煤粉粒径对密相气力输送流型影响的数值模拟

针对目前密相气力输送数值模拟过程中所存在的关键问题,提出了一种描述固相内部相互作用对颗粒运动影响的数学模型,采用该模型能够对稠密气固两相流动(乃至颗粒发生大量沉积的情况)进行数值模拟.新模型在离散颗粒模型的基础上,通过描述颗粒所在局部空间的固相浓度及颗粒群运动特征所建立,使其既能够模拟悬浮流动的稀相颗粒运动,又能模拟管内出现堆积情况的密相气固两相流.利用所建立的数学模型对高压密相煤粉气力输送的颗粒流动过程进行了数值模拟.模拟结果显示,随着颗粒粒径增大,粉体密相气力输送流型从沉积层流变化为沙丘流,进而演变成栓塞流的演变过程,并结合实验验证了典型的栓塞流、沙丘流等流型特征;并且平均栓塞长度随着粒径的增大而减小,而随着粉体粒径的增大,输送管道中固相容积份额则整体上呈增大趋势.     

固气比对超浓相煤粉气力输送流型影响的模拟

针对目前密相气力输送数值模拟过程中所存在的关键问题,提出了一种描述固相内部相互作用对颗粒运动影响的数学模型。该模型建立在离散颗粒模型的基础上,使其既能够模拟悬浮流动的稀相颗粒运动,又能模拟管内出现堆积情况的超浓相气固两相流。利用所建立的数学模型对高压超浓相煤粉气力输送的颗粒流动过程进行了数值模拟,模拟结果揭示了典型的栓塞流、沙丘流等流型特征及其流型随固气比的演变规律,并结合实验进行了验证。管道输送栓塞流波动性较为明显,波动频率向高频转化;平均栓塞长度随着进料固气比增大而增大。     

垂直弯管段高压密相煤粉气力输送特性的数值模拟

基于以氮气为输送介质的高压密相煤粉气力输送,在现有壁面模型和颗粒动力学基础上,充分考虑弯管中对气固两相流动特性起主要作用的摩擦应力的影响,建立了高压密相煤粉气力输送一体化管道(垂直管、弯管和水平管连在一起)的多相流新模型.采用该新模型模拟垂直向上转水平弯管内的气固两相流动特性,分析了补充风体积流量对弯管内固相速度、体积分数、湍动能分布以及垂直弯管压降的影响.结果表明:考虑摩擦应力后模拟所得弯管压降与试验值的误差减小至20%以内,验证了该模型的正确性;随着补充风体积流量的增大,垂直弯管压降先增大后减小,表观气速增大,垂直弯管壁面外侧煤粉堆积减少,低体积分数区范围增大,固相湍动能和固相拟温度均有所增大,当补充风体积流量达到1.0m3/h时,固相湍动能和固相拟温度均减小.     

粉体密相气力输送研究综述

物料的输送特性、输送管道压损研究、输送相图以及数值模拟等方面评述了粉体密相气力输送在近些年取得的研究进展,结合东南大学关于高压密相气力输送已有的研究成果,提出一些干煤粉加压密相气力输送的研究方向.     

高压超浓相气力输送流型及稳定性试验研究

在输送压力可达到4MPa、管路固气比可达到700kg/m3的高压超浓相气力输送试验台上进行输送物料为300μm煤粉和石英砂的流型可视化试验,获得了不同表观气速下物料的流型,研究了补充风风量对300μm煤粉输送稳定性的影响.结果表明:在试验台范围内,随着表观气速的减小,输送煤粉时获得了悬浮流、分层流和沙丘流流型,输送石英砂时获得了分层流、沙丘流和柱塞流流型,且不同流型间的区分度优于输送煤粉时流型间的区分度;随着表观气速的减小,输送系统的稳定性下降,管路的振动和各管段压降随时间的变化验证了这一事实;在输送300μm煤粉时,过小的补充风风量易发生堵管现象;沙丘流或柱塞流状态均属于间歇流动状态,且柱塞流状态是一种比沙丘流状态更不稳定的低速复杂流动状态.     

