Liquid Atomization Research Articles

Atomistic weak interaction criterion for the specificity of liquid metal embrittlement

Liquid metal embrittlement (LME) occurs in some solid–liquid metal elements’ couples (e.g., Fe-Zn and Al-Ga), called specificity. Although some material parameters like solubility and bonding energy were suggested as controlling factors, none could be attributed satisfactorily. Here we have unveiled the primary factor that governs the specificity of LME. From first-principles calculations compared with a systematic surveillance test result, we found that the grain-boundary (GB) adsorption energy shows near-zero values in all embrittling couples; the interaction between solid and liquid metal atoms is weak when an atom from the liquid state penetrates the grain boundary of the solid. Furthermore, we found that the calculated surface adsorption energy that promotes bond-breaking does not correlate to the specificity. Therefore, we consider that the penetration of a liquid metal atom surrounded by weakly interacting solid metal atoms is necessary before the bond-breaking assisted by surface adsorption occurs at a microcrack tip. This mechanism is also applicable for transgranular cracking along low-energy boundaries and crystal planes. While liquid metal atoms penetrate and diffuse into solid GB macroscopically before cracking, liquid metal’s surface adsorption stronger than GB adsorption should promote the bond-breaking of solid metal. In conclusion, the atomistic penetration precedes the surface-adsorption-assisted bond-breaking and controls the specificity of LME.

In situ synthesis mechanism of double core/rim structure Ti(C,N)-based cermets prepared by reactive hot pressing

In situ Ti(C,N)-based cermets with white core/gray rim and black core/gray rim structures were synthesized from a Co–Ti–C–TiN–W system. In addition, the reaction mechanism was explored. Results showed that the allotropic transformation of εCo into αCo initially occurred. Then, αCo–Ti solid state reaction to fabricate CoTi2 took place. Subsequently, Co–Ti liquid was formed by a peritectic reaction of CoTi2. With the spreading of Co–Ti liquid, C atoms dissolved into Co–Ti melt, resulting in the presence of Co–Ti–C ternary liquid. In the melt, C atoms reacted with Ti atoms to yield TiC. With increasing temperature, TiC and TiN particles combined to produce Ti(C,N). Continuous heating induced the dissolution of W atoms and a part of Ti(C,N) into Co–Ti–C melt. This contributed to the appearance of Co–Ti–C–N–W liquid. Once W, Ti, C and N atoms in liquid became saturated, they were precipitated. When the precipitation appeared on the surface of undissolved Ti(C,N), large Ti(C,N) particles with black core/gray rim structure were obtained. As they directly nucleated and grew from the melt, fine Ti(C,N) particulates with white core/gray rim structure were prepared. Ti(C,N)-based cermets can be produced at a low sintering temperature of 1200 °C. With an increased sintering temperature, the relative density and homogeneity of cermets were gradually improved, and more Ti(C,N) particles with core/rim structure were obtained. These effects in turn led to the increase in hardness, flexure strength and fracture toughness of cermets. Results from this work can provide a valuable reference for the formation of white core/gray rim Ti(C,N) particles, which remains controversial in traditional powder metallurgy.

Molecular dynamics study of spontaneous capillary flow and heat transfer in nanochannels

Passive cooling technology has become increasingly used in microelectronic devices, with capillary force being the most commonly used driving method. In this study, the capillary flow and heat transfer mechanisms and characteristics in nanochannels are studied based on molecular dynamics (MD), which have rarely been explored in previous studies. The capillary speed increases as the solid-liquid interaction intensity (εwl) increased, but when the wall is completely wetted, the capillary speed slows slightly an increasing εwl. By introducing wall slip, the Lucas-Washburn (L-W) equation is modified and a better fit is obtained. Furthermore, the effects of εwl, the temperature difference of the solid-liquid and surface nanostructure on capillary flow and heat transfer are analyzed. The increase in εwl and wall temperature enhances the capillary speed and heat transfer, but a continuous increase in the wall temperature will accelerate liquid evaporation and cause climb stagnation. The introduction of nanostructures slows the capillary speed, but can enhance the solid-liquid heat transfer. Finally, the microscopic mechanism of the above findings is analyzed by potential energy and density analyses. The reduction of the liquid potential energy at the solid-liquid interface can accelerate the capillary speed to a certain extent, but when the potential energy is too low, the liquid atoms will gather near the wall surface, blocking the capillary climbing. The expansion of the low potential energy region and the high mass density region enhances the solid-liquid heat transfer.

