Axial and Radial Pressure Drops of Dense Phase Plugs

ABSTRACT

An improved angle droplet collection efficiency model for the intermediate flow regime is presented in this paper, taking into account both inertial impaction and interception mechanisms. This model uses the equations of motion that has been derived by performing a force balance on a particle interacting with the flow field of a spherical collector. The fluid flow field around the collector is assumed to be the approximate solution as developed by Hamielec and Johnson for Reynolds numbers ranging between 10 and 80 and Tomotika and Aoi for Reynolds numbers less than 1.0.

The results of this work indicate that the collection efficiencies calculated by using potential flow conditions may have overestimated the overall collection of particulate matter. It was identified that the transition from intermediate to potential flow occurs when the Reynolds number is about 80. The interpolation scheme for the single droplet collection efficiency proposed in this work can be used from Stokes flow to potential flow conditions including intermediate flow regime.     

Bend Pressure Drop Predictions Using the Euler-Euler Model in Dense Phase Pneumatic Conveying

Pneumatic conveying of powdered and granular materials is a very common transport technology across a broad range of industries, for example, chemicals, cosmetics, pharmaceuticals, and power generation. As the demands of these industries for greater efficiency increases and to comply with environmental regulations there is a need for a more fundamental understanding of the behavior of materials in pneumatic conveying systems. The approach presented in this article is to develop a model of a section of pneumatic conveying line, a horizontal or vertical 90° bend, in the commercial CFD software package FLUENT and to describe the multiphase flow behavior by the mixture or Eulerian method. Models of this type have been used in the past to show qualitative and quantitative agreement between model and experiment. The model results presented were compared with experimental data gathered from an industrial-scale pneumatic conveying test system. Broad qualitative agreement in trends and flow patterns were found. Quantitative comparisons were less uniform, with predictions from around 10% to 90% different from experimental results, depending on conveying conditions and bend orientation.     

Bend Pressure Drop Predictions Using the Euler-Euler Model in Dense Phase Pneumatic Conveying

Pneumatic conveying of powdered and granular materials is a very common transport technology across a broad range of industries, for example, chemicals, cosmetics, pharmaceuticals, and power generation. As the demands of these industries for greater efficiency increases and to comply with environmental regulations there is a need for a more fundamental understanding of the behavior of materials in pneumatic conveying systems. The approach presented in this article is to develop a model of a section of pneumatic conveying line, a horizontal or vertical 90° bend, in the commercial CFD software package FLUENT and to describe the multiphase flow behavior by the mixture or Eulerian method. Models of this type have been used in the past to show qualitative and quantitative agreement between model and experiment. The model results presented were compared with experimental data gathered from an industrial-scale pneumatic conveying test system. Broad qualitative agreement in trends and flow patterns were found. Quantitative comparisons were less uniform, with predictions from around 10% to 90% different from experimental results, depending on conveying conditions and bend orientation.     

Voidage Measurement for a Moving Plug in Dense Phase Pneumatic Conveying Using Two Different Methods

Two simple ways to measure voidage in a moving plug in dense phase pneumatic conveying have been explored, along with voidage behavior and other parameters. The first method (bulk solid method) determines the average voidage for a single plug, by measuring the length and the weight of the plug, while the second method (Ergun's method) determines the voidage along the plug length from plug head to tail by using the Ergun equation. The experiments were performed under different experimental conditions of plug lengths, gas velocities, and materials. The results from both methods showed varying degrees of agreement, depending on the material transported. The measurements also found that the voidage varied only slightly with the plug velocity. However, it did vary more within the plug (e.g., plug head, middle of plug, and plug tail).     

低速高能效的浓相气力输送技术

低速浓相输送装置的出现,解决了物料在输送过程中易破碎、堵塞和磨蚀管道等难题,降低了耗气量。本文中综述了低速浓相输送的几种定义和输送过程中的相图、物料流动形态及相应的判定、影响浓相气力输送特性的因素等技术参数,并介绍了输送过程中经常遇到的堵管和磨损现象以及气力输送过程中的检测和自动控制技术,指出了今后的研究方向。     

粉体性能对浓相气力输送特性的影响

气力输送过程中物料性能是确定输送特性的重要因素,因此,粉料气力输送技术的实现要以对粉料的性能研究为基础。文中对影响气力输送的粉体基本性能及其相关参数做了较全面分析,其中粒子尺寸、粒径分布、形状是影响粉料是否可适用于浓相气力输送的关键参数,其它特性都与这3种特性相关联。介绍了几种应用广泛的粉料气力输送特性分组方法,并进行了简要评述,同时指出了今后的研究方向。     

正压浓相恒压输送系统的DCS控制设计及应用

正压浓相气力输送系统是目前国内火电厂应用最为广泛的气力输送系统,该系统对运行的控制流程要求高、逻辑性强;其关键输送技术指标均采用模拟量进行检测,并实时控制.本文介绍的恒压输送系统,除了以上特点外,其独具的输送压力的动态实时调控功能,更是对控制系统提出更高的要求;阐述了DCS用于正压浓相恒压输送系统的典型设计.     

