Local Fluidization Research Articles

Simulation and Analysis of Hydrodynamic Behavior in Different Nozzles and Its Corresponding Fluidized Beds

Uniform air distribution is the basic condition for the stable operation of circulating fluidized beds and closely related to the hole layout of nozzles and the air outlet conditions. In this paper, CAD modeling software is used to establish different opening types for nozzles and the corresponding gasifier models, and Fluent simulation software for numerical simulations (k-ε model) is introduced to the hydrodynamic behavior of the upper opening, the side opening and the combined opening types of nozzles, as well as the corresponding single-nozzle fluidized bed gasifiers. The flow field distribution under the above opening modes is obtained, including the velocity distribution, static pressure distribution, and total pressure distribution, and the influence of the boundary conditions, including the inlet gas velocity and outlet pressure, on the flow field distribution inside the nozzle and in the single-nozzle fluidized bed gasifier is also investigated. The simulation results show that the suitable optimal operating conditions for the coal gasifier can be achieved with an inlet velocity of 30 m/s and an outlet pressure of 25 kPaG. Under the above conditions, the local fluidization dead zone at the elbow and top of the nozzle is narrower, the uniformity of the wind velocity can be improved, the pressure drop of the inner core tube of the nozzle is gentle, and the pressure distribution tends to be stable. Theoretically, the anti-slag performance of the nozzle is improved, which will enhance the stability and reliability of the operation of the gasification unit.

Lipid Monolayer on Cell Surface Protein Templates Functional Extracellular Lipid Assembly.

When the ancestors of men moved from aquatic habitats to the drylands, their evolutionary strategy to restrict water loss is to seal the skin surface with lipids. It is unknown how these rigid ceramide-dominated lipids with densely packed chains squeeze through narrow extracellular spaces and how they assemble into their complex multilamellar architecture. Here it is shown that the human corneocyte lipid envelope, a monolayer of ultralong covalently bound lipids on the cell surface protein, templates the functional barrier assembly by partly fluidizing and rearranging the free extracellular lipids in its vicinity during the sculpting of a functional skin lipid barrier. The lipid envelope also maintains the fluidity of the extracellular lipids during mechanical stress. This local lipid fluidization does not compromise the permeability barrier. The results provide new testable hypotheses about epidermal homeostasis and the pathophysiology underlying diseases with impaired lipid binding to corneocytes, such as congenital ichthyosis. In a broader sense, this lipoprotein-mediated fluidization of rigid (sphingo)lipid patches may also be relevant to lipid rafts and cellular signaling events and inspire new functional materials.

Instrumented flume tests on the failure and fluidization of tailings dams induced by rainfall infiltration

Although numerous studies focus on the mechanical behaviour and instability of tailings dam materials, a framework that adequately describes all the processes involved in their failure and fluidization is still needed. Therefore, we used a series of instrumented flume tests, with pore pressure transducers, micro-seismic accelerometers, internal displacement transducers and laser scanning, to investigate the mechanisms of tailings-dam failure and fluidization. The artificial rainfall induced water infiltration was adopted to induce the failure. The test results showed that continuous rainfall expanded macro-voids and led to particle rearrangement and local collapse, producing a gradual buildup of pore pressure. Furthermore, the increasing porosity enhanced seepage forces and thus reduced the shear strength of the tailing's materials so that the dam models were close to liquefaction. However, a dam model with a higher hydraulic conductivity would not be totally damaged, even if failure and fluidization occurred. The amount of antecedent rainfall likely significantly contributed to dam failure because it raised the moisture content of the tailing's materials, which promoted internal erosion; thus, the rainfall might have been sufficient to generate local fluidization. Rainfall decreased the peak pore pressures required to initiate failure and shortened the time to failure but hardly influenced the fluidization movement distance.

