Análisis experimental del proceso de descarga de medios granulares en silos

Resumen

The behavior of granular materials passing through an orifice displays qualitative differences respect to the case of a fluid. This is due to the possibility of forming arches that block the exit. Jamming often happens when the orifice size is comparable to the beads and it is one of the most important problems in industry. In the literature, some works propose the existence of a critical value for the outlet size, above which jamming does not happen. However, this hypothesis is generally based on empiric arguments, and therefore, it is not conclusive. In this report we present an experimental work where we analyze systematically and under well controlled conditions, the microscopic and macroscopic processes involved during the discharge of a silo by gravity. In order to do that, we have designed a 2D silo made of flat transparent walls that allows us to visualize the movement of the beads. The first aim of this work is to answer the question about the existence of a critical size of the aperture. This issue has been investigated by two different strategies. On one hand, we have studied the jamming probability as function of the exit size. From these data, we have proposed a probabilistic model that relates the mean number of particles fallen between two consecutive jamming events with the typical size of the arches that block a given outlet size. From this model, it is deduced the absence of a critical size of the orifice. This implies that if we wait during a large enough time, we will always observe a jamming event although the exit was much larger than the beads. On the other hand, we have studied the behavior of the dynamics of the granular flow during the discharge. This is accomplished by characterizing the flow rate, the velocity and the packing fraction of the grains at the exit. From these results we have concluded that the outflow properties are the same independently of the size of the orifice. This fact suggests that the flow is a well defined state of the discharge process, which is consistent with the absence of a critical size of the outlet.