A discrete particle simulation study on the influence of restitution coefficient on spout fluidized bed dynamics

In this paper the influence of the normal restitution coefficient on the bed dynamics\ud during different flow regimes was studied using the discrete element model. The three flow\ud regimes comprise the intermediate/spout-fluidization, spouting-with-aeration regime and the jetin-\ud fluidized-bed. It is shown that with increase of the restitution coefficient the average bed\ud height decreases for all flow regime cases. To study the influence of the granule impact velocity\ud and the liquid film on the wall surface the free-fall experiments in the velocity range of 0.5-\ud 4.5 m/s were performed with spherical γ-Al2O3 granules. During the free-fall tests, the impact\ud angle from 0° to 80° and the viscosity of thin liquid film in the range of 1-300 mPa·s were\ud varied

Novel reactor technology for chemical-looping combustion

No Abstract

Fluid-particle interaction force for polydisperse systems from lattice boltzmann simulations

Gas-solid fluidized beds are almost always polydisperse in industrial\ud application. However, to describe the fluid-particle interaction\ud force in models for large-scale gas-solid flow, relations\ud are used which have been derived for monodisperse system, for\ud which ad-hoc modifications are made to account for polydispersity.\ud Recently it was shown, on the basis of detailed lattice\ud Boltzmann simulations, that for bidisperse systems these\ud modifications predict a drag force which can be factors different\ud from the true drag force. In this work fluid-particle interaction\ud forces for polydisperse system are studied by means of\ud lattice Boltzmann simulation, using a grid that is typically an\ud order of magnitude smaller than the sphere diameter. Two different\ud lognormal size distributions are considered for this study.\ud The systems consist of polydisperse random arrays of spheres\ud in the diameter range of 8-24 grid spacing and 8-40 grid spacing,\ud a solid volume fraction of 0.5 and 0.3 and Reynolds number\ud 0.1 to 500. The data confirms the observations made for bidisperse\ud systems, namely that an extra correction factor for the\ud drag force is required to adequately capture the effect of polydispersity.\ud It was found that the correction factor derived by van\ud der Hoef et al (J. Fluid Mech. 528 (2005) 233) on the basis of\ud bidisperse simulation data, applies also to general polydisperse\ud system

Effect of the liquid layer on the impact behaviour of particles

During a spray granulation process the moisture loading in fluidized beds has a great influence on\ud the inter-particle collision properties and hence on the flow behaviour. To study the influence of the\ud liquid layer as well as granule impact velocity on the impact behaviour free-fall experiments were\ud performed. During these experiments the g-Al2O3 granules were dropped from a predefined height\ud onto a liquid layer on the flat steel wall and the velocity-time curves were obtained using highspeed\ud video recording. The height of the liquid layer was varied from 50 mm to 1 mm. Moreover,\ud the tests were performed at different velocities and viscosities of liquid layer in the range of 1-300\ud mPa∙s. Both distilled water and water solutions of hydroxypropyl methylcellulose with different\ud concentrations (3, 6, 10 mass-%) were used.\ud The obtained restitution coefficients were compared with the experiments performed without liquid\ud film on the surface. For a granule impacted on a liquid film on the wall, the increase of liquid\ud viscosity decreases the restitution coefficient and thickness of liquid layer at which the granule\ud sticks. In the examined velocity range, with decreasing impact velocity the restitution coefficient\ud greatly decreases. To explain the obtained effects the force and energy balances for a particle\ud impacted on a liquid layer on the wall were derived. Both contributions to energy absorption\ud (granule-liquid layer and granule-wall contacts) have been taken into consideratio

Characterization of the pneumatic behavior of a novel spouted bed apparatus

Recently the importance of spouted bed technology has significantly increased in the context of drying processes as well as granulation, agglomeration or coating processes. Particulate systems concerning very fine or non spherical particles that are difficult to fluidize, often cannot be treated in conventional fluidized beds. In contrast to those fluidized beds, the spouted bed technology with its specific flow structure offers the opportunity of stable fluidization under controlled conditions. Within this work the fluid dynamics of a novel spouted bed with two adjustable gas inlets is investigated. By analysis of gas fluctuation spectra by means of a fast Fourier transformation algorithm, different operation regimes are identified and depicted graphically. Furthermore, continuum CFD-modeling of the granular solid phase motion by means of an Euler/Euler approach and comparisons with experimental obtained velocity vector fields by means of particle image velocimetry (PIV) measurements will be presented in this work

