Bed Of Particles Research Articles

Hydrodynamics of a reactor for thermocatalytic oxidation of sewage sludge

One of the promising methods for the disposal of municipal sewage sludge is its thermocatalytic oxidation in fluidized bed reactors. An important factor determining the stability of the operation of fluidized bed reactors is the uniform introduction of the air flow, which, on the one hand, fluidizes a bed of catalyst solid particles and solid sewage sludge, and, on the other hand, the oxygen of the air flow serves as an oxidizing agent in chemical reactions occurring in the reactor. A pilot plant for thermocatalytic oxidation, created according to the technology of the Boreskov Institute of Catalysis, is operated at JSC Omskvodokanal (Omsk, Russia). The plant's capacity is 50 thousand tons per year of mechanically dehydrated sludge with a water content of 70–80 wt%. The cross-sectional area of the sludge combustion zone is 3.8 m2. The heat flow from the incoming sludge (physical heat and heat of combustion) is 6527000 kcal/h. The air flow was introduced into the fluidized bed reactor with a sparger of 6 perforated pipes arranged horizontally and in parallel. The purpose of this work is to investigate the influence of inlet conditions on fluidized bed hydrodynamics. The results obtained showed that despite the uneven distribution of air flow through the holes of the sparger tubes, each pair of adjacent sparger tubes mutually compensate each other in terms of air flow per unit area of the cross section of the reactor. It is also shown that during the operation of the reactor, air cavities are formed under the sparger pipes, which are practically free of solid particles. The position of such air cavities and their approximate volume are stationary in time for a developed fluidized bed, and the volume of the cavities is approximately proportional to the dimensions of the respective sparger tubes. These air cavities periodically eject bubbles into the overlying portions of the fluidized bed. The dimensions of the bubbles are comparable to the transverse dimensions of the pipes.

3D CFD-PBM simulation of gas–solid bubbling beds of Geldart A particles with sub-grid drag correction

The importance of bubble dynamics in fluidized beds is underscored by the efforts undertaken over the past decades through experimental and numerical studies. Nonetheless, the bubble dynamic evolution and its effects on sub-grid closures were neglected in previous coarse-grid CFD models, particularly for Geldart A particles. In this article, a population balance model with improved kernels was employed to describe the bubble dynamic evolution in three-dimensional bubbling beds, whereby the EMMS drag model was closedto be incorporated into an Eulerian continuum model . Furthermore, the Extended Quadrature Method of Moments (EQMOM) was utilized to reconstruct the number density function of bubble size once their lower moments were obtained. The simulation results showed that the hydrodynamics including bed expansion characteristics, solids concentration and bubble size distribution can be well predicted by the model, indicating its capability to capture the heterogeneous structures in three-dimensional bubbling beds of fine particles.

Enteric coating of drug loaded aerogel particles in a wurster fluidized bed and its effect on release behaviour

Enteric coating of drug loaded aerogel particles in a wurster fluidized bed and its effect on release behaviour

A microscopic diffusion-induced discrete element model for swellable particles

Diffusion-induced swelling or shrinkage of particles is ubiquitous in many industrial processes and nature. Aiming to rigorously model these deformable particles, a microscopic model that considers the microstructural evolution of individual particles is developed for the first time and implemented into the discrete element method (DEM), which is experimentally validated. The robustness of this model is also evaluated by comparing its performance with a macroscopic diffusion-induced swelling model and a phenomenological swelling model. The swelling behaviours of a single particle and particle beds of various configurations in water are then analysed. It is shown that the microscopic swelling model and the phenomenological swelling model can better describe the swellings of single particle and particle beds than the macroscopic swelling model. Moreover, the microscopic swelling model can not only reproduce the phenomena of volume expansion of particles but also well predict the microstructural evolution of individual particles, as observed experimentally. Furthermore, the microscopic swelling model is capable of describing the shrinkage processes and the particle–particle diffusions of swellable particles. It is hence demonstrated that DEM with the microscopic swelling model, which captures the microscopic physical mechanisms of particle swelling and the microstructural change of swelling particles, could be a useful tool for modelling swellable granular materials in various industrial processes.