Numerical simulation of liquid jet atomization in subsonic crossflow

To explore the influence of different factors on the atomization pattern and characteristics of the crossflow, the influence of three dimensionless parameters is investigated in this paper, namely Weber number, Oh, and the momentum flux ratio q, on the atomization in subsonic crossflow. The investigations are based on the CLSVOF method and Large Eddy Simulation and developed in conjunction with adaptive mesh refinement techniques. In terms of atomization morphology, three-view drawing of the jet fragmentation morphology and cross-sectional deformation processes under different operating conditions are compared and investigated. In terms of atomization characteristics, the jet penetration depth and spraying distribution curves, as well as the fragmentation position and droplet velocity distribution are analyzed in detail. The results show that there is a pronounced effect for Weber number on the fragmentation pattern and atomization characteristics of liquid jet into a crossflow. As the Weber number increases, the column fragmentation dominated by RT instability gradually changes to surface fragmentation dominated by KH instability. The research also concluded that the momentum flux ratio q is a key element in the penetration depth, which is reflected in the fact that when q is larger, the jet bends less with a stronger penetration ability.

Possible, impossible, and expected diameters and production rates of droplets in aerosols and sprays

Liquid atomization processes generating sprays and aerosols of droplets are used in many delivery and coating systems involving pure solvents, solutions, and suspensions. In our recent experimental work, we introduced a novel liquid atomization process generating micro-sprays and aerosols of submicron-diameter droplets for pure solvents, solutions, and suspensions: gas jets disintegrate thin liquid films that are formed as bubbles approach a liquid surface. Here we develop a theoretical description of droplet sizes and flow rates, using the first principles of conservation of mass and energy, and accounting for the ratios of specific energies and the ratios of specific energy rates provided by the atomizing gas and dissipated by the atomized liquid. We introduce atomization diagrams as a graphical tool to determine possible, impossible, and expected droplet diameters and specific flow rates in aerosols and sprays produced under various conditions. We find a reasonable agreement between the theory and experiments for five different liquids converted into aerosols of submicron-diameter droplets by an atomization process where gas jets disintegrate thin liquid films that are formed as bubbles approach a liquid surface, and also for five traditional pressure nozzles that produce sprays of droplets of hundreds of microns in diameter. Our study explored the overall range of Ohnesorge number between 0.01-100, and the analysis and atomization diagrams contribute to understanding of liquid atomization and can serve as a theoretical framework for comparing different liquid atomization techniques.

Analysis of liquid column atomization by annular dual-nozzle gas jet flow

The atomization of a water column by a gas jet flow (Reynolds number $\sim O(10^{4}\unicode{x2013}10^{5})$ ) issued from a two-stage annular nozzle is investigated experimentally. Varying the nozzle geometry, the momentum flux ratio of the upper and lower jets, and the water flow rate, we measure the processes of atomization with high-speed imaging, analysed analytically into four regimes. In the bulk atomization regime, the atomization is driven by the lower jet, but it is forced to occur earlier by the stronger upper jet before the water column reaches the lower jet in the droplet atomization regime. Interestingly, the size of the atomized droplets remains unaffected by the momentum flux ratio of upper to lower jets. The atomization process is governed by the Rayleigh–Taylor instability, by which the estimated droplet size agrees well with the measurement. In the backflow regime, a strong reverse flow is induced to force a substantial portion of atomized droplets to be drawn backward to the nozzle; a floating liquid column regime is captured transitionally, i.e. the column stagnates near the lower nozzle when the water flow rate is very low. To understand the mechanisms of each regime, the single-phase jet flow is measured separately using particle image velocimetry, and implemented into the control volume analysis with which we predicted analytically and validated the conditions for the occurrence of each regime. It is found that the acceleration of gas flow (velocity gradient) experienced by the falling water is the key parameter to drive the atomization.

