Experimental investigation on atomizing characteristics of coal-water paste for pressurized fluidized bed

According to the properties of coal-water paste (CWP), a series of experimental nozzles were developed for a detailed study of the effect of nozzle design on CWPs behavior in a pressurized fluidized bed. The particle size and its distribution of atomized CWP using several types of nozzles are measured by PIV instrument and newly developed image processing software, and the effect of a number of factors on the atomizing properties are analyzed. The atomizing experimental results show that the atomization quality of CWP was found by correlating with the characteristics of CWP, gas to CWP mass ratio, atomization air motion, coal particle size distribution and the structure of nozzle etc. Mass median diameter (MMD) of atomized particles decreases with the increase of gas to CWP mass ratio. The proper type of nozzle type and proper parameters for engineering application are suggested.     

The effect of coal properties on the viscosity of coal-water slurries

Studies on coal-water slurries (CWSs) have been conducted for many years to replace fuel oil. In this research project, the effect of coal properties on CWSs have been investigated using two Turkish coals of different ranks and a Siberian bituminous coal. Physical, chemical and surface properties of coal samples were determined. Furthermore, adsorption tests were carried out in order to put forward the effect of additive adsorption on the viscosity of CWSs. Viscosity measurements were realized for CWSs of various solid ratios by weight that were prepared using coal samples having mean particle sizes of 19, 35 and 50 μm.     

CFD simulation of coal-water slurry flowing in horizontal pipelines

An Eulerian multiphase approach based on kinetic theory of granular flow was used to simulate flow of coal-water slurries (CWS) in horizontal pipelines. The RNG k-ɛ turbulent model was incorporated in the governing equation to model turbulent two-phase flow with strong particle-particle interactions. In this model, the coal particles with bimodal distribution were considered as two solid-phase components, and the moment exchange between solid and liquid as well as that between solid and solid were accounted for. The model was firstly validated with pressure gradient and concentration profile data from the open literature, and then validated with pressure gradient data of the authors’ experiments. The effects of influx velocity, total influx concentration and grain composition were numerically investigated, and the results have displayed some important slurry flow characteristics, such as constituent particle concentration distribution and velocity distribution as well as pressure gradients, which are very difficult to display in the experiments. The results suggest that both gravity difference between large and small particles and strong particle-particle interaction had significant effects on concentration distribution as well as velocity distribution. This work was presented at the 7 ^th Korea-China Workshop on Clean Energy Technology held at Taiyuan, China, July 25–28, 2008.     

Local resistance characteristics of highly concentrated coal-water slurry flow through fittings

The local resistance characteristics of high concentration coal-water slurry (CWS) flowing through three types of local fittings, namely the gradual contractions, sudden contractions and 90° horizontal elbows, were investigated at a transportation test facility. Results show that the local resistance loss of gradual contractions decreases as the contraction angle increases. When pipe diameter ratio varies little, local resistance loss of sudden contractions changes insignificantly. There is an optimal value of bend diameter ratio, at which the local resistance loss of horizontal elbows is the least. As Reynolds number increases, the resistance coefficients of all the three fittings first reduce and then stabilize, while the three pipes have different ratio of equivalent length to pipe diameter L _e /D behaviors, that is, L _e /D of the gradual contractions decreases gradually and then keeps stable; that of the sudden contractions diminishes at first and then increases, and that of the horizontal elbows increases linearly. This work was presented at the 7 ^th Korea-China Workshop on Clean Energy Technology held at Taiyuan, China, June 26–28, 2008.     

研磨对EPVC糊性能的影响

介绍了研磨前后EPVC糊黏度的变化情况,结果表明:研磨可增大EPVC糊黏度,随着研磨比例的增大糊黏度逐渐增大,但随着放置时间的延长,研磨前后糊黏度趋于一致。     

Experimental study of gas diffusion in cement paste

This paper presents an experimental study of gas diffusion in binary mixtures of hydrogen-nitrogen and xenon-nitrogen through cement pastes (CEM I and CEM V) of different water/cement ratios (0.35 and 0.45). First, the impact of water saturation on gas diffusion is investigated by performing tests on samples pre-conditioned in specific atmospheric conditions (dry, 55, 70, 82, 93 and 100% RH) by means of saline solutions. The comparison of the results obtained for the CEM I and the CEM V samples (w/c ratio of 0.45) demonstrate the importance of pore size distribution/connectivity on gas diffusion. Second, diffusion tests at different total pressures and using two different mixtures (hydrogen-nitrogen, xenon-nitrogen) are performed to study the nature of gas diffusion in cement paste. Results demonstrate that gas diffusion in cement paste is controlled by Knudsen and ordinary diffusion at pressures greater than 100 kPa and mainly by Knudsen diffusion at pressures less than 100 kPa.     

