Читать книгу Agitator Design for Gas-Liquid Fermenters and Bioreactors - Gregory T. Benz - Страница 60

Newtonian fluids are those which have a shear stress that is directly proportional to shear rate (also called velocity gradient). Mathematically, this can be represented as:

(3.19)

where μ, the coefficient on shear rate, is also known as viscosity. This can be arranged to:

(3.20)

Most low viscosity fluids (<50 cP) are Newtonian. Intermediate and even some high viscosity fluids can be Newtonian if they are true solutions, especially if they are solutions of small molecules. For example, honey and corn syrups are often Newtonian even at 30 000 cP. Some silicone solutions are Newtonian even at 20 000 000 cP, but this is quite rare. Most suspensions of more than 5–10% solids exhibit at least some non‐Newtonian behavior. High solids slurries can be very non‐Newtonian.

Low‐solids fermenter broths operating in a water‐like fluid are usually Newtonian. High solids broths may not be, and those containing both an aqueous and an oil phase usually are not. But the overall viscosity for most fermenter broths is less than 1000 cP. The exception is some filamentous broths and those containing soluble gums, such as Xanthan. Chapter 12 will cover these in more detail.