Capturing particle-particle interactions for cylindrical fibrous particles in different flow regimes

doi:10.1016/j.powtec.2018.02.050

Powder Technology

Volume 330, 1 May 2018, Pages 418-424

https://doi.org/10.1016/j.powtec.2018.02.050 Get rights and content

Highlights

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DEM is used to study the sedimentation of fibrous particles in different flow regimes.
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The dynamic behaviour of fibrous particles settling for single and multi- particle systems is studied.
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In freely falling particles, increasing the interactions leads to a decrease in the average incident angle of particles.
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At high Reynolds numbers (jet flow) the effect of interactions on particles orientation become insignificant.

Abstract

Non-spherical particles are widely used in the chemical and pharmaceutical industries. Often these particles are over-simplified as equivalent spherical particles for calculation of drag forces and particle-particle interactions. We have developed a three-dimensional discrete element method (DEM) for rigid cylindrical fibrous particles with a high aspect ratio. In this method, a cylindrical particle was represented as overlapped multiple spheres. The diameter of the spheres was the same as that of the fibrous particle, while the number of spheres was determined by the length of the fibrous particle. The simulations were carried out for different numbers of particles to study the dynamics of fibrous sedimentation and also to investigate the effect of particle-particle interactions on the average preferred orientation and terminal velocities of the particles. The simulation results are compared to experimental data with good agreement. It is shown that the particles interactions have a significant effect on the particles average orientation and terminal velocity in freely falling particles. However, for fibrous particles moving in a jet flow, the interactions have negligible influence on the particle orientation and terminal velocity.

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 ≤ Re_p ≤ 100) [10] and co-current jet flows (Re_p~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 (ρ_s/ρ_f = 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.

References (26)

G. Qi et al.
Velocity and orientation distributions of fibrous particles in the near-field of a turbulent jet
Powder Technol.
(2015)
H. Ma et al.
CFD-DEM simulation of fluidization of rod-like particles in a fluidized bed
Powder Technol.
(2017)
T. Oschmann et al.
Numerical investigation of mixing and orientation of non-spherical particles in a model type fluidized bed
Powder Technol.
(2014)
C. Yin et al.
Modelling the motion of cylindrical particles in a nonuniform flow
Chem. Eng. Sci.
(2003)
G.Q. Qi et al.
Aerodynamics of long fibres settling in air at 10 < Re < 100
Powder Technol.
(2013)
G.Q. Qi et al.
PTV measurement of drag coefficient of fibrous particles with large aspect ratio
Powder Technol.
(2012)
S.M. Iveson et al.
Nucleation, growth and breakage phenomena in agitated wet granulation processes: a review
Powder Technol.
(2001)
H.P. Zhu et al.
Discrete particle simulation of particulate systems: theoretical developments
Chem. Eng. Sci.
(2007)
Y. Chen et al.
A drag force correlation for approximately cubic particles constructed from identical spheres
Chem. Eng. Sci.
(2015)
M. Zastawny et al.
Derivation of drag and lift force and torque coefficients for non-spherical particles in flows
Int. J. Multiphase Flow
(2012)

K. Vollmari et al.

Experimental and numerical study of fluidization and pressure drop of spherical and non-spherical particles in a model scale fluidized bed

Powder Technol.

(2016)

A. Hölzer et al.

New simple correlation formula for the drag coefficient of non-spherical particles

Powder Technol.

(2008)

W. Zhong et al.

DEM/CFD-DEM modelling of non-spherical particulate systems: theoretical developments and applications

Powder Technol.

(2016)

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View full text

Capturing particle-particle interactions for cylindrical fibrous particles in different flow regimes

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Governing equations

Simulation setup

Computational algorithm

Results and discussion

Conclusions

Acknowledgements

Powder Technol.

Powder Technol.

Powder Technol.

Chem. Eng. Sci.

Powder Technol.

Powder Technol.

Powder Technol.

Chem. Eng. Sci.

Chem. Eng. Sci.

Int. J. Multiphase Flow

Powder Technol.

Powder Technol.

Powder Technol.