First-principles calculation of the resistance to lead-bismuth eutectic corrosion on Fe (111) surface of austenitic stainless steel

The corrosion of liquid metal atoms on the iron surface plays a vital role in the life of the reactor. We use density functional theory (DFT) to study the adsorption energy of Pb and Bi atoms on the Fe (111) surface and the escape energy of Fe atoms on the surface. The adsorption effects of Pb and Bi atom at the fcc site are the best, and the adsorption energies are −2.66 eV and −3.07 eV, respectively. It shows that Bi is more easily adsorbed on the Fe surface than Pb, which is consistent with the experimental phenomena. The escape energies of Fe atoms on clean surface and adsorbed Pb (Bi) surface are 5.11 eV and 4.58 (4.56) eV, respectively, indicating that the adsorption of Pb and Bi on clean Fe (111) surface makes Fe escape more easily. The influence of other alloying elements in steel (Cr, Ni, Ti, Mo, Mn, Si and Al) on corrosion behavior is also studied. The results show that these seven alloying elements all have an inhibitory effect on the adsorption effect of Pb and Bi, thereby weakening the corrosion, but their effects on the escape of Fe are different. Combining the two energies to obtain the total energy (Etot), it shows that both Si and Cr can reduce the dissolution corrosion of steel in LBE.

Finite Difference and Reinitialization Methods with Level Set to Interfacial Area Transport Equations for Gas–Liquid Two-Phase Flows

For the direct simulation of liquid atomization processes affected by surface tension, the level-set method was suitable, owing to the ease of estimating normal vectors and interface curvatures. However, the mass is not conserved in the original method because of errors in the interfacial advection evaluation. Therefore, previous studies have developed the interfacial advection equation and the interfacial evaluation method for reinitialization. The reinitialization method can be organized in terms of control parameters. Therefore, we modified and optimized the control parameters of the high-order constrained reinitialization scheme developed previously. Furthermore, the difference method in the convection term was adapted to the interfacial advection equation to evaluate the best method for simulating the droplet atomization process. Consequently, the fifth-order accuracy weighted essentially non-oscillatory scheme was observed to provide the best mass conservation for both the reinitialization and the advection equations.

Composition dependence in glass-forming ability of Cu–Ag binary alloys

Alloys around eutectic points are thought to be good glass formers. However, the glass-forming ability (GFA) as a function of composition from pure metals to the eutectic alloy has not been quantified and the underlying microstructural and dynamical origins are unclear. In this work, the GFA of Cu–Ag alloys at various compositions was thoroughly investigated by using molecular dynamics simulations. According to the results, the critical cooling rate for glass formation Rc (K/s) varies with solute content x′ (at%), following Rc ≈ Aexp(kx′) where A = 4.61 × 1012 and k = -0.25 on the Cu side and A = 4.43 × 1012 and k = -0.19 on the Ag side. The Rc of the eutectic alloy is about 5 orders of magnitude lower than that of the pure metals. Structurally, the number of densely packed icosahedral-like clusters in Cu–Ag metallic glasses increases with increasing solute content, resulting in the most densely packed structure at the eutectic composition. The dynamical properties of Cu–Ag melts are basically independent on composition at high temperatures, but obviously change with composition at low temperature. Besides the largest reduced glass transition temperature, the highest five-fold symmetry (FFS) local structure and more importantly the retarded dynamics of liquid atoms below the liquidus temperature around the eutectic composition impart the eutectic alloy with the best glass-forming ability.

Structural transformations in binary Ni62Nb38 alloy at ultrahigh temperatures

In this study, we performed the molecular dynamics simulation of the binary Ni62Nb38 alloy at heating from 300 K to 9000 K. The temperature dependences of the density and the fraction of liquid atoms were calculated. The boiling temperature of the system is determined. It has been established that near the temperature 7000 K the system undergoes phase transition from liquid to gas phase. It is shown that near the boiling temperature, the molten metal forms a branched structure. The obtained results made it possible to estimate the region of the liquid phase and to determine the probable scenario for the occurrence of structural changes near the boiling point.

