Theoretical and experimental testing of a scaling rule for air current segregation of alumina powder in cylindrical silos

Air current segregation (ACS) is one segregation phenomenon that has been identified to contribute significantly to the separation of fines (particles < 42 µm) from coarse material during the filling of industrial silos. This paper describes investigations of ACS for alumina powder based on experiments conducted in an industrial silo, in the laboratory and by computation, using the commercial computational fluid dynamics code Fluent.For the industrial silo, the extent of ACS has been measured using the accumulation of fine material at the wall as an indicator. Based on these results, modifications to the feeding system were undertaken which showed that ACS is promoted if the material is fed in a dilute form. Experiments in the laboratory confirmed this effect visually. In order to be able to compare numerically the extend of ACS, a segregation index has been developed. It was found that a dilute particle jet leads to more ACS than dense particle jet.The effects of solids feeding rate and air extraction rate on ACS have been investigated in the laboratory silo and the results clearly show that low solids feeding rates promote ACS. It was further found that an increase in the air extraction rate has a mild effect in suppressing ACS. These effects were confirmed by the Fluent simulations, which showed an unexpectedly good agreement with the experiments.