工业级水平管粉煤气力输送的最小压降速度和稳定性

采用压缩空气作为输送介质,在工业级水平管(内径50 mm)上开展了粉煤密相气力输送实验研究。在实验获得最小压降速度基础上,通过电容层析成像系统观察到,随着表观气速降低而存在分层流、沙丘流、移动床流以及栓塞流4种流型。不同流型压力信号的概率密度分布和功率谱密度分析表明,压力信号的波动特征与流型紧密联系;由于流动形态的变化,存在由稳定输送过渡到不稳定输送的临界气速,且该速度小于最小压降速度。     

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     

离散单元法数值研究水平管中密相栓流气力输送

在对密相栓流气力输送过程中气固两相运动的特点和气固两相流的数值模拟方法进行分析的基础上,将广泛应用于岩土力学中的离散单元法引入以模拟多个颗粒之间长时间的相互作用,同时考虑颗粒与流体间的相互作用(双向耦合),建立了密相栓流气力输送过程中两相流的数学模型.对多个栓的密相栓流气力输送过程进行数值模拟,结果再现了栓的滑落、形成和运动过程,与实验观察到的现象基本一致,预示离散单元法在密相气固两相流数值模拟研究中具有广阔的应用前景,为密相栓流气力输送的进一步研究提供了基础.     

Coupled DEM‐CFD Simulation of Pneumatically Conveyed Granular Media

A DEM‐CFD coupling for the simulation of gas‐solid flows was successfully implemented and simulations were performed for the application to industrial‐scale pneumatic conveying. Therefore, all particle collisions and phase interactions were considered and porosity determination was optimized. The aim of this work is to show the applicability of the presented simulation model to the different regimes of pneumatic conveying systems. As a first test case a dense vertical pneumatic conveying system was chosen and an individual plug was investigated in detail. Variations of the conveying air velocity were also considered. As a second test case dilute conveying in a horizontal‐to‐vertical pipe bend was simulated. The occurrence of roping and the reduction of particle velocity is of high interest for the design of specific pneumatic systems. It is shown that both regimes can be captured reasonably well and the results are rich in details.     

Flow regime identification in horizontal pneumatic conveying by nonintrusive acoustic emission detection

This work investigated flow regimes identification in horizontal pneumatic conveying from the scope of particle motions based on acoustic emission detection. Results showed that the variation of acoustic energy and acoustic energy fraction could be used to identify the transition between suspension flow and stratified flow. Near the transition point, the acoustic energy was maximum, and the energy fraction of particle-wall collision mutated at high mass fluxes. Furthermore, the fluctuation distribution index (FI) was constructed learning from the concept of polymer molecular weight distribution index, and the “circumferential fluctuation difference (D)” was defined. Based on this, new identification criteria and flow regime diagram were established. In suspension flow, D >0. In stratified flow, D <0. While in slug flow, D ≈ 0. According to these criteria, flow regimes can be identified independently of changing trends of measurement parameters. The universality of above criteria was further verified under different operating conditions.     

Non-Newtonian multi-phase flows: On drag reduction, pressure drop and liquid wall friction factor

The present work focuses on the non-Newtonian liquid drag reduction by gas injection. Two regimes are taken in consideration: fully stratified gas shear-thinning liquid flow and gas shear-thinning liquid slug flow regimes.Predictions of drag reduction ratio and holdup are presented for the stratified flow of gas and non-Newtonian Ostwald-de Waele liquid. Fully stratified flow is considered and the approach developed in Taitel and Dukler (1976) is used. For these regimes, CMC (CarboxyMethyl Cellulose) solution is used in order to investigate the behaviour of the gas and non-Newtonian liquids in horizontal pipes. Results have been reformulated and an extension to interfacial Andreussi and Persen (1987) correlation has been carried out for stratified regimes.For slug flow regimes, the mechanistic slug unit model is adopted in order to estimate the pressure gradients along the slug unit. The slug unit model is rearranged and reinterpreted as inviscid Burgers's equation for incompressible phases.For both stratified and slug flow regimes, three dimensional CFD (computational fluid dynamics) simulations were performed in order to compare the drag reduction ratio and pressure gradients. In stratified flows, CFD is also used in an attempt to evaluate the liquid wall friction factor and to compare the obtained values with those given by empirical standard correlations.     

