On the electrostatics of pneumatic conveying of granular materials using electrical capacitance tomography

In this work the electrostatics of the pneumatic conveying of granular materials in a non-conducting (PVC) pipe is studied using Electrical Capacitance Tomography (ECT) system. The non-conducting wall in general attains static charges arising from particle-wall collisions in the initial periods of conveying process and then reaches equilibrium with the surroundings. The polarity of particles and conveying pipe inner wall agrees reasonably well with the contact potential difference measurements. The perturbations in the capacitance signal due to charge accumulation are larger with smaller air superficial velocity. The denser flow regimes give larger wall residual charge. Wall charging process shows similar trend described by surface potential taken from electrostatic voltmeter to that revealed by ECT measurements. Also the addition of small amount (0.5% by weight) of anti-static agent (Larostat-519) in the powder form decreases the electrostatic charge generation by altering the dynamics of particle-particle and the particle-wall collisions.     

电容层析成像在高压浓相煤粉气力输送中的应用

Electrical capacitance tomography (ECT) was applied to dense-phase pneumatic conveying of pulverized coal, including the visualization of gas-solid flows in a horizontal pipeline.The pressure of experimental setup was up to 4.0 MPa, the solid-gas ratio was up to 11.73 kg·kg-1, and the diameter of conveying pipeline was 10 mm.The pipeline thickness of 8-electrode ECT system was 5 mm.An improved AC-based capacitance measuring circuit was developed.Single channel capacitance measuring circuit was adopted to si...     

Simulation of the performance of electrical capacitance tomography for measurement of dense phase pneumatic conveying

A method for the estimation of image distortion which can occur in the reconstruction of electrical capacitance tomography (ECT) data for pneumatic conveying process is presented. Image distortion is associated the finite sampling rate of tomographic instrumentation which arises from changes in the distribution of phases occurring during a measurement cycle. The method uses an algorithm which provides the solution of the inverse and forward problems for an idealized 2-D sensor and to visualize the movement of powder slugs. The influence of the geometry of the sensor electrodes and the nature of slug perturbation on the fidelity of the reconstructed image is discussed. The practical implications of the simulation for the application of capacitance tomography for monitoring and control of dense phase conveying are discussed.     

Experimental research of flow patterns and pressure signals in horizontal dense phase pneumatic conveying of pulverized coal

Understanding flow patterns and their variability is important for optimal design and trouble free dense phase pneumatic conveying of pulverized coal in a horizontal tube. Employing the electrical capacitance tomography (ECT), six flow patterns were identified and utilized for quantitative analysis based on the value and distribution of cross-sectional solid concentration. The dense-phase flow patterns in the horizontal tube of the pneumatic conveying system were somehow variable even when the operating conditions were unchanged. The probability calculation results suggest changing multiple flow patterns with one or two dominant flow for each of the seven sets of experimental conveying conditions and that a finite change in the dominant flow pattern would occur with an increasing superficial gas velocity. The power spectral density (PSD) function and the Hurst exponent of the pressure signals of the pulverized coal were well correlated with its flow patterns in a horizontal tube. The PSD functions and probability density functions (PDFs) of the void fraction signals from ECT are found to be related with flow patterns and can be used to quantitatively identify flow regimes. The ECT data may therefore be utilized for monitoring the flow patterns in a horizontal tube employed for pneumatic conveying of pulverized coal.     

Flow characteristics and pressure drop across the Laval nozzle in dense phase pneumatic conveying of the pulverized coal

Experimental studies were performed to describe the physical phenomena occurring in dense phase pneumatic conveying of the pulverize coal with a Laval nozzle installed in the pipeline. The maximal coal mass flow rate decreased from 0.87 kg/s to 0.35 kg/s and an obvious decrease in the solids loading ratio was revealed after the Laval nozzle was installed. In addition, the Laval nozzle showed a better capacity of resisting disturbance, which made it easier to control the coal mass flow rate precisely and promoted the stable conveying process. These specific physical phenomena were proved to result from the high pressure drop of the Laval nozzle. Thereby, a mathematic model was developed to predict the two-phase pressure drop across the Laval nozzle. The pressure drop model described the experimental data within the 15% deviation. The main influence factors contributing to the pressure drop of the Laval nozzle were discussed using the model. Then the effects of gas mass flow rate, solids loading ratio, convergence angle, throat diameter and throat length were revealed.     

