The simulation and experimental study of granular materials discharged from a silo with the placement of inserts

Shie-Chen Yang, Shu-San Hsiau
Department of Mechanical Engineering
National Central University,
Chung-Li, Taiwan 32054,
Taiwan
Source : http://cat.inist.fr/?aModele=afficheN&cpsidt=20126261

Abstract

The simulation method of DEM and experimental method are used to investigate the flow pattern of the filling and discharging process for two-dimensional plane silos. Two kinds of inserts conical insert and BINSERTw. are used in the silo to change the flow fields of the silo. The placement of inserts improves the flow behaviors of funnel flow type to mass flow type during discharging. The wall normal stresses are influenced by the change of the flow type. The effects of using differently shaped inserts on the flow pattern and wall stress are analyzed in this study. The controlling parameters include the silo half-angle, the orifice width, the shape of the insert and the properties of the granular materials. The simulation and experimental results are in good agreement. 2001 Elsevier Science B.V. All rights reserved.

Introduction

Considerable importance has been given in industries these days for the handling of granular materials. Roughly one-half of the products and about three-quarters of the raw materials of chemical industry are in the form of granular materials that are usually stored in silos or bunkers w1x. Although the silo is widely used, the understanding of the flow behaviors in the silos is not very established. In fact, a general and approachable research for designing a silo is not available. Fluidity of the entire materials in the silo and the load distributions on the silo walls are the two pertinent questions to be addressed while considering the material discharge from the silo. Two distinct flow types are conventionally recognized for the silo discharge problem: mass flow type and funnel flow or core flow. type w2x. If all the materials inside a silo are in flowing condition during discharge, the flow region is called mass flow, which is an ideal flowing style for running a silo. On the other hand, if there are stagnant zones or called dead zones. near the silo walls where the particles flow slowly or are stagnant, the flow type is called funnel flow or core flow. The silo geometry and the properties of the granular materials have important influences on the flow pattern. A rigid insert is generally used to improve the flowing performance and also to reduce the wall stress, which may give rise to the stable arching or dome formation during the discharging process w3,4x. Although various types of inserts are used in industries, the understanding of the mechanism with a fixed insert placed in a silo is not known with certainty still. A method to design a conical insert used in a conical hopper based on the theory of characteristic line is proposed by Johanson w4x. Johanson also suggested a critical height above the silo outlet to reduce the size of stagnant zone observed in a funnel flow hopper. Another type of insert is called conein- cone insert BINSERTw., which is a hopper-like structure allocated in the inclined hopper section in order to change the flow pattern w5x. The requirements of silo design are the flow problems and the structural stability of the silo. The wall pressure of bulk solid in a silo is different significantly to that of fluid in a container. The wall stress in silos with no inserts has been investigated both experimentally and theoretically for many years w2,7–12x. But there were few literatures to investigate the wall stress in silos with inserts. Strusch and Schwedes w12x experimentally investigated the triangular insert loads and wall stress distribution on a model silo. They also theoretically calculated the insert load by means of the slice element method. Tuzun et al. w6x experimenexperimentally investigated the effects of triangular and square inserts on the flow patterns within a tall bunker. They also measured the wall normal and shear stress. The experimental results were compared with the theories of continuum modeling. In numerical simulation, the discrete element method DEM. proposed by Cundall and Strack w13x has been successfully used to study the related problems of granular materials. There are more related literatures regarding the application of DEM to analyze the silo problems. For example, Thornton w14x applied DEM model to record the silo flow, to monitor the change of solid fraction distribution and the discharging rate. Sakaguchi and Ozaki w15x investigated the dynamic process of formation of arches, plugging phenomenon, by experiments and DEM simulations. Ristow w16x investigated the dependence of the discharging process in a two-dimensional hopper on various material and geometrical parameters. In this paper, the DEM and experimental methods are used to investigate the flow behavior of granular materials discharging from a two-dimensional plane silo. The flow patterns and the dependence of discharging rate, the wall normal stress during discharging, are studied. The influence of differently shaped inserts on the stagnant zone of the silo is also analyzed in this study. The important geometry parameters considered include the silo width D, the orifice width D , the hopper half-angle a and the 0 shape of the insert.



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