Stress-Field Modeling and Pressure Drop Prediction for Slug-Flow Pneumatic Conveying in an Aerated Radial Stress Chamber

Slug-flow pneumatic conveying is a full-bore mode of flow within the dense-phase flow regime where bulk materials are transported in the form of slugs at conveying speeds below saltation velocity. The mechanism of slug-flow pneumatic conveying consists of the particles being picked up from the stationary bed in front of a moving slug while the same amount of material is deposited behind the slug. Stress field modeling of the material slug is the first step in developing a prediction model for the pressure drop along a pneumatic conveying line. However, a reliable prediction strongly relies on an accurate assessment of several factors, including the particle properties, pipeline dimensions, and operating conditions. So far, the particle diameter has always been one of the crucial parameters, which is not desirable in regards to the limitations it imposes on the choice of bulk materials. This article focuses on one parameter, the stress transmission coefficient k_w, which relates the lateral wall stress within a slug of material to the axial stress. To date, this parameter could not be measured directly in an aerated material bed and had to be estimated. Inaccuracies within the prediction were therefore unavoidable. A newly designed test chamber now enables the measurement of the lateral and axial stresses within a slug, which leads directly to this stress transmission coefficient. This article outlines the design of the test apparatus and reports on the experimental results. For the two materials tested, an exponential correlation between the pressure on top of the slug (frontal stress) and the stress transmission coefficient was obtained. Calculating the wall friction coefficient leads to a constant value above a certain material-specific air velocity.     

Stress-Field Modeling and Pressure Drop Prediction for Slug-Flow Pneumatic Conveying in an Aerated Radial Stress Chamber

Slug-flow pneumatic conveying is a full-bore mode of flow within the dense-phase flow regime where bulk materials are transported in the form of slugs at conveying speeds below saltation velocity. The mechanism of slug-flow pneumatic conveying consists of the particles being picked up from the stationary bed in front of a moving slug while the same amount of material is deposited behind the slug. Stress field modeling of the material slug is the first step in developing a prediction model for the pressure drop along a pneumatic conveying line. However, a reliable prediction strongly relies on an accurate assessment of several factors, including the particle properties, pipeline dimensions, and operating conditions. So far, the particle diameter has always been one of the crucial parameters, which is not desirable in regards to the limitations it imposes on the choice of bulk materials. This article focuses on one parameter, the stress transmission coefficient k_w, which relates the lateral wall stress within a slug of material to the axial stress. To date, this parameter could not be measured directly in an aerated material bed and had to be estimated. Inaccuracies within the prediction were therefore unavoidable. A newly designed test chamber now enables the measurement of the lateral and axial stresses within a slug, which leads directly to this stress transmission coefficient. This article outlines the design of the test apparatus and reports on the experimental results. For the two materials tested, an exponential correlation between the pressure on top of the slug (frontal stress) and the stress transmission coefficient was obtained. Calculating the wall friction coefficient leads to a constant value above a certain material-specific air velocity.     

Particle Trajectories in Dilute Phase Pneumatic Conveying

A nonintrusive cross-correlation method to measure the particle velocities in dilute phase pneumatic conveying is described. The cross-correlation function generated gives information about the time it takes for a particle to travel between two optimally placed measurement planes. Experiments and CFD simulations are used to estimate an optimal inter-plane distance for various flow conditions.     

Effect of Dune Formation on Pressure Drop in Horizontal Pneumatic Conveying System

Pneumatic conveying is widely used in industries handling large amount of granular materials to transport the solid particles; however, the process is energy intensive as an instability of flow sets in the transportation line even in the dilute regime, causing large fluctuations in the line pressure drop, the reason of which is not clearly understood. Here, we investigate, both by experiments and by using numerical simulations, the instability transition regimes and identify the reasons of the fluctuations observed in the line pressure drop in a horizontal pneumatic transport system operating at near-saltation conditions. It is observed that the increase in the pressure drop (immediately after the saltation) is accompanied by the formation of distinct dunes. It is also observed that the line pressure drop depends on the axial location of the dune and shows large fluctuations in the regime where the dunes are unstable. Results obtained from the numerical simulations suggest that the increase in the line pressure drop in the presence of dunes is essentially due to the shear stresses at the dune surface which are larger than that for the flows in clean pipe.     