Mechanism of anomalous sinking of an intruder in a granular packing close to incipient fluidization

Objects released into a granular packing close to incipient fluidization may float or sink depending on their density. Contrary to intuition, earlier work reported that under certain conditions, a lighter sphere can sink further and slower than a heavier one. In this study, we reveal the mechanism of this intriguing phenomenon using both magnetic resonance imaging and discrete particle simulation. Our findings suggest that local fluidization around the sinking sphere and the formation and detachment of gas bubbles play a critical role in driving this anomaly.

Numerical study on pneumatic feeding characteristics of cold-flow fluidized bed reactor for biomass pyrolysis

Comprehensive exploration on the pneumatic feeding characteristics of a fluidized bed can deepen the understanding of biomass pyrolysis. An Euler-Euler model coupled with KTGF is used to simulate the pneumatic feeding process of a two-dimensional cold-flow fluidized bed. The pneumatic feeding mechanism of fluidized bed is analyzed from three aspects: feed gas, feed biomass particles and pneumatic feeding. The feed gas exerts a radial drag force on the quartz sand particles, and merges with the fluidizing gas to promote local fluidization and reduce solid hold-up, which provides feeding space for biomass particles to enter. The radial collision between feed particles and bed materials promotes the diffusion and mixing of particles. And the interaction between feed gas and feed particles promotes them to enter the bed deeper. A dynamic model of pneumatic feeding into the fluidized bed reactor is established, including particle interaction, diffusion, mixing and fluidization together.

Effects of bed size on the voidage in gas-solid bubbling fluidized beds

The effects of bed size on the voidage in gas-solid bubbling fluidized beds were statistically studied. The experiments were conducted in plexiglass fluidized beds with different sections, and glass beads were used as bed materials. The voidage signals were measured at different axial positions in the beds. The average, standard deviation, skewness, and kurtosis of the voidage signals were analyzed. The results showed that the voidage and its dispersion degree decreased with increasing bed size, while the asymmetry and steepness degree increased. The bed size had a stronger effect on the voidage at lower axial positions. The relationship between the bed size and theoretical size of some bubbles was predicted. The transition gas velocity from local fluidization to complete fluidization was obtained.

Regionalized tissue fluidization is required for epithelial gap closure during insect gastrulation

Many animal embryos pull and close an epithelial sheet around the ellipsoidal egg surface during a gastrulation process known as epiboly. The ovoidal geometry dictates that the epithelial sheet first expands and subsequently compacts. Moreover, the spreading epithelium is mechanically stressed and this stress needs to be released. Here we show that during extraembryonic tissue (serosa) epiboly in the insect Tribolium castaneum, the non-proliferative serosa becomes regionalized into a solid-like dorsal region with larger non-rearranging cells, and a more fluid-like ventral region surrounding the leading edge with smaller cells undergoing intercalations. Our results suggest that a heterogeneous actomyosin cable contributes to the fluidization of the leading edge by driving sequential eviction and intercalation of individual cells away from the serosa margin. Since this developmental solution utilized during epiboly resembles the mechanism of wound healing, we propose actomyosin cable-driven local tissue fluidization as a conserved morphogenetic module for closure of epithelial gaps.

CFD-DEM simulation of gas-solid flow of wet particles in a fluidized bed with immersed tubes

In an intensified fluidized bed processing of wet particles, it is often to use heat transfer immersed tubes to maintain reasonable temperature distribution. The wet particles could adhere to the immersed tubes, which hinders the normal fluidization. We studied the gas-solid flow in a pseudo 2D fluidized bed of wet particles with the presence of immersed tubes, by using CFD-DEM simulation with the hysteretic contact model. The gas-solid flow characteristics around the immersed tubes and the effects of immersed tubes, adhesive force and fluidization velocity on the fluidization of wet particles are analyzed. With the increase of the adhesive force the agglomeration mainly occurs near the bed wall and the channeling mainly appears in the middle of the bed. The immersed tubes can cause the particles to accumulate on the upper surface of the tubes. Also, it can break the bubbles and agglomerates when the fluidization velocity is high. As the fluidization becomes poor, the particle agglomeration becomes more obvious and coordination number increases. The local poor fluidization zone is observed mainly in the gaps between the wall and the immersed tubes.

