Experimental and simulation study of solid flows in beads mill

Nanoparticles (NPs) are widely used in various applications. NPs agglomeration will alter its physical and chemical properties. To overcome this, dispersion of NPs by beads mill is desirable to achieve good dispersion stability. Experimental works to investigate the dynamics of the system is complicated and high cost. On the other side, numerical models offer alternative method inexpensively. In this research, the effect of impeller rotation speed on the NPs by experimental and simulation study dispersion state was discussed. Experimental results showed that rotational speed of 2400 rpm could introduce a better reduction size and dispersion state than that of 1200 and 1800 rpm. Numerical modelling via Discrete Element Method (DEM) simulation was carried out to study the solid velocity distribution profile during the dispersion process. Experimental and simulation results were correlated to investigate the relation between the particle size distribution and the particle velocity distribution profile.     

Preparing samples for fullerene C60 hazard tests: Stable dispersion of fullerene crystals in water using a bead mill

A dispersion method using a bead mill was developed for preparing suspensions used in hazard testing of nanosize fullerenes. Fullerene crystals (C₆₀) of several μm to 100 μm in size were wet ground using a bead mill and dispersed in pure water containing Tween 80 as a dispersant. The particle size, dispersing condition, crystallization and fullerene oxide in the suspension prepared by bead milling were evaluated. As a result, the conditions of bead milling where the sizes of fullerene crystals were reduced to 100 nm or less were found, which made it possible to prepare a fullerene aqueous dispersion system using Tween 80. Furthermore, by centrifugation, it became possible to prepare fullerene suspensions in which the particle size of the majority of the suspended particles was 100 nm or less. The prepared suspensions remained stable for almost two months. Controlling milling conditions and milling time made it possible to achieve dispersion while maintaining the crystallized state of the fullerenes, and operating in anaerobic conditions under shade effectively suppressed the production of oxidized fullerenes. The fullerene suspensions prepared by bead milling and centrifugation were suitable for endotracheal administration tests and inhalation tests using rats.     

Modeling breakage and motion of black pepper seeds in cryogenic mill

The present investigation aims to represent three-dimensional motion and breakage phenomena of black pepper seeds in the cryogenic mill (hammer mill) using discrete element method (DEM). In DEM modeling, bonded particle model was coupled with Hertz-Mindlin contact model. Calibration method was used to select appropriate model (bond) parameters. The calibrated set of bond parameters includes 3.12?×?10¹¹?Pa?m^?1 normal stiffness; 1.56?×?10¹¹?Pa?m^?1 shear stiffness; 3.88?×?10⁸?Pa critical normal stiffness; 1.94?×?10⁸?Pa critical shear stiffness. Besides, the validity of calibrated parameters was tested in the hammer mill. The observed qualitative and quantitative results (breakage and flow pattern) of numerical and experimental approaches were in good agreement. Based on these results, a few prefatory suggestions were provided to improve the design aspects of the mill. Overall, DEM modeling offered a better understanding of particle breakage and flow pattern in the mill.     

Analysis of grinding kinetics in a laboratory ball mill using population-balance-model and discrete-element-method

Grinding processing consumes a lot of energy in mineral processing, but it is a low-efficiency process in which only approximately 1% of the total energy is used to reduce the actual particle size. Therefore, an efficient operation in the grinding process increases the competitiveness of the production and is an essential for enhancing the energy efficiency of the entire mineral processing procedure.Therefore, the study will focus on to finding a different method to predicting the particle size distribution of the ball mill, by using the PBM which reflects the actual size distributions of ground product and the DEM which can understand the internal particle behavior in the mill chamber. First, the grinding parameters were calculated by applying size distributions of ground product under various conditions to PBM and the behaviors of the particles inside the ball mill obtained through DEM were analyzed to predict the distribution of the impact energy used for grinding. Next, the relational expression between the grinding rate parameter and the normal force applied to the grinding materials was derived. Using the relational expression derived from this study, it was confirmed that the size distributions in other conditions can be predicted.     

