Relation between silico-aluminous fly ash and its coal of origin

Fly ashes are typical complex solids which incorporate at the same time intrinsic properties derived from the layers （various mineralogical and dimensional spectra） and major transformations generated during prior processing. To use fly ashes in various applications, it is necessary to characterise them completely. The first research to date carried out on silico-aluminous fly ashes in order to characterise them physically, morphologically, chemically and mineralogically, resulted in the recognition that they are relatively simple materials. In the present study, a silico-aluminous fly ash coming from the power station of Albi （France） was selected. Heat treatment at 450 and 1200℃ together with coal simulated the treatment undergone by coal in the power station in order to mimic real coal residue. In conclusion, the diversity of the particles contained in fly ash could only be exolained by the relation existing between the fly ash and its coal of origin.     

Pozzolanic reactivity of silico-aluminous fly ash

For some years it has been possible to control the particle size of fly ashes, by-products of thermal power stations. Incorporating these very fine particles （obtained by grinding and/or pneumatic selection） improves the physical-mechanical characteristics of mortars and concretes. In this study, we measured the lime consumption of the various fractions （granulometric and densimetric） and identified by X-ray diffraction the neoformed phases by the pozzolanic reaction, to show that it is not sufficient to simply define the pozzolanicity of products based on lime consumption since it does not take into account the nature of the phases formed. The size of the particles used in the test samples also has a determining effect on the quantity of lime consumed. Before comparing results, it is necessary to ensure that the size of the particles （of the global ash and its constituents） be the same. Two distinct neoformed ohases appear： CSH in the largest granular fractions （d〉 40 μm） and C3AH6 in the smaller fractions.     

Pozzolanic reactivity of silico-aluminous fly ash

For some years it has been possible to control the particle size of fly ashes, by-products of thermal power stations. Incorporating these very fine particles (obtained by grinding and/or pneumatic selection) improves the physical-mechanical characteristics of mortars and concretes. In this study, we measured the lime consumption of the various fractions (granulometric and densimetric) and identified by X-ray diffraction the neoformed phases by the pozzolanic reaction, to show that it is not sufficient to simply define the pozzolanicity of products based on lime consumption since it does not take into account the nature of the phases formed. The size of the particles used in the test samples also has a determining effect on the quantity of lime consumed. Before comparing results, it is necessary to ensure that the size of the particles (of the global ash and its constituents) be the same. Two distinct neoformed phases appear: CSH in the largest granular fractions (d>40 μm) and C<3AH6 in the smaller fractions.     

Alkali activated fly ash: effect of admixtures on paste rheology

In this paper, an investigation related to the rheological behaviour of alkali-activated fly-ash pastes (AAFA) is described. Those pastes were prepared by mixing the fly ash with an alkaline dissolution containing 85% of a 12.5 M NaOH solution and 15% of waterglass and adding some commercial chemical admixtures usually used in the Portland cement concrete fabrication, like lignosulphonates, melamines (first and second generation products) and polycarboxylates (latest generation). The fly ash rheological data were determined by rotational viscometry measurements as well as by the use of the flow table test. Results indicate that chemicals admixtures used do not work the same in the Portland cement systems than in alkali-activated fly ash systems. As a general rule, it seems that the most efficient admixtures for these new cementitious pastes (AAFA) are those based in polycarboxylates.     

A qualitative study on the pulsatile flow phenomenon in a dense fly ash pneumatic conveyor

Understanding of the dynamic particulate flow structures within a dense gas-fly ash pneumatic conveyor must be improved in order to better aid its design guidance.The complex pulsatile movement of the gas-fly ash mixture dominates the flow performance within the pipeline,and historically,non-invasive measurement devices such as the electrical capacitance tomography（ECT） were often used to sufficiently capture the flow dynamics.However,inadequate studies have been conducted on the pulsatile flow phenomenon,which directly relate to the gas-fly ash two-phase flow performance.This paper aims to investigate the pulsatile flows using an ECT device.Initially,pulsatile flow patterns under various experimental conditions were obtained through ECT.Pulses within a flow were then characterised into pulse growth and decay segments,which represent the superficial fluidisation and deaeration processes during conveying.Subsequently,structural and statistical analyses were performed on the pulse growth and decay segments.Results suggested that the increasing air mass flow rate led to the decrease of the superficial fluidisation/deaeration magnitude,however,the increase of the superficial fluidisation/deaeration durations.Also,the air mass flow rate was indicated as the dominant factor in determining the pulsing statistical parameters.This research provides fundamental insights for further modelling the dense fly ash pneumatic flows.     