Criterion for Fingering Instabilities in Colloidal Gels.

We sandwich a colloidal gel between two parallel plates and induce a radial flow by lifting the upper plate at a constant velocity. Two distinct scenarios result from such a tensile test: (i)stable flows during which the gel undergoes a tensile deformation without yielding, and (ii)unstable flows characterized by the radial growth of air fingers into the gel. We show that the unstable regime occurs beyond a critical energy input, independent of the gel's macroscopic yield stress. This implies a local fluidization of the gel at the tip of the growing fingers and results in the most unstable wavelength of the patterns exhibiting the characteristic scalings of the classical viscous fingering instability. Our work provides a quantitative criterion for the onset of fingering in colloidal gels based on a local shear-induced yielding in agreement with the delayed failure framework.

Recurrent neural network based detection of faults caused byparticle attrition in chemical looping systems

Chemical looping is a novel and promising technology that converts fossil fuels to electricity and high value chemicals with in-situ carbon capture without significant cost penalties. Smooth and controlled solid circulation is the key to the successful operation of the chemical looping system. Bridging or arching, which may occur in the moving bed standpipe of the system, caused by a combination of fines accumulation and gas flows, is a major impediment to the smooth solid circulation and can lead to the failure of the entire system operation. Thus, early detection of the tendency of bridging or arching is important. This paper describes a model that applies the long short-term memory based recurrent neural network scheme to detect the tendency of arching in the standpipe of a chemical looping system. The arching tendency can be ascertained by early detection of the bubble formation in the standpipe. The bubble movement or local fluidization is recognized to precede the arching formation due to local accumulation of fine particles generated from coarse particle attrition. The early detection of the bubbles thus renders it possible to prevent arching through the prompt action of fines removal from the system. In this study, the recurrent neural network model which detects the fines induced fault manifested as bubbles in the standpipe is developed. It is over the data generated from an experimental sub-pilot scale, cold-flow chemical looping unit. To improve the robustness of the diagnosis, a number of networks with different structures are considered, and an ensemble decision strategy is used to conduct the diagnosis. The recall value obtained of the diagnosed result, which represents the extent of the fraction of real bubbles that are detected, can reach higher than 86.7%. This result reflects a good accuracy of the recurrent neural network model in the fault detection for the chemical looping system.

Numerical and experimental study of integral multi-jet structure impact on gas-solid flow in a 3D spout-fluidized bed

Abstract In this study, the impact of multi-jet structure on the gas-solid flow in a three-dimensional integral multi-jet spout-fluidized bed was investigated using both numerical and experimental methods. Particle Image Velocimetry (PIV) was applied to analyze the difference made by integral multi-jet to the movement of particles in a 152-mm-diametered spouted bed. The Eulerian-Eulerian approach was adopted. Compared with the conventional spouted bed, the addition of the side jet facilitates particles to generate a substantial amount of secondary eddies and the movement of particles can be effectively enhanced in the cross section of the multi-jet spout-fluidized bed. When the number of side jets approached N = 18, the particle concentration in the annulus region was shown to to be extremely low. Considering the pressure drop of the spouted bed, the comprehensive optimum number of side jet of spouted bed was set to N = 24. Besides, there existed an optimum side jet diameter D = 4 mm, at which the side jet exerted the optimal fluidizing effect on the gas-solid two-phase flow in the multi-jet spout-fluidized bed. The distribution structure of the side jets with lower density, upper sparseness and symmetry contributed to the local fluidization of the inlet gas in the dead zone, and Case D demonstrated the full fluidizing effect on the particles in the multi-jet spout-fluidized bed.

The role of actin protrusion dynamics in cell migration through a degradable viscoelastic extracellular matrix: Insights from a computational model.

