粒径对煤粉云最低着火温度特性的影响

为了研究粒径对煤粉云最低着火温度特性的影响,采用粉尘云最低着火温度测试装置测试了不同粒径下煤粉云的最低着火温度,并结合ReaxFF分子动力学对其反应机理进行了微观层面的探讨。结果表明:当煤粉中位径在34 μm时,煤粉云的最佳着火质量浓度为750 g/m³,最低着火温度为550 ℃;随着煤粉粒径的增加,煤粉云最低着火温度逐渐增大,当煤粉中位径达到124 μm,煤粉云最低着火温度上升至650 ℃。通过ReaxFF分子动力学对煤粉热解过程的计算结果表明:随着反应的进行,大分子煤结构逐步分解,芳香环、C—C键、C—O键和C—H键等断裂,产生更小的分子结构,其中,H₂、H₂O、CO₂和CH₂O等小分子产生的数量逐渐增多;H·自由基和OH·自由基在反应初期有明显的数量变化,且其含量对于最终稳定产物有重要影响。     

褐煤煤尘爆炸火焰传播特性及燃烧热分解机理研究

煤尘爆炸是矿井安全开采的主要危险源之一。以褐煤煤尘为研究对象,探究煤尘粒径对煤尘火焰传播过程的影响。用高速摄影装置记录火焰的传播过程,进而分析不同粒径下煤尘爆炸火焰传播的高度和速度。为进一步分析煤尘燃烧过程中的化学反应机理,借助反应分子动力学方法对煤分子燃烧中的初始热分解过程进行了模拟。研究结果表明:爆炸火焰传播高度呈先增加、后稳定的趋势,传播速度呈先增大、后减小的趋势;随着煤尘粒径的减小,火焰传播高度和传播速度均呈增大的趋势;当煤尘粒径为10.5 μm时,火焰传播高度和传播速度的峰值分别为623 mm和4.3 m/s;煤尘热分解主要产物为H₂、H₂O、CO₂和CH₂O,这些产物进一步与氧气的结合会促进煤尘燃烧和火焰传播过程,使得整个体系燃速加快。为煤尘热分解和燃烧提供了较为充分的数据基础。     

Overall characterization of cork dust explosion

Explosibility and ignitability studies of air/cork dust mixtures were conducted in a near-spherical 22.7 L explosibility test chamber using pyrotechnic ignitors and in a furnace of 1.23 L. The suspension dust burned as air-dispersed dust clouds and the uniformity of the dispersion inside the chamber was evaluated through optical dust probes. The range of tested particle sizes went from a mass median diameter of 47.4 to 438.3 microm and the covered dust cloud concentration was up to 700-800 g/m(3). Measured explosion parameters included minimum explosible concentration, maximum explosion pressure, maximum rate of pressure rise and minimum autoignition temperature. The effect of dust particle size on flammability was evaluated and it was found that the minimum explosible concentration is around 40 g/m(3) and it is relatively independent of particle size below 180 microm. Maximum explosion pressure of 7.2 bar and maximum rate of pressure rise of 179 bar/s were detected for the smallest tested sizes. The limitations on the rates of devolatilization of the solid particles became rate controlling at high burning velocities, at high dust loadings and for large particle sizes. The effect of initial pressure on the characteristic parameters of the explosion was studied by varying the initial absolute pressure from 0.9 bar to 2.2 bar, and it was found that as initial pressure increases, there is a proportional increase of minimum explosion limit, maximum explosion pressure, and maximum rate of pressure rise. The influence of the intensity of the ignition energy on the development of the explosion was evaluated using ignition energies of 1000 J, 2500 J and 5000 J, and the experimental data showed that the value of 2500 J is the most convenient to use in the determination of minimum explosion concentration. The behavior of the cork dust explosion in hybrid methane air mixtures was studied for atmospheres with 2% and 3.5% (v/v) of methane. The effect of methane content on the characteristic parameters of the explosions was evaluated. The conclusion is that, the hazard and explosion danger rise with the increase of methane concentration, characterized by the reduction of the minimum dust explosion concentration. The minimum autoignition temperature obtained with the thermal ignition tests was 540 degrees C and the results have shown that this value is independent of particle size, for particles with mass median diameters below 80 microm.     