垂直管密相输送的数值模拟

基于稠密气体分子运动论和颗粒动力学,应用稠密气固两相流动的欧拉一欧拉双流体数学模型,利用数值方法模拟模拟垂直管内上升流动的行为,得到了气相和颗粒相速度分布以及压力损失.将该模型运用到垂直管高压密相(输送压力3 MPa、最大固气比426 kg/m3)气力输送中,结果表明模型预测的压降梯度与实验测量结果相符合.     

旋流燃烧室内煤粉多相湍流流动与燃烧的数值模拟

提出了考虑湍流-颗粒反应相互作用的颗粒随机轨道模型,以此为基础建立煤粉燃烧综合理论模型并应用于旋流燃烧室内煤粉多相湍流流动与燃烧的数值模拟.模拟结果给出了气相温度场、速度场与温度脉动均方根值分布、颗粒相温度场、速度场与表观密度场以及颗粒瞬时温度与质量随时间的变化.研究表明,考虑湍流-颗粒反应相互作用对气相与颗粒相温度场的模拟结果有一定的影响,使气相温度分布与实验数据更为接近.     

强旋湍流气固两相流动和煤粉燃烧数值模拟

该文给出一种用于强旋流的气固两相湍流模型，采用双流体模型及相应的数值解法，求解旋风炉燃烧室内气固两相流动及煤粉燃烧。模拟结果表明，前置室内湍流空气动力场分布具有强旋，中心回收流和各向异性湍流的特点。     

高压密相气力输送固相流量的实验与GRNN网络预测

煤粉高压密相气力输送是气流床加压气化的关键技术之一.在输送压力可达3.7 MPa,管路固气比可达660 kg/m3的气力输送实验台上进行系统的研究,考察输送压力、输送差压、流化风量、充压风量、补充风量、煤粉含水率等条件对固相质量流量的影响.结果表明:固相流量随着输送差压的增大而增大;随着流化风量的增大而先增大后趋向于某一定值;注入风量一定时,随着充压风量的增大而先减小后增大;与补充风量的大小基本无关;随着煤粉含水率的增大而减小.同时建立广义回归神经网络(GRNN)对固相流量进行了有效预测,最大预测误差在2.3%以内.上述工作将为系统的控制和运行提供一定的指导,同时为深化高压密相气力输送的研究奠定基础.     

煤粉复合旋流火焰燃烧特性研究（2）：数值模拟

本文数值模拟了煤粉旋流火焰燃烧过程，燃烧数值计算包括理论物理模型建立，数值方法两个大部分，计算模型处理了气相湍流与燃烧、气固两相流动、煤颗粒燃烧过程和辐射传热等物理化学过程，以ｋ－ε模型模拟湍流流动；ＰＤＦ法模拟气相扩散火焰燃烧；颗粒运动计算颗粒运动少颗粒湍流浓度方程模拟颗粒湍流扩散；通量法计算火焰辐射传热，煤粉颗粒复杂燃烧模型计算了颗粒尺寸、形状变化和颗粒孔隙内部燃烧、表面平度对整个颗粒的燃烧过程影响。计算获得了气相速度分布场、气相ｋ和ε分布场、气相温度场、气相组份场和颗粒浓度场及运动过程，揭示了煤粉复合旋流燃烧特性。     

Validation of void fraction models and correlations using a flow pattern transition mechanism model in relation to the identification of annular vertical downflow in-tube condensation of R134a

This paper reports the experimental investigation of a model for predicting flow pattern transitions and for the validation of void fraction models and correlations proposed in the authors' previous publications and for the identification of flow regimes in data corresponding to annular flow downward condensation of R134a in a vertical smooth copper tube having an inner diameter of 8.1 mm and a length of 500 mm. R134a and water are used as working fluids on the tube side and annular side, respectively, of a double tube heat exchanger. Condensation experiments are done at mass fluxes of 260 and 515 kg m^− 2 s^− 1 in the high mass flux region of R134a. The condensing temperatures are between 40 and 50 °C; heat fluxes are between 10.16 and 66.61 kW m^− 2. A mathematical model proposed by Soliman based on the models of Kosky and Lockhart–Martinelli is used to determine the condensation film thickness of R134a. Comparative void fraction values are determined indirectly using the measured data under laminar and turbulent flow conditions together with various void fraction models and correlations reported in the literature. There is good agreement between the void fraction results obtained from the theoretical model and those obtained from the void fraction models of Soliman, Chisholm and Armand, Turner and Wallis, Smith, Spedding and Spence previously proposed in the authors' publications and tested against their experimental database. Various well-known flow regime correlations from the literature are investigated to identify the flow regime occurring in the test tube, the correlations of Taitel and Dukler, Dobson, Akbar et al., Breber et al., Cavallini et al., and Sardesai et al. can provide accurate estimates of the annular flow conditions in spite of their different working conditions.     

