A soft‐sphere‐imbedded pseudo‐hard‐particle model for simulation of discharge flow of brick particles

A novel hybrid approach of soft‐sphere‐imbedded pseudo‐hard‐particle model is proposed to cope with the complex collision of nonspherical particles. In this approach, the boundary of a host hard particle is covered by a series of soft‐spheres, which are allowed to oscillate about the equilibrium position according to the position, orientation, and shape configuration of the host particle. The collision processes are twofold: as a predictive process, particle‐particle interaction takes place through the collision between the distributed soft‐spheres, which causes subspheres to deviate from the equilibrium positions; as a corrective process, relaxation is superposed to allow the soft‐spheres to move back toward the equilibrium positions quickly. Consequentially, this process generates the force and torque on the host particle and determines its movement. Finally, after validation, this new model is used to explore the effects of aspect ratio and base angle on the discharge of brick particles in hoppers. © 2016 American Institute of Chemical Engineers AIChE J, 62: 3562–3574, 2016     

3D‐Flow Analysis by Means of Streamline Calculation

Over the last several years simulation software has become more and more important for mould design and process optimisation in polymer processing. Due to the mainly thin‐walled nature of most injection moulding parts, the currently used simulation programs are predominantly based on shell‐type elements with a two‐dimensional flow field in each element. Because of some assumptions, related to this specific calculation method, these so‐called 2.5D‐programs reach their limits in the simulation of complex‐shaped, thick‐walled mouldings. The fully three‐dimensional calculation of the injection moulding process offers a high potential which leads to improvement in result quality. This paper is a report of an investigation of streamlines in three‐dimensional flow fields, which typically occur in injection moulding. The streamlines were investigated by numerical simulation using 3D‐simulation software and by experiments. For the experiments a special mould has been designed, which enables pigments to be injected into the runner system of the mould. The injection can be done at different positions over the flow channel cross section. Thus the path of different coloured particles can be directly observed. The mould can be equipped with different inserts in order to allow an investigation of different geometries. In general, simulated and observed streamlines are in good agreement. Minor differences are due to special features in mould design or the calculation algorithm. The insights gained from this investigation can be applied to find appropriate simplifications in numerical 3D‐simulation in order to cut down computing times.     

Experimental and numerical investigation of the hydroerosive grinding

This paper presents an Euler-Euler approach for the numerical simulation of the hydroerosive grinding (HE) process. It describes a two-phase slurry flow consisting of a liquid and a dispersed solid phase which causes wear at walls of devices. The continuous fluid phase is solved using a finite volume scheme in which the Large Eddy Simulation (LES) [1] model is applied to resolve large-scale turbulent structures. The solid phase is dispersed and treated as a second continuum in which drag and lift forces as well as added mass, pressure and history force are taken into account. Considering particle-particle interactions, the granular model from Gidaspow [2] is used for particle volume concentrations over 1%. Investigations of erosion processes proofed that non-spherically shaped particles as well as harder particles increase the wear on devices significantly. Consequently, non-spherical particles are utilised for the hydroerosive grinding. Their steady drag, unsteady drag and lift coefficients, depending on the particle Reynolds number, are determined by a direct numerical simulation via an in-house LES Lattice-Boltzmann solver. This Lattice-Boltzmann method was presented for laminar flows by Hölzer and Sommerfeld [3]. In this work, interpolating functions of these coefficients are implemented in the Euler-Euler approach which enables the simulation of non-spherical particle transport. Hydroerosive grinding experiments in 3D throttles and 3D planar geometries are carried out to determine an erosion model depending on particle impact velocity, particle size, particle concentration and wall hardness. Implementation of a mesh-morphing algorithm combined with the Euler-Euler scheme of the commercial solver ANSYS CFX11 [4] enables an online simulation of the hydroerosive grinding process. Additionally, the online simulation is used to validate the applied numerical methods. Very good agreements are achieved and will be presented in this paper.     

Discrete element simulation of particle flow in arbitrarily complex geometries

Conventional simulations of dense particle flows in complex geometries usually involve the use of glued particles to approximate geometric surface. This study is concerned with the development of a robust and accurate algorithm for detecting the interaction between a spherical particle and an arbitrarily complex geometric surface in the framework of soft-sphere discrete element model (DEM) without introducing any assumptions. Numerical experiments specially designed to validate the algorithm shows that the new algorithm can accurately predict the contact state of a particle with a complex geometric surface. Based on the proposed algorithm, a new solver for simulation of dense particle flows is developed and implemented into an open source computational fluid dynamics (CFD) software package OpenFOAM. The solver is firstly employed to simulate hydrodynamics in a bubble fluidized bed. Numerical results show that a 3D simulation can predict the bubble size better than a 2D simulation. Subsequently, gas–solid hydrodynamics in an immersed tube fluidized bed is simulated. Results show that bubble coalescence and breakup behavior around the immersed tubes are well captured by the numerical model. In addition, seven different particle flow patterns around the immersed tubes are identified based on the numerical results obtained.     