Thermal behavior of spiral fin-and-tube heat exchanger having fly ash deposit

This research investigates the effect of fly-ash deposit on thermal performance of a cross-flow heat exchanger having a set of spiral finned-tubes as a heat transfer surface. A stream of warm air having high content of fly-ash is exchanging heat with a cool water stream in the tubes. In this study, the temperature of the heat exchanger surface is lower than the dew point temperature of air, thus there is condensation of moisture in the air stream on the heat exchanger surface. The affecting parameters such as the fin spacing, the air mass flow rate, the fly-ash mass flow rate and the inlet temperature of warm air are varied while the volume flow rate and the inlet temperature of the cold water stream are kept constant at 10 l/min and 5 °C, respectively.

From the experiment, it is found that as the testing period is shorter than 8 h the thermal resistance due to the fouling increases with time. Moreover, the deposit of fly-ash on the heat transfer surface is directly proportional to the dust–air ratio and the amount of condensate on heat exchange surface. However, the deposit of fly-ash is inversely proportional to the fin spacing. The empirical model for evaluating the thermal resistance is also developed in this work and the simulated results agree well with those of the measured data.     

Removal of inhalable particles from coal and refuse combustion by agglomeration with solid nuclei

Airborne inhalable particles are a potent environmental pollutant. Formed via industrial processes, separation of these particles is difficult using conventional clean up techniques. In this work, solid nuclei particles of different chemical compositions were introduced into an agglomeration chamber with simulated flue gases to investigate their ability to remove these particles. Organic nuclei were able to capture more inhalable particles from coal-derived fly ash than inorganic nuclei, though these proved more effective for the agglomeration of inhalable particles in refuse-derived fly ash. Increasing the diameter of the solid nuclei benefitted the agglomeration process for both types of ash. Varying the local humidity changed adhesion between the particles and encouraged them to aggregate. Increasing the relative humidity consistently increased particle agglomeration for the refuse-derived ash. For coal-derived fly ash, the removal efficiency increased initially with relative humidity but then further increases in humidity had no impact on the relatively high efficiencies. After agglomeration in an atmosphere of 62% relative humidity, the mean mass diameter of inhalable particles in the coal-derived fly ash increased from 3.3 to 9.2 μm. For refuse-derived fly ash, agglomeration caused the percentage of particles that were less than 2 μm to decrease from 40% to 15%. After treatment at a relative humidity of 61%, the mean size of inhalable particles exceeded 10 μm.     

Energy loss at bends in the pneumatic conveying of fly ash

An accurate estimation of the total pressure drop of a pipeline is important to the reliable design of a pneumatic conveying system. The present paper presents results from an investigation into the modelling of the pressure drop at a bend in the pneumatic conveying of fly ash. Seven existing bend models were used (in conjunction with solids friction models for horizontal and vertical straight pipes, and initial acceleration losses) to predict the total pipeline pressure drop in conveying fly ash (median particle diameter: 30 μm; particle density: 2300 kg/m³; loose-poured bulk density: 700 kg/m³) in three test rigs (pipelines with dimensions of 69 mm inner diameter (I.D.) × 168 m length; 105 mm I.D. × 168 m length; 69 mm I.D. × 554 m length). A comparison of the pneumatic conveying characteristics (PCC) predicted using the seven bend models and experimental results shows that the predicted total pipeline PCC and trends depend on the choice of bend model. While some models predict trends that agree with the experimental results, other models predicted greater bend pressure drops for the dense phase of fly ash than for the dilute phase. Models of Pan, R. (1992). Improving scale-up procedures for the design of pneumatic conveying systems. Doctoral dissertation, University of Wollongong, Australia, Pan, R., & Wypych, P.W. (1998). Dilute and dense phase pneumatic conveying of fly ash. In Proceedings of the sixth International Conference on Bulk Materials Storage and Transportation (pp. 183–189), Wollongong, NSW, Australia and Chambers, A.J., & Marcus, R.D. (1986). Pneumatic conveying calculations. In Proceedings of the second International Conference on Bulk Materials Storage and Transportation (pp. 49–52), Wollongong, Australia reliably predicted the bend losses for systems conveying fly ash over a large range of air flows.     