Analysis of the fluidization behaviour and application of a novel spouted bed\ud apparatus for spray granulation and coating

Spouted beds are well known for their good mixing of the solid phase and for their intensive heat\ud and mass transfers between the fluid phase and the solid phase. Nearly isothermal conditions are\ud enabled which is of advantage for the treatment of granular solid materials in granulation,\ud agglomeration or coating processes. In this work the hydrodynamic behaviour of a novel spouted\ud bed apparatus with two horizontal and slit-shaped gas inlets is investigated by high-frequency\ud recordings of the gas phase pressure fluctuations over the entire bed. The hydrodynamic stable\ud operation domain, which is of importance for operating the apparatus, will be identified and\ud depicted in the Re-G-Ar-diagram by Mitev [1]. Another focus of this work is the simulation of the\ud spouting process by application of a continuum approach in FLUENT 6.2. The effect of the\ud frictional stresses on the hydrodynamic behaviour is examined by performing simulations with and\ud without consideration of friction. The angle of internal friction fi in Schaeffer`s [10] model will be\ud varied and the simulation results will be compared with experiments. It was found that the influence\ud of friction is not very big by application of the quite simple and empirical frictional viscosity model\ud by Schaeffer [10] basing on soil mechanical principles. Also the simulation results under negligence\ud of friction were similar to those under consideration of friction. Another part of this work is the\ud industrial application of the novel spouted bed in granulation and coating processes. Compared to\ud classical fluidized beds, a much narrower particle size distribution, a higher yield and a higher\ud product quality was obtained in the novel spouted be

Heat transfer in a membrane assisted fluidised bed with immersed horizontal tubes

The effect of gas permeation through horizontally immersed membrane tubes on the heat transfer characteristics in a membrane assisted fluidised bed was investigated experimentally. Local time-averaged heat transfer coefficients from copper tubes arranged in a staggered formation with the membrane tubes to the fluidised bed were measured in a square bed (0.15 m x 0.15 m x 0.95 m) containing glass particles (75-110 ¿m) fluidised with air distributed via a porous plate, where the ratio of gas fed or removed through the membrane bundles and the porous plate distributor was varied. The experimental results revealed that high gas permeation rates through the membranes strongly decreased the heat transfer coefficient at high superficial gas velocities for tubes at the top of the tube bundle, which was attributed to the reduced mobility and heat capacity (higher dilution) of the emulsion phase. However, the effect of gas permeation was much less pronounced for tubes lower in the tube bundle because of the lower local dilution of the emulsion phase

Natural boundary for the susceptibility function of generic piecewise expanding unimodal maps

We consider the susceptibility function Psi(z) of a piecewise expanding unimodal interval map f with unique acim mu, a perturbation X, and an observable phi. Combining previous results (deduced from spectral properties of Ruelle transfer operators) with recent work of Breuer-Simon (based on techniques from the spectral theory of Jacobi matrices and a classical paper of Agmon), we show that density of the postcritical orbit (a generic condition) implies that Psi(z) has a strong natural boundary on the unit circle. The Breuer-Simon method provides uncountably many candidates for the outer functions of Psi(z), associated to precritical orbits. If the perturbation X is horizontal, a generic condition (Birkhoff typicality of the postcritical orbit) implies that the nontangential limit of the Psi(z) as z tends to 1 exists and coincides with the derivative of the acim with respect to the map (linear response formula). Applying the Wiener-Wintner theorem, we study the singularity type of nontangential limits as z tends to e^{i\omega}. An additional LIL typicality assumption on the postcritical orbit gives stronger results.Comment: LaTex, 23 pages, to appear ETD