Statistical modeling of the geometrical structure of the system of artificial air pores in autoclaved aerated concrete

The geometrical structure of the system of artificial air pores in autoclaved concrete is studied, based on samples from laboratory and industry. The single pores are approximated by spheres, the corresponding radius distribution is determined by means of stereological methods. The resulting distribution is not a classical standard distribution, but instead requires a mixture of three components, one of them Gaussian. The pore system as a whole is approximated by the penetrable-concentric-shell or “cherry-pit” model, which consists of spheres that are partially penetrable. The corresponding hard cores form a random packing of hard spheres. The goodness-of-fit of the model is tested by means of information available from planar sections: the section profile radius distribution and the pair correlation function of profile centers.     

Derivation of pacticle size distributions from measurements on plane sections through particle beds. Effect of regularity of particle packing

When the size distribution of particles embedded in an opaque, continuous solid phase is required, the general approach is to deduce the distribution from the size distribution of particle cross-sections in a plane cut through the particle bed. When the particles are approximately spherical, this deduction can be performed by making the assumption that the distances from the plane of cut to the particle centers are rectangularly distributed. The validity of this assumption does not, however, appear to have been investigated in previously published work, and in the present contribution the assumption has been considered more closely.The distribution of the distances between the sphere centers and a random plane has been investigated both theoretically and experimentally, starting with regular packings of monosized spheres, and extending the treatment to monosized spheres packed at random and finally to packings of polysized spheres.The distribution of particle center to cutting plane distances will approach a rectangular distribution with increasing sample size even for strictly regular packings. However, for finite samples, the packing regularity may significantly affect the extent to which such a distribution is realized.     

Analysis and modeling of mesophase sphere generation, growth and coalescence upon heating of a coal tar pitch

The present study has been undertaken aiming: (1) to provide an experimental procedure for measurement of characteristics of mesophase sphere formation from a coal tar pitch upon heating at 673-723 K in terms of the time-dependent changes in the population density and the radius distribution of the spheres and (2) to prove that the nucleation, growth and coalescence of the spheres are essential for quantitative explanation of the time-dependent changes. In regard to (1), this paper describes the conditions that are required in microscopic analysis of cross-sections of the bulk of heat-treated pitch for the conversion of the population density and the radius distribution of the cross-sectional circles into those of spheres on volume basis. Regarding (2), a model analysis demonstrates that the change in the radius distribution with time, as well as that in the population density of spheres, can quantitatively be predicted. The analysis also reveals that the spheres grow without undergoing coalescence in the early period, and the sphere coalescence commences after termination of sphere generation while the growth continues.     

Numerical simulation of bidisperse hard spheres settling in a fluid

Average settling velocity of non-uniform hard spheres in a viscous fluid is determined by using a large-scale numerical simulation that is carried out for over 103 spheres in a periodic unit cell which extends infinitely. An efficient calculation scheme is used for reducing the computation cost which steeply increases with the number of the spheres. The calculation scheme is based on a fast summation method for far-field hydrodynamic interaction among spheres. It is applied in the computation of hindered settling velocity of hard spheres with bidisperse size distribution in a viscous fluid. The simulation results are compared with the theoretical predictions by Batchelor [8] and Davis and Gecol [9]. It is found that the prediction by Davis and Gecol reasonably agrees with the numerical results.     

Segregation under flow of objects suspended in a yield stress fluid and NMR imaging visualisation

This study demonstrates the segregation of spheres suspended in a gelled fluid, in laminar flow, in a sudden expansion. The flow conditions are such that gravity effects are negligible. The spheres are stable in the gel at rest. The upstream pipe diameter is in a ratio of 4:1 to the diameter of the spheres. In these circumstances, the medium is discrete, not continuous. It is shown that the initial conditions and flow kinematics lead to segregation of the matter in the downstream pipe. Nuclear magnetic resonance imaging reveals an organised distribution of the solid matter downstream of the expansion. The effects of volume concentration, geometry and flow velocity were assessed. A model of the phenomenon is proposed.     

