Numerical Simulation of Laminar Flow Field in a Stirred Tank

Stirred tanks are used extensively in process industry and one of the most commonly used impellers in stirred tanks is the R.ushton disk turbine. Surprisingly few data are available regarding flow and mixing in stirred-tank reactors with Rushton turbine in the laminar regime, in particular the laminar flow in baffled tanks.In this paper, the laminar flow field in a baffled tank stirred by a standard R.ushton turbine is simulated with the improved inner-outer iterative method. The non-inertial coordinate system is used for the impeller region, which is in turn used as the boundary conditions for iteration. It is found that the simulation results are in good agreement with previous experiments. In addition, the flow number and impeller power number calculated from the simulated flow field are in satisfactory agreement with experimental data. This numerical method allows prediction of flow structure requiring no experimental data as the boundary conditions and has the potential of being used to scale-up and design of related process equipment.

Numerical Simulation of Blood Flow in Aorta with Dilation:A Comparison between Laminar and LES Modeling Methods

Computational modeling methods have been increasingly employed to quantify aortic hemodynamic parameters that are challenging to in vivo measurements but important for the diagnosis/treatment of aortic disease.Although the presence of turbulence-like behaviors of blood flow in normal or diseased aorta has long been confirmed,the majority of existing computational model studies adopted the laminar flow assumption(LFA)in the treatment of sub-grid flow variables.So far,it remains unclear whether LFA would significantly compromise the reliability of hemodynamic simulation.In the present study,we addressed the issue in the context of a specific aortopathy,namely aortic dilation,which is usually accompanied by disturbed flow patterns.Three patient-specific aortas with treated/untreated dilation of the ascending segment were investigated,and their geometrical models were reconstructed from computed tomography angiographic images,with the boundary conditions being prescribed based on flow velocity information measured in vivo with the phase contrast magnetic resonance imaging technique.For the modeling of blood flow,apart from the traditional LFA-based method in which sub-grid flow dynamics is ignored,the large eddy simulation(LES)method capable of incorporating the dissipative energy loss induced by turbulent eddies at the sub-grid level,was adopted and taken as a reference for examining the performance of the LFA-based method.Obtained results showed that the simulated large-scale flow patterns with the two methods had high similarity,both agreeing well with in vivo measurements,although locally large between-method discrepancies in computed hemodynamic quantities existed in regions with high intensity of flow turbulence.Quantitatively,a switch from the LES to the LFAbased modeling method led to mild(<6%)changes in computed space-averaged wall shear stress metrics(i.e.,SA-TAWSS,SA-OSI)in the ascending aortic segment where intensive vortex evolution accompanied by high statistical Reynolds stress was observed.In addition,comparisons among the three aortas revealed that the treatment status of aortic dilation or the concomitant presence of aortic valve disease,despite its remarkable influence on flow patterns in the ascending aortic segment,did not significantly affect the degrees of discrepancies between the two modeling methods in predicting SA-TAWSS and SA-OSI.These findings suggest that aortic dilation per se does not induce strong flow turbulence that substantially negates the validity of LFA-based modeling,especially in simulating macro-scale hemodynamic features.

Phase-field simulations of forced flow effect on dendritic growth perpendicular to flow

The effect of supercooled melt forced laminar flow at low Reynolds Number on dendritic growth perpendicular to melt flow direction was investigated with the phase-field method by incorporating melt convection and thermal noise under non-isothermal condition. By taking the dendritic growth of high pure succinonitrile （SCN） supercooled melt as an example, side-branching shape difference of melts with flow and without flow was analyzed. Relationships among supercooled melt inflow velocity, deflexion angle of dendritic arm and dendritic tip growth velocity were studied. Results show that the melt inflow velocity has few effects on the dendritic tip growth velocity. A formula of relationship between the velocity of the melt in front of primary dendritic tip and the dendritic growth time was deduced, and the calculated result was in quantitative agreement with the simulation result.

Phase-field simulation of forced flow effect on random preferred growth direction of multiple grains

The random distribution problem of dendrite preferred growth direction was settled by random grid method.This method was used to study the influence of forced laminar flow effect on multiple grains during solidification.Taking high pure succinonitrile （SCN） undercooled melt as an example,the forced laminar flow effect on multiple grains was studied by phase-field model of single grain which coupled with flow equations at non-isothermal condition.The simulation results show that the random grid method can reasonably settle the problem of random distribution and is more effective.When the solid fraction is relatively low,melt particles flow around the downstream side of dendrite,and the flow velocity between two dendrite arms becomes high.At the stage of solidification time less than 1800Δt,every dendrite grows freely;the upstream dendrites are stronger than the downstream ones.The higher the melt flow rate,the higher the solid fraction.However,when the solid fraction is relatively high,the dendrite arm intertwins and only a little residual melt which is not encapsulated can flow;the solid fraction will gradually tend to equal to solid fraction of melt without flow.

