Computational study of the granular flow through an elbow

If a granular material inside a U-shaped tube is vertically vibrated, some interesting phenomena can be observed. Depending on the amplitude and frequency of vibration, the wall-grain interaction and the presence of an interstitial fluid, the granular material inside the tube can show a tendency to accumulate in one of his branches [1, 2]. The influence of the container walls on the granular material during the vibration and the temporal coupling between the motion of the container and the flowability of the grains, is not yet understood. The ability of the granular material to flow inside the container can vary in a single oscillation cycle, and be sensible the relation between grain size and tube diameter. In order to shed light on the role played by the walls on the grains flow and its temporal evolution during the vibration, we study through Discrete Element Method (DEM) the flow of grains in a half U-tube (L-shaped container), under vertical vibration. We investigate some rheological aspects from this system like the velocity and stress profiles, and their evolution in a single oscillation cycle. We explore how the amplitude and frequency of vibration affects the granular flow through the elbow of the tube and the influence of the walls on the net transport of grains

Study of solids conveying in single screw extruders based on flow dynamics and structure of solid pellets

Flow of granular matter is presently a subject of extensive research, due to the characteristics of this type of systems (e.g., dilatancy, segregation, arching, clustering) and relevance to various application areas, such as civil construction, agriculture, food processing, geophysics, pharmacology [1, 2]. The plasticating process in single screw polymer extrusion is one of the areas where this research can help to increase the existing knowledge. In the initial turns of an Archimedes-type screw, loose pellets are conveyed forward. However, traditional analyses assume the movement of an elastic solid plug at constant velocity. This work follows previous efforts to predict the characteristics of this flow using the Discrete Element Method (DEM) [3, 4]. Two boundary conditions are considered: a) open-discharge, implying that no compaction of the solids occurs and b) close-discharge, leading to a pressure increase. The dynamics and the structure of the flow were studied by computing the cross- and down channel velocity profiles, the coordination number distribution, the output rate, the residence time distribution and the density profile, as a function of the friction force grain-wall, screw speed and pellet size. The model is able to capture the process of plug formation towards the discharge, and its predictions provide an insight into possible flow fluctuations

Flow dynamics and structure of solid pellets along the channel of a single screw extruder

Plasticating single screw extrusion involves the progressive compaction and heating of loose solid pellets that eventually melt, form a relatively homogenous stream and are subsequently pumped through a shaping tool. Traditional analyses of the solids conveying stage assume the sliding of an elastic solid plug due to differential wall friction coefficients. However, not only the corresponding predictions may fail considerably, but it is also well known that, at least in the initial screw turns, pellets are far from compact. This work follows previous efforts to model the flow of solids in the hopper and initial screw turns using the Discrete Element Method (DEM). The model considers the development of normal and tangential forces resulting from the inelastic collisions between the pellets and between them and the neighbouring metallic surfaces. As an example of the capability of the model to capture detailed features of granular flow, the effect of pellet size on flow is discussed.Fundação para a Ciência e a Tecnologia (FCT) - SFRH/BPD/39381/2007

Modelling pellet flow in single extrusion with DEM

Plasticating single-screw extrusion involves the continuous conversion of loose solid pellets into a pressurized homogeneous melt that is pumped through a shaping tool. Traditional analyses of the solids conveying stage assume the movement of an elastic solid plug at a fixed speed. However, not only the corresponding predictions fail considerably, but it is also well known that, at least in the initial screw turns, the flow of loose individual pellets takes place. This study follows previous efforts to predict the characteristics of such a flow using the discrete element method. The model considers the development of normal and tangential forces resulting from the inelastic collisions between the pellets and between them and the neighbouring metallic surfaces. The algorithm proposed here is shown to be capable of capturing detailed features of the granular flow. The predictions of velocities in the cross- and down-channel directions and of the coordination number are in good agreement with equivalent reported results. The effect of pellet size on the flow features is also discussed

The influence of pellet-barrel friction on the granular transport in a single screw extruder

The flow of individual polymer pellets along the solids conveying zone of a single screw extruder is studied by means of a numerical model based on the discrete element method (DEM). The effect of the pellet–barrel friction coefficient (μp–b) is discussed in terms of mass output, volume fraction, residence time distribution and velocity profiles. Model predictions are compared with experimental data, showing a good match, and with calculations using traditional analyses by assuming the movement of an elastic solid plug. As μp–b is increased, three regimes with distinct behavior were identified. For up to μp–b = 0.25 (this value coinciding with that of the pellet–screw friction coefficient), particle collisions drive the granular transport. For higher values, friction determines the conveying efficiency, but when μp–b > 2.00 a plug-type behavior is anticipated.Fundação para a Ciência e a Tecnologia (FCT) - PEst-C/CTM/LA0025/2013 (Projecto Estratégico - LA 25 - 2013-2014 – Strategic Project - LA 25 - 2013-2014)