Pressure‐controlled secondary flows and mixing in sheared Platonic solid‐shaped particles

AbstractGranular materials exhibit unique secondary flow behaviors upon shearing. We demonstrate, using particle dynamics simulations, that the secondary flow patterns are controlled by a pressure exerted on particle bed. A threshold pressure, at which vortex flow transitions to disturbed or chaotic flow, depends on particle shape, that influences interparticle contacts and rheological performance. Our results show that the flow patterns are essentially determined by a dimensionless term combining pressure and granular temperature for all the spherical and Platonic solid‐shaped particles explored. Particle mixing is promoted by the vortex flow or disturbed flow with strong diffusion. The highest mixing rate under a specified pressure is obtained for cubic particles, due to the significant microstructural ordering near the boundaries causing a high gradient of packing density. These findings shed light on how applied pressure and particle shape affect secondary flows which is critical to the understanding and control of granular mixing.

Straining to find the permeability

Characterizing fluid flow in a porous and permeable material is fundamental to energy and hydrological applications, yet direct measurements of permeability are very difficult to conduct in situ. However, attending fluid flow through a material are various mechanical responses, e.g., strain fields and acoustic emissions, and these mechanical responses may hold important clues to the fluid flow in the material, specifically the permeability. Here we report results from a numerical study of fluid flow through a channel, defined by confining side blocks, that contains a particle bed. For a range of inlet velocities, we study the strain and acoustic emission in the confining side blocks. The simulations are repeated for different configurations of the particle bed. We find a one-to-one correspondence between strain and acoustic emission and the quantities that determine the permeability. Thus, strain and acoustic emission may serve as ingredients in an unconventional scheme for remote monitoring to learn the permeability.

Loop-seal flow characteristics of a circulating fluidized bed for 3 MWth scale chemical looping combustion system

This study aimed to provide a design and operational approach for a loop-seal, which is the heart of a circulating fluidized bed (CFB). Thousands of CFB systems use this non-mechanical valve to optimize performance by controlling the solids circulation rate while preventing gas mixing between reactors. Nevertheless, its design and operating guidance remains unclear because of its intricate flow characteristics, causing confusion in design and operation of the entire CFB. This study experimentally investigated the three most important elements while designing and operating loop-seal: conditions for initiating solids flow, the maximum obtainable solids flow rate, and quantitative control of solids flow rate. Experiments were conducted in a CFB, which simulated 3 MWth chemical looping combustion (CLC) system that is currently being developed. The onset of solids flow occurs when the gas drag force in horizontal passage overcomes the resistance force generated by the bed of particles. The maximum solids circulation rate was determined by solids height in the supply chamber and pressure around CFB loop. A correlation for quantitative control of solids flow in loop-seal was proposed by the relationship between the gas drag force and particle velocity. Finally, operational conditions for the loop-seal in 3 MWth CLC were proposed.

Effect of particle shape and bedding angle on suffusion in gap‐graded granular soils by coupled CFD‐DEM method

AbstractThe natural deposition of soils results in significant fabric anisotropy, and its influence on suffusion remains unclear. In this study, a series of gap‐graded soil samples with different particle shapes and bedding angles are generated and implemented into the coupled computational fluid dynamics‐discrete element method (CFD‐DEM) simulations to investigate the migration of fine particles within the coarse grain matrix. Macroscopic results, including mass loss, void ratio variation, and soil deformation, as well as microscopic particle trajectory, force chain network, and contact orientation are analyzed. In addition, drained triaxial tests on samples before and after erosion are carried out. All results turn out that the greater the bedding angle, the more severe the loss of fines and the more significant the soil deformation. Three different migration characteristics can be identified according to soil anisotropy degree, that is, when the bedding angle is 0°, the migration of fine particles remains relatively stable; when the bedding angle is 30° and 60°, the particle migration shows a gradual change throughout the erosion; when the bedding angle is 90°, the fine particles migrate significantly in the very early stages of suffusion. Besides, the aspect ratio (AR) of the particle is found to be beneficial for both erosion resistance and soil strength.

