Effective thermal conductivity of granulated two-phase systems at interstitial air pressures

Considering the htermal conduction through molecular collisions an expression for the effective thermal conductivityλ _e of loose and granular two-phase materials at different interstitial air pressure has been derived. The dependence ofλ _e on pore and particle sizes, characteristic pressure and radiative heat transfer is also discussed. Calculated values ofλ _e of glass beads and loose building materials are compared with reported results.     

Effects of Continuous Size Distributions on Pressures of Granular Gases

Direct Monte Carlo simulations are employed to investigate the granular pressures in granular materials with a power-law particle size distribution. Specifically, smooth circular discs of uniform material density are engaged in a two-dimensional rectangular box, colliding inelastically with each other and driven by a homogeneous heat bath at zero gravity. The resulting pressures are found to decrease as the widths of particle size distribution are increased. Moreover, the granular pressures in power-law systems are found to be unequally distributed among the various sizes of particles, with large particles possessing more pressure than their smaller counterparts. The width-dependent nature of the total pressures is induced by the more dispersion of smaller particles in the system as the particle size distribution is widened.     

Coalbed methane adsorption and desorption characteristics related to coal particle size

Effects of particle size on CH_4 and CO_2adsorption and desorption characteristics of coals are investigated at 308 K and pressures up to 5.0 MPa.The gas adsorption and desorption isotherms of coals with particle sizes ranging from 250 μm to 840 μm are measured via the volumetric method,and the Langmuir model is used to analyse the experimental results.Coal particle size is found to have an obvious effect on the coal pore structure.With the decrease of coal particle size in the process of grinding,the pore accessibility of the coal,including the specific surface area and pore volume,increases.Hence,coal with smaller particle size has higher specific surface area and higher pore volume.The ability of adsorption was highly related to the pore structure of coal,and coal particle size has a significant influence on coal adsorption/desorption characteristics,including adsorption capacity and desorption hysteresis for CH_4 and CO_2,i.e.,coal with a smaller particle size achieves higher adsorption capacity,while the sample with a larger particle size has lower adsorption capacity.Further,coal with larger particle size is also found to have relatively large desorption hysteresis.In addition,dynamic adsorption performances of the samples are carried out at 298 K and at pressures of 0.1 MPa and 0.5 MPa,respectively,and the results indicate that with the increase of particle size,the difference between CO_2 and CH_4adsorption capacities of the samples decreases.     

Laser-induced incandescence for soot-particle sizing at elevated pressure

This paper describes the applicability of laser-induced incandescence (LII) as a measurement technique for primary soot particle sizes at elevated pressure. A high-pressure burner was constructed that provides stable, laminar, sooting, premixed ethylene/air flames at 1–10 bar. An LII model was set up that includes different heat-conduction sub-models and used an accommodation coefficient of 0.25 for all pressures studied. Based on this model experimental time-resolved LII signals recorded at different positions in the flame were evaluated with respect to the mean particle diameter of a log-normal particle-size distribution. The resulting primary particle sizes were compared to results from TEM images of soot samples that were collected thermophoretically from the high-pressure flame. The LII results are in good agreement with the mean primary particle sizes of a log-normal particle-size distribution obtained from the TEM-data for all pressures, if the LII signals are evaluated with the heat-conduction model of Fuchs combined with an aggregate sub-model that describes the reduced heat conduction of aggregated primary soot particles. The model, called LIISim, is available online via a web interface. PACS 65.80.+n; 78.20.Nv; 42.62.-b; 47.70.Pq     

Nanopore wall effect on surface tension of methane

Local pressure is known to be anisotropic across the interfaces separating fluids in equilibrium. Tangential pressure profiles show characteristic negative peaks as a result of surface tension forces parallel to the interface. Nearby attractive forces parallel to the interface are larger than the repulsive forces and, hence, constitute the surface tension. In this work, using molecular dynamics simulations of methane inside nano-scale pores, we show this surface tension behaviour could be significantly influenced by confinement effects. The layering structure, characterised by damped oscillations in local liquid density and tangential pressures, extends deep into the pore and can be a few nanometers thick. The surface tension is measured numerically using local pressures across the interface. Results show that the tension is smaller under confinement and becomes a variable in small pores, mainly controlled by the thickness of the liquid density layering (or liquid saturation) and the pore width. If the liquid saturation inside the pore is high enough, the vapour–liquid interface is not interfered by the pore wall and the surface tension remains the same as the bulk values. The results are important for understanding phase change and multi-phase transport phenomena in nanoporous materials.     