Theoretical Study of Equilibrium Bipolar Charge Distribution on Nonuniform Primary Straight Chain Aggregate Aerosols

A general theory describing the equilibrium bipolar charge distribution for straight chain aggregate aerosols consisting of primary spheres of different diameters was derived from a theory previously developed for linear chain aggregate of uniform spheres. The present theory is based on the assumptions that (1) the individual primary particles of a straight chain aggregate are charged independently, (2) the probability that a particular primary particle has acquired q elementary charges is governed by the Gaussian distribution predicted by Boltzmann's law, based on particle size; and (3) the resultant charge of a straight chain aggregate is the algebraic sum of the charges carried by the constituent primary spheres. The present theory can be stated as follows: The equilibrium bipolar charge distribution of straight chain aggregate aerosols with nonuniform primary spheres can be expressed by Boltzmann's law with an equivalent diameter such that d_eL = Σ ⁿ _i=1 d_i . The limitations imposed by the assumptions are also discussed.     

Photocatalytic activity of titania hollow spheres: Photodecomposition of methylene blue as a target molecule

The submicrometers-sized titania hollow spheres have been synthesized by employing sulfonated-polystyrene latex particles as a template in sol–gel method. The hollow spheres have relatively smooth surface and dense arrangement of titanium dioxide layers. Photocatalytic activity of the hollow spheres was investigated by employing methylene blue (MB) as a target compound. It was observed that the particle size, size distribution, and crystallinity are important factors to get high photocatalytic activity for the decomposition of MB.     

The distribution of kth nearest neighbours and its application to cluster settling in dispersions of equal spheres

The spatial distribution of spheres in a very dilute suspension is described approximately by a Poison distribution, but much better approximations result when the volume of the spheres is taken into account. For solids fractions up to 0.32, the distribution of kth nearest neighbours calculated from the radial distribution function for a hard-sphere gas agreed closely with that determined from a computer simulation. This distribution provides insight into the concentration threshold for cluster settling and the increase in sedimentation rate with time in dilute dispersions. Two models provide rough explanations for the maintenance of cluster velocities greater than the Stokes velocity.     

不同圆球复合无序堆积床内流动传热数值分析

圆球堆积床内孔隙分布影响其内部流场及温度场分布, 且小管径-球径比堆积床由于壁面限制, 内部孔隙率变化剧烈, 其内部流动和传热不均匀现象明显。针对D/d_p为3的圆球无序堆积床构建了3种非等直径圆球复合堆积结构:径向分层复合堆积、轴向分层复合堆积以及随机复合堆积结构, 并采用DEM-CFD方法建模计算, 从径向及整体角度分析比较不同复合堆积床内流动换热特性及其流场和温度场分布的均匀性。结果表明:孔隙率及孔隙大小分布共同影响堆积床内流场和温度场分布;相对于单一等直径圆球堆积, 采用复合堆积结构能使堆积床内部孔隙率分布更均匀, 其内部流场和温度场分布也更为均匀;对于D/d_p为3的堆积通道, 径向分层堆积结构对于提高整体流动换热性能及改善内部流动换热均匀性都有显著效果。     

Packing of multi-sized mixtures of wet coarse spheres

The effect of water addition on the packing of multi-sized coarse spheres has been experimentally investigated under standard poured packing conditions. The results indicate that porosity is strongly affected by particle characteristics such as particle sizes and their distribution, in addition to water content. The packing features in the relationship between porosity and moisture content for wet multi-sized spheres are found to be similar to those for wet mono-sized spheres, implying that the same governing mechanisms apply. The comparison between the dry and wet packing systems confirms that there is a similarity between them, suggesting that the packing of wet particles can be predicted within the framework of a packing model developed for dry coarse particles. Future work in this direction is also discussed.     

Chemical reactions between calcium carbide and chlorohydrocarbon used for the synthesis of carbon spheres containing well-ordered graphite

Carbon spheres and carbon nanospheres were prepared through the chemical reactions between calcium carbide and chlorohydrocarbon without using any catalysts. The reactants were sealed in a pressure vessel and heated to temperatures of 210-250 °C. The final products were characterized using scanning electron microscopy (SEM), X-ray energy-dispersive spectroscopy (EDS) attached to SEM, transmission electron microscopy (TEM), X-ray diffraction and Raman spectroscopy. SEM and TEM examinations show that some of the obtained products are bead-like or roe-like spheres with diameters of 100-2000 nm. The carbon nanospheres have high purity (>95%) and uniform size distribution (100-200 nm). The EDS analyses reveal that the carbon contents of the spheres are 86-94%. The results indicate that the size and the size distribution of the synthesized spheres change with the change of the reactants. The experimental yields of carbonaceous materials relative to the starting materials are about 8.0-14.4% (w/w). The yields of the spheres increase whereas their size distributions decrease with the increase of H atom and the decrease of Cl atom in chlorohydrocarbon. The average graphitization degree of sample 1 and sample 2 are 81.9% and 78.7%.     