Actin protrusion dynamics plays an important role in the regulation of three-dimensional (3D) cell migration. Cells form protrusions that adhere to the surrounding extracellular matrix (ECM), mechanically probe the ECM and contract in order to displace the cell body. This results in cell migration that can be directed by the mechanical anisotropy of the ECM. However, the subcellular processes that regulate protrusion dynamics in 3D cell migration are difficult to investigate experimentally and therefore not well understood. Here, we present a computational model of cell migration through a degradable viscoelastic ECM. This model is a 2D representation of 3D cell migration. The cell is modeled as an active deformable object that captures the viscoelastic behavior of the actin cortex and the subcellular processes underlying 3D cell migration. The ECM is regarded as a viscoelastic material, with or without anisotropy due to fibrillar strain stiffening, and modeled by means of the meshless Lagrangian smoothed particle hydrodynamics (SPH) method. ECM degradation is captured by local fluidization of the material and permits cell migration through the ECM. We demonstrate that changes in ECM stiffness and cell strength affect cell migration and are accompanied by changes in number, lifetime and length of protrusions. Interestingly, directly changing the total protrusion number or the average lifetime or length of protrusions does not affect cell migration. A stochastic variability in protrusion lifetime proves to be enough to explain differences in cell migration velocity. Force-dependent adhesion disassembly does not result in faster migration, but can make migration more efficient. We also demonstrate that when a number of simultaneous protrusions is enforced, the optimal number of simultaneous protrusions is one or two, depending on ECM anisotropy. Together, the model provides non-trivial new insights in the role of protrusions in 3D cell migration and can be a valuable contribution to increase the understanding of 3D cell migration mechanics.

Impinging Fluid in Immersed Granular Material to Obtain Local Fluidization for Breaking Sedimentation

Granular material is the most abundant material type in industry. Efforts to improve the efficiency of handling of granular material are continually ongoing. Sedimentation is one of the problems in transporting this material; when sedimentation occurs, the flow of material is obstructed and requires significant energy to clean the pipelines. The problem of sedimentation in pipes is thus an issue that merits serious attention. To solve the sedimentation problem, it is proposed to use the impinging method, which is a shock flow that is inserted into the granular sediment. This experiment to impinge immersed granular material is proposed to solve this depositional problem. Shooting high-speed fluid in a short time is expected to be one of the methods of preventing sedimentation that occurs in handling granular material. The material used in this experiment varies in granule size: very fine, fine, and medium-sized granules. These experiments provide an overview of post-impinging granular behavior with fluidization movement. For very fine granular size, post-impinging fluid cavity expansion occurs, followed by slow fluidization. This fluidization movement occurs for a long time. For fine granules, fluid cavity formation happens much faster, and fluidization occurs immediately. For medium-sized granules, post-impinging fluidization occurs immediately. To measure the impinging process to produce fluidization, the Reynold Number of Impinging (Re*) is used. The fluidization process occurs at Re* < 4000. The internal fluidization movements occur mainly at Re* values 2000-4000 (i.e. transition regions).

Tissue traction microscopy to quantify muscle contraction within precision-cut lung slices.

In asthma, acute bronchospasm is driven by contractile forces of airway smooth muscle (ASM). These forces can be imaged in the cultured ASM cell or assessed in the muscle strip and the tracheal/bronchial ring, but in each case, the ASM is studied in isolation from the native airway milieu. Here, we introduce a novel platform called tissue traction microscopy (TTM) to measure ASM contractile force within porcine and human precision-cut lung slices (PCLS). Compared with the conventional measurements of lumen area changes in PCLS, TTM measurements of ASM force changes are 1) more sensitive to bronchoconstrictor stimuli, 2) less variable across airways, and 3) provide spatial information. Notably, within every human airway, TTM measurements revealed local regions of high ASM contraction that we call "stress hotspots". As an acute response to cyclic stretch, these hotspots promptly decreased but eventually recovered in magnitude, spatial location, and orientation, consistent with local ASM fluidization and resolidification. By enabling direct and precise measurements of ASM force, TTM should accelerate preclinical studies of airway reactivity.