Transportation characteristics of gas–solid two-phase flow in a long-distance pipeline

In this study, experiments on fly ash conveying were carried out with a home-made long-distance positive-pressure pneumatic conveying system equipped with a high performance electrical capacitance tomography system to observe the transient characteristics of gas–solid two-phase flow. The experimental results indicated that solids throughput increased with increasing solids–gas ratio when the conveying pipeline was not plugged. Moreover, the optimum operating state was determined for the 1000 m long conveying pipeline with a throttle plate of 26 orifices. At this state the solids throughput was about 12.97 t/h. Additionally, the transportation pattern of fly ash gradually changed from sparse–dense flow to partial and plug flows with increasing conveying distance because of the conveying pressure loss. These experimental results provide important reference data for the development of pneumatic conveying technology.     

Observations on the rheological response of alkali activated fly ash suspensions: the role of activator type and concentration

This paper reports the influence of activator type and concentration on the rheological properties of alkali-activated fly ash suspensions. A thorough investigation of the rheological influences (yield stress and plastic viscosity) of several activator parameters, including: (i) the cation type and concentration of alkali hydroxide and (ii) the alkali-to-binder ratio (n) and silica modulus (M_s), and (iii) the volume of the activation solution, on the suspension rheology is presented. The results indicate a strong dependence on the cation and its concentration in the activation solution. The viscosity of the activation solution and the volumetric solution-to-powder ratio are shown to most strongly influence the plastic viscosity of the suspension. The suspension yield stress is predominantly influenced by the changes in fly ash particle surface charge and the ionic species in the activator. A shift from non-Newtonian to Newtonian flow behavior is noted in the case of silicate-based suspensions for M_s?≤?1.5. This behavior, which is not observed at higher M_S values, or when the fly ash is dispersed in hydroxide solutions or pure water, is hypothesized to be caused by colloidal siliceous species present in this system, or surface charge effects on the fly ash particles. Comparisons of the rheological response of alkali-activated suspensions to that of portland cement-water suspensions are also reported.     

Composition dependent model for the prediction of syngas ash deposition in turbine gas hotpath

An improved physical model to predict flyash deposition is developed and discussed in this paper. This model differs from its predecessor ( [Rozati et al., in press] and [Sreedharan and Tafti, 2009] by accounting for deposition of syngas ash particles below the ash softening temperature. The modified deposition model is based on the critical viscosity approach. To test this model, deposition of ash particles impacted on a flat, 45° wedge shape geometry is computed and the results obtained from the numerical model are compared to Crosby et al. (2007). Large Eddy Simulation (LES) is used to model the flow field and flyash particles are modeled using a discrete Lagrangian framework. Results quantify deposition for 4 μm particles of various ash composition samples. Most of the deposition occurs at the stagnation region of the target plate. At 1456 K, out of all the ash samples considered in this study, WY and ND ash sample show the highest capture efficiency (15%) and KL1 ash sample exhibits the lowest capture efficiency (0.02%). In general, capture efficiencies for all ash samples followed an exponential trend with temperature. Additionally, this model is also compared to results obtained from the flat plate deposition experiments conducted here at Virginia Tech using PVC particles (Wood et al., 2010). In the case of PVC particles, the sticking probability in the deposition model assumed an exponential increase in deposition rate with temperature and was calibrated with one experimental data point. The results obtained from this model for PVC particles showed excellent agreement with the experimental measurements over a range of temperatures.     

