Modeling of shear induced coarsening effects in semi-solid alloys

In long-term rheological shear experiments with semi-solid alloys, coarsening of the particles will falsify the interpretation of the experimental results.The coarsening is intensified by the shear induced convection and the mean size of the particles is changed significantly during the experiments.A simple model has been set up which takes the influence of the convection into account.The resulting growth law has been simplified for diffusion and convection dominated growth.The growth law was verified with shear experiments in a Searl-rheometer with A356 and tin-lead alloys.The experiments demonstrated that under convection the growth follows a linear time law and that the rate constant depends on the root of the shear rate.The correction of experimental results to gain the true viscosity function is demonstrated for a shear jump experiment with A356.

The effect of side walls on particles mixing in rotating drums

In this paper, we study the effects of the presence and shape of side walls and of the overall length of rotating cylindrical drums on the mixing of particles with differing sizes by application of the discrete element method (DEM). By varying the semi-axis of the spheroidally shaped side walls and the length of the overall drum, we observe the formation of circulation patterns near the side walls. Although there is a vast amount of literature studying mixing regimes in rotating drums, little is known about the effect of the side walls of the drum on particle mixing. The results of our study demonstrate that introducing curved side walls induces a strong circulation pattern near these side walls, but has, paradoxically, a negative impact on mixing and actually promotes segregation. The cause for this segregation is the difference in velocity of differently sized particles near the curved side walls. Large particles accumulate at the curved side walls, whereas small particles move away from the curved side walls. When the length of the drum is increased, the overall effect of the side walls is decreased, although it does remain observable, even in very large drums.

A Eulerian population balance/Monte Carlo approach for simulating laminar aluminum dust flames

Recently,metal powders have been conceptualized as carbon-free recyclable energy carriers that may form a cornerstone of a sustainable energy economy.The combustion of metal dusts in oxidizing atmospheres is exothermal and yields oxide particles that could,potentially,be retrieved and,subsequently,recharged by conversion to pure metals using green primary energy sources.As a step towards a predictive tool for designing metal dust combustors,we present a fully Eulerian modelling approach for laminar particle-laden reactive flows that is,conceptually,based on a population balance description of the dispersed particles and relies on a stochastic Eulerian solution strategy.While the population balance equation(PBE)is formulated for the number-weighted distribution of characteristic properties among all particles near a spatial location,it is kinetically informed by the rates at which mass,momentum and heat are exchanged between the carrier gas and the particulate phase on the single particle level.Within the scope of the Eulerian Monte Carlo solution scheme,the property distribution is discretely represented in terms of the total number density and a finite number of property samples and the computational work is channelled towards the Eulerian estimation of mean particle properties.For the case of reactive aluminum particles,we combine a kinetic description of the gas-particle heat and mass transfer with a transport-limited continuum formulation to obtain rate expressions that are valid across the entire particle size range from the free molecular through the continuum regime.Besides velocity,the particle properties include only the particle mass,temperature and oxide mass fraction.This set of thermochemical degrees of freedom is retained also as phase transitions due to melting occur,drawing on a smooth blend of the solid and liquid thermodynamic and material properties.The particle-level formulation encompasses aluminum evaporation,surface oxidation,scavenging of oxide smoke,oxide evaporation/dissociation and radiation.After investigating how these effects translate,through the PBE,to the particle population level and affect the combustion in a homogeneous dust reactor,we analyze the combustion of an aluminum dust in a counterflow flame and validate predictions of the particles’centerline velocity profile and the flame speed by comparison with available experimental data.Concomitantly,nitrogen oxide emissions are investigated along with the particle burnout and outlet size distribution.

Experimental and numerical investigation to elucidate the fluid flow through packed beds with structured particle packings

The present paper presents an experimental and numerical investigation of the dispersion of the gaseous jet flow and co-flow for the simple unit cell(SUC)and body-centred cubic(BCC)configuration of particles in packed beds.The experimental setup is built in such a way that suitable and simplified boundary conditions are imposed for the corresponding numerical framework,so the simulations can be done under very similar conditions as the experiments.Accordingly,a porous plate is used for the co-flow to achieve the uniform velocity and the fully developed flow is ensured for the jet flow.The SUC and BCC particle beds consist of 3D-printed spheres,and the non-isotropy near the walls is mostly eliminated by placing half-spheres at the channel walls.The flow velocities are analysed directly at the exit of the particle bed for both beds over 36 pores for the SUC configuration and 60 pores for the BCC configuration,for particle Reynolds numbers of 200,300,and 400.Stereo particle image velocimetry is experimentally arranged in such a way that the velocities over the entire region at the exit of the packed bed are obtained instantaneously.The numerical method consists of a state-of-the-art immersed boundary method with adaptive mesh refinement.The paper presents the pore jet structure and velocity field exiting from each pore for the SUC and BCC packed particle beds.The numerical and experimental studies show a good agreement for the SUC configuration for all flow velocities.For the BCC configuration,some differences can be observed in the pore jet flow structure between the simulations and the experiments,but the general flow velocity distribution shows a good overall agreement.The axial velocity is generally higher for the pores located near the centre of the packed bed than for the pores near the wall.In addition,the axial velocities are observed to increase near the peripheral pores of the packed bed.This behaviour is predominant for the BCC configuration as compared to the SUC configuration.The velocities near the peripheral pores can become even higher than those at the central pores for the BCC configuration.It is shown that both the experiments as well as the simulations can be used to study the complex fluid structures inside a packed bed reactor.

Modelling of dust emissions induced by flow over stockpiles and through packed beds

Dust emissions during storage of non-moving bulk materials are studied with a numerical method.The model relies on a contact-model-free Discrete Element Method(DEM)to model the bulk particle-fluid interaction and the dust removal coupled with Computational Fluid Dynamics(CFD)to model the gas and the dust phase in a multiphase framework.Here,two storage scenarios are considered:a flown through packed bed and a flown over stockpile.For the first,the performed simulations reveal that the dust discharge can be correlated with the passing fluid pressure drop.For the second,a parameter study of factors influencing the dust emissions is performed.The parameters discussed are the stockpile size,the gas velocity,the slope angle,the particle diameter and the shape of the stockpile,taking into account conical and truncated conical stockpiles.Dust release correlations are obtained for both scenarios,which reflect very well the obtained numerical results.