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。     

表观气速对密相气力输送流型影响的模拟研究

针对目前密相气力输送数值模拟关于流型演变方面所存在的问题,提出了一种基于颗粒所在局部空间的固相浓度及颗粒群运动特征来描述颗粒间相互作用的数学模型。该模型能够对气力输送,甚至是颗粒发生大量堆积情况下的密相输送进行数值模拟,使得长期以来缺乏有效模型对密相输送流型进行数值模拟研究的问题得到一定解决。利用该模型,对水平管中煤粉高压密相气力输送的颗粒流动过程进行了数值模拟,获得了输送过程中管道内所发生的气固两相之间的分离、沉积现象,展现了沙丘流及栓塞流等流型的演变特征,模拟结果与实验观察到的现象吻合较好,从而进一步验证了新数学模型的有效性。此外,通过对不同表观气速下固相流态分布的定量分析,揭示了输送流型变化的一些内在规律。     

颗粒黏度模型对采用欧拉多相流模型模拟超密相颗粒流动行为的影响

颗粒黏度是欧拉多相流模型计算固体流动的重要参数之一,其数值大小依赖于摩擦压力和径向分布函数。通过与实验值对比,评估了常用的摩擦压力模型(Based、Johnson)和径向分布函数模型(Lun、Syamlal O’Brien)对密相颗粒流动体系的预测能力。模拟结果表明,Johnson模型的固体体积分数预测值低于Based模型;Syamlal O’Brien模型固体流率的预测值远大于Lun模型。采用根据实验结果修正的欧根系数后,BasedLun、Johnson-Lun和Johnson-SO模型组合预测的平均压降相对误差分别由68.6%、73.3%和78.2%降低至13.2%、29.7%和42.3%。综合考虑压降、固体出口质量流率、固体体积分数、壁面区域固体速度的模拟结果与实验值的偏差,发现Based-Lun模型组合的平均预测误差最小,适用于气固移动床的欧拉多相流模拟。研究还发现,欧根系数与内摩擦角对固体速度与压降有着显著的影响,而临界固含率对固体速度与压降的影响较小。     

循环流化床气固流动行为的数值模拟

采用欧拉-欧拉双流体模型,在Fluent数值模拟平台上对循环湍动流化床进行了研究。采用Gidaspow模型分段描述密相湍动和稀相循环输送区气固的相互作用,考察了反应器内压力、颗粒浓度等气固湍动特性参数的变化。计算流体力学(CFD)的模拟结果表明:反应器中压力分布较为均匀;固体颗粒相和气体相间隙成絮状分布,在轴向中心区域颗粒浓度分布较为均匀;边壁区域颗粒浓度变化较大,存在强烈的气固相互作用。     

循环流化床气固流动行为的数值模拟

采用欧拉-欧拉双流体模型,在Fluent数值模拟平台上对循环湍动流化床进行了研究。采用Gidaspow模型分段描述密相湍动和稀相循环输送区气固的相互作用,考察了反应器内压力、颗粒浓度等气固湍动特性参数的变化。CFD的模拟结果表明反应器中压力分布较为均匀;固体颗粒相和气体相间隙成絮状分布,在轴向中心区域颗粒浓度分布较为均匀;边壁区域颗粒浓度变化较大,存在强烈的气固相互作用。     

水平管气力输送的数值模拟研究

对气体相湍动能采用修正的k-ε二方程模型,颗粒相湍动动能采用颗粒动力学方法,考虑两相间的相互作用,发展建立了水平管气力输送的数学物理模型和计算方法.该模型能计算颗粒相压力、粘性系数、扩散系数、导热系数、颗粒温度等流体力学特性参数.用文献实验得到的压降和转捩速度验证了所建模型和计算方法的正确性.就水平管中圆柱坐标系下典型的三维悬浮气力输送过程进行了初步数值研究,得到了管道沿程压降、平均气体速度、平均颗粒速度和平均颗粒浓度的变化、以及输送方向上不同截面处的颗粒浓度分布.结果表明:在给定的输送条件下,颗粒在管中并不总是维持同样的悬浮状态,在入口和快速加速度段,悬浮颗粒易集中于管中心区域,从加速段到恒速段,悬浮颗粒逐渐向管底沉降;颗粒浓度在管截面上有两种分布状态.为进一步利用该法研究气力输送打下了基础.     

稠密固液循环流化床内流体湍流脉动对颗粒分散的影响

通过计算流体动力学模拟研究了固液循环流化床内流体湍流对高浓度颗粒的扩散作用。根据湍流机制下高浓度颗粒在液体中的分散机理,建立了同时考虑颗粒碰撞、流体拖拽和流体湍流扩散输运的欧拉-欧拉双流体模型。利用该模型对稠密固液循环流化床提升管内颗粒的分散运动进行了数值模拟,预测出颗粒轴向速度和体积浓度的径向分布,与相应条件下的实验数据符合很好。表明考虑流体湍流扩散输运的理论模型更能精确预测高浓度颗粒在循环流化床提升管内的运动情况;也表明即使针对以颗粒碰撞为主的稠密固液湍流体系,尺度较大的流体湍流涡也会因促使颗粒扩散输运,影响颗粒空间分布。工作为稠密固液循环流化床内两相流体动力学分析奠定了基础。     

倒置液固流化床内液固两相流动特性的数值模拟

基于欧拉-欧拉双流体模型,数值模拟倒置液固流化床内液固两相流动行为.数值模拟预测了床内颗粒的速度、浓度分布以及空隙率的变化.研究结果表明颗粒在床内分布呈现非均匀分布,床内形成局部高空隙率的流体团;随着床层高度增加,颗粒轴向速度增大:数值模拟床内空隙率与Renganthan等的实验结果相吻合.     

