Mixing of two fluids

This method examines the degree of mixing of two fluids on the molecular scale by observing the rate of a second-order reaction in the mixture. The method gives the mean square concentration of component A in a nonhomo-geneous mixture of A and B and the actual distribution of deviations from the mean concentration in the mixture. 

To this point we have limited onr consideration to mass diffitsion in a station aiy medium, and thus the only ntotion involved was the creeping motion of molecules in the direction of decreasing concentration, and there was no motion of the mixture as a whole. Many practical problems, such as the evaporation of water from a lake under the iiifliience of the wind or the mixing of two fluids as they flow in a pipe, involve diffusion in a moving medium where the hoik motion i.s caused by an external force. Mass diffusion in such c.nses is complicated by the fact that chemical species are transported both by diffusion and by the bulk motion of the medium (i.e., convection). The velocities and mass flow rates of species in a moving medium consist of two components one due to molecular diffusion and one due to convection (Fig. 14-29). 

Wilkinson and Cliff made an experimental study of the production of striations by measuring temperature profiles for the mixing of two fluids of different temperature but used a mixing index incorporating both variance and striation numbers to describe their data ... 

Enhancement of the heat transfer rate can be obtained by the generation of Dean vortices at bends in ducts. These can be bends in two-dimensional ducts [30] or in three-dimensional structures [32]. The generation of Dean vortices was originally investigated as a means of enhancing mixing of two fluid streams but applies equally well to the mixing ofhot and cold fluids. The authors cited above studied the effect of the Dean number, deflned by... 

The problem of the mixing of two fluids with the same physical properties has been discussed. Such an approach could lead to the definition of a numerical mixing time and the geometries most adapted to rapid mixing of two fluids (in particular, to find the best location for injection of the second liquid). It would be necessary to carry out the same kind of work in a real 3-D flow. 

Motionless mixers. Mixing of two fluids can also be accomplished in motionless mixers with no moving parts. In such commercial devices stationary elements inside a pipe successively divide portions of the stream and then recombine these portions. In one type a short helical element divides the stream in two and rotates it 180°. The second element set at 90° to the first again divides the stream in two. For each element there are 2 divisions and recombinations, or 2" for n elements in series. For 20 elements about 10 divisions occur. 

Miscible Mixable. Describes full mixing of two fluids without dissolution of one into the other. Soluble = dissolves in, miscible = mixes into. The converse, immiscible, refers to a lack of mixing ability, as seen in the case of oil and water. 

Figure 2 Mixing of two fluids according to the iattice fluid modei.

Figure 12.6 Different length and timescales for the mixing of two fluid streams. The precipitation process is governed by both the mixing on the large scale macromixing) and the processes on the molecular level, when the mixing is nearly complete.

Figure 7-3 Homogeneity with laminar mixing of two fluids (shown with an SMX mixer). (Courtesy of Koch-Glitsch, LP.)...

Figure 5.54 Different protocols for mixing of two fluids (a) continuous flow, (b) plug flow, and (c) open-surface manipulation...

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