Microscale, mixing

Const, (power) dissipation energy per unit vessel volume Const, impeller discharge flow energy Turbulent dispersion Gas-liquid operation Reaction requiring microscale mixing... 

Microscale shear rates, microscale mixing Any particles or fluid elements on the order of 100 fxm or less respond primarily to the fluemating velocity components... 

In addition, the turbulent fluctuations set up a microscale type of shear rate. Microscale mixing tends to affect particles that are less than 100 /xm in size. The scaleup rules are quite different for macroscale controlled process in comparison to microscale. For example, in microscale processes, the major variables are the power per unit volume dissipated in various points in the vessel and the total average power per unit volume. In macroscale mixing, the energy level is important, as well as the geometry and design of the impeller blades and the way that they set up macroscale shear rates in the tank. 

The lower horsepower is an important factor in the efficient design of axial flow or fluidfoil impellers. Such lower horsepower must be considered in the efficient design involving fluid velocity and overall macroscale mixing phenomena. On the other hand, if the process involves a certain amount of microscale mixing, or certain amounts of shear rate, then the fluidfoil impeller may not be the best choice. 

Smaller particles primarily see only the fluctuating velocity component. When the particle size is much less than 100 /zm, the turbulent properties of the fluid become important. This is the definition of the boundary size for microscale mixing. 

All of the power applied by a mixer to a fluid through the impeller appears as heat. The conversion of power to heat is through viscous shear and is approximately 2500 Btu/hr/hp. Viscous shear is present in turbulent flow only at the microscale level. As a result, the power per unit volume is a major component of the phenomena of microscale mixing. At a l-/zm level, in fact, it doesn t matter what specific impeller design is used to apply the power. 

The ratio of the rms velocity fluctuation to the average velocity in the impeller zone is about 50% with many open impellers. If the rms velocity fluctuation is divided by the average velocity in the rest of the vessel, however, the ratio is on the order of 5-10%. This is also the level of rms velocity fluctuation to the mean velocity in pipeline flow. There are phenomena in microscale mixing that can occur in mixing tanks that do not occur in pipeline reactors. Whether this is good or bad depends upon the process requirements. 

Recently, one of the most practical results of these studies has been the ability to design pilot plant experiments (and, in many cases, plant-scale experiments) that can establish the sensitivity of process to macroscale mixing variables (as a function of power, pumping capacity, impeller diameter, impeller tip speeds, and macroscale shear rates) in contrast to microscale mixing variables (which are relative to power per unit volume, rms velocity fluctuations, and some estimation of the size of the microscale eddies). 

For homogeneous chemical reactions, most of the effect of the mixer occurs at the microscale level. Microscale mixing is largely a function of the power per unit volume, and maintaining equal power per unit volume gives similar... 

If separation of the microscale mixing phenomenon from the macroscale mixing phenomenon is desired, then it is necessary to systematically vary the ratio of blade width to blade diameter. 

Analysis of correlation length and density fluctuations in pure and solvent-modified SCFs, in particular around the mixture critical curve (P - PJ. Also measurement of the mean nuclei size and microscale mixing segregation (8). 

The macroscale environment is effected by every geometric variable and dimension and is a key parameter for successful scaleup of any process, whether microscale mixing is involved or not. This has some unfortunate consequences on scaleup since geometric similarity causes many other parameters to change in unusual ways, which may be either beneficial or... 

A passive micromixer is one of the microfluidic devices. It utilizes no energy input except the mechanism (pressure head) used to drive the fluid flow at a constant rate. Due to the dominating laminar flow on the microscale, mixing in passive micromixers relies mainly on chaotic advection realized by manipulating the laminar flow in microchannels or molecular diffusion with increasing the contact surface and time between the different fluid flows. 

Paterson, A. H. J., andR. J. Kerekes (1985). Fundamentals of mixing in pulp suspensions measurement of microscale mixing of chlorine, J. Pulp Paper ScL, 11(4), J108-J113. 

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