Effect of orifice size on liquid breakup dynamics and spray characteristics in slinger atomizers

The ability of the rotary slingers to achieve fuel atomization by employing the rotational speed of the turbine shaft obviates the use of high-pressure fuel pump and makes those a popular choice for small gas turbine engines. Such atomizers contain arrays of orifices on their outer periphery for injection of the liquid into the combustion chambers. In the present work, we experimentally investigate the influence of the choice of orifice size on primary liquid breakup dynamics and spray characteristics in slinger atomizers. Three slinger discs with the same disc size and number of orifices but with different orifice size, d o (= 1 mm, 1.5 mm, and 2 mm) are examined in a high-speed slinger test rig for a range of rotational speed (2000–10,000 rpm) and liquid feed rate (0.2–1 lpm). The operating conditions are relevant to part-load and engine relighting conditions for which the selection of geometric parameters of the injector may play a crucial role in achieving the atomizer performance. In the current work, the liquid breakup process in the vicinity of the slinger orifices was visualized using front light illumination technique, whereas the pulse-shadowgraph technique was employed for droplet sizing. It is found that smaller orifice size promotes column mode of liquid breakup which results in larger primary liquid breakup length and subsequently results in larger Sauter mean diameter. The results highlight that the orifice size is an important design parameter as the liquid atomization in slinger atomizers is sensitive to the selection of orifice size especially for lower range of rotational speeds.

Thermal transport mechanism at a solid-liquid interface based on the heat flux detected at a subatomic spatial resolution.

Heat flux is a fundamental quantity in thermal science and engineering and is essential for understanding thermal transport phenomena. In this study, the heat flux in a solid-liquid interfacial region is obtained in a three-dimensional (3D) space at a subatomic spatial resolution based on classical molecular dynamics, yielding a 3D structure of the heat flux between the solid and liquid layers in contact. The results using the Lennard-Jones potential reveal the directional qualities of the heat flux, which are significantly dependent on the subatomic stresses in the structures of condensed phase systems. The heat flux and stress at the subatomic scale are related to the macroscopic transport quantities, which can be obtained using distribution functions; the stress and energy flux properties at the subatomic scale are comprehensively investigated using a single-interaction-based stress and energy flux to determine the origin of the thermal transport mechanism at the solid-liquid interface. The findings reveal that the density of states due to the stress caused by a single interaction exhibits a bandlike behavior. The net energy transport comprises positive and negative energy transport inside and outside the band. In addition, this is related to the intrinsic transport property of the atoms and molecules at the solid-liquid interface at the subatomic scale. The difference between the energy transport rates when a solid atom in the vicinity of the interface is near to or far from the liquid phase is the origin of the energy transport mechanism at the solid-liquid interface. 3D analysis of the heat flux and stress is carried out by varying the interaction strengths between the liquid molecules and solid atoms at the solid-liquid interface. This reveals that the directional quality of transport quantities is high at strong interaction strengths, thus indicating enhanced thermal transport. Furthermore, the influence of the temperature gradient in the system suggests that the energy transport imbalance between inside and outside the stress band in a high-stress field at the subatomic scale induces the net thermal transport across the interface in the nonequilibrium state.

Investigation of the droplet dispersion influence on the atomization of liquid fuel processes in view of large-scale structures formation

