Manual for the Design of Pipe Systems and Pumps - GEA ...

3.6 Fluid
behaviour
The flow curve is a diagram which shows the correlation between viscosity (η) and the shear
rate (D). The shear rate is calculated from the ratio between the difference in flow velocity of
two adjacent fluid layers and their distance to eachother.
The flow curve for an ideal fluid is a straight line. This means constant viscosity at all shear
rates. All fluids of this characteristic are "Newtonian fluids". Examples are water, mineral oils,
syrup, resins.
Fig. 6 - Flow curves
Fluids that change their viscosity in dependence of the shear rate are called
"Non-Newtonian fluids". In practice, a very high percentage of fluids pumped are
non-Newtonian and can be differentiated as follows:
Intrinsically viscous fluids
Viscosity decreases as the shear rate increases at high initial force. This means from the
technical point of view that the energy after the initial force needed for the flow rate can be
reduced. Typical fluids with above described characteristics are a.o. gels, Latex, lotions.
Dilatent fluids
Viscosity increases as the shear rate increases. Example: pulp, sugar mixture
Thixotropic fluids
Viscosity decreases with strong shear rate (I) and increases again as the shear rate decreases (II).
The ascending curve is however not identical to the descending curve. Typical fluids are a.o.
soap, Ketchup, glue, peanut butter
Viscosity
Δy
Fig. 5 - Shear rate
Viscosity
II
Shear rate
Shear rate
Fig. 7 - Thixotropic fluids
I
Δv
13
GEA Tuchenhagen
v
3 1 Newtonian
fluids
1
2
D = Δv
Δy
2 Intrinsically
viscous fluids
3 Dilatent fluids