Aerosol sample preparation methods for X-ray diffractive imaging: Size-selected spherical nanoparticles on silicon nitride foils

We are developing aerosol generating and processing methods for X-ray analyses of nanoscale materials using conventional synchrotron radiation sources and using the newly operational soft X-ray free-electron laser in Hamburg (FLASH) at the Deutsches Elektronen Synchrotron. Charge-reduced electrospray, differential mobility analysis and an electrostatic precipitator were used to prepare samples consisting of size-monodisperse spherical nanoparticles deposited on 20 nm thick silicon nitride foils supported by silicon frames. Ninety-seven and 102 nm diameter spheres were selected from a broader distribution of 98 nm spheres using differential mobility. We measured the size distribution of the spheres using forward scattering from 1.65 nm light at the Advanced Light Source (ALS) in Lawrence Berkeley National Laboratory and scanning electron microscopy (SEM). The full-width half maximum (FWHM) of the size distribution of the size-selected spheres was as narrow as 5.4 nm when measured by SEM, as compared to 16 nm for the non-size-selected distribution. Forward scattering measurements of the 97 nm diameter size-selected spheres fit a size distribution with a FWHM of 4 nm and allowed us to validate the methodology for use in future diffraction imaging experiments at FLASH.     

Near-wall porosity characteristics of fixed beds packed with wood chips

Packed beds of fuel wood chips are commonly found in thermal conversion processes such as combustion or gasification. Wood chips in particular are mostly used as fuel for small-scale domestic heating boilers but also for commercial-scale combustion units. The characterization of spatial voidage distribution inside the wood chip beds is of great importance for flow and reactor modelling. This study focuses on the radial porosity variations of cylindrical beds of three different types of commercially available wood chips including chips classified as G30 size class. The conventional technique of consolidating packed beds with a resin was chosen as the experimental procedure. The radial voidage distribution in different cylindrical beds is determined by image analysis of sections of the solidified packings. Additionally, a packing of monosized spheres was investigated in order to assess the selected procedure in comparison with widely available literature data for spheres. The results are discussed and summarized in a mathematical expression correlating the radial voidage distribution depending on average wood chip size, packing core porosity and dimensionless distance from the tube wall.     

Radial growth of vertically aligned carbon nanotube arrays from ethylene on ceramic spheres

Vertically aligned carbon nanotube (VACNT) arrays grown on ceramic spheres are obtained from ethylene using a floating catalysis process. The exhaust gas mainly contains light gaseous hydrocarbons, which decreases the contamination at the outlet of the reactor. Linear synchronous growth of the VACNT arrays is demonstrated and the morphology evolution of VACNT array grown on spheres is shown. The VACNT arrays on the spheres crack radially into a flower-like structure when the length of CNT is above 400 μm. The VACNT arrays grown on spheres still possess good flowability even when the length of the array reaches 1100 μm after a 2-h growth at 800 °C. The arrays on the spheres show good alignment, high purity and good graphitization. Meanwhile, with a decrease in temperature, the diameter of CNTs in the array correspondingly decreases, the distribution becomes narrower, and the growth rate decreases. The apparent activation energy is 180 ± 8 kJ/mol, indicating that ethylene is a good carbon source for fast and continuous radial growth of millimeter VACNT arrays on ceramic spheres.     

Radial porosity in packed beds of spheres

A functional approach has been developed to investigate the radial porosity of mono-sized spheres in cylinders. Analytical and semi-analytical equations have been developed to calculate the local radial porosity and the radial porosity distribution, respectively, within a cylindrical packing structure. The analytical equations are based upon fundamental principles and are simple, straightforward and provide highly accurate results for the radial porosity with minimal computational prerequisites. The analytical equations have been developed for the fixed packing of identical spheres in cylindrical containers with D/d ≥ 2.0. The predicted results for the local radial porosity and the radial porosity distributions are benchmarked with an existing analytical equation and available experimental data, respectively, for mono-sized spheres in cylindrical containers.     