Numerical investigation of effects of particle shape on dispersion in an isotropic turbulent flow

To investigate the effects of particle shape on the dispersion in an isotropic turbulent flow, herein two direct numerical simulations are performed. The six degrees of freedom motion of spherical and spheroidal particles in a vertical uniform flow and a gas-particle two-way isotropic turbulent flow. The former, which is investigated using a numerical simulation with the Arbitrary Lagrangian-Euler (ALE) method, shows that a spheroidal particle travels with rotating and oscillating motions, which significantly affect the pressure and the friction force on the particle’s surface. The trend of the fluid force acting on the spheroidal particle’s surface also oscillates and differs from that on a spherical particle. The time variation of the fluid force on the spheroidal particle is modeled in the $C_D$ equation, which has a sine curve’s PDF relation with ${\mathit{Re}}_p$ and the particle’s maximum and minimum $C_D$ values. The latter simulation examines the effects of the particle shape on the dispersion with the motion model developed above. The particle’s dispersion behavior, which is analyzed by the statistical variable D and the Radial Distribution Function (RDF), shows that the dispersion motion is markedly affected by particle’s sphericity, especially for particles with a relatively small sphericity. The results suggest that this difference can influence ignitability, flammability, and the concentration of combustible gases released by particles, and requires further study.     

Mathematical modeling of coating time in dry particulate coating using mild vibration field with bead media described by DEM simulation

To theoretically understand the previously reported dry particulate coating process using a mild vibration field with a bead media, a mathematical analysis model of the dry coating system was developed. In this coating process, an ordered mixture with coarse host particles (drug-loaded ion exchange resin, diameter approximately 100 µm) and fine guest particles (acrylic polymer particle, primary particle size of approximately 100 nm) is formed using a vibrating a vessel. Second, the guest particles on the host particulate surface are firmly fixed using the collision of coated particles zirconia beads (diameter 1.5 mm). Our model assumes that the unfixed guest particles are fixed by particle-to-particle collisions (C_c) provided by the apparatus, thereby increasing the coating ratio. C_c was estimated using the discrete element method and some experimental results. The model includes parameters such as the number of C_c, host particles and unfixed guest particles. The coating time simulated by the established model equation in this study fits well with the experimental results of the dry process. It depends on the ratio of the number of collisions contributing to the increased coating ratio to the number of unfixed guest particles on the surface of host particles.     

The effects of friction characteristic of particle on milling process in a horizontal rice mill

The physical and mechanical properties of rice are of significant change during milling from brown rice to white rice, especially the friction characteristics. In order to clarify the effects of roughness of rice surface on the milling process and mechanism, in this work, motion of spherical particle in a horizontal rice mill under different static friction coefficients (i.e., between particle and cylinder sieve wall μ_s,pt and between particles μ_s,pp) was simulated using the discrete element method. The uniformity of axial motion and circular motion were qualitatively and quantitatively analyzed, which are characterized by introducing the axial dispersion coefficient and uniformity index, respectively. Then, the effects of static fiction coefficients on residence time and collision energy among particles were discussed. The results indicated that the μ_s,pt mainly affects the axial motion while the μ_s,pp primarily influences on the circular motion. The residence time is strongly affected by the uniformity of axial motion while the collision energy is significantly influenced by the uniformity of circular motion. Finally, the relationship between friction characteristics and milling performance can be described based on the method of polynomial fitting. This work is useful for providing essential references to control the quality of milled rice.     

Enhancing particle swarm optimization algorithm using two new strategies for optimizing design of truss structures

This work develops an augmented particle swarm optimization (AugPSO) algorithm using two new strategies,: boundary-shifting and particle-position-resetting. The purpose of the algorithm is to optimize the design of truss structures. Inspired by a heuristic, the boundary-shifting approach forces particles to move to the boundary between feasible and infeasible regions in order to increase the convergence rate in searching. The purpose of the particle-position-resetting approach, motivated by mutation scheme in genetic algorithms (GAs), is to increase the diversity of particles and to prevent the solution of particles from falling into local minima. The performance of the AugPSO algorithm was tested on four benchmark truss design problems involving 10, 25, 72 and 120 bars. The convergence rates and final solutions achieved were compared among the simple PSO, the PSO with passive congregation (PSOPC) and the AugPSO algorithms. The numerical results indicate that the new AugPSO algorithm outperforms the simple PSO and PSOPC algorithms. The AugPSO achieved a new and superior optimal solution to the 120-bar truss design problem. Numerical analyses showed that the AugPSO algorithm is more robust than the PSO and PSOPC algorithms.     

