Brookfield DV-I Prime - Manual (Page 33)

Brookfield Engineering Laboratories, Inc. Page 33 Manual No. M/07-022-D0613
When n
> 1 the product is shear-thickening or Dilatant. It’s apparent viscosity increases as shear
rate increases.
Figure II-44
When should you use it?
The Herschel-Bulkley model should be used with non-Newtonian, time-dependent materials that
have a yield stress. Products with a yield stress only begin to flow after a certain amount of shear
stress is applied. As a result, the flow curve intersects the y-axis at a point greater than 0. After
yielding, the product creates a flow curve and behaves as a Power Law fluid so that n indicates
where there is a shear-thinning or shear-thickening tendency. (In this case, if n =1, the material
is behaving as a Bingham fluid, which is discussed next.)
An Example of the Herschel-Bulkley Model at Work
A company uses a gel-like substance as part
of their production process. Upon arrival
they test the material and apply the Herschel-
Bulkley model to ensure it will perform
correctly during process. The results in Figure
II-44 show that the consistency index is 8,550
cP, the flow index is 0.66, and the yield stress
is 51.0 dynes/cm 2 . These results indicate that
this batch of gel does not quite meet specification. While the consistency index is within
spec, the yield value is higher than normal so the fluid will not begin to flow as easily. With
a flow index of 0.66, this batch is also less shear thinning than normal. Pump and mixer
speeds must be adjusted before using this material.
Gel-Like Substance
n
= 0.66
€
t
o
= 51.0 dynes/cm 2
k
= 8550 cP