Weakening Mechanisms of Alpine Fault Gouge in High‐Velocity Shear Experiments

AbstractThe Alpine Fault, New Zealand, is a large plate boundary fault with history of major seismic events that frequently ruptured to the surface. To better understand earthquake slip at shallow depth, we analyzed the frictional properties of gouge samples collected at three field exposures distributed along 40 km of the fault trace. The samples are rich in phyllosilicates (30–55%) and quartz/feldspar (41–64%), with small amounts of calcite (4–12%). The gouge samples were sheared in a confined rotary cell under room conditions at a normal stress of 2–3 MPa, under alternating slip velocity steps from 0.002 to 1.47 m/s. We found initial friction coefficients of 0.6–0.8 and gentle velocity strengthening during early slip with short displacement and low velocity. A drastic weakening of more than 50% reduction of the frictional strength occurred after displacements of 2–5 m as slip velocity exceeds ~0.2 m/s, and there was no strength recovery even when slip velocity reduced back to low levels. This weakening was associated with marked temperature rise and intense CO2 and H2O emissions. Based on continuous monitoring of gas pressure, CO2, and H2O concentrations within the gouge chamber, our analysis revealed that the dynamic weakening was triggered by gouge‐zone pressurization, which was driven by thermal decomposition of calcite and dehydration of absorbed water. The pressurization process was enhanced by slip localization and establishment of a low‐permeability shear zone. We further envision that the gouge zone pressurization is likely to cause local overpressure and internal fluidization within of the fine‐grain gouge layer that leads to dynamic weakening.

Real-time magnetic resonance imaging of fluidized beds with internals

The effect of internals on the fluidization dynamics in three-dimensional (3D) cylindrical fluidized beds was studied using real-time magnetic resonance imaging. Instantaneous snapshots of particle velocity and particle position were acquired for gas velocities U below and above the minimum fluidization velocity Umf. Below Umf, we found local fluidization and gas bubbling in areas adjacent to an inserted horizontal tube, likely caused by the gas locally exceeding Umf as it flows around the insert. Above Umf, the presence of the insert in the bed affected the fluidization dynamics. The wake region above the insert exhibited a lower average particle velocity and reduced velocity fluctuations as well as a substantially reduced number and size of gas bubbles as compared to a reference bed without internals. The extent of this wake region was found to be approximately as wide as the insert, and it extended from the top of the insert to the free surface of the bed. Moreover, the presence of internals also affected the regions below the insert by showing an increased rate of bubble formation directly below the inserted tube. The present work can provide relevant information on the design of fluidized beds with internals. Moreover, the data gathered in this work can be used to validate the accuracy of 3D numerical models of gas-solid systems.

Phospholipid Bilayer Softening Due to Hydrophobic Gold Nanoparticle Inclusions.

Liposome-nanoparticle assemblies (LNAs) are vital in the context of novel targeted drug-delivery systems, in addition to investigating nanoparticle-lipid bilayer interactions. Quantifying membrane structural properties and dynamics in presence of nanoparticle inclusions provides a simple model to elucidate nanoparticle effects on membrane biophysical properties. We present experimental evidences of bilayer softening due to small hydrophobic gold nanoparticle inclusions. LNA structure has been investigated by a combination of cryo-transmission electron microscopy, dynamic light scattering, and small-angle neutron scattering. Neutron spin echo spectroscopy demonstrated a remarkable ∼15% bending modulus decrease for LNAs relative to pure liposomes. Clear dependence of bending modulus on gold nanoparticle diameter and concentration was observed from our observations. Our findings point toward local bilayer fluidization by nanoparticle inclusions leading to an overall bilayer softening. These findings add valuable information to liposomal drug-delivery vehicle design and membrane biophysics research.

