各向异性k-ε湍流模型在Rushton桨搅拌槽三维流场整体数值模拟中的应用

根据搅拌槽内的流动呈各向异性的特点,引入适用于强旋转流场的各向异性k-ε湍流模型,用改进的内外迭代法对有挡板的Rushton桨搅拌槽进行了整体数值模拟.利用文献中对搅拌槽内流场测定结果,给出了适用于Rushton 桨搅拌槽的各向异性湍流黏度系数值.模拟计算得到了搅拌槽内的流场分布和脉动速度分布,并同标准k-e湍流模型计算结果及文献数据进行比较.结果表明,各向异性k-ε湍流模型能成功反映Reynolds应力、湍流动能等湍流特征量,明显优于标准k-ε湍流模型.     

各向异性k-ε湍流模型在Rushton桨搅拌槽三维流场整体数值模拟中的应用

根据搅拌槽内的流动呈各向异性的特点 ,引入适用于强旋转流场的各向异性k -ε湍流模型 ,用改进的内外迭代法对有挡板的Rushton桨搅拌槽进行了整体数值模拟 .利用文献中对搅拌槽内流场测定结果 ,给出了适用于Rushton桨搅拌槽的各向异性湍流黏度系数值 .模拟计算得到了搅拌槽内的流场分布和脉动速度分布 ,并同标准k -ε湍流模型计算结果及文献数据进行比较 .结果表明 ,各向异性k -ε湍流模型能成功反映Reynolds应力、湍流动能等湍流特征量 ,明显优于标准k -ε湍流模型 .     

双层圆盘涡轮式搅拌器的CFX流场模拟

利用最新流体力学软件ANSYS-CFX12.0对双层六直叶圆盘涡轮搅拌槽内流场进行了数值模拟。采用湍流模型成功的模拟了搅拌槽内的流动,并与对应实验结果进行了对比;考察了同转速,不同搅拌层间距、底搅拌层离底高度下的流场分布,给出了湍流动能分布图,对其搅拌效果的影响进行分析,为该领域研究者提供了借鉴。     

轴流桨搅拌槽内完全湍流的研究

为研究轴流桨搅拌槽内完全湍流状况,采用相位多普勒粒子分析仪(PDPA)对MK和ZHX搅拌器进行流场测试,得到不同工况下的时均速度场分布.应用渐近不变性方法,选取适当的特征尺度,给出挡板处壁面射流的轴向速度相似剖面,确定用于评定槽内完全湍流界限的轴向速度分布曲线,建立了搅拌雷诺数与搅拌槽内完全湍流流动达到的高度之间的线性关系.结果表明,非全槽完全湍流状态下,槽上部会出现过渡流区;随雷诺数的增大,搅拌槽内完全湍流流动达到的高度增大;不同型式搅拌器的完全湍流流场所需的雷诺数不同,单层桨搅拌槽内达到全槽完全湍流需要很大的搅拌功率.     

无挡板涡轮桨搅拌槽内湍流流动的分离涡模拟

采用分离涡模型对无挡板涡轮桨搅拌槽内的湍流流动进行了研究,重点分析了流场结构和速度分布,以检验该模型模拟搅拌槽内流体流动的有效性和正确性. 为了加快收敛,先采用标准k-e模型进行稳态流场计算,并以此结果为初始值进行分离涡模拟. 与现有文献大涡模拟及实验结果对比表明,分离涡模型能捕捉槽内流体的瞬时流动特征,获得的时均速度分布与大涡模拟及实验结果吻合较好,其中对切向速度分布的预测误差不超过7%,对径向速度分布的预测精度则低一些,局部误差接近12%. 分离涡模型适用于无挡板涡轮桨搅拌槽内湍流流动的模拟,能获得与大涡模拟相近的结果,且计算量更小(约为大涡模拟的1/3).     

涡轮桨搅拌槽内流动特性的大涡模拟

利用大涡模拟方法研究了涡轮桨搅拌槽内的流动特性,采用了三种亚格子模式:标准Smagorinsky-Lilly模式(SLM)、Smagorinsky-Lilly动力模式(DSLM)和亚格子动能动力模式(DKEM),并将模拟结果与标准k-ε模型及文献实验数据进行了详细的比较.结果表明:大涡模拟方法可获得搅拌槽内的瞬态流场;对桨叶区时均速度及湍流动能的预测与实验数据相吻合,比标准k-ε模型计算结果有明显改进,三种亚格子模型中DSLM和DKEM模拟结果更好.同时分析了大涡模拟中桨叶端部附近湍流动能估计偏差的原因,发现主要是由于对轴向湍流均方根速度的预测偏差造成的.大涡模拟方法为搅拌槽内非稳态、周期性的湍流流动和湍流特性的研究提供了强有力的工具.     

