DSMC-LES方法数值模拟鼓泡流化床内气泡和颗粒流动行为

基于稠密气体分子动力学和气固两相流体动力学,建立流化床稠密气固两相离散颗粒运动-碰撞解耦模型,采用直接模拟蒙特卡罗方法(DSMC)模拟颗粒间的碰撞,采用考虑颗粒脉动流动对气相湍流流动影响的大涡模拟(LES)研究气相湍流,单颗粒运动满足牛顿第二定律,颗粒相和气相相间作用的双向耦合由牛顿第三定律确定.数值模拟流化床中颗粒流动以及气泡的生成、长大和破碎过程,获得颗粒轴向和径向速度的概率密度分布,及颗粒浓度分布.计算结果表明床内气泡的形成造成床内颗粒的循环,使得流化床内颗粒具有不同的轴向和径向脉动速度,颗粒分速度分布近似服从高斯分布.颗粒温度随颗粒浓度增加,达到最大值后,随颗粒浓度增大而下降.流化床颗粒浓度脉动主要是低频部分,高频分量较低,表明在流化床内颗粒浓度脉动频率低,能量高,颗粒浓度脉动主频率为0.4～1.0Hz,其值与Pain et al.获得的颗粒浓度脉动主频率基本吻合.     

Reducing Statistical Noise and Extending the Size Spectrum by Applying Weighted Simulation Particles in Monte Carlo Simulation of Coagulation

The direct simulation Monte Carlo (DSMC) method is widely utilized to simulate microscopic dynamic processes in dispersed systems that give rise to the population balance equation. In conventional DSMC approaches, simulation particles are equally weighted, even for broad size distributions where number concentrations in different size intervals are significantly different. The resulting statistical noise and limited size spectrum severely restrict the application of these DSMC methods. This study proposes a new Monte Carlo (MC) method, the differentially weighted time-driven method, which captures the coagulation dynamics in dispersed systems with low noise and is simultaneously able to track the size distribution over the full size range. Key elements of this method include constructing a new jump Markov process based on a new coagulation rule for two differentially weighted simulation particles, and restricting the number of simulation particles in each size interval within prescribed bounds. The method is validated by using an ideal coagulation kernel with a known analytical solution and a real coagulation kernel for which an accurate solution can be found numerically (self-preserving particle size distribution in the continuum regime).     

Monte Carlo Simulation of Kinetics of Ammonia Oxidative Decomposition over the Commercial Propylene Ammoxidation Catalyst (Mo-Bi)

Monte Carlo method is applied to investigate the kinetics of ammonia oxidative decomposition over the commercial propylene ammoxidation catalyst(Mo-Bi). The simulation is quite in agreement with experimental results. Monte Carlo simulation proves that the process of ammonia oxidation decomposition is a two-step reaction.     

Mo-Bi系丙烯氨氧化催化剂上氨分解反应动力学的Monte Carlo模拟

Monte Carlo method is applied to investigate the kinetics of ammonia oxidative decomposition over the commercial propylene ammoxidation catalyst(Mo-Bi).The simulation is quite in agreement with experimetal results.Monte Carlo simulation proves that the process of ammonia oxidation decomposition is a two-step reaction.     

Short-Range and Medium-Range Order in Lithium Silicate Glasses, Part II: Simulation of the Structure by the Reverse Monte Carlo Method

The results of X-ray and neutron diffraction experiments with (Li₂O) _x (SiO₂)_{100_x} glasses (x = 0, 20, 33.3, and 40) are discussed using the Reverse Monte Carlo method to produce atomic clusters, which are analyzed with respect to the O-O distribution, the angular correlations within the first and higher coordination spheres, and the bond-orientational multipole moments. Within Li₂O-containing silicate glasses, each lithium atom is surrounded by a ring of six SiO₄ tetrahedra in such a way that four oxygen atoms lie at the joints of a tetragonal cell centered by lithium. Thus, the lithium atoms increase the order of the glass.     

