Multi‐scale analysis of acoustic emission signals in dense‐phase pneumatic conveying of pulverized coal at high pressure

Acoustic emission technique in conjunction with multiscale processing method has been utilized to investigate the flow behavior of the dense‐phase pneumatic conveying system at high pressure. A clearly defined classification of microscale, mesoscale, and macroscale signals has been put forward with the aid of wavelet transform and V statistics analysis. The detailed signals d₁–d₄, d₅–d₇, d₈–d₁₀ were recomposed into the microscale, mesoscale, and macroscale signals, respectively, which represent microscale particle‐wall interactions, mesoscale interaction between gas phase and solid phase (such as bubbles, plugs, dunes), and macroscale flow‐induced pipe vibration. Further analysis shows that as the mass flow rate of pulverized coal increases, the energy fraction (energy of detailed signal divided by the energy of original signal) of microscale signals decreases while that of mesoscale signals increases, which indicates that particles are more likely to move as particle aggregates than individual particles when mass flow rate increases. © 2016 American Institute of Chemical Engineers AIChE J, 62: 2635–2648, 2016     

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...     

高压密相气力输送煤粉输送速率通量及神经网络模拟

在高压密相气力输送中试试验台上,研究了输送压力P₁、总差压ΔP_T和流化风量Q_f等操作条件,以及煤粉平均粒径d_p、含水率W和煤粉种类等物性参数对煤粉输送速率通量ψ的影响规律。鉴于高压密相气固两相流的复杂性,在充分试验的基础上,先后采用原始BP神经网络和两种改进算法,对ψ进行模拟和预测,并比较各算法的优劣。研究结果表明:ψ随着P₁和ΔP_T的增大而增大,随着Q_f的增大而先增大后减小;ψ随着d_p和W的增大而减小,也受煤粉种类的影响;两种改进算法的BP网络可以对研究对象进行较好的模拟和预测,但收敛速度和预测精度不可兼得。试验结果将对高压密相气力输送系统的操作运行以及关键特征参数的模拟预测起到一定的指导作用。     

HHT analysis of electrostatic fluctuation signals in dense-phase pneumatic conveying of pulverized coal at high pressure

Particle charging generated by particle-wall, particle–particle and particle–gas contacts in pneumatic transport pipelines contain rich information on gas–solid flow hydrodynamics. In this paper, experiments were performed in a dense phase pneumatic conveying system, and electrostatic fluctuation signals were detected by a ring-shape electrostatic sensor that is based on electrostatic induction theory. Power spectrum analysis and Hilbert–Huang transform (HHT) were applied to the electrostatic fluctuations, namely outputs of the electrostatic sensor, to extract and characterize the intrinsic features of dense-phase gas–solid flow. Results show that the dominant peak of the power spectrum of the electrostatic fluctuations moves toward higher frequency with the increasing gas superficial velocity. The Hilbert–Huang transform reveals the non-linear and non-stationary intrinsic nature of the electrostatic fluctuations. By using the non-linear and non-stationary signal processing method (HHT), non-linear inter-modulation characteristics in the dense phase gas–solid flow were analyzed, and the relations between the energy distribution transmissions in intrinsic mode functions (IMFs) with different orders and the flow characterization of the dense phase gas–solid flow, was investigated as well, which can represent the behavior, stability and regime transitions of the gas–solid flow in the dense-phase pneumatic conveying system at high pressure.     

Experimental investigation on blockage boundary for pneumatic conveying of powders in narrow bifurcation slits

To investigate the blockage characteristics for dense-phase pneumatic conveying in narrow bifurcation slits, a study on the blockage boundary conditions of powders was undertaken. The results show that the solid mass flow rate for blockage increases with superficial air velocity, and the variation trend can be divided into three typical stages. Besides the relationship between the solid loading ratio and superficial air velocity for blockage in the bifurcation slit displays a “S” shape with the increase of air velocity, the solid loading ratio increases, then decreases, finally increases, and in each stage above, the relationship between the two approximately meets power function, respectively. According to the “S” shape relationship, the formula used for blockage boundary [Setia, Mallick, Wypych, and Pan (2013). Validated scale-up procedure to predict blockage condition for fluidized dense-phase pneumatic conveying systems, Particuology, 11, 657–663] was modified into piecewise function for bifurcation slits. In addition, with the increase of the bifurcation angle and conveying pressure, the superficial air velocity decreases, while the solid mass flow rate and the critical solid loading ratio increase. The research work could help understand the blockage theory of the dense-phase pneumatic conveying.     

