Industrial experiences have shown that powders can
unexpectedly change from normal powder flow properties to
exhibit liquid-like flow characteristics. This change in
flow properties, known as flooding, can result in a major
loss of a powder's containment.
The prime objective of the research presented in this
thesis is to develop a method which quantifies a powder's
likelihood to flood, and to identify the conditions where
the tendency to flood becomes important. A powder is
known to exhibit liquid flow properties at high shear
rates or when aerated at or above the minimum fluidisation
velocity. The interaction of these two factors, however,
is not fully understood.
A new type of shear cell is developed which enables
the measurement of the shear characteristics of an aerated
powder. This shear cell is based on Couette geometry,
where a powder sample is sheared between two concentric
cylinders, while under controlled aeration conditions. Evaluation of the equipment with a variety of powders
identifies that the transition to liquid-like flow
properties can occur at low shear velocities and at an
aeration substantially below fluidisation. The
characterisation of a sample of flooded material shows
that additional fine particles significantly increases the
tendency for that material to flow like a liquid.
The effect of additional fine particles on a
selection of powders is studied in detail and powders with
a narrow particle size distribution are shown to be most
vulnerable to flooding. The quantities of fines required
before a powder is likely to show liquid-like flow
properties can be small, highlighting that the flooding
problem can be significantly effected by segregation.
The ability to characterise the effect of small
quantities of additional fines on the likelihood to
undergo liquid-like flow is an important step forward in
understanding the apparent random nature of flooding.
A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.
A closed access file contains Appendices D and E.