密相气力输送中气固两相流动特性多源信息分析

在加压密相气力输送煤粉实验装置上,结合静电、压力传感器与电容层析成像系统(ECT)3种方法,利用统计学和分形方法研究了不同操作条件(总输送压差、输送载气和煤粉含水率等)下密相气固两相流动力学行为。结果表明:输送载气为CO₂时,颗粒相微观运动剧烈程度随总输送压差(0.3~1 MPa)的增大而增大,气流和颗粒相之间的相互作用在总输送压差0.75 MPa下最强烈,但煤粉宏观整体的流动保持稳定;煤粉含水率增加对宏观流动稳定性没有影响,但水分增加使颗粒相的微观运动剧烈性减小;载气为N₂时,管道截面煤粉的平均浓度及分布变化明显,宏观流动状态不稳定,但颗粒相的微观运动剧烈程度与载气为CO₂时的变化规律一致。     

Evaluation of models and correlations for pressure drop estimation in dense phase pneumatic conveying and an experimental analysis

This study reviews the models and correlations for dense phase conveying in an effort to explore existing and new data on the subject and to provide guidance to the designer on the best pressure drop model. Using various data sets the Mi (Konrad)-based model was found to be best for predicting the pressure drop across dense phase plugs. A series of industrial scale tests also shows agreement with the Mi (Konrad)-based model.     

煤粉高压密相气力输送稳定性分析

在自主研发的密相气力输送实验装置上,以N₂作输送载气,以3种不同粒径的褐煤作输送物料进行输送实验,运用域重标分析方法对水平管差压信号和料罐质量脉动序列进行了Hurst指数分析,并采用统计学方法对料罐质量脉动序列进行了标准差分析,从管内流动和料罐出料出发,对整体输送稳定性进行了探讨。结果表明：水平管差压信号Hurst指数H_dp能较好地表征管内流动稳定性,而料罐质量脉动Hurst指数H_ma则需结合标准差分析才能更好地表征料罐出料稳定性。以表观气速与跃移速度比值K为特征参数进行探讨,在K=1.5左侧,随着K值减小,H_dp减小,管内流型发生转变,甚至可能发生堵管,管内流动稳定性变差,料罐质量脉动标准差增大,出料稳定性变差,整体输送稳定性变差;在K=1.5右侧,随着K值增大,H_dp减小趋势不明显,管内流动稳定性虽有变差的趋势,但不会发生堵管,而结合对料罐质量脉动序列的标准差分析,发现出料稳定性变差,甚至可能出现出料中断的情况,整体输送稳定性变差。     

Experimental study on gas-solids flows in a circulating fluidised bed using electrical capacitance tomography

It is difficult to measure the gas-solids flow in a circulating fluidised bed (CFB) because of the complicated and rapid transient process. Electrical capacitance tomography (ECT), a cross-sectional imaging technique, has been used to measure the dilute flow in a large square CFB. A sensor has been specifically designed for the measurement and a new algorithm has been developed for image reconstruction. Flow conditioning parts (internals) are designed and placed inside the CFB, aiming to enhance the contact between gas and particles in the dilute gas-solids flow. The dynamic characteristics and detailed information were obtained on two sections in the bed at different height. The performance of the internals is related to their size, combination, height in the bed and the superficial gas velocity. It has been confirmed that a particular combination of internals can increase the solids concentration in the central area of a cross-section, and can improve the probability density distribution (PDD) with a moderate gas velocity. Using a combination of a large internal at an upper location and a small one at a lower location can optimise the flow in the CFB.     

气力输送工艺在煤粉输送中的应用

针对工业煤粉输送技术效率低、不安全等问题,介绍了稀相正压输送、密相正压输送和负压输送3种气力输送工艺的原理和使用现状,研究了密相输送工艺的原理和目前主流的栓流式密相输送技术。分析了稀相正压输送、密相正压输送和负压输送工艺各自的技术特点,并在投资、能耗、环保等方面进行对比。结果表明负压输送输送距离较短,稀相正压输送能量损失大、易发热,而密相正压输送固气比高、能耗低、管道和物料磨损小、噪声低、输送过程不易发热,更适合煤粉输送。     