An efficient and reliable predictive method for fluidized bed simulation

In past decades, the continuum approach was the only practical technique to simulate large‐scale fluidized bed reactors because discrete approaches suffer from the cost of tracking huge numbers of particles and their collisions. This study significantly improved the computation speed of discrete particle methods in two steps: First, the time‐driven hard‐sphere (TDHS) algorithm with a larger time‐step is proposed allowing a speedup of 20–60 times; second, the number of tracked particles is reduced by adopting the coarse‐graining technique gaining an additional 2–3 orders of magnitude speedup of the simulations. A new velocity correction term was introduced and validated in TDHS to solve the over‐packing issue in dense granular flow. The TDHS was then coupled with the coarse‐graining technique to simulate a pilot‐scale riser. The simulation results compared well with experiment data and proved that this new approach can be used for efficient and reliable simulations of large‐scale fluidized bed systems. © 2017 American Institute of Chemical Engineers AIChE J, 63: 5320–5334, 2017     

Hybrid PIV/PTV measurements of velocity and position distributions of gas‐conveyed particles in small,narrow channels

Pneumatic conveying of particles is generally applied in large ducts. However, new applications are emerging which benefit from millimeter‐sized ducts; for example, triboelectric separators where intensive wall‐particle contact is desirable. An optical method is proposed to measure the distribution of the position and velocity of 100–1000 μm particles in such narrow ducts. Images of the system are captured using a digital camera on which a Hough transform is applied to detect the particles and their positions. The velocities are acquired by applying a hybrid particle tracking and particle image velocimetry approach. This made it is possible to overcome challenges caused by suboptimal lighting, nonsmooth background, and a large ratio between particle and duct diameter . It is shown that the algorithm is subpixel accurate when sufficient particles can be sampled. Finally, typical results are shown to illustrate the method's capabilities. © 2015 American Institute of Chemical Engineers AIChE J, 61: 3616–3627, 2015     

二元颗粒混合物在下行床反应器中局部颗粒速度的分布

采用PV-4A光导纤维测速仪在一高2m、直径30mm的下行床反应器中较系统地测定了FCC催化剂及玻璃珠各自在6个截面8个不同径向位置局部颗粒速度的轴向及径向分布,研究了操作条件及喷口位置对该两种不同物料局部颗粒速度径向分布的影响,进而将FCC催化剂和玻璃珠两种物料分别同时进入下行床反应器中,考察了以不同混合比组成的二元组分混合物的颗粒速度径向双峰分布特点.最后,分别给出了预测FCC催化剂及玻璃珠的量纲1局部颗粒速度经验关联式.实验结果表明：物性不同的粒子其局部颗粒速度径向分布差别较大,二元组分混合物的颗粒速度径向分布较FCC催化剂单一颗粒趋于均匀,且随表观气速增加,颗粒速度分布双峰特性愈为明显,可见在下行床进入细颗粒的同时加入粗颗粒引起了气固流动机制的变化与颗粒速度分布的平滑.     

Monte Carlo modeling of fluidized bed coating and layering processes

A stochastic modeling approach based on a Monte Carlo method for fluidized bed layering and coating is presented. In this method, the process is described by droplet deposition on the particle surface, droplet drying and the formation of a solid layer due to drying. The model is able to provide information about the coating coverage (fraction of the particle surface covered with coating), the particle‐size distribution, and the layer thickness distribution of single particles. Analytical solutions for simplified test cases are used to validate the model theoretically. The simulation results are compared with experimental data on particle‐size distributions and layer thickness distributions of single particles coated in a lab‐scale fluidized bed. Good agreement between the simulation results and the measured data is observed. © 2016 American Institute of Chemical Engineers AIChE J, 62: 2670–2680, 2016     

三维混合层中大涡结构与扩散颗粒的相互作用

In order to understand the interaction between large-scale vortex structure and particles, a two-way coupling temporal mixing layer laden with particles at a Stokes number of 5 with different mass loading planted initially in the upper half region is numerically studied. The pseudospectral method is used for the flow fluid and the Lagrangian approach is employed to trace particles. The momentum coupling effect introduced by a particle is approximated to a point force. The simulation results show that the coherent structures are still dominant in the mixing layer, but the large-scale vortex structure and particle dispersion are modulated. The length of large-scale vortex structure is shortened and the pairing is delayed. At the same time, the particles are distributed more evenly in the whole flow field as the mass loading is increased, but the particle dispersion along the transverse direction differs from that along the spanwise direction, which indicates that the effect by the addition of particle on the suanwise large-scale vortex structure is different from the streamwise counterpart.     

A proposal for discrete modeling of mechanical effects during drying,combining pore networks with DEM

A discrete modeling approach is introduced to investigate the influence of liquid phase distributions on damage and deformation of particle aggregates during convective drying. The approach is illustrated on a simple 3D aggregate structure, in which monosized spherical particles are arranged in a cubic packing and bonded together at their contacts; the mechanical behavior of this aggregate is simulated by discrete element method (DEM). Liquid phase distributions in the void space are obtained from drying simulations for a pore network. In a one‐way coupling approach, capillary forces are computed over time from the filling state of pores and applied as loads on each particle in DEM. A nonlinear bond model is used to compute interparticular forces. Simulations are conducted for various drying conditions and for aggregates with different mechanical properties. Microcracks induced by bond breakage are observed in stiff material, whereas soft material tends to shrink reversibly without damage. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Numerical simulation of the transient temperature distribution inside moving particles

This work is devoted to the two‐dimensional (2D) numerical simulation of heat and fluid flow by granular mixing in a horizontally rotating kiln. The heat and fluid flow in the gas phase are solved directly using a fixed Eulerian grid. At the same time, the particle dynamics and their collisions are solved on a Lagrangian grid. The no‐slip boundary condition on the particle surface is implemented using the fictitious boundary method. The heat transfer inside the particles is calculated utilising two models: the first is the direct solution of the energy conservation equation in Lagrangian and Eulerian space and the second is the so‐called linear model, which assumes a homogeneous distribution of the temperature inside each particle. Numerical simulations showed that if the thermal diffusivity of the gas phase significantly exceeds the same parameter of the particles, the linear model over‐predicts the heating rate of the particles. The analysis of the time‐averaged flow field inside the kiln showed that in the gas phase a double vortex structure is formed which increases the convective heat transfer in the upper part of the particulate bed. The influence of the particle size, the angular velocity of the drum and the fluid on the heating rates of particles is studied and discussed.