受高温空气加热的煤粉气流着火过程数学模型

在一维平面层流煤粉气流着火模型的基础上,提出了一种一维平面紊流煤粉气流的着火模型。该模型不仅考虑了火焰中的辐射传热,煤粉热解,挥发分燃烧和多相反应等过程,同时还考虑了紊流对气相质量及能量扩散的影响,使该模型更能真实地反映工程实际。此外,通过实验对所建立的数学模型进行了实验验证,两者结果吻合较好,表明所建立的数学模型是正确可信的。     

Numerical Computation of Compressible Viscous Internal Flows

Some implicit time-marching finite-difference solutions of time-averaged Navier-Stokes equations for two-dimensional compressible internal flows are presented.Five numerical examples including subsonic,transonic,supersonic and hypersonic flow fields with steady and unsteady phenomena show validity and flexibility of thepresent calculation code.The TVD scheme suggested by Harten et al is used to improve the shock resolution,and an algebraic turbulence model suggested by Baldwin and Lomax is introduced to evaluate the viscous effectin the turbulent flows.It is found that the computational results show fairly good agreement with the experi-mental data.     

Analysis of Particle Dispersion and Entropy Generation in Turbulent Mixed Convection of CuO-Water Nanofluid

In the present study, turbulent mixed convection of CuO-water nanofluid is investigated in a vertical duct. In order to simulate the nanofluid flow, the fluid phase is considered as continuous whilst the discrete particles are dispersed through it. The dispersion of CuO nanoparticles at different Reynolds numbers is studied to predict the effective mechanisms concerning nanoparticles dispersion. Results show that in the turbulent fully developed region of the duct, the effect of thermophoresis is more important than Brownian motion and the dispersion of particles is higher in the duct core region. However, in the entrance region, the particles are dispersed almost uniformly. Also, increasing the nanoparticles volume fraction augments the root mean square of turbulent velocity fluctuations and this enhances the convective heat transfer as compared with the laminar flow. Moreover, increasing the nanoparticle volume fraction decreases the thermal entropy generation and increases the frictional entropy generation and due to the dominance of thermal entropy generation, the total entropy generation therefore reduces.     

Simulation of shape dynamics of a long flexible fiber in a turbulent flow in the hydroentanglement process

This paper presents the numerical investigation of dynamics of the shape of a long flexible fiber subjected to a turbulent flow field caused by impinging jets in the hydroentanglement process. The shape change of a long flexible fiber is simulated utilizing the rod–chain model following the method developed in Wang et al. [G. Wang, W. Yu, C. Zhou, Optimization of the rod chain model to simulate the motion of a long flexible fiber in simple shear flows, European Journal of Mechanics B/Fluids 25 (2006) 337–347.]. The method is extended to simulate the fiber movement in a turbulent flow field; two-dimensional simulations are performed. The shape of the fiber is determined by forces and torques exerted on it by the surrounding fluid as well as by the neighboring fibers. Numerical results are obtained for two model representations of the same fiber which differ by the number of rods used to represent the fiber.     

紊流工况下的滑动轴承性能分析

大机组轴承越来越多运行于紊流工况下,采用不同的紊流模型计算分析机组轴承的性能存在差异。该文目的是分析采用不同的紊流模型计算结果之间的差异,为选择紊流下轴承性能分析提供依据。该文对比了采用四种紊流模型对椭圆轴承性能的分析结果,包括最小膜厚、温升,流量、刚度阻尼等,并采用单元盘弹性转子一轴承模型讨论轴承的稳定性。分析结果显示,应该以分析目的和分析范围的不同来选择紊流模型。在稳定性分析中,采用青木弘模型计算结果比其他三种模型更趋保守;在轴承流量分析,对于纯紊流下分析可采用青木弘模型,对于层紊流过渡区可采用Ng-Pan模型。     