A High Resolution Study of the Effect of Morphology On the Mass Spectra of Single PSL Particles with Na-containing Layers and Nodules

The interpretation and quantification of measurements of particle composition by laser ablation based single particle mass spectrometry is complex. Among the most difficult systems to quantify are internally mixed particles containing alkali metals and organics. The alkali atoms in such particles tend to suppress the formation of other ions sometimes to below the detection limit. Here we present a study of the behavior of single particle mass spectral peak intensities as a function of the amount of the sodium containing compounds deposited on the surface of 240 nm polystyrene latex (PSL) spheres. We generate three morphologically distinct and well defined coating types: uniform layers, cubic nodules and rounded nodules, and measure the individual particle mass spectra as a function of the vacuum aerodynamic diameter with nanometer resolution. The data show that the probability of detecting the PSL spheres depends on the amount of the alkali metal on the PSL sphere surface, its morphological distribution and the ablation laser power. The data suggest that PSL spheres with localized Na-containing nodules are easier to detect than those which are completely encapsulated. We show, for example, that at low laser power, PSL particles that are completely encapsulated with Na-containing compounds, whose weight fraction is close to 50%, cannot be detected, while 35% of PSL spheres with same amount of coating can be detected if coating is localized in nodules on a fraction of the particle surface.     

Facile size‐controllable syntheses of highly monodisperse polystyrene nano‐ and microspheres by polyvinylpyrrolidone‐mediated emulsifier‐free emulsion polymerization

A facile emulsifier‐free emulsion polymerization approach was developed to fabricate highly monodisperse polystyrene (PS) spheres using potassium persulfate as the initiator and polyvinylpyrrolidone (PVP) as a stabilizer, respectively. It was found that the size of monodisperse PS spheres tends to decrease with increase of the PVP concentration, and the size of monodisperse PS spheres could be easily controlled in a wide range from 200 to 1500 nm simply by adjusting the concentration of PVP. The increase of monomer concentration led to the increase in size of monodisperse PS spheres. In contrast, the increase of initiator concentration resulted in the decrease in size of monodisperse PS spheres. These concentration changes, however, did not significantly affect the size distributions of PS spheres. It was also found that the size of monodisperse PS spheres obtained by adding the initiator at room temperature was larger than that by adding the initiator at 70°C, and the existence of inhibitor resulted in smaller PS spheres. The mechanisms in which the above factors influence the size and size distribution were discussed. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

A study of local liquid/solid mass transfer in packed beds under trickling and induced pulsing flow

Induced pulsing flow (by cyclic liquid feeding) in packed beds, operated in the trickling flow regime, is studied as a method of overall improvement of catalytic reactor operation. In this paper results are reported of experiments aimed at determining local and global liquid/solid mass transfer rates, mainly for the so-called fast mode of ON-OFF periodic liquid feeding, with frequencies of order 0.1 Hz. Such mass transfer data for the fast mode of induced pulsing are not available in the literature. Uniform 6 mm glass spheres and alumina cylindrical extrudates, of 1.5 mm diameter and a narrow distribution of lengths, are employed in the tests. For completeness, results are also reported for single-phase (liquid) and trickling flow through the same packed beds. A well-known electrochemical technique is employed to measure instantaneous local mass transfer coefficients by means of quite a few probes distributed throughout the bed. The hydrodynamic characteristics under the above conditions, reported in companion papers, are helpful in interpreting the new mass transfer data.There is a wide spread of the time-averaged local mass transfer rates, in all cases tested, apparently due to packing and flow non-uniformities. This spread is much smaller in the case of packed uniform spheres. In general, the benefits of cyclic liquid feeding are more evident in the packed bed of spheres than in that of cylindrical extrudates; for instance, with increasing mean liquid rate, induced pulsing tends to reduce the spread of local mass transfer coefficients, which suggests that more uniform fluids distribution is promoted. The imposed liquid pulses are reflected in the observed periodic variation of local mass transfer coefficients; the latter appear to decay along the bed in the same manner as the liquid pulses. Other trends of local mass transfer rates are identified and discussed in relation to measured variation of liquid holdup, under the same conditions. For packed spheres, the measured global mass transfer rates are in fair agreement with literature correlations obtained for the trickling flow regime, unlike the case of packed extrudates where significant deviation is observed.     

乳化法制备沥青球的研究

调制沥青的球化过程是制备具有良好圆形度和高机械性能球形活性炭的关键步骤。实验以各向同性高软化点沥青为主要原料,通过加入3嘛％～60wt％的减粘剂获得调制沥青。加热调制沥青,使其处于流动态,采用乳化法实现沥青的球化。借助SMZ800体视显微镜的观测和平均圆形度的测试,确定乳化法所得PSAC圆形度好、粒径分布范围宽。