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.     

Two-Fluid, Two-Dimensional Model for Pneumatic Drying

Pneumatic drying is a widely used process in the chemical industries and includes simultaneous conveying and heat and mass transfer between the particles and the heat gas. The increase in the use of this unit operation requires reliable mathematical models to predict processes in the industrial facilities. In the present study a Two-Fluid model has been used for modeling the flow of particulate materials through pneumatic dryer. The model was solved for a two-dimensional steady-state condition and considering axial and radial profiles for the flow variables. A two-stage drying process was implemented. In the first drying stage, heat transfer controls evaporation from the saturated outer surface of the particle to the surrounding gas. At the second stage, the particles were assumed to have a wet core and a dry outer crust; the evaporation process of the liquid from a particle is assumed to be governed by diffusion through the particle crust and by convection into the gas medium. As evaporation proceeds, the wet core shrinks while the particle dries. The numerical procedure includes discretization of calculation domain into torus-shaped final volumes, solving conservation equations by implementation of the SIMPLE (Semi-Implicit Method for Pressure-Linked Equations) algorithm and controls over coupling of phases by IPSA (Interphase Slip Algorithm). The developed model was applied to simulate a drying process of wet PVC particles in a large-scale pneumatic dryer and to a drying process of wet sand in a laboratory-scale pneumatic dryer. The numerical solutions are compared successfully with the results of independent numerical and experimental investigations. Following the model validation, the two-dimensional distributions of the flow characteristics were examined.     

Two-Fluid,Two-Dimensional Model for Pneumatic Drying

Abstract

Pneumatic drying is a widely used process in the chemical industries and includes simultaneous conveying and heat and mass transfer between the particles and the heat gas. The increase in the use of this unit operation requires reliable mathematical models to predict processes in the industrial facilities. In the present study a Two-Fluid model has been used for modeling the flow of particulate materials through pneumatic dryer. The model was solved for a two-dimensional steady-state condition and considering axial and radial profiles for the flow variables. A two-stage drying process was implemented. In the first drying stage, heat transfer controls evaporation from the saturated outer surface of the particle to the surrounding gas. At the second stage, the particles were assumed to have a wet core and a dry outer crust; the evaporation process of the liquid from a particle is assumed to be governed by diffusion through the particle crust and by convection into the gas medium. As evaporation proceeds, the wet core shrinks while the particle dries. The numerical procedure includes discretization of calculation domain into torus-shaped final volumes, solving conservation equations by implementation of the SIMPLE (Semi-Implicit Method for Pressure-Linked Equations) algorithm and controls over coupling of phases by IPSA (Interphase Slip Algorithm). The developed model was applied to simulate a drying process of wet PVC particles in a large-scale pneumatic dryer and to a drying process of wet sand in a laboratory-scale pneumatic dryer. The numerical solutions are compared successfully with the results of independent numerical and experimental investigations. Following the model validation, the two-dimensional distributions of the flow characteristics were examined.     

Do Twin Boundaries Always Strengthen Metal Nanowires?

It has been widely reported that twin boundaries strengthen nanowires regardless of their morphology—that is, the strength of nanowires goes up as twin spacing goes down. This article shows that twin boundaries do not always strengthen nanowires. Using classical molecular dynamics simulations, the authors show that whether twin boundaries strengthen nanowires depends on the necessary stress for dislocation nucleation, which in turn depends on surface morphologies. When nanowires are circular cylindrical, the necessary stress of dislocation nucleation is high and the presence of twin boundaries lowers this stress; twin boundaries soften nanowires. In contrast, when nanowires are square cylindrical, the necessary stress of dislocation nucleation is low, and a higher stress is required for dislocations to penetrate twin boundaries; they strengthen nanowires.     

