Gravity flow of granular materials through a vertical channel

Abstract

Grains and powders are the second most widely used materials in industries and daily life after simple fluids. There are a variety of granular materials such as food grains, detergents, tablets and capsules, fertilizers, coal, and catalyst pellets. They show both solid-like and fluid-like behaviour. Experiments reveal unexpected features and exceptionally complex signatures in granular flows. Unlike simple fluids, their rheology is not well understood. Models based on particle dynamics are available to simulate such flows. However, simulations at an industrial scale are computationally highly expensive because of handling extremely large number of particles in a unit operation. Hence there is dire need of a constitutive theory which can predict the flow behaviour well. Since many decades, researchers have attempted to develop theories for granular flows based on soil plasticity, kinetic theory of dense gases, non-equilibrium statistical mechanics, and non-local approaches. Unfortunately, a universally accepted constitutive theory for granular flows is lacking. In conclusion, the gravity flow chosen in the present work is appropriate to examine the continuum models. None of the models, which were recently developed and are examined here, has the ability to predict all the features well. The lack of suitable boundary conditions is an another issue. This highlights the need for more work to develop a universally accepted theory for granular flow. Fluctuations in the wall stresses are observed in the simulations when periodic boundary conditions are used in two directions. It remains to be seen whether these features are preserved when such boundary conditions are not used and the actual bin problem is solved. The results may have an important bearing on the design of vertical vessels handling granular materials.