基于模糊聚类和相似度的电力系统可靠性原始参数预估

以可靠性气候影响因素作为线路模糊集元素,通过模糊聚类将可靠性原始参数已知和待求的线路进行适当分类,引入 作为考虑其他影响因素的修正系数,在分析欧氏贴近度、模糊贴近度、灰色关联度各自反映模糊集特征的差异性和局限性的基础上提出相似度指标,在同类中以可靠性参数已知的线路的可靠性影响因素序列为基准向量,以可靠性参数未知的线路的可靠性影响因素序列为待检向量,使用相似度指标进行求取。实例证明该方法的可行性和预估结果的准确性,并讨论了不同的权重系数取值会对结果产生影响。     

基于二分法的聚类负荷模型及其在电力系统可靠性评估中的应用

目前用于可靠性评估的负荷模型主要有峰荷模型、分级负荷模型以及聚类负荷模型等,这些模型难以同时满足计算精度和计算复杂性的要求.建立了基于二分法的聚类负荷模型并将其应用到电力系统可靠性评估中.该模型利用二分法原理较快地确定最佳分级数,利用快速排序算法和聚类技术将年负荷曲线中各负荷点归并到相应的负荷级,具有降低计算复杂性和提...     

Effect of crack width on chloride diffusion coefficients of concrete by steady-state migration tests

The purpose of the present study is to explore the diffusion characteristics of cracked concrete according to the width of cracks. Major test variables include crack width, concrete strength, fly ash addition, and maximum aggregate size. The diffusion characteristics have been measured by steady-state migration test. The present study indicates that the diffusion coefficients do not increase with increasing crack widths up to the so-called “threshold crack width.” The threshold crack width for diffusion is found to be around 55–80 μm. Above this threshold value, the diffusion coefficients start to increase with crack width. A composite model with the introduction of “crack geometry factor” was derived to identify the diffusion coefficient in cracked concrete. It was shown that the crack geometry factor ranges from 0.067 to 0.206. Finally, the effects of concrete strength, fly ash addition and maximum aggregate size on diffusion coefficients are also discussed.     

Restricted diffusion of residual molecules in catalyst pores under reactive conditions

The restricted diffusion of residual molecules under catalytic conversion conditions was investigated using commercial catalysts. The effectiveness factor, η, and the effective diffusion coefficient, D_e, for residual molecules were evaluated and explicitly compared based on a pseudo-first-order reaction kinetic model and the Thiele modulus. The experimental results showed that the restrictive diffusion of heavy oil molecules is affected by the joint functions of several factors, such as the operating conditions, the size and configuration of the reactant molecules, and the average pore diameter of the catalyst. The reaction temperature has a greater influence on restrictive diffusion than the other operating factors, and the ratio of reactant molecular size to catalyst pore size is the most critical factor that determines the degree of restrictive diffusion. Moreover, a mathematical expression was derived for the restrictive factors in order to describe the relationship between the effective diffusivity and ratio of molecule-to-pore size.     

Heat transfer in trickle bed column with constant and modulated feed temperature: Experiments and modeling

Heat transfer was investigated in an insulated packed bed column with co-current downflow of gas and liquid under constant and periodically modulated gas–liquid feed temperature. Bed temperatures at three axial positions were assessed at steady state for different insulating systems, different gas and liquid flow rates and system pressure. The experimental profiles recorded were modeled with a dynamic pseudo-homogeneous one parameter model to analyze the effect of operating conditions and to deduce coefficients of overall (U) and bed to wall (h_W) heat transfer. It appears that the heat transfer is strongly affected by the system pressure, whereas the liquid flow rate has a smaller influence. The experimental data of h_W were correlated with the operating conditions leading to a small average error of 7% in the correlation. Condensation of water vapor occurring in the column seems to contribute to the heat transfer inside the packed bed. Several dynamic experiments modulating the feed temperature were also conducted and described with the help of the dynamic model. Predictions with the fitted values of U were in good agreement with experiments and give confidence to apply this model in the investigation of the catalytic wet air oxidation of phenol over carbon conducted in a trickle bed reactor under temperature feed modulation.     