The influence of particle shape on flow modes in pneumatic conveying

The transportation of particles along pipes or ducts using an imposed gas flow is known as pneumatic conveying. The type of granular flow in such systems is strongly dependent on the imposed gas flow rate, and can be categorised by a distinct set of modes. These modes range from dilute flow, where the grains are entirely suspended in the gas, to moving dunes and slug flow, in which the bore of the pipe is blocked by a slow moving plug of material. Understanding the transitions between these modes is critical to the design and application of pneumatic conveying systems. Particle shape is a crucial factor in systems with gas–grain interactions but has so far been overlooked in models of pneumatic conveying. We carry out a series of simulations using the discrete element method coupled to gas flow and show that particle shape is critical to the transition between different flow modes. Particles which are spherical, or nearly spherical, transition to slug flow at high gas flow rates, whereas non-spherical particles transition instead to dilute flow. We show the lower voidage fraction in beds of non-spherical particles is crucial to explaining this behaviour.     

3‐D numerical simulations on flow and mixing behaviors in gas–liquid–solid microchannels

Three‐dimensional (3‐D) gas‐liquid–solid flow and mixing behaviors in microchannels were simulated by coupled volume of fluid and discrete phase method and simulations were validated against observations. The detachment time and length of gas slug are shortened in liquid–solid flow, compared with those in liquid flow due to higher superficial viscosity of liquid–solid mixture, which will move the bubble formation toward the dripping regime. Solid particles mainly distribute in liquid slug and particle flow shows obvious periodicity. With the increase of contact angle of the inner wall, gas slug (0–50°), stratified (77–120°), and liquid drop (160°) flows are observed. The residence time distributions of solid and liquid phases are similar because particles behave as tracers. The backmixing of solid and liquid phases in liquid drop flow is the weakest among the three flow patterns, and the backmixing of gas phase in slug flow is weaker than that in both stratified and liquid drop flows. The results can provide a theoretical basis for the design of microreactors. © 2013 American Institute of Chemical Engineers AIChE J, 59: 1934–1951, 2013     

Flow Characteristics and Shannon Entropy Analysis of Dense‐Phase Pneumatic Conveying of Pulverized Coal with Variable Moisture Content at High Pressure

Experiments consisting of dense‐phase pneumatic conveying of pulverized coal using nitrogen were carried out in an experimental test facility, with a conveying pressure of up to 4 MPa. The influences of the conveying differential pressure, the coal moisture content, the gas volume flow rate and the superficial velocity, on the solid‐gas ratios, were investigated. The Shannon entropy analysis of the pressure fluctuation time series was developed to reveal the flow characteristics. By investigation of the distribution of the Shannon entropy at different conditions, the flow stability and the evolutional tendency of Shannon entropy, in different regimes and regime transition processes, were revealed, and the relationship between Shannon entropy and the flow regime was also established. The results indicate that the solid‐gas ratio and the Shannon entropy rise with increases in conveying differential pressure. The solid‐gas ratio and the Shannon entropy reveal preferable correlation with the superficial gas velocity. Shannon entropy is different for different flow regimes, and can be used to identify the flow regimes. Both the mass flow rate and the Shannon entropy, decrease with increases in moisture content. Shannon entropy analysis is a feasible approach to researching the characteristics of the flow regime, the flow stability and the flow regime transitions in dense‐phase pneumatic conveying systems, at high pressure.     