Effect of blockage on heat transfer from a cylinder to power law liquids

The influence of planar confining walls on the steady forced convection heat transfer from a cylinder to power-law fluids has been investigated numerically by solving the field equations using FLUENT (version 6.2). Extensive results highlighting the effects of the Reynolds number (1?Re?40), power-law index (0.2?n?1.8), Prandtl number (1?Pr?100) and the blockage ratio (β=4 and 1.6) on the average Nusselt number have been presented. For a fixed value of the blockage ratio, the heat transfer is enhanced with the increasing degree of shear-thinning behaviour of the fluid, while an opposite trend was observed in shear-thickening fluids. Due to the modifications of the flow and temperature fields close to the cylinder, the closely placed walls (i.e., decreasing value of the blockage ratio) further enhance the rate of heat transfer as the fluid behaviour changes from Newtonian to shear-thickening fluids (n>1), the opposite influence is seen with the decreasing value of the flow behaviour index (n) in shear-thinning (n<1) fluids. Finally, the functional dependence of the present numerical results on the relevant dimensionless parameters has been presented in the form of closure relationships for their easy use in a new application.     

抛光膏中磨料的解析

利用SEM,EDS分析了抛光膏中磨料的形状,尺寸和成分。结果表明,进口抛光膏中采有的磨料是直径为2－5μm的柱状氧化铝。     

Rheological properties of cement paste: Thixotropic behavior and structural breakdown

In this work, a new material model is presented to simulate rheological behavior of cement paste. This material model is among others based on combined concepts by Hattori and Izumi and by Tattersall and Banfill. More precisely, coagulation, dispersion and re-coagulation of the cement particles (giving a true thixotropic behavior) in combination with the breaking of certain chemically formed linkages between the particles (giving a so-called structural breakdown behavior) are assumed to play an important role in generating the overall time-dependent behavior of the cement paste. The model evaluation is done by comparing experimental data with model prediction.     

Origin of the thermoelectric behavior of steel fiber cement paste

Carrier scattering dominates the origin of the Seebeck effect in steel fiber cement. The scattering sites include the fiber-matrix interface, which is like a pn junction, since the fiber and cement paste have opposite signs of the absolute thermoelectric power. The scattering results in positive and negative values of the absolute thermoelectric power, depending on the fiber content.     

The influence of aminosilanes on macroscopic properties of cement paste

In this work the authors present the results obtained by studying the influence of γ-aminopropyltriethoxysilane or (CH₃CH₂O)₃Si(CH₂)₃NH₂, (APTES), and N-β-aminoethyl-γ-aminopropyltrimethoxylsilane or (CH₃O)₃Si(CH₂)₃NH(CH₂)₂NH₂, (AEAPTES), on macroscopic properties of fresh and hardened cement pastes and mortars prepared from ordinary Portland cement. The mixing ability of aminosilane substances with water is an advantage in comparison to other types of organofunctional silanes when used as an admixture to the cement system. This enables direct blending with water and cement and homogeneous distribution of aminosilane molecules through the bulk of the cement material. The highly polar amine and alkoxide groups of aminosilane molecules exhibit strong chemical interactions with cement matrix which are reflected in modified macroscopic properties of the cement system. The effect of APTES and AEAPTES on (a) properties of fresh cement paste and mortar as workability, setting time, water/cement ratio, air content and density, and on (b) properties of hardened cement paste and mortar as compressive and flexural strength, was studied.     

Severs流变仪测定聚氯乙烯糊的流变性

用Severs流变仪测定聚氯乙烯糊的流动特性参数,探讨利用所得数据计算其表观黏度,确定聚氯乙烯糊的流变行为.     

Damage monitoring of cement paste by electrical resistance measurement

Electrical resistance measurement is effective for monitoring damage (due to damage infliction and subsequent microcrack opening) and healing (due to microcrack closing) of cement pastes (plain, with silica-fume, and with latex) in real time during repeated compressive loading. Damage causes the resistance to increase; healing causes the resistance to decrease.     