Bubble Loading in a Fluidized Column: Effects of Bubble Size, Particle Size, Contact Angle and Particle Density

ABSTRACT The size of bubbles and particle properties (including particle size, density and contact angle) play an important role on the degree of particle loading on bubbles and this has been studied extensively. However, experimental results on the influence of these properties on bubble loading under fluidized-bed flotation conditions (where the probability of detachment is expected to be negligible) are scarce. In this work, the loading fraction (L) of individual bubbles was studied using a high-speed video camera (Phantom Miro LC111). Particles of silica (−75 + 53 µm to −1018 + 850 µm) and magnetite (−425 + 300 µm) were fluidized in a specially designed perspex column. Specific sizes of air bubbles were generated using a combination of a syringe pump and nozzles with different internal diameters. The generated bubbles rose through a bed of fluidized particles, which resulted in bubble loading. The results indicate that fractional bubble loading decreased with an increase in both bubble and particle size. With a 2 mm bubble, the loading fraction of silica declined from circa 0.9 to circa 0.1. However, with coarse particles (−850 + 600 µm and −1018 + 850 µm), bubble loading increased with an increase in bubble size, until an optimum bubble size of 2 mm and then started to decrease as bubble size increased. An increase in particle contact angle showed a corresponding increase in loading fraction, while the density of the material seemed to play no role in terms of the loading fraction. An increase in loading fraction had a negative effect on bubble rise velocity.

CFD-DEM-VOF-phase diagram modelling of multi-phase flow with phase changes

Solid phase change in fluid–solid systems is widely encountered in chemical engineering processes, yet a reliable numerical tool to reproduce the explicit phase change process under multiple variable conditions is still lacking. In this work, a novel generic model framework is established by coupling the CFD-DEM-VOF model with a discretised thermodynamic phase diagram to simulate the solid phase change in fluid–solid systems under multiple variable conditions. The model is first validated using three cases, and then is applied to simulate the melting process of FexCy particles for demonstration where two parameters are varying simultaneously – temperature (T) and carbon composition (XC), using two scenarios – Scenario I: melting of individual FexCy particles and Scenario II: melting of FexCy particles in a packed bed. The results show that in Scenario I, the phase change process of one FexCy particle at a fixed carbon composition can be reproduced, including (i) a solid state (T < 1147 °C); (ii) a co-existence of solid and liquid phases (1147 °C < T < 1281 °C); (iii) a liquid state (T > 1281 °C). In Scenario II, the varied carbon composition indeed alters the phase change process of the FexCy particle. The horizontal extension of liquid metal and long icicle shapes of droplets are observed during the melting and dripping processes of FexCy particles in a coke bed. The present work provides a reliable tool to investigate the multi-phase flow behaviours during the phase change processes, for example, selective laser melting in additive manufacturing and ore melting in ironmaking blast furnaces.

Experimental and numerical study on characteristics and mechanism of particles attrition in fluidized bed

Particle attrition is commonly existed in gas-solid two-phase flow process. A novel CFD-DEM particle attrition model is developed, a series of experiments/simulation studies are performed to quantitatively investigate the attrition mechanism of particles in fluidized beds. The particle attrition process could generally be divided into unstable attrition stage and stable attrition stage, while the results show that unsteady attrition stage should be subdivided into high-speed attrition phase and low-speed attrition phase. Both wear and fragmentation become more intense with increasing height-diameter ratio and superficial gas velocity. The wear ratio decreases with increasing particle size, while the fragmentation ratio firstly decreases and then increases. Particle attrition mechanism also presents spatial ununiformity, particles reaching the bottom are worn most significantly, while the proportion of fragmentation based on this study for the sidewall areas, the bottom area and the center area is about 69%, 22% and 9% respectively.

A numerical study of the spectral radiative properties of packed bed with mixed bauxite and silica spheres

Bauxite and silica particles have gained increasing attention for applications in the field of concentrated solar power. In this work, a Monte Carlo ray-tracing simulation is performed to predict the radiative properties (absorptance, reflectance, and transmittance) of packed beds with mixed bauxite and silica spherical particles at wavelengths of 0.5, 2.6, 9.0, and 9.35 µm. These wavelengths are representative for the visible, near-infrared, and the mid-infrared regions that are important for solar and thermal radiation, respectively. A repeating unit-column approach is used to mathematically represent the particle bed. The effects of particle mixing ratio, volume fraction, and wavelength on the predicted radiative properties are examined. The obtained radiative properties are inputted to an inverse method to retrieve the effective absorption and scattering coefficients as well as the scattering albedo, which may be later used in a continuous-scale radiative heat transfer analysis. Furthermore, the independent scattering model is used to obtain the absorption coefficient and scattering albedo based on the absorption and scattering cross sections predicted by a Monte Carlo algorithm for a single particle. It is shown that the independent scattering model underpredicts the scattering coefficient for opaque particles but overpredicts the scattering coefficient for semitransparent particles for sufficiently high particle volume fractions. The radiative properties calculated from the independent scattering model are compared to the full Monte Carlo simulation of the particle bed to examine the influence of particle mixing on dependent scattering.

