Deformation of loose materials in convergent axisymmetri channels

Abstract

Many investigations in the USSR and elsewhere have indicated that the discharge of loose materials from tall vessels {silos) is accompanied by local rises in the loads on the walls, pulsation of the loads, and nonuniform loading round the perimeter of the vessel [1][2][3]. Although much effort has been made to elucidate the mechanism of formation of these features, so far there are no satisfactory explanations of the development of the high peak loads or of the pulsations. The rise of pressure during discharge is associated with hanging (arch formation) [4][5][6], with changes in the coefficient of friction of the material on the wall [1], and even with the influence of acoustic shock waves [7]; the asymmetric pressures on the walls are associated with inhomogeneous stacking of the material and nonuniform rigidity of the structure round its perimeter [8].Experts consider that while calculations of the static pressures are rather indeterminate, calculations of the loads arising during discharge of the materials are even more so [9]. This is why there are relatively frequent cases of serious damage to structures for the storage and treatment of loose materials [8,9]. Many investigators have remarked that the solution to this problem will involve further detailed studies of the processes occurring during movement of materials in vessels [10].in [12,13] we showed that the principal disturbances in material flowing out of a vessel involve its motion in convergent channels, i.e., in the conditions of maximally confined deformation. Such channels are components of any bunkers, whether the latter have a convergent section or a fiat bottom; in the latter case the convergent channel is formed within the material itself. Let us therefore first consider experiments on the deformation of loose materials in convergent axisymmetric channels.The experimental methods and the characteristics of the materials used in our investigations were described in detail in [11,12]. To study the kinematics of motion of material in axisymmetric vessels, there is an additional method, ngelatin freezing, n which has proved to be more convenient and less troublesome than the ~paraffin coat freezing n method. Before forming the load, all the material (both light and black sand) was heated to 100*C. After a given portion of the loose material had been discharged through the outlet, the deformed material remaining in the vessel was filled with a heated gelatin solution {water and gelation in the proportions of 10 : 1 by weight). The specimen was then cooled, removed from the vessel, and cut up along various planes.As the convergent channels we used glass funnels and also funnels to which various grades of emery paper had been glued on the inside. The angles of divergence of the funnels were 45-90 ~ and the diameters of the outlets 5-16 ram. The use of glass funnels not only reduced the coefficient of friction between the material and the wall, but also had the advantage that it was possible to make direct observations on the kinematics near the wall...

Features of the stress-strain state of loose materials in converging channels and bunkers

“…In bunkers fitted with converging channels having sloping walls B > 8 * flow develops as indicated in [3,4,8]. Below the transition in these bunkers peak values of stresses c will not be realized.…”

“…It was shown in [5] that flow in channels proceeds with formation of slip surfaces separating masses into individual "rigid" blocks, and it is characterized by deformation asymmetry. As in other cases of gravitational flow [3,4,8], the initial deformation paths during discharge from channels are dictated by dilation properties of the material and deformation conditions. Anisotropy of dilation properties provides realization of a "minimum strength surface" (MSS) AB, A~B, and separation of a "key" block ABAI (Fig.…”

“…2). We concentrate below on the reasons for this sllp surface behavior.In converging channels with angle 8 > 8 * first there is formation of a central flow channel [8]. Then its walls are eroded and a converging channel forms within the mass of loose material.…”

The second form of flow for loose materials in bunkers

“…This interest is caused by the fact that with the second form of movement horizontal pressures on the silo walls increase by a factor of 2 to 2.5 compared with static pressures, and according to some data even by a factor of 5-7 [i]. Increased pressures develop at a certain characteristic height asymmetric relative to the bunker axis of symmetry and they have a pulsating character [2,3].The increase in pressure at the~closing surface during discharge, as already noted in [4], is connected with a freely formed change in flow and even with sonic shock waves.A number of researchers are inclined to explain this increase by dynamic phenomena [5] (whence the extensively used term "dynamic" loading).There is no single view on the nature of the rest of these facts.Standards set in most countries for determining static loads on silo el~ents recommend the method proposed in 1895 by Janssen [6] in spite of the existence of many other methods. This selection is dictated by the simplicity of relationships obtained in [6], and numerous verifications of calculated results by experiments.…”

“…The SMS realized in tests for discharge (Fig. 2a) on the background of surface A were not detected since they did not develop completely (up to separation of locking blocks [4,Ii]). This is related to the size of the transverse section of the U-channel.…”