Considerations for Modeling Particle Entrainment into the Wake of a Circular Cylinder

The objective of this work is to evaluate the performance of the steady state Reynolds Averaged Navier-Stokes (RANS) computational fluid dynamics (CFD) models for estimating concentration of low Stokes number aerosols (Stk = O(10^?4)) in the wake of a bluff body. These simulations are compared with experimental data. In the simulations and experiments, particles are released upstream of the body and convected downstream, where some are entrained into the wake. The air velocity is computed using a steady state renormalized group k ～ ? model. Lagrangian particle trajectory simulations are performed in conjunction with each airflow model to calculate concentrations. The experiments are performed in an aerosol wind tunnel in which phase Doppler velocimetry measurements are obtained for the velocity field and aerosol concentration.

The RANS model yields a wake concentration deficit that extends downstream past x/D = 10, while the experiments produce elevated concentrations immediately downstream of the near wake. It is postulated that the concentration peak is at least in part attributed to particle interaction with the boundary layer by the following mechanism. Particles are transported into the boundary layer by turbulent diffusion, turbophoresis, and/or inertial forces. Particles then separate from the cylinder with the airflow and travel in a sheath around the periphery of the near wake to converge at the downstream edge of the near wake. Underestimation of the wake concentration by the RANS model is potentially due to inadequacy in the boundary layer approximation used in the model.     

圆柱绕流的数值模拟与PIV测试研究

利用计算流体力学软件Fluent对均匀来流绕固定圆柱的流动情况进行了数值模拟,考察和研究了气流绕过圆柱时的边界层的形成以及旋涡的形成及脱落过程,利用粒子图像测试(PIV)技术对绕流情况进行了实验研究.通过数值模拟结果与PIV实验结果的定性和定量分析比较,证明Fluent软件的模拟结果具有相当的准确性,能够准确的预测流体的流动特性.     

气固可压缩湍射流中颗粒扩散的直接数值模拟

A numerical method was developed to directly simulate the compressible, particle-laden turbulent jets.The fourth order compact finite difference schemes were used to discretize the space derivatives. The Lagrangian method was adopted to simulate the particle motion based on one-way coupling. It is found that the turbulent intensity profiles attain self-similar status in the jet downstream regions. At the Stokes number of 1, particles are concentrated largely in the outer boundaries of the large-scale vortex structures with the most uneven distribution and the widest dispersion in the lateral direction. Particles at the much smaller Stokes numbers are distributed evenly in the flow field, and the lateral dispersion is also considerable. Distribution of particles at much larger Stokes numbers is more uniform and the lateral dispersion becomes small. In addition, the inflow conditions have different effects on the particle dispersion. The direct numerical simulation (DNS) results accord with the previous experiments and numerical studies.     

Numerical Study of Particle Deposition in Bends of a Circular Cross-Section-Laminar Flow Regime

Particle deposition in a 90° bend has been studied numerically using a realistic three-dimensional developing flow field. In addition to the Stokes number as the impaction parameter, both the curvature ratio and the Dean number have been found to have considerable effects on the deposition efficiency. At a fixed Stokes number, the deposition efficiency increases with an increasing Dean number and a decreasing curvature ratio. The inlet velocity profile also influences the deposition efficiency. In the case of a parabolic velocity profile, the deposition efficiency is always higher than that of a uniform profile. These increases in deposition efficiency are due to the increase of secondary flow strength and the increased skewness of the axial velocity profile toward the outside of the bend.     