A study on the gas-solid particle flows in a needle-free drug delivery device

Different systems have been used over the years to deliver drug particles to the human skin for pharmaceutical effect. Research has been done to improve the performance and flexibility of these systems. In recent years a unique system called the transdermal drug delivery has been developed. Transdermal drug delivery opened a new door in the field of drug delivery as it is more flexible and offers better performance than the conventional systems. The principle of this system is to accelerate drug particles with a high speed gas flow. Among different transdermal drug delivery systems we will concentrate on the contour shock tube system in this paper. A contoured shock tube is consists of a rupture chamber, a shock tube and a supersonic nozzle section. The drug particles are retained between a set of bursting diaphragm. When the diaphragm is ruptured at a certain pressure, a high speed unsteady flow is initiated through the shock tube which accelerates the particles. Computational fluid dynamics is used to simulate and analyze the flow field. The DPM (discrete phase method) is used to model the particle flow. As an unsteady flow is initiated though the shock tube the drag correlation proposed by Igra et al is used other than the standard drag correlation. The particle velocities at different sections including the nozzle exit are investigated under different operating conditions. Static pressure histories in different sections in the shock tube are investigated to analyze the flow field. The important aspects of the gas and particle dynamics in the shock tube are discussed and analyzed in details.     

A phenomenological model for the prediction of soot formation in diesel spray combustion

A phenomenological soot model coupled with complex chemistry mechanism for the prediction of soot formation in diesel spray combustion is presented. The prototype of the model is one proposed by Leung and Lindstedt in which soot formation is treated by four global stages: particle nucleation, surface growth, surface oxidation, and particle coagulation, each of which is represented by only a few reaction steps. In the present study, the model is modified according to recent literature data. The formation of soot particles is linked with gas-phase chemistry via diacetylene and naphthalene, which are presumed to be indicative species of particle inception/nucleation. The soot surface growth is described using Frenklach et al.'s active site model, and the oxidation mechanism includes both Nagle and Strickland-Constable's O₂ oxidation and Neoh et al.'s OH oxidation models. The soot model integrated with the gas-phase kinetics is then applied in multidimensional spray simulations. The KIVA3 code that is widely used in diesel combustion studies is modified and employed for the simulations. The turbulent flow is predicted using the compressible k-ε model, and the turbulence-chemistry interaction handled by a partially stirred reactor model. The IDEA experimental data for n-heptane sprays in diesel-like conditions (800 K and 50 bar) are used for evaluation of the model. Some reaction rate constants are adjusted to achieve better agreement with the measurements. Further, sensitivity studies have been carried out and the effects of some parameters that affect the predictions are discussed. The results indicate that the model, if applied together with other models that properly describe sprays and turbulent flow, can be used for qualitative and even quantitative prediction of soot formation in diesel combustion.     

四角切圆燃烧炉内颗粒湍流扩散数值模拟

基于四角切圆燃烧锅炉内湍流流动及各向同性的假设,得出炉内和点的特征频率。同时采用特征频率－频谱随机轨道模型及随机轨道模型对炉内颗粒的湍流扩散进行了数值模拟。计算结果表明,特征频率－频谱随机轨道模型更能反映出颗粒湍流扩散中受涡结构的影响的特征,更接近实验结果。     

Bounds on Two-Phase Frictional Pressure Gradient in Minichannels and Microchannels

Simple rules are developed for obtaining rational bounds for two-phase frictional pressure gradient in minichannels and microchannels. The lower bound is based on the Ali et al. correlation for laminar-laminar flow. This correlation is based on the modification of simplified stratified flow model derived from the theoretical approach of Taitel and Dukler for the case of two-phase flow in a narrow channel. The upper bound is based on Chisholm correlation for laminar-laminar flow. The model is verified using published experimental data of two-phase frictional pressure gradient in circular and non-circular shapes. The published data include different working fluids such as air-water and nitrogen-water mixtures, as well as different channel diameters. The bounds models are also presented in a dimensionless form as two-phase frictional multiplier versus Lockhart-Martinelli parameter for different working fluids such as air-water mixture and nitrogen-water mixture. It is shown that the published data can be well bounded.