Experimental investigation on reducing diesel particulate matter using centrifugal fluidised chamber

ABSTRACT Centrifugal fluidisation with static geometry, which is a new way of fluidisation, is still not getting the attention of engine researchers for reducing the PM from diesel exhaust. In the present work, centrifugal fluidisation with static geometry is used as an after-treatment method to reduce the PM from diesel exhaust. The centrifugal fluidised chamber with SiC particles was tested on air and diesel exhaust. Experimental results show that an increase in bed stability with an increase in inlets was responsible for improving chamber efficiency for 3 and 4 inlet chambers. The number of inlets, inlet slot width and the mass of a bed particle affect the fluidisation inside the chamber. For single-cylinder diesel engines, 3 and 4 inlet chambers show the potential to remove PM with maximum chamber efficiency close to 60%. The accumulation of bed particles is the main reason for the dropping of the chamber efficiency with time.

Influence of layer thickness and substrate bed on the void fraction of powder layers for laser powder bed fusion

The spreading powder layer plays a vital role in determining the quality of Laser powder bed fusion (LPBF) -manufactured samples. In this study, the effect of layer thickness is investigated numerically and experimentally. It was determined that the void fraction at a certain height is higher than that of the entire layer, especially for thicker layers, because of gravity. The former is consistent with the experimental result for powders of the same thickness. The void fraction of the sintered part was also measured to confirm their relationship. Furthermore, the dynamics of spreading particles on a particle bed were numerically studied to investigate whether support powders around the sintered part affect the powder-spreading process. The powder layer on a particle bed is denser than that on an adjacent solid bed, suggesting that support powders will affect the powder-spreading of the sintered part, especially its borders, and contribute to the surface topography.

Data-Efficient Surrogate Model for Rapid Prediction of Temperature Evolution in a Microscale Selective Laser Sintering System

Abstract Current metal additive manufacturing (AM) systems suffer from limitations on the minimum feature sizes they can produce during part formation. The microscale selective laser sintering (μ-SLS) system addresses this drawback by enabling the production of parts with minimum feature resolutions of the order of a single micrometer. However, the production of microscale parts is challenging due to unwanted heat conduction within the nanoparticle powder bed. As a result, finite element (FE) thermal models have been developed to predict the evolution of temperature within the particle bed during laser sintering. These thermal models are not only computationally expensive but also must be integrated into an iterative model-based control framework to optimize the digital mask used to control the distribution of laser power. These limitations necessitate the development of a machine learning (ML) surrogate model to quickly and accurately predict the temperature evolution within the μ-SLS particle bed using minimal training data. The regression model presented in this work uses an “Element-by-Element” approach, where models are trained on individual finite elements to learn the relationship between thermal conditions experienced by each element at a given time-step and the element's temperature at the next time-step. An existing bed-scale FE thermal model of the μ-SLS system is used to generate element-by-element tabular training data for the ML model. A data-efficient artificial neural network (NN) is then trained to predict the temperature evolution of a 2D powder-bed over a 2 s sintering window with high accuracy.

A comparative study of scour around various shaped bridge pier

The study of scouring around bridge piers is very significant and plays a very important role in the safe and economical design of bridge piers. The term scouring is a process by which bed particles around the periphery of the pier get eroded and removed over a certain depth resulting in the formation of a scour hole around the bridge piers. This paper presents a detailed investigation of local scouring around six different shapes namely rectangular, circular, chamfered, joukowsky, oblong, and sharp-nosed of bridge pier models for selecting the best and most economical bridge pier shapes based on local scouring. For the experimental studies, local scour depth is measured around all six different shapes of bridge piers with varying discharges and velocities of 0.0169 m3 s−1 to 0.0355 m3 s−1 and 0.17 m s−1 to 0.30 m s−1 respectively. It is found that scour depth around the rectangular shape of bridge pier is larger due to development of strong intensity of horseshoe vortex caused by flat exposed frontal area and scour depth around the sharp nose shape of the pier is smaller because of flow is bifurcated due to smooth curvature caused less intensity of development of horseshoe vortex. In the comparison of obtained results, it is reported that scour depth around a pier is exactly proportional to velocity of approach flow and exposed frontal area of the bridge pier. The result of this study can be likely used to help in selecting the best shape of the bridge piers as sharp nose shape of bridge piers is better among the other six bridge pier shapes for design purposes.