The preparation of metal oxide spheres for use in neutron dosimetry

Gelation techniques for the preparation of spherical particles of pure and mixed metal oxide have been developed at Harwell for use in nuclear and non-nuclear applications. Gel precipitation processes have been used to prepare actinide oxide spheres (U, Th, Np and Pu) with chosen diameters in the range 50–1500 μm, principally for use as nuclear fuels. Sol-gel and gel precipitation processes have been developed to prepare oxide spheres (1–2000 μm diameter) of aluminium, silicon, titanium, zirconium and rare earth metals either as pure oxides or often with many other elements added. These materials have a variety of applications e.g. as catalysts and adsorbents.Oxide spheres can be used to advantage in neutron dosimetry because they can be fabricated directly from solutions and the spheres can be handled easily without becoming contaminated with impurities. The size of sphere can be chosen to contain the desired weight of nuclide in the range 5 μ g to 5 mg per sphere for pure oxide spheres. For the lower weights of nuclide required it is preferable to use an inert diluent such as alumina so that the sphere size can be kept large enough for easy handling. Here the nuclide of interest to diluent metal ratio is fixed at the solution stage and consequently each sphere produced contains an oxide mixture of the same composition. As an example of this technique indium-aluminium oxide spheres have been prepared.     

Experimental study on spontaneous imbibition characteristics of coal based on fractal theory

Coal seam water injection is a widely used dust reduction technology in Chinese coal mines. During the process of coal seam water injection, a large amount of broken coal particles will accumulate in the pore, which will affect the flow characteristics of water. It's very important for improving the on-site coal seam water injection and dust reduction technology to study the influence of coal particles on the water migration law. In this paper, a spontaneous upward imbibition experiment was used to study the effect of coal particles stacked in front fractures on water migration in coal seam water infusion. Then, a mathematical model was established to express imbibition speed and stable imbibition height. The results show when the largest particle size is selected, theoretical calculation results of stable imbibition height is closest to the experimental data; coal particles with a size of 564–1589 μm have the greatest influence on the wetting phase of coal seam water infusion; the metamorphic grade and particle size of the samples mainly affect the initial stage of water migration, and the initial imbibition speed of each coal sample is quite different. However, with increasing imbibition height, the imbibition speed difference of each coal sample gradually decreases.     

THE PSEUDOHOMOGENEOUS FLOW APPROXIMATION FOR DISPERSED TWO-PHASE SYSTEMS

ABSTRACT

The unsuitabllity of a continuum model for turbulent flow of solid-liquid mixtures has been demonstrated by a number of experimental studies which employed industrial slurries and a wide range of pipe diameters. However effective particle sizes and slurry viscosities are difficult to define for such mixtures. Using spherical glass particles of median diameter 125 μm and 240 μm, vertical flow experiments have been conducted in a test pipeline 26.17 mm in diameter. These experiments confirm earlier sand flow measurements by showing that wall friction decreases as particle size increases, in the absence of Bagnold stress or Coulombic friction effects.

Using a turbulent flow simulation, and assuming a linear increase of solids concentration in a thin layer near the pipe wall, dimensionless excess wall frictional resistances have been calculated. These predictions are likely to be valid for fine particles which do not display Bagnold stress or Coulombic friction effects.

The turbulent flow simulations confirm the effects observed in the experiments. They show that the pseudohomogeneous flow assumption is not useful for pressure drop prediction with slurries of fine particles.     

