Arseni, Angelica Maria Giovanna (2020) CFD modelling and simulation of dense granular flow in a rotating drum. [Tesi di dottorato]

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Granular materials are ubiquitous in everyday life, in nature as well as in industry; therefore the prediction of granular fluid dynamics is of great interest in both physics and engineering research. A common complex granular flow is that occurring in rotating drums, which are widely employed as mixers, separators, dryers, reactors and granulators in different industrial processes. Numerical simulations can provide a useful tool to understand the physics underlying the dynamics of these materials. In most of the works available in the literature, the commonly adopted numerical approach to study granular materials is the Discrete Element Method (DEM), where the material is modelled as an assembly of rigid particles, and the interactions among particles are explicitly considered. Although DEM has the advantage to describe the discrete nature of the flow, a relatively limited number of particles can be managed. This drawback becomes important for large-scale flow modelling, as for the case of an industrial drum, containing billions of particles; a continuum approach, where the solid phase is treated as a continuum, is possibly more suitable. In this work, we present 3D Finite Volume (FV) simulations of dense granular flow of non-cohesive beads inside a rotating cylinder, adopting the visco-plastic Jop-Forterre-Pouliquen constitutive model for the granular medium stress tensor. We investigated in our simulations different flow conditions, by changing the cylinder aspect ratio and the drum angular velocity. Moreover, the material parameters and the particle dimensions, appearing in the constitutive equation, are systematically varied, to understand their effects on the main features of flow in the cylinder. The results obtained from our simulations are also compared with several experimental results available in the literature for the mono-disperse and bi-disperse case. We reproduce the flow configurations sequence in rotating drums, ranging from rolling to centrifuging, in good agreement with experimental results. We capture some distinctive features of granular flow in a rotating drum, such as a Bagnold profile followed by an exponential tail for velocity throughout the depth of granular bed, the existence of axial components of the surface velocity, and the difference of the flow field near the lateral wall and at the symmetry plane. Moreover, we investigated the segregation patterns of bi-disperse mixture varying the filling degree of the rotating drum. This validation opens up the feasibility of characterising a wide variety of regimes by changing both physical and geometric parameters, with the possibility of discovering new dynamical regimes, and of calculating several flow quantities difficult to be accessed through experiments.

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