Bed Of Beads Research Articles

Improving Microstructural Homogeneity of Investment‐Cast Single‐Crystalline Ni‐Base Superalloys by Using Fluidized Carbon Bed Cooling

One impeding factor for faster solidification in typical investment casting processes is the limited heat removal capability. This study compares the classical Bridgman technique and the fluidized carbon bed cooling (FCBC) casting method. In particular, the influence of the thermal design is investigated in terms of microstructural and compositional homogeneity in single‐crystalline CMSX‐4. These investment casting processes mainly differ in the baffle type used to insulate the hot and cold zones inside of the casting vessel. While the Bridgman process is based on radiation cooling, the FCBC casting process relies on a dynamic baffle made of expanded graphite that is buoyant on fluidized bed of glassy carbon beads. Such a dynamic insulation layer can adapt to the cross‐sectional variation of ceramic shell molds and provide a better thermal insulation, which results in more uniform solidification conditions. This enables a reduction in the primary dendrite arm spacing by up to 26%, an increase in the homogeneity of the microstructure and composition, as well as a reduction in overall porosity and mean pore size of 41%.

Turbulent Properties of Stationary Flows in Porous Media.

In this study, we investigate the flow dynamics in a fixed bed of hydrogel beads using particle tracking velocimetry to compute the velocity field in the middle of the bed for moderate Reynolds numbers (Re=[124,169,203,211]). We discover that even though the flow is stationary at the larger scales, it exhibits complex multiscale spatial dynamics reminiscent of those observed in classical turbulence. We find evidence of the presence of an inertial range and a direct energy cascade, and are able to obtain a value for a "porous" Kolmogorov constant of C_{2}=3.1±0.3. This analogy with turbulence opens up new possibilities for understanding mixing and global transport properties in porous media.

Experimentation on stochastic trajectories: From Brownian motion to inertial confined dynamics

Statistical physics courses typically employ abstract language that describes objects too small to be seen, making the topic challenging for students to understand. In this work, we introduce a simple experiment that allows conceptualizing some of the underlying ideas of stochastic processes through direct experimentation. Students analyze stochastic trajectories of beads in a bouncing bed of smaller beads subjected to an external periodic drive. The analysis of the trajectories involves the application of a vast toolkit of statistical estimators that are useful in many fields of physics.

Countercurrent Flame Propagation and Quenching Behaviour in a Packed Bed of Spherical PMMA Beads in an Upward Flow of Pure Oxygen

ABSTRACT Countercurrent flame propagation and quenching behavior in a packed bed of spherical polymethyl methacrylate (PMMA) beads in an upward flow of pure oxygen was experimentally studied. Monosized PMMA beads of 2, 5, 8 or 10 mm in diameter (d) were packed in a vertical quartz tube of 35 mm ID to form a bed of 180 mm in height. In a typical experimental run, upon ignition from the top, a blue flame formed and propagated downwards. For d = 10, 8, or 5 mm, there existed a minimum oxygen flow rate (q) to sustain a quasi-steady state flame propagation, and the minimum q increased as d decreased from 10 to 5 mm. However, for d = 2 mm, it was not possible for the flame to propagate downwards irrespective of the q value. The apparent pyrolysis rate (m py) of PMMA was estimated by monitoring the bed mass loss rate, from which the nominal equivalence ratio upon flame quenching was also estimated. The equivalence ratio was found to be less than unity upon flame quenching, suggesting insufficient pyrolysis gases to sustain the combustion. The heat transfer between flame and PMMA bead was analyzed theoretically, and it was revealed that the minimum m py to sustain the quasi-steady state flame propagation inversely correlates with the PMMA particle size, and therefore a higher oxygen flow rate is required to sustain the flame propagation in a packed bed of smaller PMMA particles.