空气中氮气或氩气浓度对锆粉尘层着火温度的影响

为了探索抑制锆包壳剪切过程中锆粉着火的方法,采用粉尘层最低着火温度测定仪、红外热成像仪、真空手套箱等测定了不同粒径的锆粉尘层在空气和含不同浓度氮气、氩气的空气中的最低着火温度和火焰温度。结果得出:锆粉的中位粒径从2.4 μm升至71.7 μm,粉尘层最低着火温度从200 ℃升至390 ℃,表明粒径越小的锆粉着火敏感性越高;4种粒径的锆粉燃烧火焰最高温度都在1 776～1 913 ℃范围内,锆粉粒径较大时,燃烧的剧烈程度较低;氮气或氩气体积分数从60%～65%升至70% 85%时,锆粉尘层最低着火温度升至400 ℃,表明空气中高浓度的氮气或氩气对锆粉燃烧有抑制作用,且浓度越高,抑制作用越强,锆粉粒径越小,抑制效果越好。氩气的抑制效果强于氮气。     

Particle formation in ambient MALDI plumes

The ablated particle count and size distribution of four solid matrix materials commonly used for matrix-assisted laser desorption ionization (MALDI) were measured with a scanning mobility particle sizer (SMPS) combined with a light scattering aerodynamic particle sizer (APS). The two particle sizing instruments allowed size measurements in the range from 10 nm to 20 μm. The four solid matrixes investigated were 2,5-dihydroxybenzoic acid (DHB), 4-nitroaniline (NA), α-cyano-4-hydroxycinnamic acid (CHCA), and sinapic acid (SA). A thin film of the matrix was deposited on a stainless steel target using the dried droplet method and was irradiated with a 337 nm nitrogen laser at atmospheric pressure. The target was rotated during the measurement. A large number of nanoparticles were produced, and average particle diameters ranged from 40 to 170 nm depending on the matrix and the laser fluence. These particles are attributed to agglomeration of smaller particles and clusters and/or hydrodynamic sputtering of melted matrix. A coarse particle component of the distribution was observed with diameters between 500 nm and 2 μm. The coarse particles were significantly lower in number but had a total mass that was comparable to that of the nanoparticles. The coarse particles are attributed to matrix melting and spallation. Two of the compounds, CHCA and SA, had a third particle size distribution component in the range of 10 to 30 nm, which is attributed to the direct ejection of clusters.     

Determination of mean diameter and particle size distribution of acrylate latex using flow field-flow fractionation, photon correlation spectroscopy, and electron microscopy

Flow field-flow fractionation (flow FFF) was employed to determine the mean diameter and the size distribution of acrylate latex materials having diameters ranging from 0.05 to 1 μm. Mean diameters of the samples determined by flow FFF are in good agreement with those obtained from photon correlation spectroscopy (PCS). Scanning electron microscopy (SEM) yielded a mean diameter that is about 20% lower than those obtained from flow FFF or PCS, probably due to the shrinkage of particles during sample drying and high-vacuum measurements. It was found that flow FFF is particularly useful for the determination of particle size distributions of latex materials having broad size distributions. Flow FFF separates particles according to their sizes and yields an elution curve that directly represents the particle size distribution of the sample. In PCS, measurements had to be repeated at more than one scattering angle to obtain an accurate mean diameter for the latex having a broad size distribution. Flow FFF was fast (less than 12 min of run time) and showed an excellent repeatability in measuring the mean diameter with ±5% relative error.     

Leakage analysis of packages by the electrolytic test

The microhole tester is a new laboratory instrument for detection of leakage in packages. The instrument is based on electrolytic conductance measurement using direct current and it quantifies the summarized hole size in a package. In this study the reliability and validity of the microhole tester have been evaluated by testing different kinds of aseptic, retorted and gas flushed packages containing both solid and viscous products. In the case of gas flushed packages filled with a solid food product the instrument detected 100% of the leaking packages with artificially made microholes with diameters of 7–356 μm after emptying the packages of food. Retorting tended to block the holes with food and lowered the sensitivity. In the case of retorted packages with viscous food the microhole tester reliably detected hole sizes larger than 25 μm. Adding a surface active agent, poly(ethylene oxide) (PEO), to the retorted packages with totally blocked holes greatly improved the reliability of the instrument and reduced the testing time. The percentage of detected packages with totally blocked microholes with diameters between 30 and 60 μm was improved from 10–55% without PEO to 83–100% with PEO added to the package. © 1997 John Wiley & Sons, Ltd.     

