(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.