Velocity of isolated particles along a pipe in stratified gas–liquid flow

The average velocity of isolated grains of sand was experimentally measured in smooth stratified flow in slightly declined pipes. Isolated particles in smooth stratified flow behave similarly to isolated particles propelled by both hydraulic conveying and intermittent gas/liquid flow. In all three cases, particle velocity is linear with respect to the average liquid velocity of the flow (or the average fluid velocity in the slug body for intermittent flow) and has a gradient of approximately one. The data in stratified flow are successfully correlated dimensionlessly (Eq. 7). The correlation is extrapolated to zero particle velocity to estimate the conditions required to ensure sand transport in a flowline in smooth stratified flow. The experimental results suggest that particle velocity is strongly governed by the size of a particle relative to the depth of the viscous sublayer at the pipe wall. If a particle is larger than the viscous sublayer, it is exposed to more coherent turbulent structures and therefore experiences a greater drag.     

Particle Holdup Profiles in Horizontal Gas‐liquid‐solid Multiphase Flow Pipeline

Sand holdup is one of the most important hydrodynamic parameters that is needed for performance estimation, design, operation and control of oil‐gas‐sand multiphase production and pipeline transportation systems. The performance of oil‐gas‐sand multiphase flow can be reliably evaluated by measuring the sand holdup in such oil‐gas‐sand multiphase production and pipeline transportation systems. In the present work, a local sand holdup has been measured under conditions analogous to the horizontal oil‐gas‐sand three‐phase slug flow in pipelines. Accurate local sand particle holdup measurements were performed by the digital imaging technique. The results revealed the influence of operating conditions such as gas and liquid velocities and sand particle loading on the distribution of the local sand particle holdup in the horizontal air‐water‐sand multiphase slug flow pipe. Explanations for the observed trends are provided, shedding light on the general structures and mechanisms of the distribution of the local sand holdup in a horizontal oil‐gas‐sand three‐phase slug flow. Such information on the horizontal air‐water‐sand three‐phase slug flow mechanisms are essential to advance the mechanistic approach for predicting local sand holdup distribution and the subsequent effect on sand deposition during multiphase petroleum production and transfer operations.     

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。     

柱塞气固两相流输送特性

为了研究柱塞式气力输送气固两相流的输送特性,对实际的工业气力输送系统进行1∶1试验台改造,首先进行了粉煤灰在输送管内的流动模式试验;然后进行粉煤灰输送压力、输送质量流量特性试验;最后考察了主进气流量、补气流量、助吹气流量对粉煤灰输送量、固气比的影响。研究表明,柱塞式气力输送流动模式以密相栓柱流为主,其灰栓长度为0.8～2.3 m,移动速度约为2.8～11.3 m·s^-1;输送压力与输送流量呈双曲线特性,且随着气量的增加输送量增大;主进气流量起主导作用并与输送粉煤灰质量流量呈单调上升抛物线关系,与固气比呈上凸抛物线关系即先增大后减小。研究结果对柱塞式气力输送系统的工程设计、运行和理论研究提供依据并具有指导作用。     

Vertical pneumatic conveying: a flow regime diagram and a review of choking versus non-choking systems

For vertical pneumatic conveying of granular solids, a flow chart describing two different types of gas—solid systems, viz. the choking system and the non-choking system, is presented. Published equations for the prediction of whether a particular gas—solid tube system is of the choking type or non-choking type are reviewed. For the choking type system, a quantitative flow regime diagram for predicting demarcation between packed bed conveying and slugging dense phase conveying, and demarcation between slugging dense phase conveying and lean phase conveying, is developed in terms of the key parameters of loading ratio, gas and solid velocities. The usefulnes of the flow regime diagram in design is discussed. The shortcomings of an earlier flow regime diagram proposed by Leung [14] are overcome in the present diagram.     

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.     

Effects of an electrostatic field in pneumatic conveying of granular materials through inclined and vertical pipes

