Diffusers simple pipe

Figure 2-6 Simple pipe diffuser made from Schedule 80 PVC...

Solution. The transverse diffusion rate is controlled by the relatively slow crystal diffusion rate because the diffusing atoms must traverse the patches of perfect crystal between the dislocation pipes. Therefore, when the dislocations are discretely spaced, a good approximation is the simple result... 

Advection and diffusion in a uni-directional shear flow of the form v = [u(y), 0] is a classical problem studied by Taylor (1953), considered as a simple model for the dispersion of a contaminant in a river or in a flow through a pipe (Fig. 2.3). 

The eddy diffusitives for momentum and heat, and Ejj, respectively, are not properties of the fluid but depend on the conditions of flow, especially on all factors that affect turbulence. For simple analogies, it is sometimes assumed that and jf are both constants and equal, but when determined by actual velocity and temperature measurements, both are found to be functions of the Reynolds number, the Prandtl number, and position in the tube cross section. Precise measurement of the eddy diffusivities is diflScult, and not all reported measurements agree. Results are given in standard treatises. The ratio Sh/sm also varies but is more nearly constant than the individual quantities. The ratio is denoted by i/f. For ordinary liquids, where Np > 0.6, is close to 1 at the tube wall and in boundary layers generally and approaches 2 in turbulent wakes. For liquid metals is low near the wall, passes through a maximum of about unity at j/r X 0.2, and decreases toward the center of the pipe. ... 

Diffusers are used to spread the chemical and to provide mixing energy. There are many different types of diffusers. Ozone and carbon dioxide feed systems commonly use fine bubble diffusers. Chlorine diffusers (Figures 2-5, 2-6, and 2-7) are often perforated pipe or even a simple open pipe. Many liquid chemicals are delivered into the process flow with diffusers. It is important to spread the chemical evenly in the water and to provide adequate mixing. 

This simple geometry provides us with an opportunity to explore the importance of various mixing mechanisms. We have already seen the disastrous impact of poor bulk mixing. Now, taking the length scale of the pipe, the problem is broken down into simplified models of pure molecular diffusion and pure eddy dispersion to clarify the interactions between these mixing mechanisms. 

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