Unfolding events of Chymotrypsin Inhibitor 2 (CI2) revealed by Monte Carlo (MC) simulations and their consistency from structure-based analysis of conformations

We investigated the behavior of the conformations of Chymotrypsin Inhibitor (CI2) from the native to the denatured states, obtained in Monte Carlo (MC)/Metropolis simulations, where a low-resolution model is used together with knowledge-based potentials. New conformations starting from the X-ray native structure are generated by random perturbations along with a constraint to increase the radius of gyration. Unfolding is also simulated by unrestrained simulations at a higher temperature. All simulations yield a similar sequence of unfolding events. The preferred pathway starts with loss of native contacts between (N-terminal)-β₃ and continues with β₂-β₃. The persistence of the contacts between β₁ and β₂ at intermediate values of the fraction of native contacts (Q); whereas, highly unfolded conformations with only some helical contacts persisting at low values of Q, are observed. Structure-based analysis of the fluctuations of the unfolded conformations by Gaussian Network Model (GNM) reveals that the termini of the chain—C terminus being more mobile—depict relatively higher flexibility with a native-like hinge near β₂ that divides the structure into two domains. The fluctuations of the two domains are negatively correlated, with partly folded α-helix and a small hydrophobic cluster in the middle of the chain displaying positively correlated fluctuations. The most persistent short-range rotational bond correlations are observed between the residues of α-helix, C terminus of the β₁-part of the reactive site loop, and around the C terminus of the β₂. The latter regions also appear as hot spots; i.e. high frequency fluctuating regions, of the structure surviving in unfolded conformations. The results imply that the unfolded CI2 has an intrinsic ability to undergo correlated fluctuations along with some residual native structure specifically induced by its sequence, consisting at the lowest level of a single hinge.     

Conformational properties of poly(diallyldimethyl ammonium chloride) (PDDA) determined by combination of molecular dynamics, rotational isomeric states and Monte Carlo procedures

A theoretical model describing the conformational properties of poly(diallyldimethyl ammonium chloride) (PDDA) is developed with the following strategy: molecular dynamics simulations are performed on model molecules representing dimers of PDDA. The results are employed to formulate a rotational isomeric state model for these molecules in terms of short-range interactions. Furthermore, the MD trajectories permit the evaluation of conformational energies and probabilities for the 27 conformations allowed to the three bonds sequence contained in the dimers. These probabilities are then employed to generate single chains of PDDA in vacuo according to standard Monte Carlo procedures and their main squared end to end distance 〈r²〉 are computed. The procedure (MD simulations, evaluation of energies and probabilities and MC calculations) is then repeated for two more realistic systems obtained by fitting into a cubic box, having side length L and periodic boundary conditions, a forty repeating units oligomer of PDDA together with 40 Cl⁻ as counterions and 500 water molecules (referred to as water system, ) or the oligomer with its counterions, 500 water molecules and 20 NaCl molecules (referred to as salt system, ). The behavior of the three systems is noticeably different and the value of unperturbed dimensions computed for the realistic systems are in good agreement with preliminary results of experimental measurements being carried out in our laboratory.     

EDTA直接法测定Ln（NO3）3—邻二氮菲—氟脲嘧啶中稀土含量的研究

研究了在配体邻二氮菲及氟脲嘧啶存在下,直接用ＥＤＴＡ容量法测定稀土硝酸盐—邻二氮菲—氟脲嘧啶配合物（［Ｌｎ（Ｐｈｅｎ）２（Ｆｕ（ＮＯ３）］（ＮＯ３）２）（Ｌｎ＝稀土元素）［１］,终点颜色变化敏锐,结果准确。     

Grand canonical Monte Carlo simulation and volumetric equilibrium studies for adsorption of nitrogen, oxygen, and argon in cadmium (II) exchanged zeolite A

The adsorption of nitrogen, oxygen and argon has been studied in cadmium (II) ions exchanged zeolite A at 288.0 and 303.0?K. Experimentally measured adsorption isotherms are compared with theoretically calculated data using grand canonical Monte Carlo (GCMC) simulation. Nitrogen showed higher adsorption capacity and selectivity than oxygen and argon in these zeolite samples. The cadmium exchanged zeolite A showed increased adsorption capacity for nitrogen, oxygen, and argon with increase in cadmium (II) exchange levels. Isosteric heat of adsorption data showed stronger interactions of nitrogen molecules with cadmium (II) cations in zeolite samples. These observations have been explained in terms of higher electrostatic interaction of nitrogen with extra framework zeolite cations. The selectivity of oxygen over argon is explained in terms of its higher interaction of oxygen with cadmium exchanged zeolites than argon molecules. Heats of adsorption and adsorption isotherms were also calculated using grand canonical Monte Carlo simulation algorithm. Simulation studies expectedly show the proximity of nitrogen molecules to the locations of extra framework sodium and cadmium cations.     