Investigation of fluidized bed behaviour using electrical capacitance tomography

The temporal and cross-sectional distributions of particles in a 127 mm diameter fluidized bed have been obtained using a new generation, high-speed electrical capacitance tomography. Two planes of eight electrodes were used and mounted at 160 and 660 mm from the gas distributor which was a 3 mm thick porous plastic plate (maximum pore size of 50-70 μ m). 3 mm diameter, nearly-spherical polyethylene granules made up the bed. Experiments at sampling frequencies of 200-2000 cross-sections per second and gas superficial velocities from just below the minimum fluidization to 83% above minimum fluidization velocities were used. The time series of the cross-sectional average void fractions have been examined both directly and in amplitude and frequency space. The last two used probability density functions and power spectral densities. The information gathered shows that the fluidized bed was operating in the slugging mode, which is not surprising given the size of the particles. It has been found that an increase in the excess gas velocity above the minimum fluidization velocity resulted in an increase in the mean void fraction, an increase in the length and velocity of the slug bubbles as well as the bed height, and a slight decrease in the slug frequency. The results are presented in a level of detail suitable for comparison with later numerical simulation.     

水平管加压密相煤粉气力输送数值模拟

针对加压密相气力输送,对现有的颗粒静摩擦力模型进行适当修正,并将其与颗粒动理学理论相结合,建立了可以描述加压密相气力输送的气固湍流流动状况的多相流模型。该模型充分考虑了颗粒间碰撞和摩擦力作用,以及气相和颗粒团湍流脉动之间的相互作用。采用该模型对水平管内加压密相气力输送进行了三维数值模拟研究,模拟得到了气相和固相的速度、浓度和湍流强度分布,以及压降梯度的变化规律,再现了颗粒沉积层的形成和运动的动态过程。并进行了加压密相煤粉气力输送试验研究,预测的压降梯度与试验测量结果相符合。     

粉煤密相气力输送流型

基于电容层析成像技术对粉煤密相气力输送系统的流型进行了研究,获得了水平管和竖直上升管粉煤密相气力输送系统的典型流动形态。研究结果表明,水平管输送时的流型随时间复杂多变,典型流型有满管流、沉积层流、悬浮流等;统计分析发现,随表观气速不同,存在明显的主导流型;结合固相速度及管道压力信号分析,展示出其与流型之间存在一定的对应关系,进一步证实了密相气力输送系统的不稳定性特征。对竖直上升管的ECT测试结果表明,输送流型为环核结构。     

密相气力输送物料流动状态分析及供料装置的选用

综述了低速密相气力输送中的相图和物料流动形态；介绍了密相输送供料装置（充气罐供料器、螺旋供料充气罐、高压旋转阀供料器）及其应用场合。同时简要分析了物料特性对密相气力输送的性能影响。     

Application of electrical capacitance tomography to the fluidized bed drying of pharmaceutical granule

A novel calibration technique that accounts for the presence of moisture has allowed for the application of electrical capacitance tomography (ECT) to a fluidized bed dryer containing wet pharmaceutical granule. A statistical attractor comparison test denoted as the S-statistic has been used to analyze both the reconstructed and non-reconstructed ECT images to determine radial variations in hydrodynamic behavior at an axial location 10.5 cm above the distributor. This analysis has shown that variations in the bed dynamics associated with the loss of moisture during the drying process are most intense near the walls of the dryer, while the central region is characterized by a more consistent dynamic behavior. Investigation of the voidage distribution in the individual tomograms over this period in the drying process shows that the changes in the S-statistic correspond to a reduction in area of the region involved in bubbling behavior. Tomograms from early in the drying process indicate a central core of gas channeling through the very wet granule. This is due to the high cohesive forces present at high moisture. At moisture contents below 5-wt%, near the end of drying, significant divergence in dynamic behavior is identified in the S-statistic. This divergence is associated with the appearance of asymmetric bubbling behavior in the individual tomograms.     

Verification of fluidized bed electrical capacitance tomography measurements with a fibre optic probe

Previous studies aimed at determining the spatial accuracy of electrical capacitance tomography (ECT) have employed phantoms placed within the ECT measurement space. No previous studies have compared ECT with a second independent measurement technique in an operating fluidized bed. In the present work, radial voidage profiles have been measured with ECT in the 0.14-m I.D. riser of a circulating fluidized bed (CFB) and in a bubbling fluidized bed with a 0.19-m I.D. The dynamic and time-averaged radial voidage profiles have been compared with measurements taken with a fibre optic probe in the same riser and in a slightly narrower (0.15-m I.D.) bubbling fluidized bed. In spite of the intrusiveness of the latter technique, the time-averaged radial profiles in the CFB riser fall within 10% of each other when the CFB is operated at high-flux conditions that lead to a very dense wall region. Iterative reconstruction of the ECT images is not needed in this case. Similar agreement is found between the two techniques in the bubbling fluidized bed, but off-line iterative image reconstruction is clearly necessary in this fluidization regime. These results suggest that ECT, which is often described as a tomographic imaging technique with low spatial resolution, can in fact provide semi-quantitative time-averaged images of the cross-section of fluidized beds of diameter comparable to or less than that used here.     

