可压稠密气固两相流动过程的模拟计算

基于稠密气体分子运动论和颗粒动理学,建立可压稠密气固两相流动模型。采用梯度模拟来考虑气相可压缩性对气相湍流的影响。模拟计算表明气固两相射流速度沿轴向和径向减小,颗粒浓度下降。气固两相射流具有高的颗粒温度,呈现强烈的气固两相湍流流动特性。     

Geometric Phase for Two Entangled Spin-1/2 Particles in a Magnetic Field

In this paper the geometric phases of two entangled spin-1/2 particles in the presence and absence of spin-spin interaction are calculated. We also discuss the geometric phases when only one of the two particles is affected by the external magnetic field. Our results show that the geometric phase in this case is not equal to that of a single particle under the same evolution condition because of the effect of entanglement. We further study the entanglement dependence of the noncyclic geometric phases in the interacting and noninteracting spins under a time-independent uniform magnetic field. A general entanglement-dependence geometric phase is formulated.     

2维静磁场多参量微动粒子群优化算法

提出一种微动粒子群优化算法,针对2维静磁场多参量优化问题,在给出轴上目标轴向磁感应强度分布曲线的前提下,可以得到趋近于该分布曲线的磁结构。该算法分为前后两阶段:第一阶段采用前后试探法(微动),同时也参照最优粒子的信息;第二阶段采用基本粒子群优化算法。微动粒子群优化算法可以发挥多核计算机在工程设计上的潜力,而且即使粒子数目很少,也能不断趋近目标解。     

Infrared spectroscopy on size-controlled synthesized Pt-based nano-catalysts

Pt, Ru and Pt/Ru nano-particles, synthesized in ethylene glycol solutions, are studied using infrared (IR) spectroscopy and high resolution transmission electron microscopy (HRTEM). The synthesis method allows the control of the mono- and bi-metallic catalyst particle sizes between 1 and 5.5 nm. The IR spectra of CO adsorbed (CO_ads) on the Pt, Ru and bi-metallic Pt/Ru colloids are recorded as a function of the particle size. The stretching frequency of CO_ads depends on the particle size and composition. Strong IR bands due to the stretching vibration of CO_ads are observed between 2010 and 2050 cm⁻¹ for the Pt nano-particles, while two IR bands between 2030 and 2060 cm⁻¹ for linear bonded CO_ads, and at lower wavenumbers between 1950 and 1980 cm⁻¹ for bridged bonded CO_ads, are found for the Ru particles. The IR spectra for the Pt/Ru nano-sized catalyst particles show complex behaviour. For the larger particles (>2 ± 0.5 nm), two IR bands representative of CO_ads on Ru and Pt-Ru alloy phases, are observed in the range of 1970-2050 cm⁻¹. A decrease in the particle size results in the appearance of a third band at ∼2020 cm⁻¹, indicative of CO_ads on Pt. The relative intensity of the band for CO_ads on the Pt-Ru alloy vs. the Pt phase decreases with decreasing particle size. These results suggest that Ru is partially dissolved in the Pt lattice for the larger Pt/Ru nano-particles and that a separate Ru phase is also present. A Pt-Ru alloy and Ru phase is observed for all Pt/Ru particles prepared in this work. However, a decrease in particle size results in a decrease of the number of Pt and Ru atoms in the Pt-Ru alloy phase, as they are increasingly present as single Pt and Ru phases.     

Effect of particles on the flow structure and dispersion of solid impurities in a two-phase axisymmetric jet

The Euler approach is used for studying the structure of a flow and the propagation of a disperse impurity in a submerged two-phase jet for small values of the mass concentration of particles (M _L1 = 0 to 0.5) upon a variation of the size and material of particles in a wide range. The effect of particles on the propagation of a two-phase jet, gas turbulence, and solid phase dispersion is analyzed. The addition of particles decreases the jet opening angle, increases the jet range, suppresses turbulence, and deteriorates turbulent mixing with the surrounding submerged space. It is shown that at the first stage, particle accumulation effects (pinching) in the axial region of the jet appear upon an increase in the particle size and the density of the particle material. Then, upon an increase in the inertia of particles, pinching changes to intense scattering of the disperse phase in the initial cross sections of the jet. The results are compared with the results of measurements for mono- and polydisperse two-phase jet flows.     