Development of a Label-Free Immunosensor for Quantitative, Electrical Detection of Target Antigens

Point-of-care immunoassays require rapid, sensitive, and user-friendly operation. The most common example is an at-home pregnancy test, which falls under a class of diagnostics called a lateral flow assay (LFA). However, LFAs present many disadvantages such as poor sensitivity and a difficulty for users to interpret test results accurately. The gold standard immunoassay, ELISA, exhibits excellent limits of detection (LOD) and limits of quantitation (LOQ). However, barriers to the implementation of ELISA must be considered, such as the consumption of large amounts of expensive reagents, the need for trained personnel to perform the assay, and the possibility of false positives or negatives due to improper washing or procedure. A clear demand exists for a label-free immunological sensor with high sensitivity and specificity, requiring only a small amount of sample and with fully automated operation. Previous work in our group has demonstrated the successful label-free detection of nucleic acids in a microfluidic device with an embedded 3D porous electrode by their hybridization to a bed of bioconjugated microbeads. An increase in conductivity, observed by a current-voltage curve, was attributed to the negative charge added to the beads upon hybridization of the target nucleic acids. In this presentation, we demonstrate label-free sensing of antibody targets by an analogous method despite their low net charge under the conditions employed. In this system, biotinylated goat anti-rabbit IgG immobilized on streptavidin-modified beads serves as the capture antibody. The analyte, Rabbit IgG, is introduced into the channel and incubated for 30 min. A current-voltage curve (CVC) is obtained before and after incubation to detect and quantify binding of Rabbit IgG to the bead surface. We evaluated the sensitivity of this method and found that the LOD is approximately 5 nM. Following validation of this method, we examined binding kinetics by obtaining CVCs at several time points during incubation. Next, to improve the LOD in this device, faradaic ion concentration polarization (fICP) was employed to electrokinetically enrich the negatively charged IgG over the bead bed test line. After an enrichment period of 30 min, the antigen was preconcentrated over 100-fold, which resulted in greater sensitivity. Finally, we characterize the impact of ionic strength on the sensor and evaluated its performance in a simulated biofluid, saliva, under several conditions: neat, spiked with Tris, and diluted. Ongoing research in our group will translate this method to biomarkers relevant to viral diagnostics. The authors gratefully acknowledge the Roy J. Carver Charitable Trust for the funding of this project.

Hydrate formation from liquid CO2 in a glass beads bed

CO 2 sequestration in marine sediments as solid hydrates is a potential way to capture and store anthropogenic CO 2 . In this study, hydrate formation from liquid CO 2 in marine sediments was simulated in a glass beads bed, and the factors affecting the kinetics of hydrate formation were investigated. The results indicated that the rapid initial hydrate formation with a high driving force always increases the mass transfer resistance, which slows down hydrate growth. The final ratio of water conversion is higher under conditions of low temperature and higher pressure. A smaller particle size is conductive to initial CO 2 hydrate growth, but the water conversion ratio in a bed with larger particles is slightly higher. Compared with other factors, the change in water saturation has an obvious effect on the final water conversion. To inhibit the initial hydrate formation during the injection process, in this paper, a kinetic inhibitor is proposed for pre-injection into marine sediments. This work shows that at a low pressure, a low-concentration inhibitor has an obvious inhibition effect on hydrate growth. However, at a high pressure, it is necessary to increase the concentration of inhibitor to produce an obvious inhibition effect.

On bed porosity of multisized spheroidal particles / Da porosidade de partículas esferoidais polidispersas

In numerous instances of engineering the problem to quantify the porosity of polydisperse systems arises. Despite its great importance, the theoretical predictability of the bed porosity is still problematic. In the field of ceramics, classically, Furnas' studies on porosity are quoted, where he has studied void fraction resulting from blending two distinct particle sizes in various proportions. Less often, ternary diagrams plotting porosity isovalues for spherical particles beds are used to characterize ternary mixtures of distinct monosized particulate systems (usually in ceramics industry). Although similar studies using polydisperse systems have been conducted, a lot of improvement is yet to be achieved. This article falls in this context and aims at contributing to this field of technical and economic impact. Synthetic samples with controlled particle size distribution were used. The resulting porosity of those glass beads random packs (mimicking several size distributions described by Rosin–Rammler equation) has been experimentally determined under a standardized compaction level. The main result was to obtain an equation for the porosity forecast for bead beds inside spheroidal containers, as a function of the sharpness parameter, n, from Rosin–Rammler distribution. An accurate extrapolation to systems well described by the Whiten sigmoidal distribution was achieved as well. A generalization of the Ergun equation is presented at the end of the article, as an application example.

