(365c) A New Explanation to the Shear Stress of Friction During Plug Flow Pneumatic Conveying
Pneumatic conveying is commonly used for industrial loading, unloading and
for the internal transport of bulk solids. Depending on mass flow rate and
the velocity of the conveying gas different conveying phases appear.
Reducing the speed of the conveying gas will result in two phase flow
setting, i.e. dense phase conveying. Plug conveying is reached at very low
gas velocities and high pressure drop. During the last few years pneumatic plug
conveying has become more important due to lower energy consumption,
lower particle attrition and lower wear of pipelines. Therefore, for many
industrial applications dense phase pneumatic conveying is the preferred
transport mode for granular media.
A lot of research has been carried out concerning the prediction of pressure
drop of dense phase conveying. Pressure drop models for dense flow
conveying are derived corresponding to Janssen’s silo theory under
consideration of bulk solid mechanics and constant bulk solid properties along
the plug. In this context the relationship between the radial to the axial
stresses within a single plug is particularly important to understand. It reveals
the strengths acting inside the plug and the forces exerted on the wall by the
plug. So far plugs are regarded as compact bulk solid columns. Unfortunately,
these models provide strongly different results concerning the pressure loss of
single plugs and determination of pressure drop for the entire conveying
pipeline is not feasible. Furthermore most of the models are based on bulk solid
mechanics or a fixed bed column with constant properties along the plug.
A sensor was constructed to measure directly and concurrently normal
stress, wall shear stress and pressure drop for a conveyed plug. The
experimental results for vertical pneumatic conveying show a change of
porosity along the plugs. This requires a new approach independent of bulk
solid mechanics to explain wall shear stress.
Thanks to a new measuring device including both stress and pressure
sensors, also horizontal single slugs were investigated to determine porosity
and stress states within slugs of cohesionless granular material. Moreover,
a special slug-catcher was developed to investigate the porosity profile over
t h e slug height. Independent of conveying velocity, slugs were found to be
fluidized over their whole length. Nevertheless, high wall shear stresses and
normal stresses were detected within each slug, which could not be
explained by bulk solids mechanics.
The astonishing result was that the bulk porosity never exists in a slug. The
essential assumption for the existing theories could no longer be maintained.
The existing theories lost their physical bases and degenerated into pure fitting
models. Because slugs are not compact structures, no axial stress exists within
slugs. The existing wall friction can be explained by postulating a change of
momentum similar to what happens with gas friction. The molecules are
replaced by the single particles. With some simplifications of the kinetic gas
theory, it was possible to estimate the wall friction and the pressure loss.
Experimental investigations on moving slugs of granular materials with respect
to slug aspect, particle velocity, pressure gradient, slug porosity, and stress
states revealed that the wall shear stress induced by slugs can be accurately
described by applying the kinetic theory.
To improve the assumptions a twin-plane electrical capacitance tomography
(ECT) was used to investigate the dynamic porosity changes during
horizontal plug flow conveying of polypropylene granules. The results obtained
from ECT measurements were compared to the porosity calculated from
pressure drop measurements according to the Ergun equation. Using the
aforementioned measuring device, wall shear and normal stresses caused by
plugs were investigated. The calculated wall shear stresses were in good
agreement with the measured wall shear stresses. With these results,
predictions of pressure loss could be made on the basis of useful physical
assumptions and the new model.