固体粉末在聚合物基体中分散混合的数值模拟研究

根据聚合物混合理论以及固体粉末团聚体在聚合物基体中的破碎理论,提出利用累积尺度分布函数直接表征固体粉末团聚体在聚合物中分散效果的表征方法,并建立了相应的数值模型。利用这种方法得到的模拟结果与理论分析一致,说明在数值模拟中利用累积尺度分布函数表征固体粉末团聚体在聚合物中的分散混合效果是正确可行的。     

Particle Mixing and Diffusion in the Turbulent Wake of a Sphere

The mixing and diffusion of monodisperse aerosols from a turbulent free stream into the wake created by a solid sphere were studied experimentally and analytically. Experimental measurements with regard to wake velocity profiles and momentum transport were taken in a tubular flow system by hot-wire anemometry. Particle concentration profiles, measured in the wake by means of a screen collection technique, were used to calculate mixing and diffusion parameters for the aerosols used. Experimental results are presented for the mean velocity and particle concentration profiles, wake centerline velocity and particle concentration, wake velocity and concentration half widths, and momentum and particle diffusivities. It is shown that the experimental results may be reasonably described in terms of two phenomenological theories, i.e., Prandtl's mixing length and constant diffusivity in the radial cross section of the wake. Certain departures from predicted concentration behavior are empirically analyzed resulting in the ability to predict overall particle mixing and diffusion characteristics in a turbulent wake.     

Numerical Simulation of Solute Dispersion in Laminar Tube Flow

Abstract

The dispersion of a solute slug in laminar tube flow has been simulated by a numerical method (the Flux Corrected Transport Algorithm) for a set of tube lengths embracing the pure convection through Taylor dispersion regimes. The results reveal unexpected double peak breakthrough curves for tubes of intermediate length. Experimental evidence of double peaks which qualitatively confirms the simulations is discussed.     

气固颗粒系统模拟的研究进展

综述了气固颗粒系统模拟方法的发展状况。首先,从宏观、中观及直接数值模拟角度对基于欧拉-欧拉架构下的双流体模型、拉格朗日-欧拉架构下的离散气泡模型以及欧拉-拉格朗日架构下的计算流体力学和离散单元法耦合模型(CFD-DEM)及直接数值模拟方法就模型基本原理、计算量、预测精度以及应用前景等方面进行对比和分析。接着,针对工程应用前景较强的双流体模型和CFD-DEM模型进行了详细阐述,着重强调了欧拉架构下的模型植入过程,颗粒相的动力学过程和微观行为描述、两相耦合作用及相间耦合模型描述。基于欧拉-拉格朗日架构下的多相耦合模型的普适性,模型的应用已经从研究尺度的模拟逐渐走向工程尺度的模拟,成为今后研究多相颗粒系统的热点。     

滚筒干燥和气流干燥的过程的数值模拟研究

为推动本厂的特色工艺研究和技术改造工作,根据本厂实验线的设备参数和实验数据,运用最新的流体力学计算方法,对滚筒干燥和气流干燥过程进行数值模拟。该研究结果对制丝线设备改造和创新提供了一定的理论基础     

铝颗粒点火数值模拟

分析了准稳态铝颗粒在高温氧化剂中由于传热和表面的化学反应导致升温-熔化-升温-点火的过程，建立了铝颗粒点火模型，并对点火过程进行数值模拟。计算结果给出了对流传热、辐射传热、熔化热和异相表面反应热等对点火的影响。研究结果表明在点火过程中，铝的氧化在颗粒温度达到一定温度（T〉1000K）时才比较明显；点火时铝单质质量分数随环境温度的升高而略有增加，与颗粒尺寸无关；点火延时随着环境温度升高和颗粒半径的减小而缩短。     

方柱绕流中聚乙烯熔体流变行为的数值模拟

采用了DCPP和S-MDCPP两种模型分别描述低密度聚乙烯熔体的本构行为。在迭代分步算法中引入了改进的有限增量微积分(FIC)法,并采用离散的弹性黏性应力分裂技术(DEVSS)/迎风流线(SU)法解决黏弹性流动分析中的对流占优问题。讨论了S-MDCPP模型预测的速度、压力及主链拉伸分布随Weissenberg(Wi)数的变化。结果表明,两种模型预测的速度、主应力差等色条纹和主链拉伸分布吻合较好;随Wi的增大,速度及压力均增大。     

