Experimental studies and modelling of the fluidization of sands

Abstract

This paper presents the results of column tests in which the fluidization of sands was studied as a result of water flow, while the hydraulic gradient was used as the independent variable. Three stages of the experiment were distinguished: increase in pressure, fluidization (fluidization-defluidization cycle) and decrease in pressure. The obtained data were used to estimate the critical hydraulic gradient and two values of the minimum fluidization velocity for the increasing and decreasing hydraulic gradients, named fluidizing and defluidizing velocity, respectively. Modelling of the fluidization process was based on the assumption that there are zero effective stresses. Based on the model, the critical hydraulic gradient and minimum fluidization velocities were determined for the increasing and decreasing hydraulic gradients. Studies on sands of different granularity showed the consistency of the model and experimental results. Fluidization in experimental studies was observed for lower values of the hydraulic gradient than was predicted by the model. The most significant discrepancy between the model and experimental results occurred for the finest sand, while the smallest discrepancy was found for the sand with the highest porosity and a relatively narrow particle size distribution. The distinction of the fluidizing and defluidizing velocities is not only relevant for sand with grains larger than 0.5 mm.