管内充分发展流动与传热数值模拟的教学方法探讨

由于缺乏数值计算的基础理论知识,工科本科生在学习流体流动与传热过程的数值模拟方法并进行程序设计时,往往觉得难度较大。本文围绕圆管内不可压缩流体充分发展流动与传热,采用边界层积分法,推导出无量纲控制方程组;针对层流工况和湍流工况,开发出相应的数值方法。基于该方法的程序代码易于理解,且计算结果表明该方法具有预测精度高、收敛速度快的优点。本文的工作可以为计算流体力学、数值传热学及热工计算等系列课程的本科教学提供参考。

H_(2)与氨混燃增强火焰燃烧特性模拟研究

氢和氨作为清洁能源受到广泛关注,为深入探究氢-氨混燃的燃烧特性和影响因素,本文借助Chemkin仿真平台建立相关反应模型,以氢-氨混合气体为燃料,空气作为助燃剂,采用Otomo等人提出的一种氨氧化机理对其燃烧过程进行模拟计算,并模拟研究了混合气体的点火延迟时间、层流燃烧速度、绝热燃烧温度、NO排放等燃烧特性随当量比、初始压力以及燃料中H_(2)比例的具体变化规律,对不同工况下的层流火焰结构、H和OH自由基的产率(rate of production,ROP)、NO生成的敏感度进行了化学动力学分析。结果表明:纯氨气体的点火延迟时间长、层流燃烧速度慢,掺氢后燃烧特性均有所改善,且提高了火焰的绝热燃烧温度,但掺氢比例越大,NO排放越多。NO摩尔分数随当量比变化的趋势先增后减,在当量比为0.8左右达到峰值。综合考虑氢-氨混燃的一系列燃烧特性以及掺氢、加压的成本和收益情况,推荐H_(2)占比15%、当量比φ=1.1、压力P=0.2 MPa为氢-氨混合燃烧的最优条件。

环形间隙式层流元件设计及流量特性研究

为了解决传统层流流量计(LFM)线性度不佳、长径比较大、加工使用不便和结构易受流体影响等诸多问题,受双锥流量计的启发,提出了一种环形间隙式层流元件结构,介绍了测量原理和流道内非线性压力损失来源。保持该结构外套筒体和圆柱锥体同轴心,其流道截面为同心圆环,通过CFD仿真确定了锥形导流结构的合理锥形角度,确定了层流元件的尺寸参数。将取压孔设置在流道中充分发展的层流段,理论上消除了传统毛细管式LFM进出口处流动局部损失和层流发展段的动能损失。制作3种不同间隙大小的试件并进行试验,结果显示,当测量流量值小于53 mL/min时,层流元件的测量误差在3%以内;当测量流量值在(130~6189)mL/min时,测量误差在±2%以内;层流元件的压差和流量之间具有优秀的线性关系。说明环形间隙式层流元件结构可有效克服传统LFM的非线性影响,同时测量流量范围可随间隙大小变化而改变。

基于深紫外消毒的隔离层流床的研制

目的为满足临床上对隔离层流床的需求,设计并制造一款基于深紫外消毒的隔离层流床。方法利用深紫外消毒代替传统消毒方法,床帘选用磁吸开关,床帘和床顶连接选用带亚克力背胶的环保PVC材质的连接条达到床体全封闭,从整体上部进气、底部排气,利用ANSYS流体仿真,检测床体内部的层流效果。结果本设计实现床体全封闭隔离的效果。同时流体仿真结果表明床内层流效果良好。结论基于深紫外消毒的隔离层流床既满足隔离要求,又实现了良好的层流效果,具有一定的临床应用价值和意义。

不同截面肋柱-软尾结构单相流动传热比较

基于任意拉格朗日-欧拉(ALE)方法,利用动网格技术和重叠网格技术数值模拟研究了不同Reynolds数、截面形状以及长径比下的肋柱-软尾结构在通道中的双向流固耦合换热问题。模拟工况为:Reynolds数Re=200,275,351;肋柱截面形状:圆形和方形;长径比k=2,3,4。研究表明:Re=275时圆形截面肋柱-软尾结构的流动换热综合能力优于方形截面结构,k=3时圆形截面肋柱-软尾结构的流动换热综合能力最佳,肋柱周围的局部换热能力方形截面结构好于圆形截面结构;k=3时,随着Reynolds数的增大,不同截面形状的肋柱-软尾结构的流动换热综合能力逐渐升高,并且高Reynolds数、高长径比的圆形截面综合流动换热能力最佳;与不加软尾肋柱结构的流动换热能力进行对比发现,对于圆形截面形状的肋柱结构,增加软尾后综合换热能力增大了19.46%。

