Mixing eddy motion

Turbulent flow, by means of the chaotic eddy motion associated with velocity fluctuation, is conducive to rapid mixing and, therefore, is the preferred flow regime for mixing. Laminar mixing is carried out when high viscosity makes turbulent flow impractical. 

Mixing is accomplished by the rotating action of an impeller in the continuous fluid. This action shears the fluid, setting up eddies w hich move through the body of the system. In general the fluid motion involves (a) the mass of the fluid over large distances and (b) the small scale eddy motion or turbulence which moves the fluid over short distances [21, 15]. 

In polymer processing, because of the very high viscosities of polymeric melts, the flow is laminar and eddy motion due to turbulence is absent therefore, it cannot contribute to mixing. Similarly, molecular diffusion does not contribute much to mixing because it occurs extremely slowly. We are therefore left with convection as the dominant mixing mechanism.2... 

Turbulent and laminar mixing are quite different. Turbulent mixing takes place by three mechanisms turbulent eddy motion, bulk or convective flow, and molecular diffusion. Laminar flow has no eddies to assist in mixing. Laminar mixing first depends on creating very thin layers between initially unmixed components, followed by molecular diffusion. 

Molecular diffusion results from concentration gradients and takes place spontaneously. Eddy motion is caused by turbulent flow, and bulk flow is caused by large-scale matter flows. In polymer processing, the viscosity of polymer melts is very high. How is laminar and eddy motion owing to turbulence is absent. Molecular diffusion is too slow. Convection is thus the dominant mixing process. 

It is an empirical fact that a fluid flowing in a small tube or at low velocity does so by the mechanism of laminar flow, also called viscons, or streamline, flow. The layers of fluid slide over each other with no macroscopic mixing, and the velocity in macroscopic steady flow is constant at any point. At higher velocities flow becomes tnrbulent there is mixing by eddy motion between the layers, and even in overall steady flow the velocity at a point fluctuates about some mean value. 

Macromixing The phenomenon whereby residence times of clumps are distributed about a mean value. Mixing on a scale greater than the minimum eddy size or minimum striation thickness, by laminar or turbulent motion. 

For an incompressible viscous fluid (such as the atmosphere) there are two types of flow behaviour 1) Laminar, in which the flow is uniform and regular, and 2) Turbulent, which is characterized by dynamic mixing with random subflows referred to as turbulent eddies. Which of these two flow types occurs depends on the ratio of the strengths of two types of forces governing the motion lossless inertial forces and dissipative viscous forces. The ratio is characterized by the dimensionless Reynolds number Re. 

In the mid-latitude region depicted in Fig. 7-5, the motion is characterized by large-scale eddy transport." Here the "eddies" are recognizable as ordinary high- and low-pressure weather systems, typically about 10 km in horizontal dimension. These eddies actually mix air from the polar regions with air from nearer the equator. At times, air parcels with different water content, different chemical composition and different thermodynamic characteristics are brought into contact. When cold dry air is mixed with warm moist air, clouds and precipitation occur. A frontal system is said to exist. Two such frontal systems are depicted in Fig. 7-5 (heavy lines in the midwest and southeast). 

In seawater, physical processes that transport water can also cause mass fluxes and, hence, are another means by which the salinity of seawater can be conservatively altered. The physical processes responsible for water movement within the ocean are turbulent mixing and water-mass advection. Turbulent mixing has been observed to follow Pick s first law and, hence, is also known as eddy diffusion. The rate at which solutes are transported by turbulent mixing and advection is usually much faster than that of molecular diffusion. Exceptions to this occur in locations where water motion is relatively slow, such as the pore waters of marine sediments. The effects of advection and turbulent mixing on the transport of chemicals are discussed further in Chapter 4. 

The pattern and rate of exchange of stratospheric air with the mesosphere, with the troposphere, and between hemispheres is still poorly known. Within the stratosphere a complex pattern of eddy mixing and mean motions governs the redistribution of trace constituents. In the lower stratosphere the adsorption or attachment of trace constituents to natural sulfate particles influences their subsequent transport. Some main features in stratospheric transport processes based on tracer studies are reviewed here. 

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