Identifying the Type of Flow Behavior

Depending on the kind of process you work with, you may have facilities on hand to check the rheology (flowing characteristics) of the fluids involved in your work. If so, you can plot the shear stress against shear rate over your working range of shear rates to see which type of flow behavior you have as compared to those shown in Fig. 48.7. There are many different kinds of instruments (viscometers) to measure these properties. Further discussion of these may be found in references 4,5, and 6 at the end of this chapter. 

Otherwise, bear in mind that very often the range of shear rates to which a process fluid is exposed in pip e flow is often cjuite limited, and because of this it is often possible to adecjuately represent the flow behavior of a process fluid over a limited range of shear rates by the power law or Ostwald de Waele model, and the Metzner-Reed Reynolds number, which we shall shortly discuss. Meanwhile, in the absence of any lab equipment to provide you with shear-stress versus shear-rate data, we suggest that you vary the flow rate to provide several flow rates, and run pressure drop surveys over the pipeline in question. Tabulate the data and use this to develop a power law relationship (see following sections) as a first approximation for your pijjeUne and process fluid. Most likely you will not need to perform any more elaborate study than that 

We have said that for a Newtonian fluid the viscosity can be expressed 

When we consider non-Newtonian liquids, the viscosity is not constant, because of the influence of shear rate and time. So for non-Newtonian fluids we refer to their apparent viscosity i, or effective 

That is, the apparent viscosity of a non-Newtonian liquid can be expressed as the ratio of shear stress to shear rate at a specific point within the fluid, and perhaps even at a specific point in time, or within a specific shear-rate range, depending on the nature of the liquid. 

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