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.     

Fluid dynamics characterization of a pneumatic bed using a spouted bed type solid feeding system

The development of a bed is reported, in which characteristics from spouted beds and pneumatic conveying are combined. The apparatus can be described as a pneumatic transport bed modified by the introduction of a recirculation process for inert materials, which leads to a final configuration akin to that of spouted beds. The dependency of the particle recirculation rates on the air flow rates and on the distance between the air inlet and the lower end of the central tube (z₀) was investigated. Also studied was the variation of the total pressure drop with air flow rate, for different loads of inert materials (glass spheres, d_p = 2.8 mm) and different z₀ values.     

炭黑密相气力输送的节能试验

密相气力输送系统的输送效率是人们关注的焦点,从工程意义来讲,降低其输送能耗具有相当大的空间和重要的意义。以炭黑为输送物料进行了密相气力输送节能试验,对比了可控旁通管和双辅管输送系统的耗气量;验证了可调式LAVAL管的节能效果及其对降低粒子破碎率的作用。     

Investigation on characteristics of pulverized coal dense-phase pneumatic conveying under high pressure

Experiments of dense-phase pneumatic conveying of pulverized coal were carried out in a test facility with a conveying pressure up to 4 MPa. The influence of fluidization nitrogen flow rate, the flow rate of supplementary nitrogen, and the pressure difference between sending hopper and receiving hopper on the solids to gas ratio and the solid mass flow rate was investigated. Test results indicate that with the increase in fluidization nitrogen flow rate, the solid mass flow rate increases, and the solids to gas ratio increases at first and then declines. When the fluidization of pulverized coal in the sending vessel becomes intensive, with the increase in supplementary nitrogen flow rate, the solids to gas ratio declines and the solid mass flow rate increases. And the solid mass flow rate increases linearly with the increase in pressure difference between two hoppers. The experimental results provide a database for the design and operation of a dense-phase pneumatic conveying system. This work was presented at the 6 ^th Korea-China Workshop on Clean Energy Technology held at Busan, Korea, July 4–7, 2006.     

补充风对水平管高压密相气力输送影响的模拟研究

针对水平管高压密相气力输送数理模型的缺陷与不足,引入Savage径向分布函数修正的颗粒动理学理论、基于Berzi摩擦压应力模型构建的摩擦应力模型以及修正的三段式曳力模型,在欧拉-欧拉方法的基础上建立了一个能同时兼顾水平管高压密相气力输送中稀相流、过渡流以及密相流输送特性的三维非稳态数理模型。并采用该数理模型考察了补充风对水平管高压密相气力输送的影响,模拟结果精准地预测了水平管压降及其随补充风的变化规律,而且其预测的水平管固相体积浓度分布与ECT图也是相吻合的,从而验证了数理模型的可靠性。模拟结果表明：随着补充风的增加,气固两相速度和湍动能以及颗粒拟温度增大,固相体积浓度减小。     

Pressure drop prediction for horizontal dense-phase pneumatic conveying of pulverized coal associated with feeding to gasifier

Based on extensive bench-scale data derived from the horizontal dense-phase pneumatic conveying of pulverized coal, a correlation of solids friction factor λ_z was proposed in an effort to establish a model to predict the pressure drop when coal fed to the gasifier. Further, it was also an attempt to modify some public models to verify their availabilities. Then, based on the data collected from an industrial-scale horizontal pipeline under the high pressure up to 2.0 MPa, the proposed model was found to be possibly among the best ones for predicting the pressure drops of the dense flow of pulverized coal. The modified Mallick and Wypych model can also provide satisfying predictions. The results suggest that the two models are both suitable for scale-up of dense-phase pneumatic conveying of pulverized coal at high pressures.     