纵向肋强化波纹螺旋通道流动与传热性能

螺旋通道换热器广泛应用于农业工程领域,为了进一步提升螺旋通道的传热性能,该研究提出了一种带纵向肋的矩形截面波纹螺旋通道,数值研究了波纹数和纵向肋几何参数对通道内流体流动与传热性能的影响。结果表明:增加波纹螺旋通道的波纹数,使得通道在层流状态下产生了多涡结构,增加了通道的综合换热性能。在雷诺数Re=750,波纹数n=18时,相比于光滑螺旋通道,波纹螺旋通道内综合换热评价指标最高增加了27.66%。在此基础上,在通道内增加纵向肋,通过数值模拟观察到纵向肋诱导产生了纵向涡。在纵向肋高度不变的情况下,随着纵向肋宽度的增加,通道的综合换热评价指标存在最大值。当Re=250,肋宽为1/3通道宽度,肋高为1/16通道高度时,相比于波纹螺旋通道,纵向肋波纹螺旋通道的综合换热评价指标最高为1.157。在纵向肋宽度一定的情况下,增加纵向肋高度使通道的综合换热评价指标呈现先增大后减小的趋势。研究结果可为纵向肋波纹螺旋通道的设计及优化提供参考。

NUMERICAL SOLUTION OF VISCOUS FLOW PAST A SOLID SPHERE WITH THE CONTROL VOLUME FORMULATION

The control volume formulation with the QUICK finite difference scheme is used to solveincompressible liquid flow past a solid sphere in terms of stream function and vorticity.Several tech-nical points are addressed on improving the accuracy and efficiency of numerical simulation of similarproblems of fluid flow.In particular,the importance of suitable specification of the distortion func-tion to enforcing the far field boundarv conditions is emphasized.

Investigation of a Counter Flow Microchannel Heat Exchanger Performance with Using Nanofluid as a Coolant

In this paper the performance of a counter flow microchannel heat exchanger (CFMCHE) is numerically investigated with a nanofluid as a cooling medium. Two types of nanofluids are used Cu-water and Al2O3-water. From the results obtained it’s found that thermal performance of CFMCHE increased with using the nanofluids as cooling medium with no extra increase in pressure drop due to the ultra fine solid particles and low volume fraction concentrations. The na-nofluids (Cu-water and Al2O3-water) volume fractions were in the range 1% to 5%. It’s also found that nanoflu-id-cooled CFMCHE could absorb more heat than water-cooled CFMCHE when the flow rate was low. For high flow rates the heat transfer was dominated by the volume flow rate and nanoparticles did not contribute to the extra heat absorption. Also the performance of CFMCHE can be increased considerably by using nanofluids with higher thermal conductivities.

Influence of velocity profile on calibration function of Lorentz force flowmeter

A Lorentz force flowmeter is a noncontact electromagnetic flow-measuring device based on exposing a flowing electrically conducting liquid to a magnetic field and measuring the force acting on the magnet system. The measured Lorentz force is proportional to the flow rate via a calibration coefficient which depends on the velocity distribution and magnetic field in liquid. In this paper, the influence of different velocity profiles on the calibration coefficient is investigated by using numerical simulations. The Lorentz forces are computed for laminar flows in closed and open rectangular channels, and the results are compared with the simplified case of a solid conductor moving at a constant velocity. The numerical computations demonstrate that calibration coefficients for solid bodies are always higher than for liquid metals. Moreover, it can be found that for some parameters the solid-body calibration coefficient is almost twice as high as for a liquid metal. These differences are explained by analyzing the patterns of the induced eddy currents and the spatial distributions of the Lorentz force density. The result provides a first step for evaluating the influence of the laminar velocity profiles on the calibration function of a Lorentz force flowmeter.

层流搅拌槽内柱状单颗粒运动特性的研究

为了完善固液搅拌槽内非球形颗粒运动特性的基础数据,利用摄像法对带有圆盘桨的层流搅拌槽内柱状单颗粒的运动过程进行可视化实验,使用图像处理方法量化颗粒运动,分析了颗粒的尺寸对颗粒运动特性的影响,并采用基于格子玻尔兹曼方法的直接数值模拟解析颗粒运动和槽内流场。研究结果表明:颗粒净重力和颗粒周围流场分布共同影响颗粒临界悬浮所需的搅拌雷诺数和黏性Shields数;颗粒偏离槽底中心运动是颗粒悬浮前的必要步骤;由颗粒周围压力梯度引起的悬浮力是颗粒上升的重要原因;颗粒在桨盘底部区域的运动轨迹、颗粒-桨盘间距和运动跟随性受颗粒长径比的影响。

A Computational Solution of Natural Convection Flow in a Rotating Fluid with Radiative Heat Transfer

An analysis of the hydromagnetic free convective flow past a vertical infinite porous plate in a rotating fluid is carried out. The temperatures involved are assumed to be very large so that the radiative heat transfer is significant, which renders the problem very non-linear even on the assumption of a differential approximation for the radiative flux. The temperature and velocity fields are computed using a generic software tool based on the Nakamura finite difference scheme. The genericity of the software tool is in the sense that it is a common solution to the category of time dependent laminar fluid flows expressed in one spatial coordinate. The input equations, together with other relevant parameters, are transformed into postfix code which will be farther interpreted in the computation process. The influence of the various parameters entering into the problem is shown graphically followed by a discussion of results.

