Flujo De Fluidos

Páginas: 5 (1018 palabras) Publicado: 1 de octubre de 2012
Shear rate: Shear rate is reflection of the process characteristics as opposed to the process liquid properties themselves and must be quantified separately. Imagine that the fluid you want to test is sandwiched between two plates separated by a now distance. Keep the bottom plate stationary and move the top plate at a defined velocity. Shear rate is the ratio of the moving plate velocity, V , tothe distance separating the plates , X.
Shear rate= V/X
The unit of measurement for shear rate is reciprocal second or S^-1 . This approach to quantifying shear rate assumes that the fluid behaves in a uniform way as shown by the arrows in figure 7. The layers of fluid molecules slide over each other in a repetitive fashion such that the closer you get to the moving plate , the faster thelayer is moving .
Table 2 gives several examples of shearing action and the shear rate associated with each. The thing to recognize is that you can easily compute the relevant shear rates for your own situation. Consider the shearing action in a mixer. The blades are equivalent to the moving plate and the side wall is the stationary plate. The angular velocity of the blades is easily computed basedon the rotational speed of the mixer; the distance from the blade to the side wall is a known dimension provided by the manufacturer. The equivalent shear rate is simply the ratio of those two numbers.
A similar calculation can be done for the shear rate of fluid flow in a pipe, as shown in figure 8, assuming that the velocity profile of the fluid changes linearly from the pipe wall to the centerof the pipe where maximum velocity occurs. Although a linear change in velocity profile is not the usual case, it provides a convenient starting point for an initial calculation.
Material behavior.
With these fundamental concepts in place, the important conclusion to recognize is that many CPI materials are non-Newtonian and will change in viscosity as the shear rate is adjusted, in otherwords, viscosity is not a single number for them.
Pseudoplasticity: The most common type of viscosity behavior is pseudoplasticity, where a material is viscosity decreases as the shear rate increases. This behavior can be characterized by a flow curve or rheogram (figure 9). Many of the materials mentioned in this article, with the exception of water and select petroleum refining products, arepseudoplastic. Water is always 1 cP at 20°C no matter what shear rate are referred to as Newtonian (in honor of Isaac newton who fist postulated the parametric relationships discussed earlier in this article), those which do change are called non-Newtonian.
Thixotropy: a related issue that affects measured viscosity values is the length of time for the shearing action.
If a material is sheared at aconstant rate and the viscosity decreases over time, we call that thixotropy. Some materials exhibit thixotropic behavior, but recover completely once the shearing action stops. This is important because the material will not behave as expected by the end user if the loss in viscosity is permanent. For pseudoplastic materials , the way to observe whether thixotropy occurs is to run an up/downtest (increase the spindle speed, then decrease the spindle speed ) and see if the data for the flow curves are separate , as shown in figure 10. The spatial area between the curves is an indicator of how significant the thixotropic behavior may be.
If the curves lie on top of each other or are relatively close together , there is little to no thixotropy.
Temperature: temperature is another keyparameter that must be considered when measuring viscosity . As temperature increases , most materials exhibit a decrease in viscosity . So it is important to define the temperature at which the viscosity measurement is made to ensure consistency of results .
When new materials are evaluated , it is common to perform a temperature profile test. Run a series of up/down shear ramps at discrete...
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