CFD investigation on dust dispersion pollution of down/upwind coal cutting and relevant countermeasures for spraying dustfall in fully mechanized mining face

To explore the effects of downwind and upwind coal cutting on dust pollution in a fully mechanized mining face, the airflow-dust migration was simulated by using ANSYS-FLUENT software, and the follow-up spraying dust control method was optimized according to the differences in dust concentration distribution during downwind and upwind coal cutting. The results show that during downwind and upwind coal cutting, the average airflow velocity of the fully mechanized mining face is about 1.053 m/s. During downwind coal cutting, the dust generated by the shearer mainly pollutes the mining area and the area within 0–50 m on its downwind side, while the dust generated by the advancing support mainly pollutes the mining area and the sidewalk area within 0–20 m on the downwind side of shearer. During upwind coal cutting, the dust generated by the shearer mainly pollutes the vicinity of the drum and the area within 0–45 m on its downwind side, while the dust generated by the advancing support mainly pollutes the sidewalk area within 0–30 m on the downwind side of shearer. In view of the differences in dust concentration during downwind and upwind coal cutting, the dust concentration in the working face was divided into four areas and the follow-up spraying dust control method was optimized. The field measurement results indicate that compared with the original spraying facility, the optimized opening mode raises the average dust removal rates of total dust and respirable dust in the working face by 52.1% and 43.8% during downwind coal cutting and by 53.6% and 42.3% during upward coal cutting.     

Quantitative ultrasound estimates from populations of scatterers with continuous size distributions

Although quantitative ultrasound imaging based on backscattering coefficients has proven potential for tissue characterization, the scattering models used in most studies assume distributions of identical scatterers. However, actual tissues may exhibit multiple levels of spatial scales. Therefore, the objective of the present study is to analyze the effects of scatterer size distributions when using a fluid-sphere model for estimating values of effective scatterer diameter (ESD) through both simulations and experiments. For simulations, ESD estimates were obtained at several analysis frequencies between 1 and 40 MHz from populations of scatterers with diameters ranging between 25 and 100 μm, 25 and 50 μm, 50 and 100 μm, and 50 and 75 μm. For sufficiently high analysis frequencies, the ESD estimates obtained through simulations were approximately inversely proportional to frequency and mostly independent of the underlying scatterer size distribution. Asymptotic expressions for the expected ESD estimates at low- and high-frequency limits were derived. Experiments were conducted using two gelatin phantoms with contrast agent spheres ranging in diameter from 30 to 140 μm and 70 to 140 μm, and 5-, 7.5-, 10-, and 13-MHz focused transducers. Not only was the asymptotic behavior of ESD versus frequency estimates observed experimentally, but also the experimental ESD estimates using the 10- and 13-MHz transducers were lower than the smallest scatterers present in the second phantom. These results may have a direct impact on how scatterer size estimates corresponding to specimens with different subresolution spatial scales should be interpreted.     