Rheological effects of a gas fluidized bed emulsion on falling and rising spheres

To enable the mechanistic description of the mixing of larger particles in gas-fluidized beds in models (e.g. fuel particles in combustors), knowledge about the rheology of the bed emulsion is required. Here, it is crucial to determine the drag on large fuel-alike particles.This work presents the experimental work on the fate of 13 different solid spheres falling or rising through a bed of air and glass beads at minimum fluidization. The trajectories of the tracer are highly resolved (sampling rate of 200 Hz) by means of magnetic particle tracking, this previously unmet accuracy allows disclosing the complex rheological behavior of gas-solids fluidized bed emulsions in terms of drag on immersed objects. The trajectories reveal that none of the tracers reach terminal velocity during their fall and rise through the bed. The shear stress is obtained through the drag force by solving the equation of motion for the tracer. The data reveal particularities of the bed rheology and clear differences of its effect on rising and falling particles. When studying the shear stress over the characteristic shear rate of each tracer, it can be seen that the stress of the bed on the tracers is dominated by a yield stress, with a somewhat smaller contribution of the shear stress. For rising tracers this last contribution is almost negligible.The falling tracers show strong interaction with the bed emulsion, resulting in a fluctuating shear stress, which increases with tracer size and density. The stagnation of some tracers at low shear rates reveals a viscoplastic behavior of the bed emulsion, exhibiting a typical yield stress that showing a clear dependence on the tracer diameter and buoyant density. The concept of yield gravity is used in order to introduce a normalized shear stress which provides additional verification of the experimental observations in relation to the influence of tracer size and relative density on the shear stress.

Velocity characterization of solids in binary fluidized beds

Owing to challenges in velocimetry techniques, flow hydrodynamics of binary fluidized beds have remained largely unexplored. Thus, the present study seeks to address unraveling the velocity distributions in the binary fluidized bed of glass beads (0.5 and 2 mm) in fluidized with 10, 30, and 40% of coarser fractions. In order to decipher the velocity distributions, RPT was implemented, which yielded velocity distributions of both the 0.5 and 2 mm fractions. Distinct features of solids flow were observed with varying bed compositions. The mean velocity of finer flotsam (0.5 mm particles) fractions were found to diminish upon increasing the jetsam fraction (2 mm) due to variation in bed voidage. On the contrary, an enhanced coarser fraction of 2 mm particles in the bed diminished solids’ fluctuation, which led to a corresponding reduction in the RMS velocities. Similar trends were also noted when solids’ diffusivity was estimated for various bed compositions. Finally, the efficacy of this technique was independently verified qualitatively through mixing patterns obtained from ‘bed freezing’.

Sensitive Electrochemical Detection of Nucleic Acids Following Electrokinetic Enrichment in a Bed of Bioconjugated Beads for Point-of-Care Testing

In this presentation, we demonstrate that ion concentration polarization (ICP) in the presence of fluid flow drives focusing and efficient capture of target nucleic acids within a bed of oligoprobe-conjugated beads. A key finding is that ion conduction along the surface of the bioconjugated beads is the dominant contributor to current to the electrode that drives ICP, under the condition of ICP-induced depletion of ions from the bulk solution. Therefore, hybridization of a target nucleic acid (a polyanion) to the bead surface leads to a shift in the slope of the current-voltage curve. This approach is versatile in that a target nucleic acid can be detected electrically, in the absence of a label, or electrochemically via a label that generates an electroactive product. The resulting approach allows for a plug-and-play nucleic acid sensor using off-the-shelf bioconjugated beads and simple electronics, making it advantageous for point-of-care testing.

Kinetic Modeling of Gas Phase Sugar Cracking to Glycolaldehyde and Other Oxygenates

Sugars can be cracked in a fluidized bed of glass beads to glycolaldehyde and various other oxygenate products that are valuable molecules on their own or as platform molecules for conversion into ...

