Static contact statuses between conical silos and granular materials and its influential factors

Finite element models, which employ the Drucker-Prager yield criterion, have been developed to simulate the static contact statuses between conical silos and granular materials in 3 forms: the near contact, the sliding contact and the sticking contact. Contact conditions are established when 2 separated surfaces touch at normal direction while maintaining tangential relative movement. In general physical meaning, the surfaces in contact status have the following characteristics: 1) No penetration between each other; 2) The normal pressure and the tangent friction force may be transferred during contact; 3) Generally the normal pulling force cannot be transferred when surface separation occurs. Due to the symmetric property of conical structures, simplified two-dimensional contacting simulations are carried out in this paper, nonlinear finite element software ANSYS is used and the contacting surfaces between granular materials and conical silos are defined with rigid-to-flexible surface-to-surface contact pair. The target surfaces of conical silos are modeled with TARGE169 element and the contact surfaces of granular materials are modeled with CONTA171 element. During finite element analysis, conical silos and granular materials are meshed with two-dimensional solid element, PLANE42. The static contact statuses are investigated with conical silos containing different granular materials. The silo geometries vary at a dip angle of 20°, 33.7° and 45°. Sunflower seeds, corn, coal, rounded gravel and wheat are selected as the granular materials. Results show that the mechanical properties of granular materials (including bulk density, elastic modulus, Poisson's ratio, dilation angle, internal friction angle, cohesion) and silo designs (especially dip angle) have significant effects on the contact statuses at the interface between conical silos and granular materials: 1) For various granular material, 3 contact statuses, i.e. the form of near contact, sliding contact and sticking contact, can be found between granular materials and conical silo walls; 2) The contact statuses between conical silos and granular materials do not depend on (or not mainly depend on) any mechanical property of granular materials. The contact statuses are a combined effect of all mechanical properties of granular materials. Those granular materials with very small dilation angle may have the near contact statuses. Those granular materials with higher cohesive force usually present a smaller sticking contact area, and those granular materials with higher elastic modulus and bulk density usually present a larger sticking contact area than those with opposite material properties; 3) With the decreasing of conical silo depth, the near contact area disappears, the sliding contact area decreases and the sticking contact area increases. 4) Under the sliding contact status, the friction energy dissipation is mainly due to the relative motion between contact surfaces. Under the sticking contact status, the friction energy dissipation is mainly due to the elastic deformation because of the contact. The greater the sticking contact area, the more difficultly the silo discharges. The greater the sliding contact area, the more seriously the silo internal surfaces could be damaged. Since larger sticking/sliding contact area inevitably causes unloading difficulties or friction damage, contact statuses between granular materials and conical silos should be optimized in the silos design in order to boost storage efficiency.