Deep learning-based tomographic imaging of ECT for characterizing particle distribution in circulating fluidized bed

The gas and solids in a circulating fluidized bed (CFB) are heterogeneously dispersed and a multiscale flow regime may form both in time and space. Accurate measurement of the fluidizing process is significant for investigating the multiscale gas–solid flow characteristics and the design, optimization, and control of CFBs in various applications. This article develops a deep learning-based tomographic imaging of electrical capacitance tomography (ECT) to characterize the particle concentration distribution in a CFB. The deep tomographic imaging approach is realized through training a well-designed convolutional neural network (CNN) with the numerically built dataset. Simulation results demonstrate that the average values of the relative image errors reconstructed by CNN in the test set are 0.1110 and 0.1114 for the 60 and 100 mm pipes, respectively, which are better than the average values of 0.1819 and 0.2519 by the Landweber algorithm. With the verification of the trained model based on the prepared data can image the unseen typical flow patterns better than Landweber, it is further used to investigate the particle flow characteristics of a lab-scale CFB. Experimental results reveal that the developed deep tomographic imaging of ECT can successfully measure the fluidized particle distribution in both the 60 and 100 mm pipes, showing good prediction and generality of the designed CNN model. A flow regime transformation from “annular” flow to “core-annular” flow and pneumatic conveying is observed under the tested conditions. Besides, the flow regime would be highly affected by the fluidized gas flow rate and the initial bed height.     

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。     

Three‐Dimensional Numerical Simulation of Dense Pneumatic Conveying of Pulverized Coal in a Vertical Pipe at High Pressure

A two‐fluid model based on the kinetic theory of granular flow was used to study three‐dimensional steady state flow behavior of dense phase pneumatic conveying of pulverized coal in a vertical pipe, where the average solid concentration ranges from 11 % to 30 %, and the transport pressure ranges from 2.6 Mpa to 3.3 Mpa. Since the solid concentration is rather high, a k–?–k_p–?_p model which considers the turbulence interaction between the gas and particle phase, was incorporated into the two‐fluid model. The simulation results including profiles of gas and particle phase axial velocity, profiles of solid concentration, profiles of the turbulence intensity of the particle phase, as well as the value of the pressure gradient were reported. Then, the influences of solid concentration and transport pressure on the flow behaviors were discussed. The experiment was also carried out to validate the accuracy of the simulation results which showed that the predictions of pressure gradient were in good agreement with the experimental data. Simulation results indicate that the location of maximal solid concentration deviates from the pipe center and the deviation becomes more obvious with the solid concentration increasing, which is analogous to the phenomenon in the liquid/solid flow. Besides, pressure gradient declines as the transport pressure decreases, which is validated by experiment described in the paper. Moreover, the analysis indicates that it is necessary to consider the turbulence of particles for the simulation of dense phase pneumatic conveying at high pressure.     

Pneumatic transport of granular materials with electrostatic effects

The methodology of coupling large eddy simulation (LES) with the discrete element method was applied for computational studies of pneumatic transport of granular materials through vertical and horizontal pipes in the presence of electrostatic effects. The LES numerical results obtained agreed well with the law of the wall for various y⁺‐ranges. The simulations showed that a thin layer of particles formed and remained adhered to the pipe walls during the pneumatic conveying process due to the effects of strong electrostatic forces of attraction toward the pipe walls. Particle concentrations were generally higher near the pipe walls than at the pipe center resulting in the ring flow pattern observed in previous experimental studies. The close correspondence between particle velocity vectors and fluid drag force vectors was indicative of the importance of fluid drag forces in influencing particle behaviors. In contrast, the much weaker particle–particle electrostatic repulsion forces had negligible effects on particle behaviors within the system under all operating conditions considered. The electrostatic field strength developed during pneumatic conveying increased with decreasing flow rate due to increased amount of particle‐wall collisions. Based on dynamic analyses of forces acting on individual particles, it may be concluded that electrostatic effects played a dominant role in influencing particle behaviors during pneumatic conveying at low flow rates, whereas drag forces became more important at high flow rates. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

