume Methods (FVM) and implement both the volume-of-fluid (VOF) methods for liquid-gas interface tracking as well as the volume-fraction methods to account for the concentration of embedded solid nano-particles in the liquid phase. Water is used as the base-liquid and the solid phase is modelled via metallic nano-particles (both aluminium oxide and titanium oxide nano-particles are considered) that are homogeneously mixed within the liquid phase. The gas phase is considered as a vapour arising from the bolling processes of the liquid-phase. The finite volume methodology is implemented on the OpenFOAM software platform, specifically by careful modification and manipulation of existing OpenFOAM solvers. The governing fluid dynamical equations, for the three-phase boiling problem, take into account the thermal conductivity effects of the solid (nano-particle), the momentum and energy equations for both the liquid-phase and the gas-phase, and finally the decoupled mass conservation equations for the liquid- and gas- phases. The decoupled mass conservation equations are specifically used to model the phase change between the liquid- and gas- phases. In addition to the FVM and VOF numerical methodologies for the discretization of the governing equations, the pressure-velocity coupling is resolved via the PIMPLE algorithm, a combination of the Pressure Implicit with Splitting of Operator (PISO) and the Semi-Implicit Method for Pressure-Linked Equations (SIMPLE) algorithms. The computational results are presented graphically with respect to variations in time as well as in the nano-particle volume fractions. The simulations and results accurately capture the formation of vapour bubbles in the two-phase (particle-free) liquid-gas flow and additionally the computational algorithms are similarly demonstrated to accurately illustrate and capture simulated boiling processes. The presence of the nano-particles is demonstrated to enhance the heat-transfer, boiling, and bubble formation processes. Full article

This work demonstrates the chemical synthesis of two-dimensional nanoflakes of mesoporous nickel/nickel (II) hydroxide (Ni/Ni(OH)₂-NFs) using double templates of surfactant self-assembled thin-film and foam of hydrogen bubbles produced by sodium borohydride reducing agent. Physicochemical characterizations show the formation of amorphous mesoporous 2D nanoflakes with a Ni/Ni(OH)₂ structure and a high specific surface area (165 m²/g). Electrochemical studies show that the electrocatalytic activity of Ni/Ni(OH)₂ nanoflakes towards methanol oxidation in alkaline solution is significantly enhanced in comparison with that of parent bare-Ni(OH)₂ deposited from surfactant-free solution. Cyclic voltammetry shows that the methanol oxidation mass activity of Ni/Ni(OH)₂-NFs reaches 545 A/cm² g_cat at 0.6 V vs. Ag/AgCl, which is more than five times higher than that of bare-Ni(OH)₂. Moreover, Ni/Ni(OH)₂-NFs reveal less charge transfer resistance (10.4 Ω), stable oxidation current density (625 A/cm² g_cat at 0.7 V vs. Ag/AgCl), and resistance to the adsorption of reaction intermediates and products during three hours of constant-potential methanol oxidation electrolysis in alkaline solution. The high-performance electrocatalytic activity of Ni/Ni(OH)₂ nanoflakes is mainly derived from efficient charge transfer due to the high specific surface area of the 2D mesoporous architecture of the nanoflakes, as well as the mass transport of methanol to Ni²⁺/Ni³⁺ active sites throughout the catalyst layer. Full article

Resistive random access memory (RRAM) has two distinct processes, the SET and RESET processes, that control the formation and dissolution of conductive filament, respectively. The laws of thermodynamics state that these processes correspond to the lowest possible level of free energy. In an RRAM device, a high operating voltage causes device degradation, such as bends, cracks, or bubble-like patterns. In this work, we developed a statistical simulation of the switching mechanism in a ZnO-based RRAM. The model used field-driven ion migration and temperature effects to design a ZnO-based RRAM dynamic SET and RESET resistance transition process. We observed that heat transport within the conducting filament generated a great deal of heat energy due to the carrier transport of the constituent dielectric material. The model was implemented using the built-in COMSOL Multiphysics software to address heat transfer, electrostatic, and yield RRAM energy. The heat energy increased with the increase in the operating power. Hence, the reliability of a device with high power consumption cannot be assured. We obtained various carrier heat analyses in 2D images and concluded that developing RRAM devices with low operating currents through material and structure optimization is crucial. Full article

Femtosecond laser pulses have been successfully used for film-free single-cell bioprinting, enabling precise and efficient selection and positioning of individual mammalian cells from a complex cell mixture (based on morphology or fluorescence) onto a 2D target substrate or a 3D pre-processed scaffold. In order to evaluate the effects of higher pulse durations on the bioprinting process, we investigated cavitation bubble and jet dynamics in the femto- and picosecond regime. By increasing the laser pulse duration from 600 fs to 14.1 ps, less energy is deposited in the hydrogel for the cavitation bubble expansion, resulting in less kinetic energy for the jet propagation with a slower jet velocity. Under appropriate conditions, single cells can be reliably transferred with a cell survival rate after transfer above 95% through the entire pulse duration range. More cost efficient and compact laser sources with pulse durations in the picosecond range could be used for film-free bioprinting and single-cell transfer. Full article

