Capturing particle-particle interactions for cylindrical fibrous particles in different flow regimes
Graphical abstract
In this work discrete element method is used to model the aerodynamic behaviour of cylinder–like fibrous particles in different flow regimes. The main aim of this work was to study the influence of particle-particle interactions on their orientation and terminal velocity. The simulation results are validated using experimental data.
Introduction
Cylinder-like fibrous particles transport in fluids can be seen in different industrial processes including biomass combustion, polymer suspension, fluidized beds and papermaking [1,2]. Understanding the aerodynamic behaviour of fibrous particles is very important to design, scale-up, and improve the performance of these industrial systems [1,3]. Some studies have been conducted on the motion of cylindrical particles in two-phase flows [[3], [4], [5]]. Yin et al. [5] have studied the motion of a single PVC cylindrical particle in stagnant water. Their simulation results for both high and low speed flows show a good agreement with experimental data. Fan et al. [2] have conducted an experimental study on settling of single slender particles with different aspect ratios. They concluded that slender particles tend to orient horizontally and during settling, they oscillate around the stable horizontal orientation as they interact with the fluid. Most of the studies, including the above-mentioned ones, on the motion of cylindrical particles have been conducted on single-particle systems, and there are very few experimental and numerical investigations on multi-particle systems for non-spherical particles [[6], [7], [8]].
Recently Qi et al. [1,9,10] have experimentally investigated the aerodynamics of fibrous particles with a large aspect ratio (L/d = 40) in different flow regimes. They studied the effect of the volume fraction of fibrous particles on the average terminal velocity (the velocity of a falling particle when the drag force equals to the gravity force) and orientation both in free fall (10 ≤ Rep ≤ 100) [10] and co-current jet flows (Rep~70, 000) [1]. The experimental results of their study revealed that, by increasing the particles volume fraction, the average particles terminal velocity increases while the average orientation angle decreases [9,10]. However, in jet flows both the average orientation and terminal velocity appear to remain unchanged with an increase in the particles volume fraction [1]. This behaviour is ascribed to the ‘aerosol cloud’ effect where the particles tend to move together as a cloud at a velocity greater than individual particle [11]. Nevertheless, another hypothesis is that, by increasing the number of particles, the interactions between the particles increase, which may also contribute to the velocity increase. Experimental studies suffer from some limitations, therefore, a combination of theoretical analysis and experimental studies will offer a better insight into this particulate system. The aim of this research paper is to simulate the motion of fibrous particles in air flow, with particular interest on the effect of particle-particle interactions on particles terminal velocity and orientation in the multi-particle system.
Section snippets
Governing equations
To simplify the model, the following assumptions have been applied:
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The fluid (air) velocity is not affected by the motion of particles, since the volume fraction of the solid phase in the system is very small (<10−6).
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The effect of electrostatic force and Van der Waals force are neglected, because these forces are insignificant for particles larger than 10 μm [12].
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The buoyancy and Basset history forces are neglected as for a large solid to fluid density ratios (ρs/ρf = O (103)), Basset term and
Simulation setup
In this simulation, fibrous particles are introduced randomly from a plane on top of a pipe (see Fig. 1). As the particles move down under the gravity force they can interact with each other and with the pipe walls. When they reach a distance of 1.5 m from the releasing point, their velocity and orientations are saved into a file.
The particles properties and the model parameters used in the simulation are given in Table 1. To represent the cylindrical fibrous particles the multi-sphere method
Computational algorithm
A three dimensional discrete element method (DEM) is developed using FORTRAN programming language to calculate particles movement and interactions. After reading the required initial data such as particle properties (density, size, and initial locations), the simulation domain is specified by defining the geometry (walls) using rectangular planes. Then, after defining the initial particle positions and velocities, a contact search algorithm is applied to determine whether there are
Results and discussion
The Simulations are carried out using a range of numbers of particles (from 1 to 10,000 particles) and each simulation repeated four times using random initial positions and orientations, to ensure that the results are independent of initial conditions such as initial orientation of the particles.
Conclusions
In this work, the orientation angle and the falling velocities of fibrous particles with a large aspect ratio (L/D = 50) have been studied by the DEM method. The simulation results show that a single falling particle or particles without interactions prefer to fall with a horizontal orientation, which is consistent with the experimental observations. By increasing the number of particles, the probability of particle-particle interactions increases, which leads to a decrease in the average
Acknowledgements
This work was supported by an Australian Government Research Training Program (RTP) Scholarship; and Mayne Pharma International Pty Ltd.
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