Modeling Solids Friction for Dense-Phase Pneumatic Conveying of Powders

This article results from an ongoing investigation aimed at developing a new validated test-design procedure for the accurate prediction of pressure drop for dense-phase pneumatic conveying of powders. Models for combined pressure drop coefficient (“K”) for solids-gas mixture were derived using the concept of “suspension density” by using the steady-state “straight pipe” pressure drop data between two different tapping locations of the same pipe and also for two different diameter pipes. It was observed that the derived models were different depending on the location of tapping points (for the same pipe) and selected pipe diameters. The derived models were then evaluated by predicting the pressure drop for pipelines with various diameters or lengths (69 mm I.D. × 168 m, 105 mm I.D. × 168 m, 69 mm I.D. × 554 m) for the conveying of power station fly ash. A comparison between the predicted pneumatic conveying characteristics (PCC) and the experimental plots showed that the models resulted in significant over-predictions. In the second part of the article, the “system” approach of scaleup was evaluated. “Total” pipeline pressure drop characteristics for test-rig pipelines were scaled up to predict the PCC for larger/longer pipes. It was found that the “system” approach generally resulted in grossly inaccurate predictions. It was concluded that further studies are needed for a better understanding of the solids-gas flow mechanism under dense-phase conditions.     

Experimental and Simulation Studies of Dilute Horizontal Pneumatic Conveying

Dilute horizontal pneumatic conveying has been the subject of this experimental and numerical study. Experiments were performed utilising a 6.5 m long, 0.075 m diameter horizontal pipe in conjunction with a laser-Doppler anemometry (LDA) system. Spherical glass beads with three different sizes 0.8–1 mm, 1.5 mm, and 2 mm were used. Simulations were carried out using the commercial discrete element method (DEM) software, EDEM, coupled with the computational fluid dynamics (CFD) package, FLUENT. Experimental results illustrated that, for mass solid loading ratios (SLRs) ranging from 2.3 to 3.5, the higher the particle diameter and solid loading ratio, the lower the particle velocity. From the simulation investigations it was concluded that the inclusion of the Magnus lift force had a crucial influence, with observed particle distributions in the upper part of the conveying line reproducible in the simulation only by implementing the Magnus lift force terms in the model equations.     

A Pressure Drop Model for Positive Pneumatic Conveying of Flour Through a Vertical Pipeline

A differential equation of motion for gas-flour two-phase flow in a vertical pipe was first derived based on the momentum conservation and by adopting two empirical expressions for the velocity ratio of flour to gas and frictional coefficient between flour and pipe wall, and then a pressure drop model for dilute positive pneumatic conveying of flour through a vertical pipeline was developed by employing the continuity and state equations for gas. The conveying tests were conducted on a positive pneumatic conveying system of flour in a flour mill. Under each of the six different flow conditions, the conveying parameters, such as the flour and gas mass flow rates and the pressure drop between two selected cross sections on the vertical pipeline were measured. The pressure drop between the two selected cross sections was evaluated using the pressure drop model for each of the six flow conditions. The calculated values of pressure drop agree well with the measured data, and it is demonstrated that the model is applicable to vertical positive pneumatic conveying systems of flour.     

Numerical investigations of acoustic agglomeration of liquid droplet using a coupled CFD-DEM model

Acoustic agglomeration is widely considered a potentially effective technology for application in artificial defogging and precipitation. A coupled three-dimensional Computational Fluid Dynamics and Discrete Element Method (CFD-DEM) model was constructed to investigate the agglomeration performance of liquid droplets in the acoustic field. The acoustic field is calculated by solving the Linearized Navier-Stokes Equations (LNSEs) in the time domain, and the background flow is initially obtained using the Reynolds-averaged Navier-Stokes (RANS) equations with a k–ε turbulence model. The motion of the droplet aerosol follows Newton’s second law with fluid-particle and particle-particle interactions, including collision, agglomeration, and fragmentation. The agglomeration performance of liquid droplets under high-intensity acoustic waves was numerically investigated in terms of the effects of the acoustic properties as well as the droplet characteristics.The numerical results show that it is necessary to consider droplet fragmentation in the process of acoustic agglomeration under the action of high-speed jet. The sprayed droplets are more likely to collide and condense than those without a breakup model, which has rarely been reported in previous studies. Acoustic frequency has a significant effect on agglomeration behavior, with optimal frequencies of about 225 Hz, 150 Hz, and 125 Hz corresponding to droplets with mode diameters of 15.97 μm, 25.85 μm, and 42.88 μm, respectively. However, despite the fact that most studies favoured large acoustic intensity for agglomeration performance, the agglomeration performance of aerosol particles is not always positively correlated with acoustic intensity, especially for large droplets. The optimal intensity of droplet with d_p = 42.88 μm is in the range of 120-130 dB, which is smaller than the maximum operation pressure of 150 dB used in this study. In addition, an effective approach to increase the agglomerate size is to extend the residence time that liquid droplets are exposed in the acoustic and flow field, especially because the typical acoustic intensity of actual operation is usually not that high.     