In this work, we have investigated the influence of the transfer process on the monocrystalline graphene in terms of quality, morphology and electrical properties by analyzing the data obtained from optical microscopy, scanning electron microscopy, Raman spectroscopy and electrical characterizations. The influence of Cu oxidation on graphene prior to the transfer is also discussed. Our results show that the controlled bubbling electrochemical delamination transfer is an easy and fast transfer technique suitable for transferring large single crystals graphene without degrading the graphene quality. Moreover, Raman spectroscopy investigation reveals that the Cu surface oxidation modifies the strain of the graphene film. We have observed that graphene laying on unoxidized Cu is subject to a biaxial strain in compression, while graphene on Cu oxide is subject to a biaxial strain in tension. However, after graphene was transferred to a host substrate, these strain effects were strongly reduced, leaving a homogeneous graphene on the substrate. The transferred single crystal graphene on silicon oxide substrate was used to fabricate transmission line method (TLM) devices. Electrical measurements show low contact resistance ~150 Ω·µm, which confirms the homogeneity and high quality of transferred graphene. Full article

Why the bubbles are negatively charged? This is almost 100 years old question, which many scientists have striven and still are striving to answer using the latest developments of the MD simulations and various physical analytical methods. We scrutinize with this paper the basic literature on this topic and conduct our own analysis. Following the philosophical law of parsimony: “Entities should not be multiplied without necessity”, we assume that the simplest explanation is the right one. It is well known that the negative change of the Gibbs free energy is a solid criterion for spontaneous process. Hence, we calculated the energies of adsorption of OH⁻, H₃O⁺ and HCO₃⁻ ions on the air/water interface using the latest theoretical developments on the dispersion interaction of inorganic ions with the air/water interface. Thus, we established that the adsorption of OH⁻ and HCO₃⁻ ions is energetically favorable, while the adsorption of H₃O⁺ is energetically unfavorable. Moreover, we calculated the change of the entropy of these ions upon their transfer from the bulk to the air/water interface. Using the well-known formula ΔG = ΔH − TΔS, we established that the adsorption of OH⁻ and HCO₃⁻ ions on the air/water interface decreases their Gibbs free energy. On the contrary, the adsorption of H₃O⁺ ions on the air/water interface increases their Gibbs free energy. Thus, we established that both OH⁻ and HCO₃⁻ ions adsorb on the air/water interface, while the H₃O⁺ ions are repelled by the latter. Therefore, electrical double layer (EDL) is formed at the surface of the bubble–negatively charged adsorption layer of OH⁻ and HCO₃⁻ ions and positively charged diffuse layer of H₃O⁺ ions. Full article

Microbubbles with slow rising speed, higher specific area and greater oxygen dissolution are desired to enhance gas/liquid mass transfer rate. Such attributes are very important to tackle challenges on the low efficiency of gas/liquid mass transfer that occurs in aerobic wastewater treatment systems or in the aquaculture industries. Many reports focus on the formation mechanisms of the microbubbles, but with less emphasis on the system optimization and assessment of the aeration efficiency. This work assesses the performance and evaluates the aeration efficiency of a porous venturi-orifice microbubble generator (MBG). The increment of stream velocity along the venturi pathway and orifice ring leads to a pressure drop (P_atm > P_abs) and subsequently to increased cavitation. The experiments were run under three conditions: various liquid velocity (Q_L) of 2.35–2.60 m/s at fixed gas velocity (Q_g) of 3 L/min; various Q_g of 1–5 L/min at fixed Q_L of 2.46 m/s; and free flowing air at variable Q_Ls. Results show that increasing liquid velocities from 2.35 to 2.60 m/s imposes higher vacuum pressure of 0.84 to 2.27 kPa. They correspond to free-flowing air at rates of 3.2–5.6 L/min. When the system was tested at constant air velocity of 3 L/min and under variable liquid velocities, the oxygen dissolution rate peaks at liquid velocity of 2.46 m/s, which also provides the highest volumetric mass transfer coefficient (K_La) of 0.041 min⁻¹ and the highest aeration efficiency of 0.287 kgO₂/kWh. Under free-flowing air, the impact of Q_L is significant at a range of 2.35 to 2.46 m/s until reaching a plateau K_La value of 0.0416 min⁻¹. The pattern of the K_La trend is mirrored by the aeration efficiency that reached the maximum value of 0.424 kgO₂/kWh. The findings on the aeration efficiency reveals that the venturi-orifice MBG can be further optimized by focusing on the trade-off between air bubble size and the air volumetric velocity to balance between the amount of available oxygen to be transferred and the rate of the oxygen transfer. Full article