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size was chosen to be the time necessary to rotate the inner
impellers one degree (one revolution required 360 time
steps). This criterion was used to determine the time step
size for all cases studied in this work. Further reductions of
the time step did not show any significant influence on the
calculated torque.
A number of revolutions of the impellers were necessary to
yield a quasi-steady state solution. In most cases, after three
revolutions of the inner impellers, a periodic solution was
achieved. For example, when the inner impellers were rotating
at 93 rpm, the time step was set to 0.0018 s. This required 48 h
computational time to yield the periodic flow field solution,
when ran in parallel on a XEONw 3.2 GHz dual processor,
3.25 GB of RAM and Windowsw XP as operational system.
RESULTS AND ANALYSES
Impeller power curves are determined by measurement of
torque and impeller rotational speed. On a log–log plot, the
Newton number (Ne) decreases linearly with the Reynolds
Number (Re) in the laminar regime. The slope of the curve
is 21, so that KP ¼ Ne . Re.
Because the Reynolds number is dependent on the vis-
cosity, and the viscosity of non-Newtonian fluids is a function
of the shear rate, and thus of the rotational speed of the
impellers, the determination of an apparent viscosity is an
important task in the design and characterization of agitation
in non-Newtonian fluids. The Metzner and Otto (1957)摘要：一项实验和数值方案对同轴搅拌系统的设计和混合性能特性进行了探索和确定。牛顿和非牛顿流体层流制度对同轴搅拌机在化学过程中的构造进行了讨论并分析了功率消耗。锚叶轮与双斜叶涡轮在同向旋转模式中组合使用。据研究表明接近叶轮的功率消耗受叶尖速比的影响，但也有人认为接近叶轮的速度对斜叶涡轮的功率没有任何影响。计算流体动力学（CFD）被用来计算由同轴搅拌机所创造的流场。CFD 是能够很好预测同轴搅拌系统的功率消耗。功率和雷诺数被用于获得同轴搅拌机的功率特性。采用该方法以成功获得同轴搅拌机构造的一个主功率曲线，并且是一个有用的工程工具用来预测同轴搅拌系统的功率消耗。
关键词：同轴搅拌机；非牛顿流体；计算流体动力学（CFD）；功率消耗；锚；A200
1介绍
在化学加工行业中搅拌槽是最常用的流体混合装置。有效的混合对于产品质量是至关重要的，减少副产物的形成，悬浮固体，传热和传质。当流体粘度在制造过程中增加，搅拌机的设计在工业应用中尤其具有挑战性。同轴叶轮系统非常有前途替代这个过程因为两个独立转动的叶轮上相同的反应器轴的协同流体动力学效应。一个常见的同轴结构包括高速叶轮组合和关闭间隙叶轮。在传统的混合系统中，关闭间隙叶轮或接近叶轮主要用于粘性流体的混合并且增加频繁交换靠近器壁的材料的传热率。在低粘度流体中，他们的混合效率是非常差因为切向运动其主要的促使作用。这将导致一个实心体旋转由于缺乏挡板而使切向流偏转。固体转动也会发生在低粘度无涡流槽的开式叶轮中。在一个同轴混合系统中，有令人困惑的低粘度，接近叶轮可以作为开式叶轮的挡板并初级混合的工作。在高粘度时，内叶轮将逐渐失去效率而外叶轮将会起主要的混合工作。