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

Chapter 5 Discussion Page 5.2
Fur<strong>the</strong>r investigation needs to be conducted and a large data base established to determine
whe<strong>the</strong>r <strong>the</strong> roughness <strong>effect</strong> will:
• follow <strong>the</strong> Nikuradse trend;
• continue increasing, following more closely <strong>the</strong> oblique asymptote;
• or if it does follow <strong>the</strong> horizontal asymptote as proposed by Slatter.
5.3 ROUGHNESS FUNCTION CORRELATION USING OPTIMUM PARTICLE
SIZES
If, however, <strong>the</strong> optimum <strong>particle</strong> <strong>size</strong>s are used to plot <strong>the</strong> roughness function correlation
as shown in Figure 4.11 <strong>the</strong> data points for mixture 2 tend to lie just above <strong>the</strong> horizontal
asymptote. On <strong>the</strong> whole <strong>the</strong> data points tend to follow more closely <strong>the</strong> assumptions on
which <strong>the</strong> Slatter model is based. What this does bring into question is that <strong>the</strong> d S5 <strong>size</strong> does
not necessarily give <strong>the</strong> best representative <strong>particle</strong> <strong>size</strong>. These optimum <strong>particle</strong> <strong>size</strong>s
indicate that <strong>the</strong> representative <strong>particle</strong> <strong>size</strong> will vary depending on <strong>the</strong> PSD <strong>of</strong> <strong>the</strong> slurry
being transported. In fact it would seem from Table 4.1, looking at <strong>the</strong> optimum <strong>size</strong>s for
kaolin clay and mixture 2, that a slurry with a steep PSD for <strong>the</strong> smaller <strong>particle</strong>s (eg. kaolin
clay test sets) would have a higher representative <strong>particle</strong> <strong>size</strong> than a slurry which has a PSD
shape that is less steep from small to large <strong>particle</strong>s. In Table 4.1 <strong>the</strong> kaolin clay test sets
have an average <strong>particle</strong> <strong>size</strong> <strong>of</strong> d x =d 91 as opposed to mixture 2 tests sets which have an
average representative <strong>particle</strong> <strong>size</strong> <strong>of</strong> d. =d 71 • Therefore <strong>the</strong> higher fraction <strong>of</strong> larger
Particles present in <strong>the</strong> slurry <strong>the</strong> lower <strong>the</strong> representative <strong>particle</strong> <strong>size</strong>, as <strong>the</strong> best
representation <strong>of</strong> <strong>the</strong> roughness <strong>effect</strong> is reached at a lower d x value.
5.4 SLURRY TEMPERATURE
As reported in section 4.2.7 an increase in temperature <strong>of</strong> between goC to 10°C made no
significant difference and no temperature <strong>effect</strong>s were found. It is surprising to see this trend
since this would cause a decrease in <strong>the</strong> viscosity for water <strong>of</strong> approximately 20%. This
Could influence <strong>the</strong> rheology <strong>of</strong> <strong>the</strong> slurry and might lead one to question <strong>the</strong> method <strong>of</strong>