TWO-FLUID MODEL FOR PNEUMATIC DRYING OF PARTICULATE MATERIALS

The Two-Fluid model has been used for modeling the flow of particulate materials through pneumatic dryer. The model was solved for a one-dimensional steady-state condition and was applied to the drying process of wet PVC particles in a large-scale pneumatic dryer and to the drying process of wet sand in a laboratory-scale pneumatic dryer. A two-stage drying process was implemented. In the first drying stage, heat transfer controls evaporation from the saturated outer surface of the particle to the surrounding gas. At the second stage, the particles were assumed to have a wet core and a dry outer crust; the evaporation process of the liquid from a particle assumed to be governed by diffusion through the particle crust and by convection into the gas medium. As evaporation proceeds, the wet core shrinks while the particle dries. The drying process is assumed to stop when the moisture content of a particle falls to a predefined value or when the particle riches the exit of the pneumatic dryer. Our developed model was solved numerically and two operating conditions, adiabatic and given pneumatic dryer wall temperature, were simulated. Comparison between the prediction of the numerical models of Rocha and DryPak, (Pakowski, 1996), which were presented by Silva and Correa (1998), with the prediction of our numerical simulation reviled better agreements with DryPak then with the models of Rocha. The results of the developed model were also compared with experimental results of Baeyens et al. (1995) and Rocha.     

压缩机蒸汽轮机及真空系统改造总结

介绍压缩机蒸汽轮机及真空系统节能技术改造工艺方案,提出各真空系统改造的主要内容和设备选型要求.改造措施实施后耗能明显降低、经济效益提高、机组自动化程度提高、安全稳定运行增强,但技术改造又产生了新问题,针对遗留问题提出相应的解决办法.     

TWO-FLUID MODEL FOR PNEUMATIC DRYING OF PARTICULATE MATERIALS

The Two-Fluid model has been used for modeling the flow of particulate materials through pneumatic dryer. The model was solved for a one-dimensional steady-state condition and was applied to the drying process of wet PVC particles in a large-scale pneumatic dryer and to the drying process of wet sand in a laboratory-scale pneumatic dryer. A two-stage drying process was implemented. In the first drying stage, heat transfer controls evaporation from the saturated outer surface of the particle to the surrounding gas. At the second stage, the particles were assumed to have a wet core and a dry outer crust; the evaporation process of the liquid from a particle assumed to be governed by diffusion through the particle crust and by convection into the gas medium. As evaporation proceeds, the wet core shrinks while the particle dries. The drying process is assumed to stop when the moisture content of a particle falls to a predefined value or when the particle riches the exit of the pneumatic dryer. Our developed model was solved numerically and two operating conditions, adiabatic and given pneumatic dryer wall temperature, were simulated. Comparison between the prediction of the numerical models of Rocha and DryPak, (Pakowski, 1996), which were presented by Silva and Correa (1998), with the prediction of our numerical simulation reviled better agreements with DryPak then with the models of Rocha. The results of the developed model were also compared with experimental results of Baeyens et al. (1995) and Rocha.     

真空喷射器替代K02101压缩机油雾风机技改简介

K02101压缩机油箱上部油雾风机电机出现异常后,为保障压缩机正常运转,提出使用真空喷射器在线替换油雾风机的改造方案,抽出油箱内的油气并维持负压,消除了轴承箱大量漏油的隐患,避免了装置的停车。     

双螺杆非啮合并列型螺纹元件三维非等温流场数值模拟

利用ANSYS软件包对双螺杆挤出机非啮合并列型螺纹元件进行了三维非等温非牛顿流场分析,给出了速度场、压力场、温度场的基本规律以及螺杆特性曲线的分布。计算结果表明：螺棱处的物料具有较大的压力梯度和速度梯度,啮合区的物料温度较高,应当采取适当的降温措施;流量与螺杆转速成正比,与流道两端的压差成反比。     

啮合同向双螺杆3种构型的模拟分析和实验研究

通过数值模拟方法对3种含捏合盘元件螺杆构型的剪切速率、停留时间和回流量等指标进行了分析,通过实验对3种螺杆构型的混合性能进行了对比。结果表明,90°捏合盘元件与反向捏合盘元件组合可以达到较好的分布混合效果,45°捏合盘元件与反向捏合盘元件的组合可以达到较好的分散混合能力,捏合段加入反向螺纹元件并不能提高分布混合能力,但可以延长停留时间。     