Compression moulding and injection over moulding of porous PEEK components

A simple and adaptable process for the production of porous PEEK has been demonstrated herein, which uses compression moulding to infiltrate molten PEEK into of a packed bed of salt beads. The process has the capacity to vary the pore size and porosity within the range suitable for materials to replace bone, but compressive testing showed the stiffness to be well below the target to match trabecular bone. This issue was addressed by creating a hybrid structure, integrating “pillars” of solid PEEK into the porous structure, by the injection over-moulding of compression moulded PEEK-salt inserts that contained drilled holes. Good bonding between the moulding and the insert was demonstrated and it was found that as little as 35 mm2 of support, in the form of PEEK “pillars” was required to achieve the target performance.

Charge transfer and bipolar charging of particles in a bubbling fluidized bed

Particle–particle and particle–wall collisions in gas–solid fluidized beds lead to charge accumulation on particles. This work evaluated the effect of fluidization time on charge transfer and bipolar charging (charge separation) and their influence on hydrodynamic structures in a fluidized bed. Experiments were performed with glass beads and polyethylene particles in a glass column. The pressure fluctuations and net electrostatic charge of particles were measured during fluidization. Wavelet and short-time Fourier transforms were used to analyze pressure fluctuations. The results revealed that bipolar charging is the dominant tribocharging mechanism in a bed of glass beads. Bipolar charging in a bed of particles with a narrow size distribution does not affect either hydrodynamic structures or the transition velocity to the turbulent regime. A large difference between the work functions of the wall and particle in the bed of polyethylene particles leads to high charge transfer. Formation of a stagnant particle layer on the wall eventually causes the energy of macro-structures to increase to its maximum. At longer fluidization times, the macro-structural energy decreases and bubbles shrink until the electrostatic charge reaches the equilibrium level. These results well describe the effect of fluidization time on hydrodynamic structures.

Thermal Cracking of Sugars for the Production of Glycolaldehyde and Other Small Oxygenates.

Thermal cracking of sugars for production of glycolaldehyde, a potential renewable platform molecule, in yields up to 74 % with up to 95 % carbon recovered in the condensed product is demonstrated using glucose as the feed. The process involves spraying an aqueous sugar solution into a fluidized bed of glass beads. Continuous operation is carried out for more than 90 h with complete conversion and stable product selectivity. Besides glycolaldehyde, the other identified condensed products are pyruvaldehyde (9 %), formaldehyde (7 %), glyoxal (2 %), acetol (2 %), and acetic acid (1 %). The effects of temperature, glucose feed concentration, and type of sugar feedstock are investigated. Cracking the monosaccharides fructose and xylose leads to very different product distributions from glucose, but similar carbon recovery. A reaction network in agreement with the main observed products from cracking of monosaccharide sugars is proposed.

Permeability and flow through a packed bed of beads in a rectangular cross‐section affected by overlapping wall effects

AbstractThe beads in a packed bed are usually distributed randomly. Therefore, on a macroscopic scale, a fluid flows uniformly through such a porous medium. For thin porous media, deviations are known near the regions of confinement. This so‐called wall effect is extensively investigated in literature for circular confinements. In this work, the wall effect and the flow through porous media in a rectangular domain is studied. This has great importance during the initial filling process of lithium‐ion batteries with the liquid electrolyte. Such batteries consist of collector foils, on which the porous electrodes are attached. Their thickness is only 5 – 10 times a particle diameter dP and may be modeled as a monodisperse bed of spheres. The filling flow is extremely slow and may be in the pre‐Darcy flow region, wherefore the validity of Darcy‘s law is experimentally checked. Then, the local porosity function depending on the distance to the wall is experimentally determined. For very thin porous media, the wall effect of each wall may overlap each other, which is taken into account by using an overlapping model from the literature. The widely used model of Carman‐Kozeny [1] relates the permeability with the porosity. Its validity in the near‐wall region is checked by comparing experimentally‐obtained mean pore diameters to those related to the model of Carman‐Kozeny. The flow through a porous media is computed and compared to experimental results from literature for circular confinement and preliminary optically‐matched PIV measurements.