Relation between extrudate swelling of polypropylene and molecular weight

Six samples of polypropylene produced by Montepolimeri were carefully characterized in dilute solution and studied, with a capillary extrusion rheometer, in large ranges of shear and temperature. The extrudate swelling behaviour is found to be regular as far as its shear stress and temperature dependence is concerned, but the influence of molecular weight is abnormal. With increasing molecular weight the amount of elastic recovery (as measured by the extrudate swell) decreases markedly and the shear modulusG (obtained from the data by making use of simplifying assumptions) appears to be an increasing function ofM _w at all the shear stress investigated. No similar results appear to be reported in the literature.Presented in part at the VIIIth Intern. Congress on Rheology, Naples, September 1–5, 1980.     

Meander of a fin trailing vortex and the origin of its turbulence

The low-frequency meander of a trailing vortex shed from a tapered fin installed on a wind tunnel wall has been studied using stereoscopic particle image velocimetry in the near-wake at Mach 0.8. Distributions of the instantaneous vortex position reveal that the meander amplitude increases with downstream distance and decreases with vortex strength, indicating meander is induced external to the vortex. Trends with downstream distance suggest meander begins on the fin surface, prior to vortex shedding. Mean vortex properties are unaltered when considered in the meandering reference frame, apparently because turbulent fluctuations in the vortex shape and strength dominate positional variations. Conversely, a large peak of artificial turbulent kinetic energy is found centered in the vortex core, which almost entirely disappears when corrected for meander, though some turbulence remains near the core radius. Turbulence originating at the wind tunnel wall was shown to contribute to vortex meander by energizing the incoming boundary layer using low-profile vortex generators and observing a substantial increase in the meander amplitude, while greater turbulent kinetic energy penetrates the vortex core. An explanatory mechanism has been hypothesized, in which the vortex initially forms at the apex of the swept leading edge of the fin where it is exposed to turbulent fluctuations within the wind tunnel wall boundary layer, introducing an instability into the incipient vortex core.     

气态ClO2体积分数与爆炸压力的关系

采用20 L柱状爆炸罐研究了气态ClO2的爆炸特性,得出了气态ClO2分解爆炸的体积分数下限为9.5%,不存在上限。在实验条件下,气态ClO2的爆炸压力随体积分数的增加而增大,体积分数为90%时,最大爆炸超压达到0.64 MPa,且气态ClO2的爆炸压力与其体积分数梯度有关。最大爆炸超压出现的时间随气态ClO2体积分数的增加而缩短,体积分数为10%时,最大爆压在2 195 ms时出现,当体积分数达到70%时,最大爆压出现的时间在10 ms以内,体积分数继续增加,最大爆压出现的时间基本维持在8 ms。     

高龄混凝土表面碳化与应力腐蚀关系的研究

对多栋在役80年的混凝土建筑进行调查,检测了不同受力状态下的高龄混凝土构件的表面回弹值,并设计了一组混凝土构件老化室内模拟实验,用以分析高龄混凝土结构表面碳化发展机理。研究结果表明:普通混凝土的表面碳化速度与结构应力状态密切关联,处于拉应力状态的区域,其碳化深度明显大于压应力或无应力区域,表面碳化成为微观裂纹形成的基础,而微观裂纹的扩展将导致钢筋的锈蚀,最终导致结构承载力下降。只有处于低应力状态的混凝土才有可能达到理想的在役年限。该结论对于正确评估混凝土结构的寿命提供了实验依据。     

