锚式搅拌槽中高粘牛顿流体的流速分布及其粘性耗散

本文用照相法测定了锚式搅拌槽中高粘度流体的流型和速度分布.利用最小二乘法获得速度分布拟合方程以及槽中剪切率γ和耗散能Φ分布方程.比较了槽壁区域剪切率和全槽平均剪切率、Metzner常数k_S和搅拌器特性参数C_3,指出k_S是表征槽中高剪切区域剪切率大小的特征量,C_3表征了全槽平均剪切率的大小.对于均匀剪切场,两者是一致的.可以用单位体积剪切率分布系数φ_V来描述槽中剪切率分布的均匀程度.本文还研究了Re数对速度分布流型的影响,发现在低Re数下,流型是对称的.高Re数下,流型则不对称.     

折叶涡轮桨搅拌器流型及温度分布数值模拟

采用MIXSIM对折叶涡轮桨搅拌器搅拌槽内流体流型及紊流状态下的温度分布进行模拟.结果显示:随Re数的增大,流体流型由径向流逐渐向轴向流发展;在温度变化趋于稳定时,温度在搅拌轴两侧基本呈对称分布,且分别存在一个低温区域.     

多层搅拌桨流动场的测量与数值模拟

应用商业计算流体力学软件CFX对搅拌槽内多层搅拌桨的流场进行了模拟,并与LDV测试结果进行了比较.低Reynolds数k-ε模型和代数应力模型流型吻合较好;标准k-ε双方程模型和RNG k-ε模型下层桨流场出现不同程度的扭曲,与实验测试结果不符.速度分量的对比结果表明,低Reynolds数k-ε模型和代数应力模型能较准确地模拟搅拌槽内的流动场,其中尤以代数应力模型最为准确.     

基于分形桨的湍流耗散强度特征研究

针对搅拌过程中湍能耗散造成的能量损失问题，提出了一类新型分形几何涡轮桨，并基于计算流体动力学，对系列分形桨进行湍能耗散强度分布数值模拟实验。通过分析分形桨的湍能耗散分布强度规律，探究了分形维数及迭代次数对搅拌设备各部分湍能耗散以及搅拌功率的影响。结果表明：流场湍能耗散强度分布特征为横向呈风轮状，纵向呈扇状；相对于无迭代桨，分形迭代桨湍流耗散强度降低；分形维数增加会降低各处的湍能耗散强度。迭代次数增加会降低流场中湍能耗散，但会增大搅拌桨上的耗散强度；分形迭代桨会降低搅拌功率，并随着分形维数增大而效果加强。研究结果为高速离心搅拌桨设计提供新的设计思路。     

锚式搅拌槽中高粘弹性流体的流速分布及其功率消耗

用照相法测定了锚式搅拌槽中高粘弹性流体的流型和流速分布,另测定了搅拌功率消耗,结果发现:1.与牛顿流体相比,在低Re数下,粘弹性流体的切向速度较大,而径向速度则较小.2.转速相同时,在高剪切率区域,粘弹性流体的剪切率大于牛顿流体.由CEF方程导出功率计算式N_pRe_af_s~(1-n)=k_pf_vf_s~2[1+F_1avf_s~(m-n-3)Wi/K_s~2]用实验数据确定f_(?)和F_(1av),得到可适用于牛顿流体、假塑性流体和粘弹性流体的普适功率计算式,计算结果与实验值比较接近.     

锚式搅拌槽中高粘弹性流体的流速分布及其功率消耗

用照相法测定了锚式搅拌槽中高粘弹性流体的流型和流速分布,另测定了搅拌功率消耗,结果发现:1.与牛顿流体相比,在低Re数下,粘弹性流体的切向速度较大,而径向速度则较小.2.转速相同时,在高剪切率区域,粘弹性流体的剪切率大于牛顿流体.由CEF方程导出功率计算式N_pRe_af_s~(1-n)=k_pf_vf_s~2[1+F_1avf_s~(m-n-3)Wi/K_s~2]用实验数据确定f_(?)和F_(1av),得到可适用于牛顿流体、假塑性流体和粘弹性流体的普适功率计算式,计算结果与实验值比较接近.     