Dynamics and stability of a linear cluster of spherical magnetic nanoparticles

Magnetic particles freely moving in a fluid may organize themselves into dense phases, bulk clusters, or linear chains. The dynamics of particles forming a chain is analyzed theoretically taking into account the magnetic dipole interaction as well as the Van der Waals molecular interaction. The vibrational spectrum has two branches (the magnetic branch associated with the rotation of the magnetic moment of a particle and the elastic branch associated with the displacement of particles). In the case of particles with constant mass density and magnetic moment, which is interesting for applications, these two modes are in fact independent; i.e., the effects of mode hybridization are weak. However, these effects can be manifested for hollow particles. From analysis of the vibrational spectrum, the criterion for the chain stability to a transition to a denser phase is established.     

Interaction of a Particle‐Laden Gaseous Jet with a Confined Annular Turbulent Flow

A numerical analysis of polydispersed glass particles interacting with a confined turbulent bluff‐body flow was performed by combining the finite‐volume method for the gaseous flow with a mesh‐free Lagrangian approach for the particulate flow. Three turbulence‐closure models, namely the Reynolds‐stress, the standard k‐ϵ, and the nonlinear k‐ϵ models, were first comparatively studied for the single‐phase flow. The second‐moment Reynolds‐stress model was then selected for the prediction of the turbulent gaseous flow in a gas‐particle system, where an improved eddy‐interaction model was used to predict turbulence‐induced particle dispersion. The interaction between the two phases was accounted for through coupling source terms. Numerical predictions of two‐phase mean and fluctuating velocities for particle sizes ranging from 15 to 115 μm were compared with corresponding experimental data. Reasonably good agreement was achieved for the mean properties of both the gaseous and particulate flows.     

Spontaneous symmetry breaking on a mutiple-channel hollow cylinder

This Letter investigates coupled asymmetric exclusion processes with two types of particles on multiple parallel channels of a hollow cylinder. The model is inspired by the structure of microtubules, along which motor proteins such as kinesins and dyneins move in opposite directions. Interactions between two-species particles are assumed to take place only on the left and right boundaries where a rule of narrow entrances is applied. Narrow entrances mean that a particle cannot enter the system if either of two nearest-neighbor sites on the neighboring channels is occupied by a particle of the other species. This rule is similar to, but different from, that in [E. Pronina, A.B. Kolomeisky, J. Phys. A 40 (2007) 2275] since the narrow entrance rule in our model involves two neighbors. The phase diagram of our model is studied theoretically and via Monte Carlo simulations. The spontaneous symmetry breaking (SSB) is observed in the system. There are four possible phases in the system and with SSB occurring in two of them: high/low density and asymmetric low/low density. Bulk density and particle currents are also computed. Theoretical calculations deviate from Monte Carlo simulation results due to the neglecting of correlations in particles dynamics in mean-field analysis.     

On the restoration of the magnetic phase diagram of a superparamagnetic material

The measurement reduction method is used to restore the magnetic phase diagram for a system of superparamagnetic particles with different dispersion abilities. It is shown that using this method makes it possible to considerably reduce the consequences of the particle size spread and determine the domains of existence of different magnetic phases at the phase diagram.     

Evolution of crystallization structure during deep deformation and annealing of single crystals of austenitic carbon steel

The distribution of alloying elements, impurities, and secondary phases in axial and interaxial regions was studied for single crystals of the austenitic carbon steel, 04Kh17N14M3BG2, with the classical cross-shaped dendritic structure. It was shown that the interaxial regions are enriched with chromium, molybdenum, and carbon and contain precipitates of Cr₂₃C₆ and Mo₂C. The chemical and phase inhomogeneities, connected with dendritic crystallization, are stable with respect to the effect of deep deformation and annealing, and in the finished product evolve into bands with a different content of impurities and second phase particles.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 62–66, November, 1991.     