Numerical simulation on dense phase pneumatic conveying of pulverized coal in horizontal pipe at high pressure

A kinetic–frictional model, which treats the kinetic and frictional stresses in an additive manner, was incorporated into the two fluid model based on the kinetic theory of granular flow to simulate three dimensional flow behaviors of dense phase pneumatic conveying of pulverized coal in horizontal pipe. The kinetic stress was modeled by the kinetic theory of granular flow, while the friction stress is from the combination of the normal frictional stress model proposed by Johnson and Jackson [1987. Frictional–collisional constitutive relations for granular materials, with application to plane shearing. Journal of Fluid Mechanics 176, 67–93] and the modeled frictional shear viscosity model proposed by Syamlal et al. [1993. MFIX documentation and theory guide, DOE/METC94/1004, NTIS/DE94000087. Electronically available from http://www.mfix.org], which was modified to fit experimental data. For the solid concentration and gas phase Reynolds number was high, the gas phase and particle phase were all treated as turbulent flow. The experiment was carried out to validate the prediction results by three kinds of measurement methods. The predicted pressure gradients were in good agreement with experimental data. The predicted solid concentration distribution at cross section agreed well with electrical capacitance tomography (ECT) image, and the effects of superficial velocity on solid concentration distribution were discussed. The formation and motion process of slug flow was demonstrated, which is similar to the visualization photographs by high speed video camera.     

Characterization of pneumatic transportation of pulverised coal in a horizontal pipeline through measurement and computational modelling

Pneumatic conveyors which feed the burners at coal fired power plants are not designed for the injection of an increasingly wide variety of modern fuels. The objective of this study is to understand and predict the stable delivery of fuel to the burners. The pneumatic transportation of pulverised coal in a horizontal pipeline has been investigated using a classic dual approach: measuring a set of characteristic parameters of the dispersed flow as well as its computational modelling. The Large Eddy Simulation approach is used for the modelling of the air/fuel two-phase flow. A particular treatment for the solid phase has been designed in order to cope with gravity effects. A good qualitative agreement between the modelling results and the experimental data has been found. The importance of gravity effects compared to inter-particle collisions is addressed.     

Measurement of fluidised bed dryer by different frequency and different normalisation methods with electrical capacitance tomography

To investigate the effect of moisture content, excitation frequency and normalisation model on image reconstruction with a fluidised bed dryer, an electrical capacitance tomography (ECT) sensor was mounted near the bottom of the drying chamber. An ECT system based on an HP4128 impedance analyser was used to measure capacitance and loss conductance between the electrode pairs in the sensor. It has been found that the capacitance depends on not only the particle moisture but also the excitation frequency. With a low moisture content, the relationship between capacitance and frequency is simple and linear. With a high moisture content, however, the relationship becomes more complex and non-linear. For image reconstruction, different normalisation models have been used: series, parallel, Maxwell and Böttcher models. The results show that with a low moisture content, these models give nearly the same image errors. With the increase in moisture content, the difference between these models becomes more and more obvious. With different gas-solids flow patterns, the four models also give slightly different images. In the end of this paper, solids distribution and averaged solids concentration profile for dynamic test with different excitation frequencies in a fluidised bed dryer are given.     

New insights into transient behaviors of local liquid-holdup in periodically operated trickle-bed reactors using electrical capacitance tomography (ECT)

Transient behavior of local liquid-holdup of air-kerosene fluids in periodically operated trickle-bed reactor (TBR) is investigated in an acrylic column (140 mm ID and a height of 980 mm) packed with 3.6 mm glass spheres using a nonintrusive technique of electrical capacitance tomography (ECT). Local liquid-holdups determined from ECT images of normalized permittivity are experimentally calibrated under the steady-state operation with that from the classic drainage method. The instantaneous ECT images are captured at several axial positions along the column for the periodically operated TBR with slow-mode. The effect of periodic operation parameters (split, period, and time-average flow rate) on the instantaneous profiles of local liquid-holdup is firstly examined compared with the previous results. Transient variations of radial distributions of liquid and their maldistribution factor are calculated and further analyzed to provide more liquid distribution information under periodic operations.