Oxygen sites active in H-abstraction at a Cu(110)-O surface: Comparison of a Monte Carlo simulation with imide formation studied by XPS and VEELS

A Monte Carlo simulation of the development of the topographic structure of the Cu(110)-O overlayer has provided quantitative data for the distribution of different oxygen sites during formation of the overlayer. Those oxygen atoms present at the ends of copper-oxygen chains are shown to be able to account for the observed activity of the overlayer in imide formation through H-abstraction from ammonia. The Monte Carlo simulation also mimics closely features of the Cu(110)-O system observed by scanning tunnelling microscopy.     

Novel Diglycolamic Acid Functionalized Iron Oxide Particles for the Mutual Separation of Eu(III) and Am(III)

Iron oxide (Fe₃O₄) particles functionalized with diglycolamic acid (Fe-DGAH) were synthesized and characterized by TG-DTA, X-Ray diffraction,¹H NMR, and scanning electron microscopy (SEM). The extraction behavior of Am(III) and Eu(III) in Fe-DGAH was studied from dilute nitric acid medium to examine the feasibility for the mutual separation of trivalent actinides and lanthanides using Fe-DGAH. For this purpose, the effect of various parameters such as the duration of equilibration and concentrations of europium, nitric acid, and diethylenetriaminepentaacetic acid (DTPA) in the aqueous phase on the distribution ratio (K_d) of Am(III) and Eu(III) was studied. The conditions needed for the efficient separation of Am(III) from Eu(III) were optimized using DTPA. The distribution ratio of ?10⁴ mL/g was obtained for both Am(III) and Eu(III) at pH 3, and it decreased with an increase in the concentration of nitric acid. However, a separation factor of Eu(III) over Am(III) of ?150 was achieved in the presence of DTPA. Rapid sorption of metal ions in the initial stages of equilibration followed by the establishment of equilibrium occurred within 2 h. The sorption data were fitted to the Langmuir adsorption model, and the apparent europium sorption capacity was determined to be ?50 mg/g. The study indicated the feasibility of using Fe-DGAH particles for magnetic separation of Eu(III) from Am(III) with high separation factors.     

The Nano-Particle Mass Classifier (Nano-PMC): Development,Characterization, and Application for Determining the Mass,Apparent Density,and Shape of Particles with Masses Down to the Zeptogram Range

Existing aerosol particle mass classifiers (PMCs) can classify particles having masses down to ca. half an attogram (i.e., 10^?18 g), which corresponds to a diameter of ca. 10 nm for spherical particles with standard density (1 g/cm³). Here, we describe an improved design of such a classifier, namely, the nano-PMC, which can classify particles with masses down to 20 zeptograms (10^?21 g). The response of the classifier was characterized with spherical polystyrene-latex and ammonium sulfate particles, produced by atomization and mobility classification. Measured responses were compared with predictions by a numerical trajectory-based model that considers particle diffusivity. Measurements and predictions of the mean mass of the particles penetrating the classifier agreed within experimental uncertainty (<6%). Differences in the spectrum width could be attributed to recirculation flows occurring in the classification channel.

To demonstrate the capabilities of a nano-PMC, we used it in a tandem configuration with a differential mobility analyzer to determine (1) the size-dependent shape factor of cubic sodium chloride particles having diameters from 15 to 120 nm, and (2) the apparent density and mass–mobility coefficient of coalesced and aggregated silver particles generated by spark ablation. Measurements of the shape factor of the cubic sodium chloride particles show good agreement with previous observations. Coalesced silver particles exhibited an apparent density that was lower compared with that of bulk silver, suggesting a slightly non-spherical particle shape. The mass–mobility scaling exponent of aggregated silver particles determined by the measurements was 2.3 ± 0.1.

Copyright 2015 American Association for Aerosol Research     

Field-Flow Fractionation (FFF) and Related Techniques for the Separation of Particles,Colloids and Macromolecules

Thin-channel flow techniques can be used for the separation of samples ranging over 15 orders of magnitude in molecular weight, namely, macromolecules, colloids, and particles. The first part describes the group of field-flow fractionation techniques. Flow system components are illustrated and described, as well as mode of operation and theory behind retention in the thin channel. An update of current topics under research is given, namely, normal retention mode of Brownian particles (under 1 μm in diameter) as compared to the steric retention mode of non-Brownian particles (above 1 μm in diameter). Optimization of the separation by field programming is briefly discussed. Another group of thin-channel techniques for small-scale processing, the split-flow thin (SPLITT) cells, in which fractionation is based on mass transport across the channel thickness, is presented in the second part. Principles of separation in this technology as well as possible applications are illustrated and discussed.     