Analysis of the breakage characteristics of rice particle beds under confined compression tests

Analysis of the breakage characteristics of rice particle beds under confined compression tests

Application of particle-scale modelling to the combustion of a char particle in a fluidised bed of CLOU particles

This research concerns the combustion of a biomass char in a fluidised bed of either inert silica particles or particles of a material capable of supporting combustion using chemical looping oxygen uncoupling (CLOU). An analytical solution for combustion using CLOU was developed, accounting for the chemical reactions occurring within a mass-transfer boundary layer of a finite thickness, surrounding a char particle. The reactions considered were: (1) combustion of char with oxygen to CO and CO2, (2) homogenous reaction of CO with O2, and (3) oxygen release from CLOU particles. The thickness of the boundary layer, γ, was evaluated using correlation in the literature and the combustion of the char particle was modelled as a shrinking particle. Results from the model were compared with experiments performed by combusting char from birch-wood and activated carbon in a fluidised bed (i.d. 30 mm) of either particles capable of CLOU (CuO supported on mayenite) or inert silica sand. Both types of experiments were undertaken with a partial pressure of oxygen, pO2, close to the equilibrium pressure of O2 over CuO at 1173 K, i.e. ∼0.016 bar of O2. The results from the analytical model agreed with the experimental observations from both beds.In addition to the analytical solution, fuller, numerical models were developed, in which a char particle and its surroundings were discretised in 1D to solve the mass balance equations and to assess the validity of the assumptions made in the analytical solution. To establish the range of conditions in which the analytical solution is valid, various cases were simulated. For example, char combustion and gasification in a bed of CuO-particles fluidised by CO2-rich gas. Hence, besides the validation of the analytical solution, the results of the numerical solutions provided insight into the importance of char gasification with CO2 on the overall particle conversion in CLOU at the industrial scale.

Effective thermal conductivity of metal hydride particle bed: Theoretical model and experimental validation

It is essential to accurately predict the effective thermal conductivities of metal hydride (MH) beds for simulating the dynamic hydrogen storage of MH reactors. In this study, a 3-D analysis model was established to detailedly analyze the contributions of different heat transfer pathways on overall heat transfer of particle bed. The results indicate more than 95% heat quantity is transferred through the particle-gas film-particle pathway. Besides, taking the LaNi5 particle bed (λs equals 12.5 Wm−1K−1) as an example, only 26.2% gas volume contributes to transferring 90% heat quantity, proving the existence of heat transfer concentrating (HTC). Thus, a heat transfer concentrating model for predicting effective thermal conductivities of particle beds was established. The predicted thermal conductivities of HTC model were compared with six different models and the experimental data in references. The prediction accuracy and applicability of HTC model are the best. Then, the thermal conductivities of LaNi5 and Fe particle beds were experimentally measured. The predicted thermal conductivities by HTC model agree great with the experimental data, and the mean prediction errors are less than 7%. Finally, the HTC model is applicable within the porosity and thermal conductivity ratio from 0.33 to 0.68 and 1 to 8915, respectively.

Minimum fluidization velocity and bed expansion of the inverse fluidized bed

To define the fluid-dynamic properties of inverse fluidized beds, studies of the minimum fluidization velocity and bed expansion were performed in this work. The experiments were performed in an inverse fluidized bed of polypropylene and polystyrene particles in a cylindrical column using water and an aqueous glycerol solution of different concentrations as fluidizing medium. The dependence of the bed porosity on the superficial velocity of the fluid was determined experimentally for all the particles studied and different fluidizing media. The influence of fluidizing medium viscosity and particle density on the expansion of the inverse fluidized bed and on the minimum fluidization velocity was investigated. Comparison of the experimental data for the expansion of the inverse fluidized bed and the values calculated by the Richardson-Zaki correlation for the conventional fluidized bed showed some deviation, but the experimentally determined data were found to follow the shape of the Richardson-Zaki curve. In this paper, a correlation for the expansion of inverse fluidized beds is proposed.