This paper is devoted to an important research from the point of view of modern combustion physics of the problem of numerical modeling of spraying, ignition and combustion of liquid fuel at different values of the initial radius of the droplets. The need for a detailed study of the physicochemical processes taking place in the combustion of liquid fuels is determined by the increased requirements for the efficiency of various technical devices, the accuracy of the prediction of ignition, the burning rate and is because of modern environmental requirements for environmental protection. The efficiency of various technical devices, in particular internal combustion engines, is largely based on the results of a fundamental study of the processes of physics and combustion chemistry. Purpose of the work is to use methods of mathematical modeling, investigate the process of atomization, ignition and combustion of liquid fuel droplets at various initial radii, in the presence of turbulence, chemical transformations, and determine the optimal parameters for liquid fuel combustion. Computer simulation of the spray and combustion of liquid fuel injection was carried out using differential equations describing turbulent flow in the presence of chemical reactions. The theoretical significance of the research is that the fundamental results, which are got in this work, can be applied in the combustion's construction theory to achieve a deeper understanding of complex physical and chemical phenomena, something in the combustion chambers. The practical significance of computational experiments is that the results obtained can be used in the design of various technical devices using combustion, which would solve simultaneously the problem of process optimization, increasing the efficiency of fuel combustion and minimizing emissions of harmful substances.

Diffusion-Controlled Grain Growth in Liquid-Phase Sintering of W–Cu Nanocomposites

Abstract The kinetics of W grain growth during liquid-phase sintering (LPS) of W-15 – 25 wt.% Cu nanocomposite powder compacts was investigated in terms of microstructural development as well as theWatom diffusion process in liquid Cu. It was found that W grains grew rapidly while maintaining a round shape during LPS at 1623 K, being inversely proportional to the Cu matrix contents. This indicates that the diffusion-controlled Ostwald ripening (DOR) mechanism dominates the growth process in W–Cu nanocomposites. The result of this investigation on the kinetics of W grain growth definitely identifies the occurrence of DOR growth. Namely, the time exponent of the growth equation was close to the value of 3 and the kinetic constant forWgrain growth decreased with decreasing volume fraction of the W solid phase. This is basically due to the fact that the increase of a liquid matrix lengthens the diffusion path, as a result decreasing the growth rate. The diffusivity of the W atoms in liquid copper was calculated from the growth kinetics to be lower within an order of magnitude than the self-diffusion of the liquid atoms. The presence of Watom solubility in liquid Cu, as well as atom diffusivity, is thought to be responsible for diffusion-controlled growth of W grains during LPS of W–Cu nanocomposite powder.

Численное моделирование образования температурного факела неизотермических жидких впрысков

В данной работе представлены результаты вычислительных экспериментов по исследованию образования распыла и температурного факела неизотермических жидких впрысков при высокой турбулентности. Проведено численное моделирование распыла и горения жидких впрысков топлив и влияния начальной температуры газа в камере сгорания на данные процессы. Температура в камере сгорания менялась от 700 до 1500 К для двух видов жидких топлив: октана и додекана. Построены распределения капель жидких топлив по радиусам и по температуре в различные моменты времени. Выявлено оказываемое влияние начальной температуры на концентрационные характеристики топлива различного вида топлив. В результате установлено, что при давлении более 80 бар при высокой турбулентности за оптимальную принимается температура газа равная 900 К. При данной температуре камера прогревается до высоких значений температур и концентрация образующихся продуктов реакции наименьшая. Полученные результаты могут быть применены при построении теории горения жидких топлив и будут способствовать более глубокому пониманию сложных физико-химических явлений, которые происходят в камерах сгорания.

A study on the degassing mechanism of Pb-based alloy under ultrasonic treatment

In recent years, the ultrasonic technique is being developed at a pretty faster speed, whose assisting tactic has been applied in the electronic packaging field. What we need is to determine the mechanisms of ultrasonic treatments on electronic packaging fillers. In this paper, Pb-based alloy is typically chosen as the filler metal, in this paper, it is found that the flowability of which can be improved under ultrasonic vibration. Also, ultrasonic degassing efficiency is greatly promoted, with the best parameter for degassing that ultrasonic power and duration time of 1000W and 60s, a molten alloy’s treating temperature of 340°C. The assistance of ultrasonic is deeply explained from the thermodynamic aspect. Understanding of the atomic level interactions of liquid Pb atoms and vacuum is crucial, so we make an ultrasonic-assisted molecular dynamic simulation of melting Pb to room temperature to 330°C in 10ps, and the diffusion behavior of Pb atom under periodic vibration is revealed.