桨叶型式对中心龙卷流型搅拌槽内流动场的影响

采用计算流体力学(CFD)对中心龙卷流型搅拌槽内部流场进行了数值模拟,从速度、压力、湍动能、功耗等方面研究三种不同的搅拌桨叶对搅拌槽工作性能的影响。研究结果表明:6PBDT在整个搅拌槽内速度分布相对较均匀;6SBDT在轴截面处的压力分布较为均匀,湍动能分布面积相对较大;在相同条件下,6SBDT所消耗的功率大,即槽内流体的混合性能最好。     

管式搅拌反应器停留时间分布的数值模拟

对一种新型管式搅拌反应器的停留时间分布(RTD)进行了实验研究,运用商业计算流体力学(CFD)软件FLUENT进行流场模拟及RTD计算,证实管式搅拌反应器的停留时间分布受搅拌转速及入口流量等因素影响,带搅拌装置的管式反应器适当增加转速能使RTD曲线变窄,改善流型和抑制返混;转速过大会使RTD曲线变宽,导致死区的产生,不利于反应器的混合.RTD曲线的数值模拟计算结果与实验结果基本稳合.     

气液两相流混合设备流场特性

利用有限元软件ANSYS Workbench 17.0对气液两相搅拌釜,在通气量分别为0.2 m~3/h和0.4 m~3/h以及同一搅拌转速120 rpm下的液相速度矢量分布进行分析。结果表明:上层桨附近的流场是相当经典的混合流型的特征,釜内底部下层桨附近流场是明显的径向流,釜内的整个流场分布是以搅拌轴作为中心轴呈轴对称分布。     

过氧化氢异丙苯分解反应器的改进

通过对现有反应器流型、工况的反应和传热计算分析,找到了CHP分解反应器中温度分布不均匀的原因。实验研究证明,通过改进搅拌的型式,改变反应器流场,能改善CHP分解反应器的温度分布。工业应用表明,采用“螺旋桨 导流筒”型式的搅拌后,能明显改善CHP分解反应器的温度分布。     

应用三维PDA研究多层桨在搅拌过渡区的流场

在直径为0.5ｍ的搅拌釜内,应用三维激光颗粒动态分析仪（简称PDA）测定了多层搅拌桨在搅拌过渡流区的三维流场．研究了不同的桨叶安放角、桨径、桨层间距、雷诺数及不同桨型对流场的影响规律,为在过渡流区操作的多层搅拌反应器的优化设计提供参考．     

CFD analysis of a rotor-stator mixer with viscous fluids

The characterization of the hydrodynamics of a rotor-stator mixing head has been carried out in the laminar regime with viscous Newtonian fluids. The rotor-stator considered is a very common design composed of a flat blade rotating in a fixed slotted cage. A numerical methodology has been used based on the virtual finite element method to model the velocity patterns, estimate the distribution of shear stress and the flow rate through the head. We have found that the numerical prediction of the power consumption and flow profiles compare well with experimental data. The generation of a pseudo-cavern around the mixing head and how it scales with the Reynolds number have also been investigated, showing that there is a minimum speed limit below which the rotor-stator cannot be used.     

CONFINED COAXIAL JET FLOWS INTO A COLD MODEL OF CVD CHAMBER

The length of recirculation zone is an important factor in the design of CVD chambers for manufacturing monolithic infrared optical materials. The recirculation length of confined coaxial air jet flows, which consist of a central tubular flow and a surrounding annular flow, into a cold model of CVD chamber at atmospheric pressure and room temperature were studied numerically by a two-dimensional Galerkin finite element method and experimentally by observing the streamlines of methylamino chloride particles. In the experiments, the tubular Reynolds number varies from 180 to 600 and the annular Reynolds number varies from 37.0 to 165.1. The simulation and experimental results show that the length of recirculation zone increased initially with the increase of either tubular or annular Reynolds number. Nevertheless, when either one of the tubular and annular Reynolds numbers was relatively high, or both Reynolds numbers were at relatively medium values, small waves occurred, laminar flow could not be maintained, and the observed recirculation length dropped dramatically from 10.4-15.7 Dh to 3.7-4.6 D,, where Dk is the hydraulic diameter of the chamber. Suggestions of the flow conditions and chamber length for the design of CVD chambers are given to make most of the chamber length occupied by the recirculation zone so that an uniform deposition of materials could be obtained.     