球形纳米氧化铝颗粒制备微晶陶瓷及传质动力学研究

采用球形纳米氧化铝颗粒制备氧化铝微晶陶瓷,研究造粒、烧结等工艺过程对陶瓷微观结构和力学性能的影响,并结合动力学计算分析球形颗粒在烧结过程中的传质特性。结果表明:通过液相造粒掺入0.8%(质量分数)的PVA能够优化球形颗粒的压制成型并提高坯体密度。烧结温度从1400℃提高至1550℃,陶瓷相对致密度由74.1%增大至97.5%,而晶粒尺寸由0.6μm仅增至1.4μm,这与球形颗粒自身稳定的形态及其堆积形成的均匀孔隙有关。在1550℃下烧结时间由30 min延长至120 min时,气孔率由4.8%降低至0.4%,晶粒尺寸则由1.2μm增至2.7μm。另外,通过动力学计算得出球形颗粒的烧结活化能为788 kJ/mol,证实球形颗粒在烧结前期和中期具有生长惰性,利于获得微晶结构。经1550℃保温90 min的陶瓷,其密度达到98.9%,平均晶粒尺寸仅为1.6μm,硬度达到26.4 GPa,弯曲强度为574 MPa。     

Influence of coal properties on dust suppression effect of biological dust suppressant

In order to study the effect and influencing factors of biological dust suppressant on secondary dust of coal dust, the biological dust suppressant was prepared with bacillus pasteurii and 0.6 mol/L urea-CaCl₂ solution. The experimental result shows that the yield of mineralized products of CC is the highest, reaching 81.11%, in bonding solutions with different calcium source and different concentration. For 5.0 g coal dust, 8 mL is the suitable spraying amount of bacteria solution and bonding solution. Biological dust suppressant has the best secondary dust suppression effect on coal dust with particle size above 200 μm. Moreover, it has the strongest effect on inhibiting secondary dust emission of lignite, and the weight loss rate of coal dust about 200 μm is only 0.84%. At the same time, the proportion of calcite crystals with the strongest thermal stability in mineralized products produced by MICP is higher. When the coalification degree of coal is lower, the effect of biological dust suppressant on secondary dust emission is better.     

西北戈壁地区气溶胶的浓度及粒度分布特征

为了获取西北戈壁地区气溶胶随着粒径变化的概率分布函数,对该地区的2次现场取样进行调查。结果表明,采样点气溶胶的质量浓度为0.03~0.1 mg/m3;气溶胶的粒径分布遵循双峰分布的规律,第1个峰值出现在约0.5μm处,第2个峰值出现在约3μm处,其差异主要表现在第2个峰值的面积大小;大多数采样点气溶胶粒子数量中位粒径小于1μm,质量中位粒径位于2~5μm之间。     

煤粉超细化技术及设备研究

根据双向旋转球磨机的超细粉碎原理,研究开发制备超细煤粉的技术及设备,并采用该设备进行煤粉的超细粉碎实验,分析各种工艺参数对超细煤粉产品粒度影响。结果表明:采用双向旋转球磨机制备超细煤粉的最佳工艺条件为:筒体及搅拌器转数25r/min,球料质量比为8,研磨时间为4～5h,磨球尺寸为10～20mm,制备出超细煤粉产品的粒度为d50=1.15μm,d90=2.67μm。     

Characterizing organic monolithic columns using capillary flow porometry and scanning electron microscopy

Polyethylene glycol diacrylate monoliths prepared using different amounts of monomer, porogen ratio, and capillary dimensions were characterized using capillary flow porometry (CFP) and scanning electron microscopy (SEM). Our results reveal good agreement between SEM and CFP measurements for through-pore size distribution. The CFP measurements for monoliths prepared by the same procedure in capillaries with different diameters (i.e., 75, 150, and 250 μm) clearly confirmed a change in through-pore size distribution with capillary diameter, thus, certifying the need for in-column measurement techniques over bulk measurements (e.g., mercury intrusion porosimetry). The mean through-pore size varied from 3.52 to 1.50 μm with a change in capillary diameter from 75 to 250 μm. Consistent mean through-pore size distribution for capillary columns with the same internal diameter but with different lengths (1.5, 2, and 3 cm) confirms the high interconnectivity of the pores and independence of CFP measurements with respect to capillary length. CFP and SEM measurements not only allow pore structure analysis but also prediction of relative column performance. Monoliths with narrow through-pore size distribution (0.8-1.2 μm), small mean through-pore size, and thin skeletal size (0.55 μm) gave the best performance in terms of efficiency for polyethylene glycol diacrylate monoliths.     