静电陀螺监控器六次校准与赤道陀螺方位环随动性能关系分析

静电陀螺监控器可以全天候提供连续高精度的位置及航向信息,且不受天气制约。但是在低纬度海域静电陀螺监控器会出现六次校准频繁重置的现象,导致静电陀螺无法获得准确的漂移模型系数,进而影响到设备的正常工作。针对上述现象,通过分析设备结构和工作原理,构建了赤道陀螺随动性能数学模型,推导出赤道陀螺方位环随动速率与所在纬度相关性的计算方法,研究了六次校准重置与赤道陀螺随动性能的关系,提出了低纬度地区六次校准重置现象的规避方法。研究结果有效拓展了静电陀螺监控器的启动纬度范围,提高了设备应用效益。     

Li2SiO3 fast microwave-assisted hydrothermal synthesis and evaluation of its water vapor and CO2 absorption properties

A series of lithium metasilicate (Li₂SiO₃) powder materials has been successfully synthesized by the microwave-assisted hydrothermal route using lithium hydroxide and tetraethyl-orthosilicate-derived sol precursors. Ceramic powders were obtained under hydrothermal conditions of autogenous pressure in the presence of a nonionic surfactant. The production of pure and well-crystallized Li₂SiO₃ using very short reaction times at low temperatures was shown by X-ray diffraction, scanning electron microscopy, and N₂ adsorption-desorption analyses. Synthesized Li₂SiO₃ particles were nanocrystalline and exhibited different morphologies and specific surface areas depending on the synthesis conditions. Additionally, the capability of selected Li₂SiO₃ samples to absorb H₂O and CO₂ was evaluated via thermogravimetric analyses by varying the temperature, carrier gas, and water vapor concentration. Li₂SiO₃ particles exhibited interesting textural and morphological characteristics that make them suitable for use as a CO₂ absorbent and which suggest that they also have the potential to be used in other applications.     

On the Relation between Fronts and High-Shear Layers in Wall Turbulence

Direct numerical simulation results of turbulent channel flow are analyzed in order to examine the relation between two kinds of near-wall flow structures, namely the instantaneous shear layers and the fronts which are derived from two-point statistics of the streamwise velocity component. The near-wall shear layers are analyzed by flow visualizations and conditional sampling, while the fronts are examined by means of space-time correlations and spatial two-point correlation functions. The present study focuses on the analysis of the propagation speed and the spatial shape of the structures. Concerning the propagation speed it is shown that the results obtained from flow visualizations are in close agreement with the propagation velocities derived from space-time correlation functions. The comparison of VISA results for the instantaneous shear with spatial structures obtained from two-point correlations of the streamwise velocity and the shear gives evidence that the fronts are intimately related to the pronounced near-wall shear layers.     

Relation between the atomistic picture and continuum mechanics description of detonating solid-state explosives

In the present work, using the molecular-dynamics approach, we examine the initiation and development of detonation, initiated by a pulse of an external force, in a 3-D solid-state explosive. It is found that, prior to the onset of the chemical reaction, the substance passes from crystalline into the liquid state. Based on a mesoscale analysis, we analyze the applicability of main conservation equations, written in their integral, most general form or in the stationary form for the entire computation domain, including the undisturbed region of the crystal, reacting zone, and detonation products, to detonating solid explosives.Received: 30 August 2002, Accepted: 3 July 2003, Published online: 5 August 2003     

多位置寻北误差与陀螺数据采样时间的关系

为了在规定寻北时间内确定最佳采样时间和提高寻北精度,针对多位置陀螺寻北仪寻北误差与采样时间的关系问题,以FOG二位置寻北方案为例,采用FOG寻北误差理论推导结果与寻北实验结果相比较的方法,结合FOG数据的自相关性理论分析结果,分析得到了多位置寻北误差随采样时间的变化规律:随采样时间的增加,寻北结果会产生寻北精度不稳定、基本稳定、寻北精度较高和寻北误差增大等四种情况;分析了寻北实验误差小于理论计算结果以及寻北精度并非一直随采样时间增加而提高的原因;提出采样时间极限的概念,并得到实验样机采样时间极限为120 s,提出确定采样时间的原则和选择最佳采样时间的方法,这对多位置陀螺寻北仪设计和提高寻北精度具有一定指导意义.