Description of TiO2 thin films treated in NH3 atmosphere by optical dispersion models

The influence of a reductive ammonia atmosphere on TiO₂ sol-gel film structural and optical characteristics was investigated. X-ray Diffraction technique, X-ray photoelectron spectroscopy and Rutherford Back Scattering analysis were applied to study the crystallinity, oxidation state and element concentration profile of the modified films. Their optical properties were investigated by UV-Vis spectroscopy. The refractive index and extinction coefficient were obtained by fitting theoretical transmittance curves to experimental ones using Forouhi-Bloomer (FB) and Tauc-Lorentz (TL) physical models. Both models revealed slight decrease (up to 2.6-2.7 eV) of the FB and TL band gaps with increase of the treatment temperature. These results were discussed in terms of the additional energy levels creation due to the defect TiO₂ structure formation during thermal treatment in reductive atmosphere.     

Understanding production of fines in batch ball milling for mill scale-up design using the population balance model

The rate of production of fine material in the batch mode of grinding operation forms the basis for determination of the grindability parameter of the Bond approach and the breakage distribution function of the population balance model (PBM) approach to the mill scale-up design. For a given set of mill operating conditions, the rate of production of fines is determined by the breakage characteristics and production history of the material being ground. Another important aspect is the variation in the rate of production of fines with grinding time. With a view to developing a clear understanding of these aspects, a detailed analysis of variations in the rate of production of fines was carried out using the PBM framework and two well-known functional forms for the specific breakage rate and breakage distribution parameters. In this paper, it has been shown how the results of this analysis can be used for: (i) obtaining more accurate estimates of the breakage distribution parameters by performing just one short-duration batch grinding experiment, and (ii) explaining variation in the Bond Work index with the product size in terms of the exponent of particle size in the expression for the specific breakage rate function: $k_j=A^1{x}_j^{\alpha }$.     

Multi-objective optimization design using amended particle swarm optimization and Taguchi method for a six-phase copper rotor induction motor

A multi-objective optimization design technique for a six-phase copper rotor induction motor is proposed. The amended particle swarm optimization (PSO) and Taguchi methods combined with finite element analysis are used in this design technique. The objectives in the first-stage optimization are the minimization of manufacturing cost and starting current. In the second-stage optimization, the objectives are the maximization of efficiency, power factor and output torque. The Taguchi method can optimize the machine parameters of performance characteristics in electrical discharge machining. The experimental results are further transformed into the signal-to-noise ratios and amended PSO coefficients based on amended PSO analysis with regard to multiple performance characteristics index values. The results of the optimizations showed significant reduction in terms of the use of magnets as well as improvement in the machine performance. Finally, the experimental results confirm the validity of the proposed optimization design approach.     

Influence of dry grinding in a ball mill on the length of multiwalled carbon nanotubes and their dispersion and percolation behaviour in melt mixed polycarbonate composites

Ball milling of carbon nanotubes (CNTs) in the dry state is a common way to produce tailored CNT materials for composite applications, especially to adjust nanotube lengths. For Nanocyl^TM NC7000 nanotube material before and after milling for 5 and 10 h the length distributions were quantified using TEM analysis, showing decreases of the mean length to 54% and 35%, respectively. With increasing ball milling time in addition a decrease of agglomerate size and an increase of packing density took place resulting in a worse dispersability in aqueous surfactant solutions. In melt mixed CNT/polycarbonate composites produced using masterbatch dilution step, the electrical properties, the nanotube length distribution after processing, and the nano- and macrodispersion of the nanotubes were studied. The slight increase in the electrical percolation threshold in the melt mixed composites with ball milling time of CNTs can be assigned to lower nanotube lengths as well as the worse dispersability of the ball milled nanotubes. After melt compounding, the mean CNT lengths were shortened to 31%, 50%, and 66% of the initial lengths of NC7000, NC7000-5 h, and NC7000-10 h, respectively.     

Modeling of the mechanical treatment of a solid reactant under active gas in the high-energy mill on the example of the titanium-gaseous nitrogen system

A mathematical model for calculating the dynamics of product formation (titanium nitride) in the gas (nitrogen) medium during milling and activation of a solid reactant in the high-energy mill was built based on a new macrokinetic theory of mechanochemical synthesis. For the first time, the model uses an approach based on the theory of short-lived active sites to take into account the intensification of chemical processes in the volume of the solid and on the surface formed during destruction. The model can calculate the following macroscopic parameters: temperature, depth of chemical transformations, size of particles, gas pressure, structural defects of particles (excess energy). An analytical formula for determining the mechanochemical reaction on an active surface was derived. The modelling results were compared with the known experimental data.     