MOWChIP-seq for low-input and multiplexed profiling of genome-wide histone modifications.

Epigenetic mechanisms such as histone modifications play critical roles in adaptive tuning of chromatin structures. Profiling of various histone modifications at the genome scale using tissues from animal and human samples is an important step for functional studies of epigenomes and epigenomics-based precision medicine. Because the profile of a histone mark is highly specific to a cell type, cell isolation from tissues is often necessary to generate a homogeneous cell population, and such operations tend to yield a low number of cells. In addition, high-throughput processing is often desirable because of the multiplexity of histone marks of interest and the large quantity of samples in a hospital setting. In this protocol, we provide detailed instructions for device fabrication, setup, and operation of microfluidic oscillatory washing-based chromatin immunoprecipitation followed by sequencing (MOWChIP-seq) for profiling of histone modifications using as few as 100 cells per assay with a throughput as high as eight assays in one run. MOWChIP-seq operation involves flowing of chromatin fragments through a packed bed of antibody-coated beads, followed by vigorous microfluidic oscillatory washing. Our process is semi-automated to reduce labor and improve reproducibility. Using one eight-unit device, it takes 2 d to produce eight sequencing libraries from chromatin samples. The technology is scalable. We used the protocol to study a number of histone modifications in various types of mouse and human tissues. The protocol can be conducted by a user who is familiar with molecular biology procedures and has basic engineering skills.

Bedforms Produced on a Particle Bed by Vertical Oscillations of a Plate.

We describe a new mechanism that produces bedforms and characterize the conditions under which it operates. The mechanism is associated with pressure gradients generated in a fluid saturated particle bed by a plate oscillating in the water above it. These vertical pressure gradients cause oscillatory bed failure. This facilitates particle displacement in its interior and transport at and near its surface that contribute to the formation of a heap under the plate. Flows over erodible beds generally cause shear stresses on the bed and these induce bed failure. Failure driven by pressure gradients is different from this. We report on bedforms in a bed of glass beads associated with such fluctuating pressure gradients. We measure the development of the profiles of heaps as a function of time and determine the tangential and normal motion of areas on the beds surface and estimate the depth of penetration of the tangential transport. The measurements compare favorably with a simple model that describes the onset of failure due to oscillations in pressure.

Frictional hysteresis and particle deposition in granular free-surface flows

Shallow granular avalanches on slopes close to repose exhibit hysteretic behaviour. For instance, when a steady-uniform granular flow is brought to rest it leaves a deposit of thickness $h_{stop}(\unicode[STIX]{x1D701})$ on a rough slope inclined at an angle $\unicode[STIX]{x1D701}$ to the horizontal. However, this layer will not spontaneously start to flow again until it is inclined to a higher angle $\unicode[STIX]{x1D701}_{start}$, or the thickness is increased to $h_{start}(\unicode[STIX]{x1D701})>h_{stop}(\unicode[STIX]{x1D701})$. This simple phenomenology leads to a rich variety of flows with co-existing regions of solid-like and fluid-like granular behaviour that evolve in space and time. In particular, frictional hysteresis is directly responsible for the spontaneous formation of self-channelized flows with static levees, retrogressive failures as well as erosion–deposition waves that travel through the material. This paper is motivated by the experimental observation that a travelling-wave develops, when a steady uniform flow of carborundum particles on a bed of larger glass beads, runs out to leave a deposit that is approximately equal to $h_{stop}$. Numerical simulations using the friction law originally proposed by Edwards et al. (J. Fluid Mech., vol. 823, 2017, pp. 278–315) and modified here, demonstrate that there are in fact two travelling waves. One that marks the trailing edge of the steady-uniform flow and another that rapidly deposits the particles, directly connecting the point of minimum dynamic friction (at thickness $h_{\ast }$) with the deposited layer. The first wave moves slightly faster than the second wave, and so there is a slowly expanding region between them in which the flow thins and the particles slow down. An exact inviscid solution for the second travelling wave is derived and it is shown that for a steady-uniform flow of thickness $h_{\ast }$ it produces a deposit close to $h_{stop}$ for all inclination angles. Numerical simulations show that the two-wave structure deposits layers that are approximately equal to $h_{stop}$ for all initial thicknesses. This insensitivity to the initial conditions implies that $h_{stop}$ is a universal quantity, at least for carborundum particles on a bed of larger glass beads. Numerical simulations are therefore able to capture the complete experimental staircase procedure, which is commonly used to determine the $h_{stop}$ and $h_{start}$ curves by progressively increasing the inclination of the chute. In general, however, the deposit thickness may depend on the depth of the flowing layer that generated it, so the most robust way to determine $h_{stop}$ is to measure the deposit thickness from a flow that was moving at the minimum steady-uniform velocity. Finally, some of the pathologies in earlier non-monotonic friction laws are discussed and it is explicitly shown that with these models either steadily travelling deposition waves do not form or they do not leave the correct deposit depth $h_{stop}$.

