Tesis doctoral de Manuel Francisco Acevedo Escalante
In this report we present experimental results about the effect that faceted particles have in the charge and discharge of granular material from a bidimensional silo. In particular, we analyze the effect of the feed rate and the particles aspect ratio (defined as the ratio between their longest and shortest side) on the deposits morphology. In previous works it was found that the aspect ratio of faceted particles strongly affects the deposits configuration. Indeed, the longest particles tend to align horizontally, a tendency that is attenuated as the particles become shorter. For the limit case of square particles numerical simulations predicted a configuration where most of the particles align with a diagonal in the direction of gravity, i.E. Pointing downwards. This behavior has been experimentally confirmed in this work. we show that the effect of increasing the feed rate -which was varied by controlling the initial packing fraction of the configuration from which the deposits were performed- was different depending on the type of particles utilized. For elongated particles the feed rate has a negligible effect on the particles orientation. Nevertheless, increasing the feed rate causes an augment of the number of defects (holes) within the structure, leading to a reduction of the packing fraction. For the square particles the behavior is completely different. Effectively, the deposits morphology displays a well marked dependence on the feed rate. Indeed, the strong preference of the squares for the pointing down alignment is reduced as the feed rate is increased. At the same time, we observe a non monotonous dependence of the final packing fraction of the sample on the feed rate (or initial packing fraction of the sample). Hence, the reported behavior for low feeding rates resembles the obtained for elongated particles: increasing feed rate causes a reduction of the final packing fraction. But if the feed rate is sufficiently high, this tendency is reversed and the packing fraction increases again. The origin of this phenomenon is discussed in relation with results reported in other granular systems which show that granular media are history-dependent. concerning the silo discharge process, we have observed that the shear stress developed within the silo seems to lead to the alignment of particles in the direction of such stress. This alignment seems to facilitate the flowability of the particles and occurs independently on their aspect ratio. For the case of square particles this behavior is specially important as it gives rise to the development of columnar arrangements of particles. These columns are proved to be extremely stable, a phenomenon that is known as ratholing which, before this work, was only observed in cohesive particles. finally, we explore the effect that the lateral walls geometry has on the systems of square particles. To this end, we compare the original results using flat lateral walls with additional ones using triangular sawtooth walls. The deposits obtained with sawtooth lateral walls reveal an increase of the number of particles perfectly aligned pointing. Nonetheless it seems that the obtained structure is more rigid that the one obtained with flat walls causing also an augment of the number of defects where the particles get aligned horizontally. During the discharge process, the triangular sawtooth walls clearly prevent the columnar arrangements. Still, in some regions within the silo the horizontal alignment is the most likely, a behavior that is related with the shear stress.
Datos académicos de la tesis doctoral «Depósitos y flujo de partículas con caras planas en un silo.«
- Título de la tesis: Depósitos y flujo de partículas con caras planas en un silo.
- Autor: Manuel Francisco Acevedo Escalante
- Universidad: Navarra
- Fecha de lectura de la tesis: 13/06/2013
Dirección y tribunal
- Director de la tesis
- Iker Zuriguel Ballaz
- Tribunal
- Presidente del tribunal: ángel Garcimartín montero
- marcos Salazar cruz (vocal)
- álvaro Janda galán (vocal)
- Fernando Alba elías (vocal)