The pneumatic transport of granular materials through an inclined and vertical pipe in the presence of an electrostatic field was studied numerically using the discrete element method (DEM) coupled with computational fluid dynamics (CFD) and a simple electrostatic field model. The simulation outputs corresponded well with previously reported experimental observations and measurements carried out using electrical capacitance tomography and high-speed camera techniques in the present study. The eroding dunes and annular flow regimes, observed experimentally by previous research workers in inclined and vertical pneumatic conveying, respectively, were reproduced computationally by incorporating a simplified electrostatic field model into the CFD-DEM method. The flow behaviours of solid particles in these regimes obtained from the simulations were validated quantitatively by experimental observations and measurements. In the presence of a mild electrostatic field, reversed flow of particles was seen in a dense region close to the bottom wall of the inclined conveying pipe and forward flow in a more dilute region in the space above. At sufficiently high field strengths, complete backflow of solids in the inclined pipe may be observed and a higher inlet gas velocity would be required to sustain a net positive flow along the pipe. However, this may be at the expense of a larger pressure drop over the entire conveying line. In addition, the time required for a steady state to be attained whereby the solids flow rate remains substantially constant with respect to time was also dependent on the amount of electrostatic effects present within the system. The transient period was observed to be longer when the electrostatic field strength was higher. Finally, a flow map or phase diagram was proposed in the present study as a useful reference for designers of inclined pneumatic conveying systems and a means for a better understanding of such systems.     

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。     

Estimation of particulate velocity components in pneumatic transport using pixel based correlation with dual plane ECT

This article presents a technique developed to estimate the velocity components of two phase solid/gas flow using electrical capacitance tomography (ECT). The pixel by pixel correlation method for consecutive frames in a given sensor plane has been used to trace the particle velocity profile in the transverse direction. The transverse movement of solid particles in slug flows has been reported recently in the literature. The transverse velocity of the particles is probably caused by the picking up mechanism experienced by single particles, to form a slug body. Rest of the particles following the slug forms a stationary layer thus exhibiting no transverse component. These phenomena have also been observed in earlier studies using high-speed video camera. The pixel-based correlation using ECT confirms these observations and also helps to detect the slugging phenomena. The same technique is implemented to trace the path of rotational motion of an object inside the sensor plane and also to detect the transverse motion of particulates in dilute phase vertical pneumatic conveying system. Both axial and transverse velocity components estimated by ECT are verified using Laser Doppler Anemometer (LDA).     

Flow regime determination in horizontal pneumatic transport of fine powders using non-intrusive acoustic probes

A variety of flow regimes may be observed in dilute phase pneumatic transport of fine powders. As the gas flow rate is reduced or the solids flow rate increased, particles may settle on the bottom of the horizontal sections, forming either a stagnant layer or slowly moving dunes. This change in flow regime leads to such problems as flow instabilities and very long residence times for some particles. Maintaining a consistent operation and product quality requires rapid detection of any change in flow regime. In many applications, particularly in pharmaceutical processes, the installation of intrusive sensors is undesirable. The objective of this study was to develop reliable flow regime detection through the on-line analysis of signals from non-invasive acoustic sensors.Non-intrusive microphones were used to record acoustic emissions generated by powder flow through a horizontal, 0.1 m diameter, stainless steel, pneumatic transport pipe, at various solids fluxes and superficial gas velocities. Measurements were recorded simultaneously on the top and the bottom of the pipe, to record the flow of solids as they hit and are reflected from the pipe walls. To confirm the flow regimes, high speed video imaging in a section of clear acrylic pipe allowed for detailed analysis of the flow structure. Two flow regimes were observed: dilute phase flow and conveying over settled solids. Cycle or frequency analysis of the acoustic measurements recorded from the top or the bottom of the pipe provides reliable, on-line detection of these flow regimes.     

Resistance Properties of a Bend in Dense‐Phase Pneumatic Conveying of Pulverized Coal under High Pressure

Experiments of high‐pressure dense‐phase pneumatic conveying of pulverized coal with different mean particle sizes using nitrogen were carried out in an experimental test facility with a conveying pressure of up to 4 MPa. The effects of three representative operating parameters (solids‐to‐gas mass flow ratio, conveying pressure, mean particle size) on the total pressure drop were examined. The pressure drops across the horizontal and vertical bends were analyzed by experimental and analytical calculation. The results show that the pressure drop due to gas friction is of much less significance, while the pressure drop due to the solids friction component of the total pressure drop dominates. There exists a relationship between the pressure drop due to solids kinetic energy loss and mass flux of solids.