Phase transitions in a system of unstable particles

A study is reported of phase separation in a system of particles created at a constant rate and having a finite lifetime. It is shown that (1) phase separation is possible if the particle lifetime exceeds a certain critical value, (2) the particle-density difference between the phases depends on particle lifetime, and (3) the correlation function in the two-phase region oscillates (with damping) as a function of spatial coordinates, which implies correlation between the phase locations. Fiz. Tverd. Tela (St. Petersburg) 40, 741–745 (April 1998)     

受碾区域内颗粒轴向流动特性的离散元模拟

为探讨受碾状态颗粒的稳定流动, 在碾辊轴与筛筒组成的受碾区域内, 建立了轴向运动的颗粒流离散元物理模型. 研究结果表明: 受碾区域内各颗粒沿轴向运动能力的差异造成了颗粒流密度不均匀; 颗粒与筛筒间的静摩擦系数影响颗粒轴向流动的形态、速率及集散程度, 受碾区域内单层颗粒的轴向均方偏差与流动时间的平方正相关, 属于“super”扩散; 整体分析受碾区域发现, 颗粒的轴向平均速度沿轴向坐标逐渐增大, 而颗粒的三轴合成平均速度沿轴向坐标逐渐降低; 受碾区域内各轴向位置处颗粒运动的剧烈程度不同, 沿轴向坐标颗粒的波动速度平方呈现先增大后降低而后又增大的趋势; 单颗粒的碰撞总能量损失能谱也表明了颗粒运动程度不同, 即轴向流动时在受碾区域的前半段碰撞剧烈, 能量损失多, 在后半段碰撞程度弱, 能量损失较少. 通过对受碾区域内颗粒流动的数值模拟分析, 明晰了颗粒在受碾条件下稳定流动特性, 有益于碾磨工业对产品品质控制及设备参数优化的研究.     

Chemically selective NMR imaging of a 3-component (solid-solid-liquid) sedimenting system

A novel magnetic resonance imaging (MRI) technique which resolves the separate components of the evolving vertical concentration profiles of 3-component non-colloidal suspensions is described. This method exploits the sensitivity of MRI to chemical differences between the three phases to directly image the fluid phase and one of the solid phases, with the third phase obtained by subtraction. 19F spin-echo imaging of a polytetrafluoroethylene (PTFE) oil was interlaced with 1H SPRITE imaging of low-density polyethylene (LDPE) particles. The third phase was comprised of borosilicate glass spheres, which were not visible while imaging the PTFE or LDPE phases. The method is demonstrated by performing measurements on 2-phase materials containing only the floating (LDPE) particles, with the results contrasted to the experimental behaviour of the individual phases in the full 3-phase system. All experiments were performed using nearly monodisperse particles, with initial suspension volume fractions, phi(i), of 0.1.     

Phase separation and structure in a concentrated colloidal dispersion of uniform plates

The structure and phase behaviour of a colloidal dispersion of plate-like particles are described. The plates are nickel (II) hydroxide and have short-range, repulsive interactions and a low polydispersity. As the concentration of the plates is increased, an equilibrium phase separation between a columnar phase and a less ordered phase is observed. Complementary measurements using small-angle neutron and small-angle X-ray scattering have been used to distinguish the columnar phase from other possible ordered structures. Previously isotropic-nematic phase transitions have been observed [#!ref1!#], however this dispersion forms the more highly ordered columnar phase, due to the aspect ratio and the low polydispersity of the plate-like particles. The concentration at which phase separation occurs, increases as the range of the particle interactions is reduced. This system provides an interesting model for comparison with theory and calculations of structures in liquid crystal and mesophase in which the particle interactions can be altered. Received 24 February 1999     

Population growth in random media. I. Variational formula and phase diagram

We consider an infinite system of particles on the integer lattice Z that: (1) migrate to the right with a random delay, (2) branch along the way according to a random law depending on their position (random medium). In Part I, the first part of a two-part presentation, the initial configuration has one particle at each site. The long-time limit exponential growth rate of the expected number of particles at site 0 (local particle density) does not depend on the realization of the random medium, but only on the law. It is computed in the form of a variational formula that can be solved explicitly. The result reveals two phase transitions associated with localization vs. delocalization and survival vs. extinction. In earlier work the exponential growth rate of the Cesaro limit of the number of particles per site (global particle density) was studied and a different variational formula was found, but with similar structure, solution, and phases. Combination of the two results reveals an intermediate phase where the population globally survives but locally becomes extinct (i.e., dies out on any fixed finite set of sites).     

