Self-fluidization of Fastly-Moving Granular Gravity Currents with Implication on Pyroclastic Flows

Fluidization of pyroclastic solids has long been indicated as one key to explain the enhanced mobility of dense pyroclastic gravity currents and their associated hazard. However there is a lack of characterization of the actual pattern and extent of fluidization establishing in real pyroclastic flows and some authors still raise arguments about the relevance of fluidization to the mobility of dense pyroclastic gravity currents. The present paper addresses the fluidization of pyroclastic granular solids with a specific focus on the analysis of factors that may promote homogeneous fluidization and retard solids de-aeration and consolidation. These factors include fines content, particle polydispersity and the establishment of shear flow. The influence of fines content and particle polydispersity has been assessed by analyzing the fluidization behaviour of mixtures of pyroclastic solids characterized by different and properly tailored particle size distributions and fines content. Pyroclastic mixtures, extracted from a Neapolitan Yellow Tuff deposit, were nearly homogeneous as to the lithological composition. The experimental campaign has been carried out with a lab-scale segmented fluidization column 120 mm ID. Experiments included quasi-steady fluidization of granular mixtures at variable gas superficial velocity as well as collapse tests from pre-set fluidized states. Results of the collapse tests were independently analyzed by looking at the time-series of the pressure drop across the bed as well as at the bed surface level as a function of time, recorded with a high-speed video camera. The study has been pursued further to analyze the effect of shear on the fluidization behaviour of pyroclastites. To this end the experimental apparatus has been modified and equipped with a rotating cylinder which could be used to impose a given shear rate to the fluidized bed. Shear rates were comparable to those that are likely to establish in real pyroclastic flows. Quasi-steady fluidization experiments and collapse tests were carried out under either stationary or sheared fluidization conditions. We compared the fluidization behaviour of mixtures characterized by different fines contents. Moreover we compared the fluidization behaviour recorded under either stationary or sheared flow conditions. Results of experimental tests highlight that both the presence of a fine fraction and the establishment of shear flow significantly influence the fluidization behaviour of pyroclastic mixtures, promoting homogeneous fluidization of the granular mixtures. The range of gas superficial velocities within which bubble-free fluidization occurs is significantly broadened by the presence of fines, by the polydisperse nature of the mixtures and by the establishment of shear. The de-aeration/consolidation times determined by the collapse tests are correspondingly much longer.