TURBULENT FLOW IN STIRRED TANKS: SCALE-UP COMPUTATIONS FOR VESSEL HYDRODYNAMICS

A numeric model for turbulent flow was used to compute the flow patterns in Rushton turbine agitated vessels. The cases considered cover two orders of magnitude for the three different scaling criteria of constant impeller Reynolds number, power input per unit mass, and impeller tip speed. The constant power input scale-up criteria maintains the turbulence levels throughout the vessel during scale-up. The circulation times, however, increase with vessel size for this scaling criteria. The other scaling criteria of constant impeller tip speed and impeller Reyonds number lead to decreasing turbulence levels in the tank along with further increases in circulation times.     

Laminar Flow and Heat Transfer Characteristics in Jackets of Triangular Flow Channels

Laminar flow and heat transfer characteristics of jacketed vessel with triangular flow channels were numerically studied under hydrodynamically and thermally fully developed conditions. Constant heat flux at theheated wall was assumed. The numerical program code interms of vorticity, stream function, axial velocity com ponent and energy equations was written based on a finite volume method. Based on the numerical results, the flow and temperature field were given, and the effects of Dean and Prandtl numbers on flow and heat transfer were ex amined, and the correlations of flow resistance and mean Nusselt number were developed for the jacket. The results show that the structure of secondary flow is steady two vortices in the investigated range of dimensionless curvatureratio and Reynolds number. Two peaks of local Nusselt number increase significantly with Prandtl and Dean num ber increasing, but the local Nusselt numbers near two ends and at the center of the heated wall increase only slightly. The center and two ends of heated wall are the poor positions for heat transfer in the jacket. Compared with the outer half coil jacket at the same area of heated wall, curvature radius, Reynolds number and Prandtl number, e jacket of triangular flow chmnel has lower flow resistance and less mean Nusselt number.     

柔性Rushton搅拌桨的功耗与流场特性研究

基于传统的Rushton桨,开发了一种柔性叶片Rushton搅拌桨。采用数值模拟方法研究了柔性桨的功耗及层流和湍流流场特性,并分别采用扭矩测量法和粒子图像测速法进行了实验验证。结果表明,对于实验规模的搅拌容器,当介质黏度与甘油接近时,可用橡胶作为柔性桨叶制作材料。Reynolds数≤100时,柔性桨的功耗大于刚性桨;Reynolds数大于该值后,柔性桨的功耗小于刚性桨。柔性桨叶对被搅拌流体具有自适应特性,流固耦合作用下产生的变形增加了流体的径向流动能力。搅拌低黏度流体时,柔性桨能提升近桨区流体的速度,增加桨叶远端流体的循环流动能力;搅拌高黏度流体时,近桨区和桨叶远端流体的速度均大于刚性桨。就尾涡而言,柔性桨产生的涡量较小,耗能少。     

端面微尺度效应和热黏效应对干气密封性能的影响

考虑干气密封环端面微尺度效应及气膜的热黏效应,基于气体多变过程理论和气体润滑理论,对可压缩流体条件下的扩展平均雷诺方程进行修正,并应用有限元法进行求解;通过改变密封的操作参数,分析研究了端面微尺度效应和热黏效应对干气密封性能的影响规律。结果表明,干气密封在正常工作状态下,压力的升高使气体滑移流和表面粗糙度之间的耦合效应减弱,对泄漏率有固定的影响。若量纲1膜厚H_s<3.5,应考虑表面粗糙度对密封性能的影响;当逆Knudsen数B≥21或Knudsen数Kn≤0.0476时,可忽略气体滑移流效应的影响;热黏效应和表面粗糙度能提高密封动压效应,降低泄漏率,而气体滑移流效应却使泄漏率增大,端面承载能力降低;在低速高压下,表面粗糙度比滑移流效应对密封性能的影响大;当外压p_o≥2.02 MPa时,热黏效应在对密封性能影响中占主导地位。     