Simulation Studies of Reconstruction Algorithms for the Determination of Optimum Operating Parameters and Resolution of Graded Screen Array Systems (Nonconventional Diffusion Batteries)

Graded screen array (GSA) systems (nonconventional wire screen diffusion batteries) are increasingly finding application in the measurement of radioactive ultrafine cluster aerosols (0.5–5 nm in diameter), where conventional diffusion batteries are of limited utility. The GSA systems typically consist of varying mesh number, single/multiple wire screen stages operated either in series or in parallel, with the choice of a wide range of wire screen parameters and sampling flow rates. Numerical simulations studies of the expectation-maximization and Twomey size distribution reconstruction algorithms have yielded concepts that permit the optimization of GSA operating parameters, together with the determination of the potential size resolution of experimental GSA systems, the effects of random errors in the data, and algorithm stopping criteria. In general, a choice of wire screen operating parameters that result in the uniform fractioning of the sampled distribution among the various GSA stages and the inference of size intervals with successive midpoint diameters differing in diffusion coefficients by about one order of magnitude were found to be beneficial to the reconstruction process.     

Synthesis and Characterization of Magnesium Alloy Surface Composite (AZ91D - SiO2) by Friction Stir Processing for Bioimplants

Silicon - Magnesium shows potential for bioimplant applications because of its high biocompatibility and equivalent bone strength. However, the high corrosion rate of magnesium and magnesium alloys...     

Determination of the glass-rubber transition of poly(vinyl chloride) and poly(ethylene terephthalate) by small-angle X-ray scattering

Semicrystalline polymers can be regarded as systems of two phases, an amorphous and a crystalline phase, with different electron densities and different thermal expansion coefficients. The small-angle X-ray scattering of these systems is proportional to the square of the difference between the electron densities of the phases and so increases with temperature. As Fischer and Kloos have shown¹, this increase is discontinuous at the glass-rubber transition temperature, T_g, so providing a method for determining T_g and the difference α_ar-α_ag between the thermal expansion coefficients of the amorphous phase above and below T_g. This method was applied to poly(vinyl chloride) (PVC) and poly (ethylene terephthalate) (PETP). For the latter, small-angle X-ray scattering in addition constitutes a method for following and interpreting physical aging.     

Cysteine-Functionalized Chitosan Magnetic Nano-Based Particles for the Recovery of Uranium(VI): Uptake Kinetics and Sorption Isotherms

Magnetic-chitosan nanoparticles, functionalized with cysteine, were synthesized and characterized by element analysis, FT-IR, XRD, TEM, and vibrating sample magnetometry. The sorbent was tested for U(VI) recovery, considering:

• a. pH effect,

• b. sorption isotherms (fitted by Langmuir equation), and

• c. uptake kinetics (modeled using the PSORE).

Maximum sorption capacity approached 100 mg U g^?1. The nanometric size of sorbent reduces the impact of resistance to intraparticle diffusion; this may explain the fast kinetics (equilibrium within 50 min). The reaction is exothermic, spontaneous. The metal could be desorbed using acidified urea solution and the sorbent could be recycled for 5 cycles.     

Direct Evidence for the Modulation Caused by Ti Substitution of Ta in a (Ta2O5)0.92(TiO2)0.08 Ceramic by Analytical Electron Microscopy

The high‐temperature tetragonal structure of Ta₂O₅ was observed in a Ta₂O₅ ceramic sample doped with 8 at.% TiO₂. Direct evidences for the modulation along [] direction caused by Ti replacing Ta are obtained based on atomic‐resolution high‐angle annular dark‐field lattice imaging and electron energy loss spectroscopy analysis. The atomic mechanism of Ti substitution of Ta is explicitly described based on the structure model along [110] direction projection.     

Characterization and Electrophoretic Deposition of Poly(Phenylsilsesquioxane)–Titania Hybrid Particles Prepared by the Sol–Gel Method

Poly(phenylsilsesquioxane)–titania (PhSiO_3/2–TiO₂) hybrid particles were prepared from phenyltriethoxysilane and titanium tetra- n -butoxide by the sol–gel method. Fourier transform infrared spectra showed that PhSiO_3/2 and the TiO₂ components were hybridized through Si–O–Ti bonds. The refractive index of the particles was monotonically increased from 1.57 to 1.62 with an increase in the TiO₂ content. The PhSiO_3/2–TiO₂ particles were electrophoretically deposited on indium tin oxide (ITO)-coated glass substrates to form opaque, thick films about 3 μm in thickness. When the mole ratio x in (1− x )PhSiO_3/2· x TiO₂ was equal to or less than 0.05, the deposited PhSiO_3/2–TiO₂ films became transparent with a heat treatment at 400°C because of the thermal sintering of the particles.