基于Aspen Plus的原油常减压蒸馏装置的模拟

在Aspen Plus软件平台上,以标定数据为主要输入数据,以产品控制指标为主要约束条件,建立了某厂原油常减压蒸馏装置的稳态模拟流程。在建立过程中,采取对装置流程进行简化处理、将总板效率作为调节变量等方法,使模拟流程的工艺参数、物料平衡和产品恩氏蒸馏数据与生产基本相符。     

Spherical particle movement in dilute pneumatic conveying

A numerical simulation of a particle in a horizontal pipe has been carried out, and the variation of aerodynamic forces is described. The major forces that control particle motion are drag in the axial direction, and lift due to air velocity gradient and due to spin in the transverse direction. An elastic contact model based on rigid body sliding has been incorporated, which avoids particle settlement without having to use any form of irregular bounce. The results from the simulation agree closely with experimental time-of-flight measurements.     

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.     

组合式同向旋转双螺杆特性曲线分析

应用有限元分析软件ANSYS对同向旋转双螺杆挤出机螺纹输送元件中的三维非牛顿流动进行了模拟,计算出在一定转速和挤出量等工艺条件下螺纹元件的建压能力,确立了一定导程螺纹元件的特性曲线。在此基础上,对组合式同向旋转双螺杆挤出机工作特性进行了分析,并对双螺杆计量段充满长度和排气段脱挥长度的确定作了初步探讨。     

DYNAMIC INTERACTION COMPENSATORS FOR MULTILOOP CONTROL STRUCTURE

This work presents a design procedure of a dynamic compensator for multivariable IMC control structure. This compensator can yield both faster transient response and zero steady state error to interaction effects. Several examples are given to demonstrate the availability of this methodology. They are chosen with different degrees of interactions, based on RGA (relative gain array) and RDG (relative disturbance gain). In addition, an 1 MC structure optimally decoupled is used as an upper bound for the performance obtainable to compare with the compensated interactive system. Finally, it is shown that this; procedure is easily extended to classical feedback controllers PI and P1D.     

亚临界气体喷射器的设计计算

计算气体压缩喷射器可达到喷射系数的研究已很成熟,但对亚临界气体喷射器可达到喷射系数,大多采用气体喷射压缩器的方法进行计算。针对亚临界气体喷射器的特性,给出了3种计算方法:气体喷射压缩器计算方法;喷射泵计算方法;气体喷射器计算方法。研究表明:对膨胀比和压缩比都小于临界压力比的亚临界气体喷射器,膨胀比较大时,工作介质的弹性影响不能忽略;压缩比较小时,引射介质的弹性影响应该忽略;进而得出在膨胀比和压缩比都接近临界压力比时,适宜采用把工作介质视为弹性,引射介质视为非弹性的气体喷射器计算方法;而在膨胀比和压缩比都非常小且接近于1时,应该采用把工作介质和引射介质都视为非弹性的喷射泵计算方法。     

Modelling of polymer fluid flow and residence time distribution in twin screw extruder using fictitious domain method

Abstract

The flow behaviour of a polymer melt in the conveying region of an intermeshing corotating twin screw extruder was studied using the combination of mixed finite element and fictitious domain method. The model was a combination of the governing equations of continuity and momentum with Carreau rheological model in a three-dimensional Cartesian coordinate system. The equations were solved by the use of a mixed Galerkin finite element technique. The Picard’s iterative procedure was used to handle the non-linear nature of the derived equations. The particle tracking technique was used to obtain residence time distribution and analyse distributive mixing in conveying region. The shear rate distribution was investigated as a criterion for dispersive mixing. The applicability of this model was verified by the comparison of experimentally measured pressure and simulation results for high density polyethylene melt. This comparison shows that there is a good adequacy between experimental data and model predictions.     

带二次顶出装置的吸尘器筒式分离器注射模设计

摘要：根据吸尘器筒式分离器的特点,以Moldflow模流分析结果为依据,确定了最佳的浇注系统,并对模具脱模力、顶针大小和数量进行了详细计算,通过脱模力、顶针直径的计算分析,采用二次顶出方式解决产品顶出问题。合理设计了模具冷却水路,减少了热缩现象,保证了注射成型工艺窗口最大化和产品质量,缩短了成型周期,提高了生产效率。