Determination of the solid concentration in a binary mixture from pressure drop measurements

A new and simple method to estimate the concentration of solids in a binary solids mixture is proposed and experimentally tested in a small fluidized bed set up. In the proposed method, it is necessary to measure the pressure drop in different sections of the bed at different gas velocities so as to be able to determine local minimum fluidization velocities using the Ergun equation. Because the minimum fluidization velocity of a solids mixture is known to be dependent on its weight fraction, a calibration curve can be derived by means of the Cheung equation and this is shown to be sufficient to obtain reasonably good estimates of the axial solids concentration profiles inside the bed. This methodology has been applied to three different mixtures of iron oxide ore of varying size with limestone or glass spheres. Satisfactory agreement between the experimental and calculated concentrations as well as pooled relative standard deviations lower than ±7%, are obtained in all three cases. This technique could facilitate future experimental research on solids mixing and segregation phenomena in fluidized beds, as it is a very low-cost, non-destructive and very fast method to determine solids concentration bed profiles.

Characteristics of non-spherical fluidized media in a fluidized bed–membrane reactor: Effect of particle sphericity on critical flux

Abstract The fluidization of millimeter-sized media to mitigate the membrane fouling by micron-sized particles has received increasing attention, but a question that remained unanswered is the effect of the sphericity of the fluidized media on the energy-efficiency of fouling mitigation, which motivated the current study. Four millimeter-sized fluidized media types, of which three had similar equivalent volume diameters, were investigated, with micron-sized polystyrene particles serving as the particulate foulant. The direct observation through the membrane (DOTM) technique was used to determine the local critical flux, while a high-speed video camera was used to capture the local fluidization hydrodynamics (namely, particle velocity, particle rotation velocity, particle contact area and particle momentum). The results indicate that (i) although smaller media mitigated fouling better among higher-sphericity media, the smallest, low-sphericity media here performed similarly or worse; (ii) lower-sphericity media gave lower particle rotation, greater contact area with the membrane, and lower particle momentum; (iii) in contrast to higher-sphericity media, the extent of local fouling mitigation decreased as the particle velocities increased and the local critical flux was not the most strongly correlated with particle momentum; (iv) lower-sphericity media generally gave lower overall critical flux for the same hydrodynamic characteristic; (v) particle sphericity is a negligible factor in the energy-efficiency of fouling mitigation by similar-sized fluidized media, but, between low-sphericity media, the larger media was more energy-efficient. The findings here highlight the non-negligible influence of the particle sphericity of the fluidized media, and are expected to be valuable in the optimization of the properties of the fluidized media towards more energy-efficient membrane fouling mitigation.

Generation and evaluation of an artificial optical signal based on X-ray measurements for bubble characterization in fluidized beds with vertical internals

The performance of fluidized bed reactors strongly depends on the bubble behavior, for which reason knowledge concerning the bubble properties is important for modeling and reactor optimization. X-ray measurements can be used to characterize bubbles within the cross-section of a fluidized bed on a laboratory scale, but cannot easily be extended to hot, pressurized large scale plants. For future measurements at hot conditions in a fluidized bed methanation reactor, we have developed an optical probing system that can be employed under these conditions. However, optical sensors are only able to investigate the local fluidization patterns at a defined position in the bed. The objective of this study is to characterize differences in bubble properties between local optical measurements and an X-ray tomography method that is able to detect bubbles over the entire cross-section. Therefore, an artificial optical signal is created out of existing hydrodynamic X-ray measurement data obtained at a cold flow model of a pilot scale methanation reactor. The determined bubble properties of both methods (i.e. evaluation of the derived artificial optical probe signal and image reconstruction based on the evaluation of original X-ray tomographic data) are compared with regard to the bubble rise velocity and the bubble size (for the X-ray method) or pierced chord length (for the optical evaluation method), respectively. The comparison shows that for the evaluation of the optical probe data, statistical effects have to be considered carefully. The detected mean chord length of the optical method does not immediately correspond to the mean bubble size determined by the X-ray method. Moreover, also differences regarding the bubble rise velocity were detected for some fluidization states. The reason for the discrepancies between both methods could be identified and corrected, amongst others by means of a Monte Carlo simulation in which rising bubbles in a fluidized bed were simulated and characterized by a local virtual optical sensor.