Brookfield DV-III Ultra Rheometer - Manual (Page 59)

Brookfield Engineering Laboratories, Inc. Page 59 Manual No. M98-211-E0912
Figure IV-49
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
VI-2 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