涡轮桨搅拌槽内湍流尺度的数值模拟

本文采用了多重参考系法结合标准κ~ε湍流模型模拟了槽径为270 mm、叶轮直径为93 mm的搅拌槽内的湍流流场。通过考察搅拌槽内的湍流强度、湍流尺度及两者之间的相互关系,是以一个新的角度从更深层次上研究搅拌槽内的湍流特性。研究表明了湍流强度及湍流尺度在搅拌槽内的分布;并得到在湍流耗散区尺度大的区域,湍流强度大;而在含能区尺度大的区域,湍流强度小;耗散区尺度与湍流强度相关性大。     

搅拌槽内单相湍流流场数值模拟研究进展

搅拌槽内的流场是决定混合、传热及传质等操作的基础,对流场的研究具有十分重要的意义,计算流体动力学是研究流场的重要方法。本文回顾了搅拌数值模型的发展历程,阐述了三十年来各种搅拌流场数值模拟方法的特点及其应用情况,对比分析了各种湍流模型的优缺点,并展望了未来搅拌槽内单相湍流模型的发展方向。     

搅拌槽内湍流流场的数值模拟

搅拌槽在过程工业中的应用非常广泛,对搅拌槽内流场的研究具有十分重要的意义。实验研究方法具有局限性,基于计算流体动力学的数值模拟方法是研究流场的重要方法。文中介绍了搅拌流场的数值模拟方法和湍流模型,分析了各自的特点及其应用情况,总结了搅拌模拟时边界条件的处理情况,并指出了未来的发展方向。     

涡轮桨搅拌槽内湍流特性的V3V实验及大涡模拟

分别用体三维速度测量技术（volumetric three-component velocimetry measurements,V3V）和大涡模拟（large eddy simulation,LES）方法对涡轮桨搅拌槽内流场进行研究,发现在完全湍流状态下,涡轮桨搅拌槽内流场的量纲1相平均速度及湍动能分布同Reynolds数无关。用V3V方法实现了Rushton桨叶附近三维流场的重构;探讨尾涡的三维结构及运动规律;分析了叶片后方30°截面轴向、径向和环向速度沿径向分布规律。用V3V实验结果对比了2D-PIV（particle image velocimetry）数据中的尾涡涡对位置和涡量,涡对位置吻合度较好,但2D-PIV中涡量较V3V小37.5%;通过大涡模拟得到完整的尾涡结构,发现在叶片上边缘后侧存在一个和尾涡形成方式相同但不成对出现的涡结构;将大涡模拟结果和2D-PIV及V3V实验结果对比发现,大涡模拟在速度分布及尾涡运动轨迹方面均同实验结果吻合较好,表明大涡模拟能较好地预测涡轮桨搅拌槽内流场。     

Extraction into a flow of a fluid with a velocity gradient

Unsteady extraction from a system of semi-infinite capillaries to the flow of a fluid with a linear velocity gradient is studied. An assumption that the diffusion in the flow is a quasi-steady-state process is taken to obtain a linear equation for the local mass flux from the pore space, which involves fractional derivatives with respect to time and flow coordinate. Limiting solutions for the total diffusion flux at small and large times that cover the entire time interval are found.     

Electrolytes: From a Thorn Comes a Rose,and from a Rose,a Thorn

In this article we take a stroll through the metaphorical rose garden, part of the flower garden, that is electrochemical storage systems. We take a closer look at batteries as the center‐pieces – the roses – and especially at the system component of batteries, the electrolyte and its development, throughout history. Just as a rose can only develop its unique beauty when taken good care of and provided with clean water and suitable nutrients, a battery cell cannot function without its electrolyte; the seemingly trivial component that has to juggle a multitude of requirements in order to make batteries truly bloom.     

Propagation of a burning front over a condensed material from a focus

Measured propagation speeds are given for the combustion front in a blind gap in a condensed substance, which are related to gap height and mean pressure. The front may propagate monotonically, which is characteristic of relatively high pressures and wide gaps, or as a result of additional burning foci arising at a certain distance from the main front. At low pressures and particularly with small gap heights, the front propagates in an unstable fashion over the surface.Moscow. Translated from Fizika Goreniya i Vzryva, Vol. 28, No. 4, pp. 49–53, July–August, 1992.     

Mesomorphic transitions in a mixture of a flexible and a semi-rigid polymer

Phase studies on a mixture of two polymers are presented, one of which is mesomorphic. We have systematically examined the influence of the molecular weight of the flexible polymer (including the oligomers) and of the semi-flexible polymer. In addition to the effect of the molecular weight, specific interactions are important for compatibility and formation of a homogeneous mesomorphic phase. The nature of this phase is demonstrated to be cholesteric and the pitch is determined.     

When is a Beam a Plate?

Formulas associated with simple beam or plate theories are used extensively in structural design, to determine You's modulus, and to determine fracture strength of brittle nonmetallic materials. The question may arise as to whether the proper ratio of beam-width-to-beam-depth is such that the bent structural element can be considered as a beam and, if not, what is the correction factor that should be used with the simple beam formula. These questions are answered for a range of structural metallic and brittle nonmetallic materials.     

Laminar flow deformation of a droplet adhering to a wall in a channel

We study the deformation of oil droplets of varying viscosity adhering to a solid, rigid wall in a rectangular channel induced by a pressure-driven, laminar flow of water. Our experimental investigation involves the systematic study of the effects of initial droplet volume, flow rate and viscosity ratio on the various modes of droplet motion and deformation; this is performed via direct visualisation of the flow in a glass cell using a high-speed camera. Our results are used to construct flow maps that delineate the critical conditions for droplet ‘sliding’, ‘crawling’ and detachment from the channel wall as a function of system parameters. Comparisons of our experimental results with numerical predictions obtained using a procedure based on the diffuse-interface method yield reasonably good agreement for certain flow regimes.     

Detonation of a Column of a Chemically Active Bubbly Medium in a Liquid

The problem of a detonation wave propagating in a cylindrical column of a chemically active bubbly medium screened by a liquid from the tube walls is formulated and numerically solved within the framework of the Iordanskii–Kogarko two-phase model with allowance for energy dissipation due to acoustic radiation of bubbles. The wave structure of the reaction zone and the detonation velocity of the bubbly medium column are calculated. It is found that the self-sustaining wave can propagate with a velocity greater than the velocity of one-dimensional bubble detonation by a factor of 1.5–2.5.