DEM simulation for optimal design of powder mixing in a ribbon mixer

A ribbon mixer is often employed in powder mixing in a wide range of engineering fields. The structure of the ribbon mixer is extremely complicated. This structure makes it difficult to understand the mixing mechanism by experimental approaches due to problems related to accurate sampling. At present, the mixing mechanism in the ribbon mixer is empirically identified as convection, despite a lack of precise assessment. Additionally, experimental investigations to find the optimal design of the ribbon mixer have not been sufficiently conducted because of its prohibitive cost. As such, there is a lack of sufficient discussion concerning the design for better mixing in the ribbon mixer. Numerical technologies represent a promising approach for solving the aforementioned problems. Significant improvements in computer hardware have enabled numerical models such as the discrete element method (DEM) to be positively employed in powder mixing. In the current study, an identification approach is developed for convective mixing, and besides, the study explores an effective parameter for better mixing in the ribbon mixer using the DEM. A swept volume measurement approach due to paddle movement is newly developed to identify the main mixing mechanism as convection. Sensitivity analyses are performed to find an effective parameter for better mixing. Through the sensitive analyses, the blade width is indicated as an important factor for achieving better mixing. Moreover, this study shows that the relationship between the swept volume and mixing index remains, even if the paddle width changes. Thus, the swept volume measurement method is revealed as useful for identifying the mechanism as convection in the ribbon mixer. Thus, not only novel finding regarding the blade width for better mixing but also the development of an approach for identifying convective mixing in the ribbon mixer is presented herein. Incidentally, convection being the dominant mechanism is consistent with the novel finding regarding blade width achieving better mixing.     

Direct numerical simulation of ignition of a single particle freely moving in a uniform flow

In this study, a direct numerical simulation (DNS) of the ignition of a single particle freely moving in a uniform flow is performed to investigate the particle’s ignition behavior in detail. The Arbitrary Lagrangian-Eulerian (ALE) method is employed to compute the six degrees of freedom motion of a particle (Zhang et al., 2015). The computational setting follows the experiment designed by Lee and Choi (2015). The volatile gas that is composed of methane blows out at the particle surface. Its velocity is calculated by Ex-CPD model (Umemoto et al., 2017) and its direction is set perpendicular to the particle surface. The ignition behavior is compared with that observed in the experiment. The effect of the particle’s shape is also investigated. Results show that the ignition delay of the particle and the flame inclined angle are in good agreement with that of the experiment. While examining the combustion of the gas phase by considering the variation of Flame Index (FI), it is found that a premixed and diffusion regions are formed around the particle after the devolatilization starts. The gas phase ignites at the boundary of the premixed and diffusion regions and the flame propagates towards the particle. This causes a rapid increasing of the temperature and the volatile velocity on the particle surface. Finally, a diffusion flame is formed and reaches a stable state around the particle. It is also revealed that the flame keeps spherical despite the spheroidal shape of the particle.     

Smoothed particle hydrodynamics for numerical simulation of underwater explosion

 Underwater explosion arising from high explosive detonation consists of a complicated sequence of energetic processes. It is generally very difficult to simulate underwater explosion phenomena by using traditional grid-based numerical methods due to the inherent features such as large deformations, large inhomogeneities, moving interface and so on. In this paper, a meshless, Lagrangian particle method, smoothed particle hydrodynamics (SPH) is applied to simulate underwater explosion problems. As a free Lagrangian method, SPH can track the moving interface between the gas produced by the explosion and the surrounding water effectively. The meshless nature of SPH overcomes the difficulty resulted from large deformations. Some modifications are made in the SPH code to suit the needs of underwater explosion simulation in evolving the smoothing length, treating solid boundary and material interface. The work is mainly focused on the detonation of the high explosive, the interaction of the explosive gas with the surrounding water, and the propagation of the underwater shock. Comparisons of the numerical results for three examples with those from other sources are quite good. Major features of underwater explosion such as the magnitude and location of the underwater explosion shock can be well captured. Received: 2 April 2002 / Accepted: 20 September 2002     

Analysis,design and optimization of structures using force method and genetic algorithm

In the first part of this paper, the energy formulation of the force method is presented and analysis is performed using genetic algorithm. Two simple examples are provided to show the accuracy of the approach. In the second part, an efficient method is developed for designing structures with prescribed stress ratios for its members. The genetic algorithm performed very well and designs with specified stress ratios were achieved with a good convergence rate. A unit value of c_i for all the members of a structure corresponds to the well known fully stressed design. In the third part, minimum weight design is formulated by the additional conditions being imposed on the design process. Again, genetic algorithm showed to be a powerful means for optimization. Copyright © 2005 John Wiley & Sons, Ltd.