A MICROSTRUCTURAL MODEL OF FLOWING TERNARY MIXTURES OF EQUAL-DENSITY GRANULES IN HOPPERS

Size segregation during gravity flow of ternary granular mixtures was monitored by measuring discharge rates at various stages of the batch emptying of a funnel-flow hopper. Average static and flowing bulk densities as well as discharge rates of the mixtures were determined experimentally as a function of particle size ratios and weight fractions in the mixture. A simple geometric model is proposed, based on the relative number density of the different size particles to explain the observed phase transitions of the flowing-bed microstructure. The model is used to predict the critical weight fractions in the mixture for particles of a given size to make up a continuous phase as a function of particle shape and size ratio. The other two particle sizes are treated as discrete or dispersed phases of the flowing microstructure. Size segregation of the discrete phases of particles during flow is minimised when the ternary mixture is continuous in the finest particle size while coarse continuous mixtures are shown to be prone to size segregation as a function of size ratio and pore geometry. The discharge rates are found to correlate closely with the predicted phase transitions of the flow fields.