Nonsteady discharge of granular media from a silo driven by a pressurized gas

We studied experimentally and numerically the effect of an imposed gas pressure on the discharge flow of granular media from a cylindrical silo. This study is motivated by a nuclear safety related phenomenology of fuel fragments displaced from a fuel rod under several accidental conditions, the flow being potentially driven by pressurized fission gases within the rod. We imposed a moderate constant air pressure at the top of the granular column (???3000 Pa) and we varied the size and type of the particles and the surrounding fluid where the discharge occurs, using air and water to test the role of the coolant fluid in the nuclear safety problem. The measured parameters are the particle mass flow rate, the volumetric flow rate of air, and the pressure along the silo. The particle and air flow rates are found to be nonsteady and to increase with time. To model these behaviors, we use a twophase continuum model with a frictional rheology to describe particle-particle interactions, and we propose a simple quasisteady analytical model considering the air-pressure gradient at the orifice as an additional driving force to the gravity. We implemented numerically the two-phase continuum model in an axisymmetric configuration which reproduces the experimental results.