The study of lithium ion de-insertion/insertion in LiMn2O4 and determination of kinetic parameters in aqueous Li2SO4 solution using electrochemical impedance spectroscopy

In this work, we report a basic study on the mechanism of lithium ion de-insertion/insertion process from/into LiMn₂O₄ cathode material in aqueous Li₂SO₄ solution using electrochemical impedance spectroscopy (EIS). An equivalent circuit distinguishing the kinetic parameters of lithium ion de-insertion/insertion is used to simulate the experimental impedance data. The fitting results are in good agreement with the experimental results and the parameters of the kinetic process of Li⁺ de-insertion and insertion in LiMn₂O₄ at different potentials during charge and discharge are obtained using the same circuit. The results indicate that the de-insertion/insertion behavior of lithium ions at LiMn₂O₄ cathode in Li₂SO₄ aqueous solution is similar to that reported in the organic electrolytes. The charge transfer resistance (R_ct), warburg resistance, double layer capacitance and chemical diffusion coefficient (DLi+) vary with potentials during de-insertion/insertion processes. R_ct is lowest at the CV peak potentials and the important kinetic parameter, DLi+ exhibits two distinct minima at potentials corresponding to CV peaks during de-insertion–insertion and it was found to be between 10⁻⁸ and 10⁻¹⁰ cm² s⁻¹during lithium de-insertion/insertion processes.     

A predictive model based on tortuosity for pressure drop estimation in ‘slim’ and ‘fat’ foams

The use of porous structures with high external surface area represents an important breakthrough in several industrial applications. Foam structures have received an increasing scientific and industrial interest since the last decade. Knowledge of pressure drop induced by these foam structures is thus essential for successful design and operation of high performance industrial systems. In this context, an analytical investigation was conducted for the determination of the permeability and the inertial coefficient in foams. The theoretical model is based on modified cubic lattice, which allows to take into account the presence of matter at the junction of struts. The existing model developed in the literature is then modified to incorporate this geometrical approach for determining the tortuosity of the foam. Finally, the permeability and inertial coefficient analysis are performed in order to derive the pressure drop on foams. The modeling procedure is based only on physical principles and geometrical considerations with no adjustable parameters in order to reconcile the theoretical work with the experimental data of the literature. Finally, this model is validated for two marginal cases (i.e. ‘slim’ and ‘fat’ foams).     

Dy掺杂对α-Al2O3的烧结及热膨胀行为的影响

研究了Dy掺杂对氧化铝恒速无压烧结行为及热膨胀行为的影响。烧结行为研究表明Dy掺杂促进了氧化铝的致密化,提高了样品开始收缩的温度,抑制了α-Al2O3烧结过程中的颗粒粗化。计算得到的未掺杂及掺杂坯体样品的平均线膨胀系数分别为6.78×10-6/℃和7.89×10-6/℃,烧结体分别为7.54×10-6/℃和9.98×10-6/℃,因此Dy掺杂对氧化铝坯体和烧结体的热膨胀系数影响不明显,分析表明镝铝石榴石化合物的产生是膨胀系数发生变化的主要原因。     

Extraction of biologically active compounds from Sideritis ssp. L.

In this study extraction of polyphenols and flavonoids from cultivated hybrid Sideritis scardica × Sideritis syriaca, known for its rich content of phenolics and flavonoids with antioxidant activity, was investigated. Extractions have been done by ethanol and water-ethanol, respectively. High equilibrium values of the extracted species were obtained—17.55 mg/(g solid) total phenolics and 5.7 mg/(g solid) total flavonoids with ethanol as solvent. The influence of the solvent on the total yield and the content of biologically active compounds were studied. Maximum polyphenolics and flavonoids extraction was observed for water-ethanol solvent ratio 20/80. Increase of the content of ethanol in the solvents led to lower total yield of extracts but higher percentage of polyphenolics. The extraction kinetics showed that 90% of the phenolic compounds were extracted during the first 2.5 h. The experimental kinetics was described by a constant effective diffusion coefficient D_e = 1.5 × 10⁻¹² m²/s in the solid, accounting for the actual particle size distribution.     

PVDF/CB composites prepared by novel solvent casting method for low voltage PTC temperature regulation applications

A novel solvent casting preparation technique utilizing three variants of poly(vinylidene fluoride) (PVDF) to achieve a thermal cut off and a self regulation effect at a low applied voltage is reported in this study. The positive temperature coefficient (PTC) composites were prepared by dissolving PVDF in 1‐methyl‐2‐pyrrolidone (NMP) solvent, blending with Vulcan® XC72 carbon black (CB) filler, crosslinking with vinyl trimethoxysilane (VTMOS) and quenching in water. All composites displayed a highly macrovoidal structure that promoted a PTC effect when subjected to a thermal expansion effect via an electrical current. Subsequently the current was cut off and self regulation behavior was exhibited. Kynar® 761A PVDF resulted in the strongest PTC effect, and displayed temperature regulation at around 100°C which may be attributed to the highly semi crystalline nature and the larger molecular weight of this polymer in comparison with the other PVDF composites studied. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011