Chemistry Reference
In-Depth Information
3
Doppler Ultrasound-Based Rheology
Beat Birkhofer
3.1
INTRODUCTION
3.1.1
Overview
The principle of the technique described in this chapter is simple: the
shape of the symmetric velocity profile in laminar, stationary pipe flow
depends on the rheological characteristics. For example, Newtonian
fluids have a parabolic flow profile, while for shear-thinning fluids it is
steep close to the wall and flat towards the centre (Fig. 3.1). Thus, being
able to measure this velocity profile allows a characterisation of the
rheological properties of the fluid. Combining the velocity profile with
pressure drop results in a non-invasive in-line rheometer, which allows
to measure the shear rate-dependent viscosity.
Pulsed ultrasound
1
, a flow measurement method applied since the
early 1970s in the medical field, can be used to obtain the velocity
profile. Compared to magnetic resonance imaging, it is relatively simple
and far less expensive. In contrast to laser Doppler anemometry (LDA)
or particle image velocimetry (PIV), the technique can be applied in
opaque fluids.
The main benefit of the combination of ultrasonic velocity profiling
(UVP) and pressure drop (PD) is the applicability as in-line rheometer
for complex fluids. The necessary basic installation - a straight pipe
with the fluid flow - is quite ubiquitous in the relevant industries such
as food, pharmaceutical, cosmetic or chemical. As the rheological
properties are measured directly in-line, there are the obvious advan-
tages of avoiding complicated measurements in the laboratory and
1
The question as to whether pulsed Doppler ultrasound makes use of the Doppler effect depends
on the definition of the Doppler effect (Cobbold, 2007, Chapter 10). For Jensen (1996, Chapter 6),
the classic Doppler effect is only an artefact in pulsed systems.
