Volume SH 4: Part
1 Gravity Transfer of Slurries in Open Channels
This part
is a state-of-the-art review and considers the transport
of slurries along open or closed ductwork using the
material head as the driving force for flow. This occurs
for both open channel (or flume) flow of either "settling"
or "non-settling" slurries in either the laminar
or turbulent regimes. The transfer of slurries in open
channels or flumes, while not as important as pipeflow,
is commonplace in the mining and water industries and,
as with pipeflow, can be conveniently categorised into
flow of "non-settling" slurry and of "settling"
slurry. For the turbulent flow of water the Chezy formula,
with an appropriate Chezy coefficient depending on the
flume geometry, has found widespread use for the prediction
of the head/flowrate relationship. This approach can
be adapted for use with "non-settling" slurries.
Methods are also available for Newtonian and non-Newtonian
laminar flow.
Large-scale
experimental studies have been undertaken for "settling"
slurries in a wide range of flume geometry. Some of
these studies have resulted in procedures for correlating
available head with slurry flowrate but only Wilsons
theory has provided a mechanistic insight into the flow.
The choice of materials of construction for chutes and
flumes is critical for a good engineering design. For
chutes, linings are often employed to reduce adhesion
and frictional forces between paste or cake and inner
wall, although cooled steel surfaces have also been
employed. For open channel flow, abrasion is generally
more of a problem, particularly in the mining industry
and plastic linings have been used to reduce wear rates.
Test methods are available for assessing the degree
of adhesion and wear for a bulk solid on different alternative
linings. Good mechanical design if of prime important
for continuous reliable performance of either chutes
or flumes. Different chute cross-section geometries
can be used including closed circular pipe and rectangular/square
cross-section, while for flumes the choice generally
widens to include trapezoidal and V-shapes. Chutes can
be installed vertically, near vertically or deliberately
inclined. They can be convergent, constant cross-sectioned
or divergent. In addition, chutes can have smooth bends
and much of the Roberts et al theory focuses on this
situation.
The type and quality of
connections between sections of chute or flume are crucial
in ensuring progressive build-up and eventual blockage
do not occur and in minimising wear rates at these joints.
Metering of flow in chutes is occasionally required
but is more important in flume flow where some type
of weir can be used or alternatively an ultrasonic level
detector.