A Generic Computational Solution of a Natural Convection Flow past an Infinite Vertical Porous Plate

A simulation was carried out on an unsteady flow of a viscous, incompressible and electrically conducting fluid past an infinite vertical porous plate. A generic computer program using the Galerkin finite element method is employed to solve the coupled non-linear differential equations for velocity and temperature fields. The diffusion equation, the energy equation, the momentum equations and other relevant parameters are transformed into interpretable postfix codes. Numerical calculations are carried out on the flow fields both in the presence of cooling and heating of the plate by free convection currents. The effects of the dimensionless parameters, namely, the Prandtl number, the Eckert number, the modified Grashof number, the Schmidt number and the time on the temperature and velocity distributions are discussed.

高Re数层流管道中颗粒聚集特性的数值研究

该文基于“相对运动模型”对高Re数层流管道中颗粒的惯性聚集特性进行了数值模拟.为了解决高Re数流长管道问题,对管道进、出口施加了周期性边界条件.研究结果表明,采用周期性边界条件可以有效地减小计算域,选用L=4D的管道便可计算出高Re数管流中颗粒的受力特性.与低Re数不同的是:随着Re数的不断增大,颗粒在径向上的升力不再呈类抛物线分布,升力曲线在r+=0.5~0.7之间出现一个下凹的区域,在这个区域内有出现新聚集点的趋势,并且用a+=1/17的颗粒在Re>1000时得到了这个新聚集点的位置.此外,通过对流场进行分析,发现颗粒的周围有二次流产生,其强度随着Re数的增大以及颗粒向壁面靠近逐渐增强,而二次流的产生影响了颗粒升力空间分布.

通道层流场中单颗粒惯性横向迁移的数值研究

为研究通道层流场中单颗粒惯性横向迁移规律,以圆管和方管为例,对层流场中性单颗粒惯性横向迁移现象开展数值模拟研究。将颗粒放在圆管和方管进口截面的若干位置并释放,采用离散颗粒模型,计算粒径d=0.8、1.0、1.2 mm的3种球形颗粒在雷诺数Re=100、125、150条件下,由不同初始位置释放后在流场中的迁移轨迹及横向迁移后所达到的平衡位置。结果表明,对于圆管,颗粒在初始半径分别为0.7R与0.8R(R为圆管半径)处释放时,惯性迁移的平衡位置均在0.6R附近;随着粒径增加,颗粒迁移的平衡位置向管道轴心偏移;随着Re的增加,颗粒迁移的平衡位置向管壁方向偏移。对于方管,颗粒的初始释放位置设定在z0=0.22L与0.33L(L为方管截面边长)处,当颗粒释放位置越靠近通道轴心时,颗粒的横向迁移距离就越短;随着Re的增加,颗粒的平衡位置会更加偏向通道中心;相对于大颗粒,小颗粒的平衡位置更靠近壁面。

错位碰撞型微混合器混合性能的模拟分析与优化设计

微混合器作为微流控设备的重要组成部分,广泛应用于生化领域,由于在微通道下流体流动为层流,混合较差,对于快速反应,混合是影响效率的主要因素。本文对影响混合效率的3种微混合器结构进行了数值模拟,通过变化微混合器的通道宽高比、发散处最大宽度、碰撞处错位高度3个结构参数,模拟研究其在层流下的混合性能。结果表明,流体碰撞处错位高度对混合性能影响最明显。对模拟结果最佳的微混合器(MST)进一步优化其结构得到新的微混合器(MTT),将MTT与MST及普通T型微混合器(MT)进行比较。MTT微混合器出口处的混合指数达到81%,而普通T型微混合器在相同的条件下只有5.3%。通过模拟分析混合过程,有效改善了错位碰撞型微混合器的结构,有利于提高流体混合效率,提高反应速度。

NUMERICAL SIMULATION OF 3-D CORNER FLOWS IN A CIRCULATING WATER CHANNEL

A Navier-Stokes procedure was developed based on a finite volume method to simulate corner flows in circulating water channel (CWC). The standard 2nd-order central scheme together with a deferred correction method was applied for the convective terms. All the other terms were discretized using 2nd order central differencing. The standard k-Ε model was used for the approximation of turbulent flows. First, this method used to calculate the turbulent flows in a 90° bend and the computed results are in good agreements with the experiment. This method was also employed to calculate the flows in a model CWC corner.