(152e) Effect of Fine Particles on the Flow of Narrow-Sized Coarse Slurry through 90° Bend

Pipe bends are
an integral part of any pipeline network system as they provide the flexibility
for pipeline routing. In the case of single-phase fluid flows, secondary flows
are generated in bends as a result of forces caused by change in flow
direction, which result in additional pressure losses across them. For
solid-liquid flows, the secondary flows generated in bends also affect the
solid distribution in the downstream side in addition to the increase in
pressure loss. Limited studies have been carried out for solid-liquid flows to
establish the pressure drop across the bend and hardly any study is reported on
the effect of bend on the redistribution of solids downstream of the
conventional 90° horizontal circular
bend.

It is expected that any data provided on these two aspects will be of
great relevance to the slurry pipeline designers for optimizing the life of the
pipe bends. Hence, in the present study an attempt has been made to establish
the effect of a conventional 90°
horizontal circular bend with radius ratio of 5.6 on the re-distribution of
narrow-sized as well as bi-modal solid particles downstream of the bend for
fully developed flow at the inlet. The effect of addition of fines on the
pressure drop has also been established.

The pilot plant test loop used for the present study is a
re-circulating loop of 53mm diameter pipe and consists of mixing chamber, pump
with motor, pipe loop and measuring tank. The total test loop length is 30m.
The test bend is located in the loop downstream of one bend with upstream
length being 6m. This ensured a fully developed flow at the inlet of the test
bend. The straight length downstream of the test bend is 7.6m. The solid
concentration is measured at 6D downstream of the test bend in two orthogonal
planes namely mid-horizontal plane and mid-vertical plane for narrow-sized as
well as bi-modal slurry with average afflux concentration of 20.4% (by weight)
at three average flow velocities [1.78, 2.56 and 356 m/s]. The bi-modal slurry
is prepared by mixing water to narrow-sized coarse silica sand particles [mean
diameter < 448.5 mm and specific
gravity of 2.65] and the fine fly ash particles [mean diameter < 75 mm and specific gravity of 2.17] in different
proportions. The ratios in which the particles are mixed to coarser particles
are approximately 10, 20, 30 and 40% for 20% average efflux concentration of
bi-modal slurry. The concentration is measured near iso-kinetic conditions
using a sampling tube. The sampling tube is traversed either from bottom of the
pipe or from inner wall of the pipe depending on the plane of measurement. The
pressure drop across the bend is measured between different pairs of pressure
taps located across the test bend for the above said values of efflux
concentrations and flow velocities.

Based
on the present study, the following major conclusions are drawn:

1. In bi-modal slurry, at any given Reynolds number, the bend loss
   coefficient decreases with increase in percentage of the fines mixed.
2. The re-distribution of solid particles takes place downstream of the
   bend due to the interaction of centrifugal force and pressure driven
   secondary flows.
3. The concentration distribution is more uniform just downstream of
   the bend.
4. The re-distribution of solid particles is also seen in the
   mid-horizontal plane for both mixture as well as individual coarse
   particles.
5. The finer fraction of the particles is distributed uniformly across
   the pipe cross section at all velocities of the flow. This uniform
   suspension of finer particles provides better suspension to coarse
   particles.