Why the presence of insert above the outlet can enhance silo discharge: A tentative answer

Although the enhancement of insert on the discharge rate of particles drained from silo has been recognized and reported by different researchers, a thorough understanding of the proper mounting position of insert and also the underlying mechanism of the discharge-rate enhancement is still elusive. In this work, the drainage of particles from a flat-bottomed silo mounted with cylindrical insert was investigated by performing three-dimensional Discrete Element Method (DEM) simulations. The DEM simulations successfully represented the non-monotonic variation of discharge rate with the axial position of the insert and demonstrated that there existed an axial region mounted in which the insert induced the increase of discharge rate. Parameter analyses indicated that this axial region was within a special intermediate zone in the converging part of the flowing zone, whose lower boundary corresponded to the location of the commonly assumed free-fall arch and upper boundary was featured as the most violent fluctuation of particle compressive force. The discrete Fourier transforms of the time signal of particle vertical velocity show that the installation of insert in this axial region changed the fluctuating characteristics of particles. Our DEM simulation results thus suggest that the proper installation position of insert should be within the intermediate zone, and the enhancement of discharge rate might be ascribed to the influence of insert on the dynamic response of particles in the special intermediate zone.