Explosion dust particle size measurements

In situ measurements of the sizes and concentrations of dust particles generated by the detonation of high explosives in clay soil near Leesville, La., sandy clay soil near Huntsville, Ala., and sandy soils near Orogrande, N.M. are reported. Measurements were generally made within 1 m of the surface (in one case 10 m) at distances ranging from 10 to approximately 50 m from the detonation point with a combination of Knollenberg lightscattering counters (for particles with equivalent radius in the submicron to 15-microm range) and a Knollenberg optical array probe (for particles of 10-150 microm). Measurements were made for periods of several tens of seconds following detonation. All dust size distributions, irrespective of soil or explosive type, exhibit a bimodal character with mass mean radii of approximately 7 and 70 microm. Peak aerosol mass loadings inferred from the distributions have values ranging from 0.05 to 10 g gm-3 with the larger mode of particles contributing most to the mass loading. Predictions of dust extinction coefficients at visible (0.55-microm) and IR (10.4-microm) wavelengths were made using the measured size distributions together with estimates of dust refractive indices. These predictions suggest that extinction should be nearly neutral (wavelength independent) in agreement with transmission measurements made during some of the tests. Predicted mass extinction coefficients, under the assumption of dust material density of 2.5 g cm-3, are of the order of 0.05 m2 g-1 at both visible and IR wavelengths. This value is also in good agreement with a test-averaged measured value of 0.03 m3 g-1 (at lambda = 10.6 microm) obtained using a short path transmissometer and hi-vol sampler.     

Drop tower experiments on sticking, restructuring, and fragmentation of preplanetary dust aggregates

For the determination of the aggregation efficiency of preplanetary dust, we performed impact experiments with fractal dust aggregates in the drop tower Bremen. We found that for the lowest impact velocities, the dust aggregates, which consisted of micron-sized, monodisperse SiO₂ spheres, hit and stuck with no measurable impact restructuring. For intermediate collision velocities, compact aggregate structures formed, and at the highest impact velocities, aggregates were fragmented. Our experimental results are in quantitative agreement with a numerical dust aggregate collision model (Dominik, C., Tielens, A. G. G. M. 1997, Astrophysical Journal vol. 480, p. 647), when the latest experimental values for the rolling-friction and break-up energies are used. However, the presence of a rarefied gas flow, in which the incoming aggregates were embedded, increased the threshold velocity for sticking. Although the impinging aggregates were disintegrated at high impact velocities, the resulting fragments were dragged back to the target on which they could stick due to a then considerably lower collision velocity. This aerodynamically-supported aggregation process might be responsible for the rapid growth of preplanetary bodies in the size range from ∼0.1 m to ∼10 m. Such a rapid growth is necessary to prevent a loss of most of the solid bodies of these sizes due to gas-drag-induced fast orbital decay.     

Pickering乳液聚合制备聚苯乙烯/纳米SiO2复合微球与其性质

以两亲性纳米SiO2为稳定剂构筑Pickering乳液,分别采用偶氮二异丁腈(AIBN)及过硫酸铵(APS)作为引发剂引发苯乙烯单体聚合,制得了不同尺寸及形貌的聚苯乙烯/纳米SiO2(PS/SiO2)复合微球,并分析了Pickering乳液聚合的机理。通过傅里叶变换红外光谱仪(FT-IR)、凝胶渗透色谱-多角度激光光散射联用仪(GPC-MALLS)、热失重分析(TGA)及差示扫描量热仪(DSC)对产物进行了表征,结果表明,AIBN引发所得复合微球为微米级(0.5μm~2.0μm),而APS引发产物为亚微米级(0.1μm~0.5μm),两者引发所得聚合物重均分子量(-Mw)分别为4.780×104g/mol及3.411×105g/mol;复合微球的热性能因纳米SiO2的存在而得到增强。