Performance of new viscosity modifying admixtures in enhancing the rheological properties of cement paste

The use of viscosity modifying admixtures (VMA) has proved to be very effective in stabilizing the rheological properties and consistency of self-compacting concrete (SCC). SCC is known for its excellent deformability, high resistance to segregation and use, without applying vibration, in congested reinforced concrete structures characterized by difficult casting conditions. Most of the commercial VMAs currently available in the market are costly and increase the price of such a concrete. Identification or production of new low-cost VMA is then essential. This paper presents the performance of four new polysaccharide-based VMAs in enhancing the rheological and consistency properties of cement paste. The study of the rheological properties and consistency of cement paste to screen the dosage and type of new VMA to be used in SCC is a promising approach. Investigation was carried out on cement pastes with combinations of various dosages of new VMAs and of a superplasticizer (SP) to study the influence on rheology, consistency and washout mass loss. A commercial VMA designated in this paper as “COM” was tested for comparison. The study on new VMAs is encouraging and confirms that pastes with satisfactory rheological and consistency properties comparable with or even better than commercial VMA can be developed. The combined use of proper dosages of VMA and SP is shown to clearly contribute to securing high-performance cement pastes that is highly fluid yet cohesive enough to reduce water dilution and enhance water retention. Attempt has also been made to correlate rheological properties (yield stress) to consistency (slump) of pastes.     

Microstructure-based micromechanical prediction of elastic properties in hydrating cement paste

Elastic properties of hydrating cement paste can be successfully predicted by combination of the hydration model, percolation theory and micromechanical analysis. Reconstruction of hydrating microstructure is based on the 3D digital NIST model of cement hydration, which is enhanced for the prediction of two C-S-H types. Chemical phases in a percolated microstructure served as an input in a two-level analytical or one-level 3D FEM or FFT elastic homogenization. Special mesh generation for the percolated microstructure is discussed as well as its numerical implementation. Good results from FEM and FFT were found for the size of the representative volume element of 50 × 50 × 50 μm, considering water-to-cement ratio in the range from 0.25 to 0.5. While good predictions in well-hydrated cement pastes were obtained for both analytical and numerical approaches, numerical homogenization was found more accurate and versatile for the whole hydration time.     

流体阻力摩擦系数关联式的比较与改进

着眼于流体阻力摩擦系数的计算关联式。在众多基础教材中,选取常用的三本进行比较。在通用工具莫迪图上,用三本教材中的关联式对图中七条不同相对粗糙度的曲线,进行特定雷诺数范围内的流体阻力摩擦系数的计算。并应用Excel的函数库和图表向导功能作出大量的图表,以便对这些关联式进行定量的误差分析。进而又运用数形结合的方法,将计算值与实验值的相对误差,同摩擦系数的相互作用参数-雷诺数结合在一起,并给出校正关联式的拟合公式。使得计算精度在很大程度上得到提高。     

Effect of the variations of clinker composition on the poroelastic properties of hardened class G cement paste

The effect of the variations of clinker composition on the poroelastic properties of class G oil-well cement pastes is studied using a multiscale homogenization model. The model has been calibrated in a previous work based on the results of a laboratory study. Various compositions of class G cements from literature are used in a hydration model to evaluate the volume fractions of the microstructure constituents of hardened cement paste. The poroelastic parameters such as drained bulk modulus, Biot coefficient, and Skempton coefficient are evaluated using the homogenization model. The results show that the variations in chemical composition of class G cements have no important effect on the variations of the poroelastic properties.     

新型改性丁腈橡胶对PVC糊树脂力学性能的影响

通过试验配方设计、样条制备及性能检测,讨论了不同含量的改性粉末丁腈橡胶对PVC糊树脂力学性能的影响.结果表明,随着弹性体用量的增大,软质PVC材料的拉伸强度下降,断裂伸长率得到明显改善.新型改性粉末丁腈橡胶能够改善PVC糊树脂的性能.     

Impedance spectroscopy study of hardened Portland cement paste

In this paper, the differential impedance analysis (DIA) has been applied to the study of the dielectric properties of hardened Portland cement paste. Two time constants are found in the impedance spectra obtained in the frequency region form 100 kHz to 15 MHz. One time constant has been attributed to the solid matrix and the other one to the liquid phase filling the pores. The effect of the cement paste-electrodes interface has been quantified using two different experimental set-ups. Measurements using direct contact between electrodes and cement paste have been compared with measurements using an air gap technique in which the specimen “floats” between the electrodes. The two referred time constants have been found in both types of measurements. The influence of drying on the dielectric parameters is also studied.     

Application of a microwave impulse technique to the measurement of free water content in early hydration stages of cement paste

A microwave impulse method is designed and employed to the monitoring of residual moisture content in early hydration stages of cement paste. Complementary experiments consisting in determination of the times of beginning and end of setting and of the course of bending strength, compression strength, Young's modulus and hydration heat production during the early hydration period are performed in order to find possible correlation between the measured moisture changes, development of mechanical parameters and hydration heat release. It is observed that the fastest decrease of residual moisture content roughly agrees with the early stages of the hardening process, i.e., with the time period between the end of setting and the moment when first measurable values of mechanical parameters are obtained.