Measurement of the Stress Transmission Coefficient of Material Slugs in an Aerated Radial Stress Chamber

In dense-phase pneumatic conveying, particles are transported along a pipeline at relatively low conveying speeds. Due to the relatively gentle handling characteristics of this mode of flow, it is suitable for conveying fragile and brittle bulk materials used in the food and chemical industries. The simulation of the stress field within a slug aims at developing an accurate prediction model for the pressure drop along a pneumatic conveying line. A reliable prediction of the pressure drop strongly depends on an accurate assessment of the particle properties, the pipeline dimensions, and the operating conditions. In past decades, a few models have been developed to serve this purpose, most of them including the mean particle diameter as a crucial parameter. This generally limits the selection of materials to those of nearly spherical particle shape, as it is extremely difficult to obtain a representative diameter for irregularly shaped particles or bulk commodities comprising differently sized and/or shaped particles. Another previously conflicting parameter is the so-called stress transmission coefficient k w , which relates the lateral wall stress within a slug of material to the axial stress. Previously, this parameter could not be measured directly in a test rig and had to be estimated; therefore, inaccuracies within the prediction were unavoidable. Consequently, a new test chamber was developed to measure the lateral and axial stresses within a slug, which leads directly to the stress transmission coefficient. The design of the test apparatus is outlined and the initial tests undertaken are reported. A strong dependence of the radial stress measurements on temperature changes of the test rig induced by the airflow was discovered. Possible solutions to compensate for this influence are addressed and further discussed.