Minima of shear viscosity and thermal conductivity coefficients of classical fluids

The shear viscosity and thermal conductivity coefficients of various liquids exhibit minima along certain trajectories on the phase diagram. These minima arise due to the crossover between the momentum and energy transport mechanisms in gas-like and liquid-like regimes. We demonstrate that the magnitudes of the minima are quasi-universal in appropriately reduced units, especially for the viscosity coefficients. The results presented in support of this observation concern the transport properties of three simple model systems with different pairwise interaction potentials (hard spheres, Lennard–Jones, and Coulomb) as well as seven important real atomic and molecular liquids (Ne, Ar, Kr, Xe, CH4, CO2, and N2). The minima in viscosity and thermal conductivity represent useful reference points for fluid transport properties.

Effect of copper nanoparticles on thermal behavior of two-phase argon-copper nanofluid flow in rough nanochannels with focusing on the interface properties and heat transfer using molecular dynamics simulation

A comparison between the efficacy of surface boundary structure and presence of nanoparticles on the condensation two-phase flow inside rough nanochannels has been accomplished by applying molecular dynamics procedure to evaluate the thermal conductivity and flow characteristics. Simulation is performed in a computational region with two copper walls containing rectangular rough elements under different saturated temperatures. The main properties of liquid-vapor interface including density and the number of liquid atoms, are obtained. It is observed that the density profile is more affected by nanoparticles than the roughness. Also, compared to the condensation of nanofluid in a smooth nanochannel, the rough wall causes a greater drop in the temperature at the early time steps and by development of liquid films, effects of the wall roughness reduce. At the first of the condensation process, adding nanoparticle causes that transferring argon particles to the liquid phase increases with a steeper slope. Furthermore, HCACF for nanofluid condensation flow over considered correlation time is analyzed and following that the thermal conductivity for different saturated conditions is calculated. It has been represented that at lower temperatures the roughness makes more significant influence on the heat transfer of two-phase flow, while at higher temperatures the importance of nanoparticles prevails.

Molecular dynamics simulation of nonisothermal crystallization of a single polyethylene chain and short polyethylene chains based on OPLS force field

Abstract Molecular dynamics simulations on the nonisothermal crystallization of a single polyethylene chain and short polyethylene chains based on the all-atom model and optimized potentials for liquid simulations-all atom (OPLS-AA) force field are conducted in this article. Four all-atom single chain models with different chain lengths (C1000, C2000, C3000, and C4000) and four all-atom short chain models with the same chain length and different number of chains (2C500, 4C500, 6C500, and 8C500) are constructed. The collapse process at a high temperature of 600 K and the nonisothermal crystallization process with different cooling rates at the temperature range of 600–300 K are simulated. Roles of chain length, number of chains, cooling rate on the potential energy, van der Waals (V dw) energy, radius of gyration, root mean square deviation, and crystallinity are explored. By comparing with the existing results obtained by the united atom model, the validity and accuracy of this study are proved. Results show that in the collapse process, the chain length is the major factor, whereas the cooling rate has the greatest influence during the nonisothermal crystallization process. As the cooling rate decreases, a “platform” appeared in the V dw energy curve, which has a profound impact on the crystallization.

Prediction of the adsorption properties of liquid at solid surfaces with molecular scale surface roughness via encoding-decoding convolutional neural networks

Molecular dynamics (MD) simulation can effectively analyze the transport properties of liquid at the solid surface with different nanoscale roughness, while high computational costs are required. Herein, a deep learning encoding-decoding convolutional neural network is proposed to predict the adsorption density distribution of atomic and organic liquids under different molecular scale surface roughness. Compared with the previous deep learning studies focusing on simple macro adsorption parameters, our deep learning method realizes the prediction and visualization of micro scale adsorption behavior with very high accuracy. The data-driven deep learning algorithm replaces the MD extensive sampling and simplifies the operation process, which improves the computational efficiency of a single model 36000-fold. This study proves the good coupling between MD and deep learning method, which is helpful for designing surface geometry to obtain desirable interfacial transport properties of molecular liquid and complementing the nanoscale model system enabling the interactive visualization.