Wall stress analysis in an unsteady hopper flow with ellipsoidal particles

Design of bulk solid storage vessels requires the knowledge of wall stress distribution in static and moving beds of granular materials. Most previous studies have shown that for spheres, side wall stress developed in hoppers increases with the distance from the top surface but there is hardly any limited value. Theoretically, the wall stress profile can be predicted by Jessen-Walker-Walters theory. However, wall stress is affected by many variables, particularly the shape of granular materials. In this work, ellipsoids which can represent a large number of particle shapes are used to investigate the effect of particle shape on the wall stress profile developed in a cylindrical hopper. The results show that due to the better flowabilities, spheres exert higher maximum stress on walls than ellipsoids. Wall stress for ellipsoids approaches a constant value quickly. It is also found that the wall stress distribution is not affected much by orifice size, but solid bed height. Further, an attempt is made to characterize the two parameters of wall friction coefficient μ and the lateral stress ratio K_A in Jessen-Walker-Walters theory, indicating that both μ and K_A vary with aspect ratio of ellipsoids. The assumption of the constant μ and K_A may bring significant error in the wall stress prediction.