the effect of the particle size distribution on non-newtonian turbulent ...

Chapter 4 Results and Analysis Page 4.2
Mooney (1931) (Section 2.6.2 - Figure 2.5) in that <strong>the</strong> relationship between <strong>the</strong> shear stress
and shear rate is independent <strong>of</strong> <strong>the</strong> diameter in larninar flow. This was true for all tests
conducted, where pipediameter had no influence on wall shear stress at a given pseudo-shear
rate in <strong>the</strong> laminar regime. This indicates that <strong>the</strong> slurry properties were time independent.
In larninar flow, on physical observation <strong>of</strong> <strong>the</strong> slurries in <strong>the</strong> transparent sections <strong>of</strong> <strong>the</strong>
pipeline, <strong>the</strong> slurry <strong>particle</strong>s near <strong>the</strong> tube wall could be seen to be travelling in straight lines.
As <strong>the</strong> velocity increased <strong>the</strong> <strong>particle</strong>s took on a more random, swirling or eddy motion and
<strong>the</strong> transition from larninar to turbulent flow could clearly be seen. The differential
transducer output reading showe,d more variation in <strong>the</strong> transition region and confIrms <strong>the</strong>
physical observation. This evidence also supports <strong>the</strong> fact that true turbulence is occurring.
Settling <strong>of</strong> <strong>particle</strong>s was not observed at any <strong>of</strong> <strong>the</strong> concentrations tested. The intersection
<strong>of</strong> <strong>the</strong> laminar and turbulent data can be taken as <strong>the</strong> critical point at which turbulence begins
and hence <strong>the</strong> critical velocity can be determined.
The 25mm pipeline only shows <strong>the</strong> first few data points for <strong>the</strong> onset <strong>of</strong> turbulence. This
is due to <strong>the</strong> fact that <strong>the</strong> back pressure <strong>of</strong> <strong>the</strong> l50mm pipeline was used as <strong>the</strong> driving force
for <strong>the</strong> 25mm pipe. Hence, <strong>the</strong>re was insufficient driving force to enable <strong>the</strong> recording <strong>of</strong>
<strong>the</strong> full turbulent range. Low velocities were also recorded in <strong>the</strong> 200mm pipeline due to
<strong>the</strong> set-up <strong>of</strong> <strong>the</strong> pipeline system. In a looped pipeline system one is normally limited to two
pipeline diameters due to <strong>the</strong> available head-capacity. Using <strong>the</strong> looped pipeline system at
UCT, testing was conducted using as wide a range <strong>of</strong> diameters as was possible on <strong>the</strong> one
system, but in doing so low velocities in <strong>the</strong> 200mm pipeline were inevitable.
4.2.1 Pipe Roughness
There is quite a marked difference in <strong>the</strong> pipe roughness for <strong>the</strong> three smaller pipelines as
oPposed to <strong>the</strong> 200mm pipeline (Table 3.II). The smaller pipelines (ie. 25mm, 80mm &
lS0mm pipelines) are PVC tubing whereas <strong>the</strong> 200mm pipeline is constructed <strong>of</strong> steel. The
inside <strong>of</strong> <strong>the</strong> 200mm pipe was corroded thus accounting for <strong>the</strong> high pipe roughness value.