高压浓相粉煤气力输送特性及信息熵分析

在输送压力可达4.0MPa,固气比高达450kg/m3的高压气力输送试验台上,用氮气进行粉煤高压浓相气力输送试验研究。分别在不同的输送差压、浓度和速度等条件下进行了输送试验,考察操作参数对煤粉固气比等气力输送特征参数的影响,用信息熵分析试验过程中采集到的压力波动时间序列,探讨流动稳定性和流型变迁过程中信息化趋势,建立信息熵和流型之间的关系。结果表明在输送差压增大的过程中,固气比和Shannon信息熵均增大;气体流量与Shannon信息熵和固气比之间呈现较好的规律性;不同流动形态的Shannon熵差异较大,不同流型之间的Shannon熵区分度较好。Shannon信息熵分析为研究高压浓相气力输送流型及其转变特性提供了一种行之有效的方法。     

Minimum pickup velocity: The transition between nano‐scale and micro‐scale

The transport of nano‐scale particles has become increasingly important, but the knowledge base available is limited. This study aims to bridge the knowledge gap between the nano‐ and micro‐scales for pneumatic conveying. A key parameter is the minimum pickup velocity (U_pu), which is the minimum fluid velocity required to initiate motion in a particle originally at rest. The U_pu values of nine alumina particles with particle diameters (d_p) ranging from 5 to 110,000 nm were determined using the weight loss method, then compared against the established pickup Zones (analogous to the Geldart Groups). Results indicated that: (1) U_pu varied non‐monotonically with increasing d_p, thus revealing the missing link between the nano‐ and micro‐scales; (2) the intermediate particle diameters surprisingly did not agree with any pickup Zone; (3) Zone III (analogous to Geldart Group C) is inadequate for all the nano‐scale particles, so new boundaries and a new Zone are proposed. © 2016 American Institute of Chemical Engineers AIChE J, 63: 1512–1519, 2017     

粉煤灰中浓度气力输送管道压降研究

对于粉体气力输送流动区域划出中等浓度区，使浓相输送与稀相输送的区分变得清楚、分析了粉体气力输送管道压降的主要影响因素，在量纲分析基础上，对中浓度气力输送提出一个比较简洁的管道压降关联式，与文献的实验数据进行拟合，结果表明该式可以很好地描述粉煤灰中浓度气力输送管道的总压降。对于给定的一个相图，该式的系数k、有一固定的数值；此值与物料质量流率（或气体质量流率）、料气比无关。     

不同载气供料对工业级竖直上升管粉煤气力输送的影响

采用Air和CO₂作为供料和输送载气,以工业级竖直上升管(内径50 mm)粉煤输送系统为实验平台,开展了粉煤密相气力输送实验研究。实验中选用Air→Air、Air→CO₂、CO₂→Air和CO₂→CO₂作为典型工况,进行了输送特性参数的对比分析。研究结果表明,供料载气与输送载气组合方式的变化,会引起发料罐中粉煤流化状态的改变,进而导致粉煤输送流型和稳定性方面的差异。借助电容层析成像系统(ECT)获得了竖直管的ECT信号和流型图像,并采用标准差和功率谱函数对ECT信号进行分析。分析结果表明,Air→Air工况为稳定的环状流;Air→CO₂输送过程稳定性最差,输送由无定态流型转为栓塞流;CO₂→Air和CO₂→CO₂输送过程均为气栓流,但CO₂→CO₂工况形成的气栓较长,其ECT信号的功率谱峰值较大。     

高压密相气力输送垂直弯管阻力特性

在输送压力可达4 MPa的气力输送实验台上,进行不同平均粒径煤粉的密相输送实验。从理论与实验两方面研究气固两相流流经弯管的压损随煤粉体积分数以及表观气速的变化规律。结果表明,高压密相输送中煤粉运动造成的能量损失是总压损的主要部分,且总压损随着煤粉体积分数的增加而增加。研究表明,相同质量流量下,随着表观气速的增加,弯管动能压损增加,摩擦压损降低;相同表观气速时,随着煤粉体积分数的增加,摩擦压损增加;固相摩擦系数与煤粉平均粒径及煤粉质量流量无明显关系,随着表观气速的增加略有下降。