Numerical Simulation of Particle Segregation in Vibration Fluidized Beds

The influence of vibration parameters on the segregation phenomenon of a binary mixture in a vibration fluidized bed is investigated. Initially, the mixture composed of spherical balls with different densities but same diameter is in a perfect mixing state in the bed. The motion of particles is simulated based on the discrete element method. The effects of friction coefficient, vibration frequency, amplitudes, and gas velocity are analyzed. The coefficient of segregation to the degree of particle segregation is calculated for different operating conditions. The segregation degree in the vibration fluidized bed is found to be higher than that in the bed without vibration. The curve for the segregation degree exhibits a single peak value which represents the optimal segregation result.     

数值研究液-液水力旋流器内颗粒浓度分布特性

根据计算流体力学(CFD)的原理和方法,应用Fluent软件对液-淡水力旋流器内部颗粒浓度分布进行数值模拟,得到了不同进口流量和不同进料浓度下旋流器内各个截面上浓度分布情况.结果表明:旋流器内各截面上浓度变化曲线呈现抛物线型;进口流量越大,在管壁处浓度值越小,油滴不断向管芯聚集;进料浓度越大,在旋流器的大部分区域中,浓...     

含铝炸药圆筒试验与数值模拟

采用圆筒试验研究了两种直径(50 mm和100 mm)含铝炸药的作功能力,获得了圆筒壁膨胀位移与时间的关系.利用有限元动力学程序LS-DYNA,采用Lee-Tarver点火增长三项式模型对两种含铝炸药的圆筒实验进行了数值模拟.通过与实验结果相比较,得到了含铝炸药的爆轰产物JWL状态方程和反应速率函数的参数,较好地再现了两种含铝炸药圆筒试验结果的参数.Lee-Tarver点火增长三项式模型能够较好地反映含铝炸药后期能量释放驱动圆筒壁膨胀的过程.     

三螺杆分散混合数值模拟及实验分析

为了研究平行同向啮合三螺杆挤出机的分散混合效果,采用离散元法对三螺杆挤出机中聚丙烯(PP)/低密度聚乙烯(PE-LD)两种塑料颗粒在加料段的混合情况进行数值模拟.首先建立了三螺杆挤出机的三维模型,在EDEM软件中仿真出不同螺杆转速下挤出机的混合效果,通过对收集盒分层处理计算得出混合过程中混合均匀度的变化趋势.分析结果表...     

三种类型战斗部破片飞散的数值模拟

为了得到可变形战斗部的整体性能,用有限元动力学程序LS-DYNA模拟了相同结构口径、相同装药类型的可变形战斗部、偏心起爆和传统周向均匀战斗部的飞散过程,得到三种结构战斗部沿目标方位角和破片飞散角内的破片空间和速度分布.结果表明,在给定毁伤目标范围内(-30°～30°),与传统周向均匀战斗部相比,偏心多点起爆战斗部的平均速度增益为124.6%,密度增益为103.5%;可变形战斗部的平均密度增益为175.9%,平均速度增益为110.2%.可变形战斗部对目标的整体毁伤性能优于偏心起爆战斗部.     

流体中气泡微细化与分散过程的数值模拟

采用CFD商用软件模拟了搅拌加喷吹情况下溶池中流场和相的分布.对比分析了水模型内单相流和两相流时液面和轴向迹线图、矢量图和相分布图,重点考察了中心搅拌和偏向搅拌对漩涡和相分布的影响规律.模拟结果表明,中心搅拌模式下在搅拌桨轴附近形成很大的漩涡,偏心时虽然也有漩涡形成,但偏离轴心.两相流偏心搅拌模式下由于漩涡较小且偏离轴心,有利于气体的分散;偏心搅拌与中心搅拌相比可以抑制漩涡在几何中心的形成,有利于气体的分散和气泡的细化,同时偏心搅拌也可以有效地抑制水模底部的死区.研究结果对喷气机械搅拌装置的设计具有参考价值.