3D printed Ni/Al2O3 based catalysts for CO2 methanation - a comparative and operando XRD-CT study

Ni-alumina-based catalysts were directly 3D printed into highly adaptable monolithic/multi-channel systems and evaluated for CO2 methanation. By employing emerging 3D printing technologies for catalytic reactor design such as 3D fibre deposition (also referred to as direct write or microextrusion), we developed optimised techniques for tailoring both the support’s macro- and microstructure, as well as its active particle precursor distribution. A comparison was made between 3D printed commercial catalysts, Ni-alumina based catalysts and their conventional counterpart, packed beds of beads and pellet. Excellent CO2 conversions and selectivity to methane were achieved for the 3D printed commercial catalyst (95.75% and 95.63% respectively) with stability of over 100 h. The structure-activity relationship of both the commercial and in-house 3D printed catalysts was explored under typical conditions for CO2 hydrogenation to CH4, using operando ‘chemical imaging’, namely X-Ray Diffraction Computed Tomography (XRD-CT). The 3D printed commercial catalyst showed a more homogenous distribution of the active Ni species compared to the in-house prepared catalyst. For the first time, the results from these comparative characterisation studies gave detailed insight into the fidelity of the direct printing method, revealing the spatial variation in physico-chemical properties (such as phase and size) under operating conditions.

Determination of the Effective Viscosity of Non-newtonian Fluids Flowing Through Porous Media

When non-Newtonian fluids flow through porous media, the topology of the pore space leads to a broad range of flow velocities and shear rates. Consequently, the local viscosity of the fluid also varies in space with a non-linear dependence on the Darcy velocity. Therefore, an effective viscosity is usually used to describe the flow at the Darcy scale. For most non-Newtonian flows the rheology of the fluid can be described by a (non linear) function of the shear rate. Current approaches estimate the effective viscosity by first calculating an effective shear rate mainly by adopting a power-law model for the rheology and including an empirical correction factor. In a second step this averaged shear rate is used together with the rheology of the fluid to calculate. In this work, we derive a semi-analytical expression for the local viscosity profile using a Carreau type fluid, which is a more broadly applicable model than the power-law model. By solving the flow equations in a circular cross section of a capillary we are able to calculate the average viscous resistance directly as a spatial average of the local viscosity. This approach circumvents the use of classical capillary bundle models and allows to upscale the viscosity distribution in a pore with a mean pore size to the Darcy scale. Different from commonly used capillary bundle models, the presented approach does neither require tortuosity nor permeability as input parameters. Consequently, our model only uses the characteristic length scale of the porous media and does not require empirical coefficients. The comparison of the proposed model with flow cell experiments conducted in a packed bed of monodisperse spherical beads shows, that our approach performs well by only using the physical rheology of the fluid, the porosity and the estimated mean pore size, without the need to determine an effective shear rate. The good agreement of our model with flow experiments and existing models suggests that mean viscosity is a good estimate for the effective Darcy viscosity providing physical insight into upscaling of non-Newtonian flows in porous media.