TWO DIMENSIONAL MODEL ANALYSIS OF TURBULENT FLOW IN AN AGITATED VESSEL WITH PADDLE IMPELLER*

A two-dimensional model for the turbulent flow in an agitated vessel is developed by modifying a two-dimensional laminar flow model. The proposed model is based on the assumption that the tangential velocity adjacent to the vessel wall satisfies the slip condition. Specifically, it is assumed that the tangential velocity is inversely proportional to the radius. The eddy diffusivity of momentum is assumed to be constant. The eddy diffusivity calculated from the model is in good agreement with the available experimental data, while the calculated tangential velocity depends on the Reynolds number in a manner similar to that observed experimentally.     

Mass/heat transfer from a neutrally buoyant sphere in simple shear flow at finite Reynolds and Peclet numbers

Theoretical analyses of mass/heat transfer from a neutrally buoyant particle in simple shear flow indicate that mass/heat must diffuse across a region of closed streamlines of finite thickness at zero Reynolds number, whereas spiraling streamlines allow the formation of a thin mass transfer boundary layer at small but non‐zero Reynolds numbers (Subramanian and Koch, Phys Rev Lett. 2006;96:134503; Subramanian and Koch, Phys Fluids. 2006;18: 073302). This article presents the first numerical results for mass/heat transfer at finite Reynolds and Peclet numbers. The simulations indicate that fluid particles in the flow‐gradient plane spiral away from the particle for Reynolds numbers smaller than about 2.5 while they spiral toward the particle for higher Reynolds numbers. Solutions of the Navier‐Stokes equations coupled with a boundary layer analysis of mass transfer yield predictions for the rate of mass transfer at asymptotically large Peclet numbers and Reynolds numbers up to 10. Simulations of mass transfer for zero Reynolds number and finite Peclet numbers confirm Acrivos' (Acrivos, J Fluid Mech. 1971;46:233–240) prediction that the Nusselt number approaches a finite value with increasing Peclet number. Simulations at finite Reynolds numbers and Peclet numbers up to 10,000 confirm the theoretical predictions for the concentration gradient at the particle surface at angular positions away from the flow‐gradient plane. However, the wake near the flow‐gradient plane remains too large at this Peclet number to yield a quantitative agreement of the overall rate of mass transfer with the theory for asymptotically large Peclet number. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Velocity profiles and frictional pressure drop for shear thinning materials in lid-driven cavities with fully developed axial flow

A finite element numerical study has been carried out on the isothermal flow of power law fluids in lid-driven cavities with axial throughflow. The effects of the tangential flow Reynolds number (Re_U), axial flow Reynolds number (Re_W), cavity aspect ratio and shear thinning property of the fluids on tangential and axial velocity distributions and the frictional pressure drop are studied. Where comparison is possible, very good agreement is found between current numerical results and published asymptotic and numerical results. For shear thinning materials in long thin cavities in the tangential flow dominated flow regime, the numerical results show that the frictional pressure drop lies between two extreme conditions, namely the results for duct flow and analytical results from lubrication theory. For shear thinning materials in a lid-driven cavity, the interaction between the tangential flow and axial flow is very complex because the flow is dependent on the flow Reynolds numbers and the ratio of the average axial velocity and the lid velocity. For both Newtonian and shear thinning fluids, the axial velocity peak is shifted and the frictional pressure drop is increased with increasing tangential flow Reynolds number. The results are highly relevant to industrial devices such as screw extruders and scraped surface heat exchangers.