The adsorption of water in finite carbon pores

Grand canonical Monte Carlo simulations were applied to the adsorption of SPCE model water in finite graphitic pores with different configurations of carbonyl functional groups on only one surface and several pore sizes. It was found that almost all finite pores studied exhibit capillary condensation behaviour preceded by adsorption around the functional groups. Desorption showed the reverse transitions from a filled to a near empty pore resulting in a clear hysteresis loop in all pores except for some of the configurations of the 1.0?nm pore. Carbonyl configurations had a strong effect on the filling pressure of all pores except, in some cases, in 1.0?nm pores. A decrease in carbonyl neighbour density would result in a higher filling pressure. The emptying pressure was negligibly affected by the configuration of functional groups. Both the filling and emptying pressures increased with increasing pore size but the effect on the emptying pressure was much less. At pressures lower than the pore filling pressure, the adsorption of water was shown to have an extremely strong dependence on the neighbour density with adsorption changing from Type IV to Type III to linear as the neighbour density decreased. The isosteric heat was also calculated for these configurations to reveal its strong dependence on the neighbour density. These results were compared with literature experimental results for water and carbon black and found to qualitatively agree.     

气压对颗粒物质振动分离的影响

文章作者系统地研究了垂直振动颗粒床中，在不同气压、颗粒尺寸以及密度情况下大球的运动规律。实验发现，系统处于真空状态或低气压时，大球总是向上运动。然而，当在通常大气压下，大球则会出现上升和下降两种运动状态。大球下降这种运动状态，只在大球的密度和颗粒床中颗粒尺寸足够小时才会出现，颗粒床中的负气压梯度导致大球下降。     

Effects of air on the segregation of particles in a shaken granular bed

Effects of interstitial air on the motions of a large intruder in a shaken granular bed are studied experimentally as a function of ambient air pressure, particle size of the bed, and the density of the intruder. It is found that the intruder always rises from the granular bed in the absence of air. However, the intruder can acquire both positive and negative buoyancy in the presence of air. Negative buoyancy can be observed only when both the density of the intruder and the particle size of the bed are small enough. This negative buoyancy can be explained by the unusual air pressure distribution found in the bed.     

Impacts of air pressure on the evolution of nanosecond pulse discharge products

Based on the nonequilibrium plasma dynamics of air discharge, a dynamic model of zero-dimensional plasma is established by combining the component density equation, the Boltzmann equation, and the energy transfer equation. The evolution properties of nanosecond pulse discharge (NPD) plasma under different air pressures are calculated. The results show that the air pressure has significant impacts on the NPD products and the peak values of particle number density for particles such as O atoms, O3 molecules, N2(A3) molecules in excited states, and NO molecules. It increases at first and then decreases with the increase of air pressure. On the other hand, the peak values of particle number density for N2(B3) and N2(C3) molecules in excited states are only slightly affected by the air pressure.     

Mechanical resistance of a compact nanoparticle in a gaseous medium

Resistance of fullerenes in air is investigated under standard conditions. The general problems of resistance of compact nanosized particles in gaseous media are solved at different temperatures and pressures. The resistance of the nanosized particle moving in a gaseous medium is calculated using the deterministic approach, direct methods of molecular dynamics, and main theorems of classical mechanics (including the theorem on the momenta of a particle and a particle ensemble). Theoretical results obtained are in complete agreement with the Lapple experimental data on the resistance of ultradispersed particles. Moreover, they describe well the resistance of particles having much larger sizes for which the molecular dynamics model disregarding collisions of molecules seems to be inapplicable.     

NMR magnetization transfer as a tool for characterization of nanoporous materials

The application of nuclear magnetic resonance magnetization transfer experiments to probe the surface-to-volume ratio and pore morphology of porous materials with characteristic pore sizes of 1-100 nm is described. The method is based on the phenomenon of incomplete freezing of liquids in small pores where a few monolayers adjacent to the pore walls remain liquid. Sufficient difference between the transverse relaxation times in the solid frozen core and liquid surface layer allows the initial preparation and subsequent re-equilibration of a solid-liquid magnetization grating. The method is demonstrated using model nanoporous materials with known characteristics. The ensuing problems of the mechanism of the magnetization transfer through the interface and within the frozen core are discussed and elucidated by pulsed-field-gradient NMR experiments.     

压缩空气微波等离子体发射光谱的实验研究

利用微波等离子体发生装置,以压缩空气为工作气体,在1～5 atm气压下激发了微波等离子体。使用光谱测量系统,对不同气压和不同入射微波功率情况下的压缩空气微波等离子体的发射光谱进行了实验研究。结果显示,在其他条件不变时,随着气压升高,压缩空气微波等离子体的带状连续谱特征逐渐减弱;随着入射微波功率降低,带状连续谱强度逐渐减弱而带状连续谱特征依然显著。实验结果为了解压缩空气微波等离子体的光谱特性和NO活性基团的产生条件提供了实验依据。     

颗粒增强金属基复合材料变形强化中的应变梯度效应

 针对单轴压缩实验,根据颗粒增强金属基复合材料中颗粒和基体两相的局部变形协调条件,并通过简单的位错模型,确定出与变形协调相应的几何必需位错密度,进而导出一种颗粒强化-应变梯度律。从中可以清楚地看出,颗粒增强金属基复合材料的强化由材料的微结构特征几何参数l和基体应变梯度联合控制。对于颗粒含量一定的复合材料,颗粒越小,应变梯度越高,强化效果越好。这一结果揭示了,颗粒强化及尺寸效应主要是通过应变梯度效应来表现的。这也同时说明,应变梯度可能是控制材料变形与断裂的重要因素之一。     