Experimental and numerical study of a horizontal-vertical gas-solid two-phase system with self-excited oscillatory flow

To better explore the energy-saving mechanism and flow characteristics of the self-excited oscillatory flow, the experiment is performed by a new self-excited oscillatory flow generator that the 45° oblique sheet is mounted through the pipe axis in a horizontal-vertical closed pneumatic conveying system. The experimental study focuses on the optimum air-velocity and power consumption, and results shows the maximum reduction of the optimum air-velocity and the coefficient of power consumption are approximately 8.2% and 16.4%, respectively. In addition, the CFD-DEM coupled approach is first developed to investigate the interaction of gas-solid flow in terms of the gas turbulent kinetic energy and spatial particle flow characteristics. Compared with the conventional pneumatic conveying, it is found that the optimum air-velocity and power consumption are reduced by the new self-excited oscillatory flow at lower air-velocities. The numerical results show that the approximately symmetric distribution of axial velocity and the intensive tangential velocity is emerged in the self-excited oscillatory flow at upstream. Particles is efficiently dispersed and suspended by the self-exited oscillatory flow reflecting in the smaller particle variation coefficient and the lager particle suspension coefficient. And since the airflow kinetic energy is utilized more fully to promote particles flowing, the spatial particle axial velocity is accelerated and reached early steady state. As a result, the developed numerical model is further explained the mechanism of energy saving with the self-excited oscillatory flow.     

大气压交流气液两相滑动弧放电特性

滑动弧放电可以在大气压下产生低温等离子体,在能源、环境及医学等领域具有广阔的应用前景。本工作对大气压交流气液两相滑动弧放电图像和多因素影响放电特性的规律进行了实验研究,通过对电信号进行快速傅里叶变换(FFT),分析放电过程中能量注入特点。研究结果表明:气液两相滑动弧放电稳定,与气相滑动弧相比放电强度减弱,但是等离子体分布均匀,出现明亮斑点;放电电流信号特征发现,滑动弧放电过程包括击穿伴随滑动模式和稳定滑动模式,与气相滑动弧放电相比,气液两相滑动弧的滑动周期变长;放电电压和电流信号的频谱分析发现,气液两相滑动弧放电电压和放电电流谐波的含量相比气相滑动弧放电明显减少,放电稳定性提高;气体流量和峰值电压对平均放电功率的影响规律与相同条件下气相滑动弧放电基本一致,增加液体流量和水溶液电导率,雾化液滴在放电过程中对高能电子的吸附作用增强,使得平均放电功率下降。     

密相气力输灰管道的设计与应用

对气力输灰工程中密相输灰管道的设计和计算确定管内压力和速度的公式进行整理 ,同时可以得到较合理的管径和不同管径的长度。运行参数当已知时 ,给出的计算公式可作为设计的依据     

车轮半自动径,轴向跳动谐波检测仪的研制

本文介绍“车轮半自动径、轴向跳动谐波检测仪”的用途，测量原理及主要技术参数，应用计算机完成了测试数据在线处理及测试过程的自动化。     

Numerical Simulation for Hydrodynamic Analysis and Pressure Drop Prediction in Horizontal Gas-Solid Flows

Two fluid or Eulerian modeling incorporating the kinetic theory for granular particles and accounting for four-way coupling was performed to investigate the hydrodynamics and pressure drop characteristics of gas-solid flows in horizontal pipes. The model was validated by comparison with the experimental data found in literature and the predictions agreed reasonably well with experimental results. It was found that lift force along with particle-wall collision and specularity coefficient play significant role in the simulation of horizontal gas-solid flows. Granular temperature model by Ding and Gidaspow (1990 Ding , J. , and D. Gidaspow . 1990 . A bubbling fluidization model using kinetic theory of granular flow . AIChE Journal 36 ( 4 ): 523 – 538 .[Crossref], [Web of Science ®] , [Google Scholar]) predicts the velocity profiles of both phases accurately. The gas-solid two-phase flow in the horizontal pipe generally has an asymmetric structure in the vertical direction, which is due to the effect of gravity. An extensive investigation was also done to study the effect of various flow parameters like particle properties, gas velocity, and solid concentration on pressure drop prediction. Finally a simplified correlation was proposed for fully developed pressure drop in horizontal gas-solid flows. Unlike the existing correlations, this correlation is valid for a wide range of particle size, pipe diameter, and mass loading.