February 19, David Kelly Non-Newtonian fluids are encountered in a wide and varied range of industrial processes including food, mining and minerals, pharmaceuticals, pulp and paper processing and other products in the form of pastes, slurries, concentrated solid suspensions or emulsions. Examples include coal slurries, mud, sludge, paint, ketchup, mayonnaise, blood, shaving cream, toothpaste, dilute solutions etc. These fluids regularly exhibit non-Newtonian viscous properties and it is important for the designer to be familiar with the flow behaviour of such fluids, how to characterise the fluid physical properties of these fluids and how to use these properties to predict flow behaviour in process piping systems. Newtonian fluids exhibit a linear relationship between shear rate and shear stress. Non-Newtonian fluids on the other hand typically exhibit either shear thinning or shear thickening behaviour and in some cases, exhibit a yield stress, i.
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Before this publication, viscosity corrections were done using viscosity correction charts. The new method is much easier to use because it calculates the viscosity corrections automatically by using the formulas provided.
The need to "eyeball" from charts has been removed, so the new method is more accurate. The standard also provides a nice background description of the method as well as its limitations and the pump types and pump design characteristics for which the procedure is applicable. The following came as a result of a discussion between several of our readers. What Are Newtonian Fluids? Can we assume that crude oil or refined products behave as Newtonian fluids? A key statement is "The viscosity is a function only of the condition of the fluid, particularly its temperature.
Examples of non-Newtonian fluids are slurries, suspensions, gels and colloids. Perhaps one would feel comfortable with this assumption for crude oil and refined products. But does heavy crude at some point low temperature stop behaving as a Newtonian fluid? Most of the fluids are Newtonian with the basic definition being that viscosity is constant with the shear rate. What is shear rate? It is the relative stress imposed on the fluid by the moving fluid. Consider, for example, a 10—inch, outside diameter OD closed impeller, with 0.
If it rotates at 3, rpm, then the metal peripheral velocity is 78 feet per second. The fluid in direct contact with the ring is spinning at the same velocity, according to a no-slip condition.
The stationary ring is 0. Now the peripheral velocity changes along the radial position of the vane it is at a little less than 5 inches as compared to the OD of, say, 10 inches. An average value can be estimated the calculation is similar to the one above , and the shear rate will be similar. Glue, for example, gets sticky. Glue, however, is not pumped by centrifugal pumps but more typically by gear pumps. Shear rate, however, still works there as well: in a clearance between the spinning gear and the wall, and such clearance is also typically in the range of 0.
Such a shear rate, however, is usually not an issue, as the amount of product in the clearance is small and the overall "dilution" by the sticky, damaged fluid is negligible.
In certain cases, though, it does matter. For example, if the fluid pumped by a gear pump is an emulsion deposited on photography film, then even minor imperfections may cause specks and blemishes, and the film ends up defective.
Pumping food—such as cherries, applies, etc. The pump requirement is gentle pumping with low shear. Progressing cavity pumps work best for these cases. In a centrifugal pump with clearance between the impeller wall and casing of 0. If pumped oil has a cSt viscosity, for example, it stays so at 3, rpm or 1, rpm. The shear rate changes, but the viscosity is still cSt. However for some fluids non-Newtonian , viscosity does change—either up or down, as shown in Figure 1. These fluids are dilatent or thixotropic.
This has an effect on the power required and also may cause fluid degradation, in addition to having an effect and usually does on power.
Power is force times speed. Force is stress times area. Stress is viscosity times shear rate. For dilatent fluids shear stress goes up always, as both viscosity and shear rate increase, but for thixotropic fluids it can go either way. Shear rate may not increase as fast as the decrease in viscosity, and the product stress can increase, decrease or stay about the same.
It all depends on the fluid being pumped. Usually, however, shear stress decreases. That means that power to the pump also decreases with the shear rate. In other words, fluid is first viscous, but once it begins moving, it becomes less viscous, which means less power is required to pump it.
Neglecting the driver rating motor selected too small is common. It gets sized for the viscosity of the fluid in motion, but then a motor keeps tripping upon start-up because more power is needed to get things going. Ketchup is one example, and this is why you must shake the bottle like crazy at a restaurant to get it flowing.
However once it flows, it does so quickly. As always—a parting quiz. Why is a "typical" rule-off-thumb that centrifugal pumps do not operate well above the magic number of centistokes viscosity?
A correct answer gets you a free admission ticket to the next Pump School session:.
Experimental results show that blood is not a purely viscose fluid, but possesses significant viscoelastic properties The viscosity of blood is a determinant shear stress where the shear stress is the energy transferred to the vessel wall due to interaction with a fluid in motion. Electrical properties of blood Hemoglobin Hb solution was prepared according to the method of Trivelli The cell has two parallel squared black electrodes of 0.
Catégorie : V) Fluide Newtonien
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