Experimental characterization of ultrafine particle emissions from a light-duty diesel engine equipped with a standard DPF

A detailed experimental characterization of the particle emissions of a EURO 5 light-duty Diesel engine, equipped with a wall flow Diesel Particulate Filter (DPF), is presented. Particle Size Distributions (PSDs) in the range from 4.5 up to 160 nm have been measured at the engine exhaust by means of a Scanning Mobility Particle Sizer, during both DPF accumulation and regeneration phase. During accumulation, the size-dependent DPF removal efficiency has been evaluated by PSDs measurements at both inlet and outlet of the filter. The maximum efficiency was found in the range 10-40 nm. Nevertheless, the DPF ensures a strong reduction in the emissions for the sub-23 nm nanoparticles too, with an efficiency between 91–95%. Unless for the warm-up phase, the engine operating conditions do not have a great impact on the particles emitted due to the high DPF removal efficiency. The measurements carried out during the regeneration phase evidenced that particle emissions strongly depend on DPF temperature. Actually, once the soot burnout temperature is reached, soot oxidation starts and a great amount of particles in terms of PN is released. Overall, PSDs showed an increase in particle number concentration up to two order of magnitude with respect to the emissions measured during the accumulation phase.     

Nonlinear responses of electronic-excitation-induced phase transformations in GaSb nanoparticles

We studied electronic-excitation-induced phase transformations in nanoparticles using transmission electron microscopy. GaSb particles excited by 75 keV electrons transform to two phases consisting of an antimony core and a gallium shell or an amorphous phase, or remain in the original crystalline phase, depending on particle size and/or temperature. It is suggested that such nonlinear responses of the phase transformations may arise from synergistic effects of bond instability, localized excitations, enhanced diffusivity, or thermal equilibrium in reactions.     

The kinetics of homogeneous nucleation of silver nanoparticles stabilized by polymers

The formation of Ag nanoparticles synthesized by homogeneous nucleation, stabilized by polymers (PVA and PVP) was monitored by UV–Vis spectrophotometry and transmission electron microscopy. Our aim was to differentiate between the two main phases of particle formation, i.e. nucleation and growth and to characterize their rates with the help of appropriate kinetic equations. Time resolved spectrophotometric measurements revealed that particle formation is an autocatalytic process: a slow, continuous nucleation phase (3–5 min) is followed by a rapid, autocatalytic growth phase where the maximal particle size is 5–7 nm. By freezing the reaction mixture, the process of particle growth can be followed from 5 to 40 min on TEM pictures. The first order rate constants were calculated and they are strongly depend on the polymer concentration. If the growing particles are attached by PEI to the surface of a solid support, the formation of silver nanoparticles can also be followed by atomic force microscopy (AFM) and we can control the particle growth on mica surface. The cross section analysis of the pictures show, that the particle growing process can be also monitored at solid–liquid interface.     

The robustness in dynamics of out of equilibrium bidirectional transport systems with constrained entrances

Macroscopic and microscopic long-distance bidirectional transfer depends on connections between entrances and exits of various transport mediums. Persuaded by the associations, we introduce a small system module of Totally Asymmetric Simple Exclusion Process including oppositely directed species of particles moving on two parallel channels with constrained entrances. The dynamical rules which characterize the system obey symmetry between the two species and are identical for both the channels. The model displays a rich steady-state behavior, including symmetry breaking phenomenon. The phase diagram is analyzed theoretically within the mean-field approximation and substantiated with Monte Carlo simulations. Relevant mean-field calculations are also presented. We further compared the phase segregation with those observed in previous works, and it is examined that the structure of phase separation in proposed model is distinguished from earlier ones. Interestingly, for phases with broken symmetry, symmetry with respect to channels has been observed as the distinct particles behave differently while the similar type of particles exhibits the same conduct in the system. For symmetric phases, significant properties including currents and densities in the channels are identical for both types of particles. The effect of symmetry breaking occurrence on the Monte Carlo simulation results has also been examined based on particle density histograms. Finally, phase properties of the system having strong size dependency have been explored based on simulations findings.