Effect of the particle sizes of the components on the leakage threshold of the electrical conductivity of a dispersed mixture

The electrical conductivity of a dispersed mixture of sand and graphite of various granulometric compositions (filling factor k=0.74) was investigated. It is shown that for mixtures which have pore sizes in a structure of dense packing of one component do not exceed the particle sizes of the other component and the threshold volume concentration of the conducting phase is completely determined by the ratio of the particle sizes of both phases. An analytic expression relating the volume threshold concentration of the conducting component and the particle sizes of the components is obtained by solving the bond problem.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 10–14, July, 1982.     

Size Measurement of Small Magnetic Particles

Many different methods for size measurement are known and since they differ in their physical principles, different results are also obtained. In the recent past, there were rapid developments in new measurement techniques and one can now quickly and routinely determine particle sizes in the very fine range. Smaller particle sizes, on the other hand, tend to increase the probability of agglomeration. The measurement of the particle sizes for magnetic materials is not well understood and this paper reports results regarding the size determination of very fine magnetic materials. Because of their dipole moment, these particles tend to agglomerate even more, which in turn causes certain difficulties during the measurements. Wet and dry laser measurement systems were compared and also magnetic materials with different permeabilities in order to establish the influence of individual factors on the measurement process and on the accuracy of the results obtained.     

Analytical estimates of permeability and possible convective heat transfer in compact porous fluid–saturated rocks

The possible occurrence of convection in fine-grained (low-porous <15%) fluid-saturated (argon, water) rocks under hydrostatic pressures of up to 100 MPa is studied. Calculations of the Rayleigh filtration number and the Darcy number are presented. It is shown that the convective motion of fluid does not occur in pore-saturating fluids within the considered range of pressure variation, pore sizes, and temperature differences.     

Pressure-sensitive blackbody point radiation induced by infrared diode laser irradiation

Ultrabroadband radiation from Yb(2)O(3) at ambient and low air pressures was investigated under the excitation of a 980 nm diode laser. The radiation was confirmed to be blackbody radiation, and it is sensitive to environmental air pressure in the way that the integrated radiation intensity decreases linearly with increasing air pressure. An ideal gas model may be employed to interpret the linear dependence. The pressure-sensitive radiation characteristic provides a potential method for noncontact measurement of air pressure with high accuracy.     

Some theoretical considerations in modeling laser-induced incandescence at low-pressures

Laser-induced incandescence (LII) of nanoparticles at low pressures has received some attention in recent years as a particle sizing technique or a technique for inferring the mean value of the absorption function of the particle material. In this paper, we are concerned with some fundamental issues in the theory of LII with particular attention paid to those encountered at very low pressures. The commonly adopted Rayleigh approximation for particle laser energy absorption and subsequent thermal emission is critically evaluated against the Mie solution in the range of size parameter relevant to LII. The Rayleigh approximation can cause significant error in particle laser energy absorption rate, especially when shorter wavelengths are used, and potentially in the particle temperature inferred from the two-color LII. We also demonstrate that claims that low-pressure LII can be used for particle sizing are flawed, due to the use of an incorrect expression for radiation heat loss rate from the particles in this regime, and unjustified neglect of particle sublimation heat loss. Using the currently best available carbon sublimation rate expression and physical parameters, the relative importance of heat conduction, thermal radiation, and sublimation heat loss from an isolated carbon particle was investigated for different ambient pressures, particle temperatures and particle diameters. To ensure particle radiation heat loss is dominant over conduction and sublimation the ambient pressure and the particle temperature should be kept respectively lower than 10^-4 atm and below about 2800 K. Under these conditions the effective temperature of a particle ensemble containing non-aggregated polydisperse primary particles to the power of -4 is proportional to the mean value of the particle absorption function, provided the particles are in the Rayleigh regime in the near infrared. The effect of aggregation on particle absorption and emission is briefly discussed. PACS 44.10.+i; 44.40.+a; 61.46Df     

Characterization of Powder Beds by Thermal Conductivity: Effect of Gas Pressure on the Thermal Resistance of Particle Contact Points

The thermal conductivity of ceramic powder packed beds was measured at temperatures below 100 °C for various powder sizes and compositions and under different gas atmospheres. Measurements at low pressures (down to 10 Pa) combined with a theoretical model allowed the elucidation of geometrical and thermal resistance parameters for the contact points between granules. The gap thickness and contact point size were found to be well correlated with the mean particle size. The thermal conductivities of all powders at low pressure were found to differ at most by a factor of two, whereas the solid‐phase conductivities of the powder materials differed by more than one order of magnitude. A theoretical model accounting for the size‐dependence of contact point conductivity is incorporated to rationalize this trend.