A TOMOGRAPHIC STUDY OF VOIDAGE PROFILES IN AXIALLY SYMMETRICAL GRANULAR FLOWS

A specially constructed multiple-source scanner is used to produce consecutive tomographic images of the horizontal planes at different heights of the granular beds flowing in both cylindrical and conical vessels. In a series of experiments involving batch discharge of mono-sized and binary fills of materials, the transients in the cross-sectional voidage profiles are quantified to within a spatial accuracy of 1 mm in Cartesian and 1-degrees in polar coordinates in time steps of 5-10 s. The plane mean values of the voidage are calculated in each case to produce height profiles of voidage within the vessel at different stages of discharge. The height profiles of voidage obtained within a converging flow field such as that occurring in a conical vessel are all found to exhibit a well-defined maximum value close to the discharge orifiCe. Following the flow initiation, the position of the voidage maximum is found to move up the cone, resulting in a counter-current voidage wave front accompanying the steady material discharge from the hopper orifice. In a vessel with a cylindrical top section, the plane mean voidage is found to teach its minimum value above the plane of bin-hopper transition.