Characterisation of mixing processes using PEPT/fluid mixing.
PEPT (positron emission particle tracking) is a technique for tracking a small
radioactive tracer in Lagrangian co-ordinates. The technique was used to study the
flow patterns of non-Newtonian CMC (Carboxy Methyl Cellulose) solutions inside
a vessel agitated by an axial flow impeller. The 'non-intrusive' PEPT technique
uses two position-sensitive detectors to track a radioactive particle in space and
time. The particle is labelled with a positron emitting isotope. Once emitted from
the nucleus a positron annihilates with an electron releasing energy in the form of
two 511 keV back-to-back gamma-rays travelling in opposite directions, 180
degrees apart. The tracer particle is introduced into the stirred vessel which is
mounted between the two detectors of the positron camera.
Three axial flow impellers produced by Lightnin Mixers Ltd were used to carry out
the experiments. Results showed that the discharge from the three impellers was
radial when agitating non-Newtonian viscous solutions of CMC. Trajectory
analysis was used to compare the performance of the impellers using the agitation
index and the efficiency of circulation.
A limited number of experiments was carried out to compare the effect of baffles
on the circulation of the fluids in a mixing tank. The results showed that mixing of
these non-Newtonian liquids in an unbaffled tank is better than in a baffled tank
when using axial flow impellers.
Other experiments were carried out to suspend solid particles in viscous fluids.
Results showed that the minimum speed required to suspend large particles is
lower than that required to suspend small particles. There are many correlations
and models in the literature to determine the minimum speed required to suspend
all the particles in a fluid; some of these correlations and models were compared
with experimental results from this work. The correlation of Zweitering (1958)
agreed with experimental data after modification. The Geisler et al. (1993) model
agreed with the data provided that the power consumption is correctly substituted.
The last part of this work concerned the flow of non-Newtonian viscous materials
through industrial equipment. Yoghurt was chosen as the test fluid as one of the
companies sponsoring this project was Eden Vale, a yoghurt manufacturer. A
method was proposed using rheological measurements to simulate the flow
through the dispensing pipeline and distributing nozzles; this method allows the
designer to predict the final properties of yoghurt after passing through the paching
head. Measurements were also carried out to determine the final gel structure of
yoghurt in the delivery pots.
This data of this thesis is useful in designing stirred tanks when non-Newtonian
fluid is present, either for agitation or when suspending solids. Also, a method was
provided to design yoghurt manufacturing line.