粗糙颗粒动理学及流化床内气固流动的数值模拟

基于气体分子动理学和颗粒动理学理论,考虑颗粒旋转流动对颗粒碰撞能量交换和耗散的影响,建立粗糙颗粒动理学。采用Chapman-Enskog颗粒速度分布函数,提出了颗粒相应力、热通量和颗粒碰撞能量耗散计算模型。采用欧拉-欧拉气固双相流模型,数值模拟鼓泡流化床内气体-颗粒两相流动特性。模拟结果得到了床内颗粒相速度和脉动速度分布,与Yuu等实验结果相吻合。分析不同的切向弹性恢复系数对颗粒相拟总温的变化规律,结果表明在低颗粒浓度时颗粒拟总温随切向弹性恢复系数而增加。     

水平气力输送管中颗粒浓度分布的数值模拟

以商业软件FLUENT为平台,通过UDF将DEM程序编译进FLUENT,利用DEM与CFD耦合的方法研究了水平管气力输送过程中颗粒粒径和密度对颗粒浓度分布的影响.结果表明,水平管道内颗粒浓度沿轴向方向不断降低,在管中心浓度逐渐扩散,使径向分布逐渐变得均匀;与双欧拉模型的计算结果不同,在管下部的高颗粒浓度区呈现出中心浓度高于边壁浓度的分布,而在管上部则表现为中心浓度低于边壁浓度的分布;随着颗粒密度及颗粒体积的增加,颗粒的跃动距离减少.     

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。     

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.     

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。     

宁东灵武矿区煤粉密相输送管道压降研究

采用Barth气力输送理论,通过实验在质量流率1 550—1 700 kg/h的输送范围内,研究了宁东灵武矿区煤粉密相输送的压降和表观气速的关系。结果表明:随着表观气速的增加,水平管道和竖直管道的压降都是先降低后升高,但竖直段的压降变化速度比水平段变化快,水平段的经济气速(4 m/s)小于竖直段的经济气速(7 m/s)。通过计算值与实验值比较,发现理论计算值与实验值偏差在30%以内,说明基于Barth附加压降法对宁东灵武矿区煤粉密相气力输送管阻力特性的计算具有较好的适应性。     

Horizontal pneumatic conveying from a fluidized bed

In this paper the feasibility of feeding a horizontal pneumatic conveying line directly from a fluidized bed is explored by investigating the relationships governing the solids mass flow rate in such a pipe as a function of both pressure drop and pipe length. Three different materials were fluidized by air and discharged though a 25 mm internal diameter pipe. Materials used were turnip seeds of mean diameter 1.5 mm, carbon steel shot of mean diameter 0.73 mm and plastic pellets of mean diameter 3.76 mm. Several pipe lengths were used, from 0.75 to 1.77 m. The experiments showed that it is feasible to feed directly from a fluidized bed to a horizontal pneumatic conveying line. The flow regime in the pipe was that of dense phase conveying also called slug flow. The data collected show that there is a clear relationship between the pressure drop down the conveying line and the discharge rate of solids from the line. The discharge rate is also dependent on the pipe length.In previous studies of pneumatic conveying, the solids and gas mass flow rates have been independently set, which cannot be done if the conveying line is fed from a fluidized bed. For a pipe fed from a fluidized bed, the solid and gas mass ratio are coupled and this was modelled using the theory for air-augmented granular discharge through an orifice in a hopper or silo of Nedderman et al. [1983. The effect of interstitial air pressure gradients on the discharge from bins. Powder Technology 35, 69-81], but as modified by Thorpe [1984. Air-augmented flow of granular materials through orifices. Ph.D. Thesis, University of Cambridge] for horizontal discharge. This was then combined with a modification of the theory of Konrad [1981. Ph.D. Dissertation, University of Cambridge] to give a prediction of the total pressure drop and the gas and solid mass flow rates. This combined model for dense-phase conveying from a fluidized bed was found to give an excellent fit to the data using the standard values for the constants in every equation. The predictions of the combined model also agree well with the experiments of Konrad [1981. Ph.D. Dissertation, University of Cambridge] for discharge from a hopper into a horizontal conveying line.     

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.     

Die Grundlagen der pneumatischen Förderung

Principles of pneumatic conveying . The design of pneumatic conveying systems is critically dependent upon a knowledge of the pressure drop and the minimum necessary velocity of the conveying gas. Starting from the areas of application of pneumatic conveying systems, the conveying states occurring are discussed and appropriately assigned to the diagram of state. The pressure loss equation is reported for conveying in form of a fully suspended flow and strand-type conveying and the theoretically predicted pressure losses are compared with experimental values for the conveying of coarse and finegrained materials. After mentioning the calculation of the pressure drops due to acceleration and to pipe bends, the article discusses the problem of pipe enlargement for long conveying distances. Equations are also given for the calculation of the so-called blocking limit and of the conveying gas velocity at which the minimum pressure drop occurs. The influence of the compressor on the choice of the operating point is also discussed. New calculation equations are then given for horizontal and vertical conveying in form of a fully suspended flow conveying and for horizontal strandtype conveying which permit reliable scale-up. Moreover, a new equation is reported for the blocking limit in horizontal conveying.     

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.