Bed Structure Research Articles

A conjugated heat and mass transfer model to implement reaction in particle-resolved CFD simulations of catalytic fixed bed reactors

Modelling catalytic fixed bed reactors with a small tube-to-particle diameter ratio requires a detailed description of the interactions between fluid flow, intra-particle transport, and the chemical reaction(s) within the catalyst. Particle-resolved computational fluid dynamics (PRCFD) simulations are the most promising approach to predict the behaviour of these reactors accurately, since they take into account the local packed bed structure explicitly. In this work, a conjugated heat and mass transfer model for use in PRCFD simulations is presented in order to couple the fluid flow through the fixed bed with transport and reaction in the porous catalyst, while guaranteeing the no-slip boundary condition at the fluid–solid interface. For this purpose, the solutions of the solid and fluid domain are computed separately and are coupled by calculation and updating the boundary condition at the particle surface. Owing to the consideration of secondary gradients, the developed transfer model is also valid for unstructured calculation meshes containing non-orthogonal cells at the fluid–solid interface. Such meshes are often used to resolve complex geometries, such as a packed bed, in a computationally efficient manner. The coupling approach is validated using cases for which an analytical solution or literature correlations derived from experimental data are available. The simulation results of a short catalytic packed bed with rings catalysing the partial oxidation of n-butane to maleic anhydride exemplify the potential of PRCFD involving reactions to analyse the catalyst performance in great detail.

Shifts in biodiversity and physical structure of seagrass beds across 5 decades at Carriacou, Grenadines.

Caribbean seagrass beds are facing increasing anthropogenic stress, yet comprehensive ground-level monitoring programs that capture the structure of seagrass communities before the 1980s are rare. We measured the distribution of seagrass beds and species composition and abundance of seagrass and associated macroalgae and macroinvertebrates in 3 years over a 47-year period (1969, 1994, 2016) at Carriacou, Granada, an area not heavily impacted by local human activity. Seagrass cover and physical parameters of fringing beds were measured in transects at high (HWE) and low wave energy (LWE) sites; frequency of occurrence of all species, and biomass and morphology of seagrasses, were measured at 100 m2 stations around the island. Losses in nearshore seagrass cover occurred at HWE but not LWE sites between 1969 and 2016 and were associated with increases in the seagrass-free inshore zone (SFI) and erosional scarps within beds. Total biomass did not vary across years although there were progressive changes in seagrass composition: a decline in the dominant Thalassia testudinum and concomitant increase in Syringodium filiforme, and establishment of invasive Halophila stipulacea in 2016 at LWE sites. Species richness and diversity of the seagrass community were highest in 1994, when 94% of macroalgae (excluding Caulerpa) were most abundant, and sea urchins were least abundant, compared to 1969 and 2016. Multivariate statistical analyses showed differences in community composition across the 3 years that were consistent with trends in urchin abundance. Increases in SFI and scarp number in seagrass beds at HWE sites occurred mainly after 1994 and likely were related to increased wave forcing following degradation of offshore coral reefs between 1994 and 2016. Our observations suggest that landward migration of seagrass beds with rapidly rising sea level in future will not be realized in reef-protected seagrass beds at Carriacou barring reversal in the processes that have caused reef flattening.

A note on a transverse magnetic field controlled co-current bubble column

An experimental study has been carried out investigating the fluidization behavior of a bubble column with a bottom magnetic particle bed controlled by an external transverse magnetic field. The magnetization-first/gas-scanning mode was applied, at up to 45 kA m-1 field intensity, with liquid superficial velocities of up to 20 mm s-1 and with a gas flowrate of up to 8 m3 h-1. Particle fractions of two different sizes of up to 1 mm were used. The focus has been both on the three-phase magnetic particle bed expansion playing the role of a gas distributor and the gas holdup of the abovepositioned two-phase section, as well as related column parameters. Piezometric measurements have been performed that provided detection of the position of the interface between the two column sections without visual observation, as well as the gas holdup in the two-phase zone. The bed expansion was strongly affected by the bed state created by the initially established liquid flow rate. The results showed that the intensity of the field applied to the magnetic solids allows control both of bed expansion and internal bed structure, so the applicability of magnetically assisted three-phase beed as a gas distributor in bubble column seems promising.

Non-invasive temperature measurement in fixed bed reactors using RFID technology

Measuring axial temperature profiles in fixed bed reactors is essential for an optimal and safe operation. Typically, thermocouples are used which influence the bed structure - especially in arrangements with a low ratio between tube and particle diameter - and thus local transport phenomena and reaction rates. In this publication, Radio-Frequency Identification (RFID) tags are used for temperature measurement in fixed beds without disturbing the bed structure. Therefore, the temperature is compared in a tubular fixed bed (▪) of ceramic rings (▪) measured with RFID tags and thermocouples at similar axial positions at different temperature levels 20<T<140▪. By integrating an RFID tag into a particle using 3D printing, the disturbance of the fixed bed by the measurement device can be even completely avoided. It is shown that the overall values of temperature and the dynamic measurement with RFID tags is comparable to measurements using thermocouples. However, significant measurement gaps can occur, especially with unfavorable antenna-tag alignment or small antenna spacing. As a summary, it is possible to measure temperature in fixed bed reactors using RFID technology without requiring direct access and thus disturbing the bed structure. Finally, current shortcomings of RFID are discussed, and research needs presented.

Prediction criterion and numerical validation for the interaction between hydraulic fractures and bedding planes.

Shale is a kind of sedimentary rock with an obvious bedding structure. The effect of the bedding plane on hydraulic fracture initiation, propagation, and complex fracture network formation is remarkable and a major problem in hydraulic fracturing and shale oil and gas development. In this study, a criterion is established to predict the evolution behavior of hydraulic fractures (HF) under different confining pressure differences and intersection angles. This criterion is intended to predict the types of interaction between HFs and bedding planes (BPs): penetrating, slipping, or dilating. The dependence of crossing on the intersection angle and the principal stress difference is quantitatively presented using the criterion. Meanwhile, 20 simulations with principal stress differences of 2, 4, 6, and 8 MPa and intersection angles of 30°, 45°, 60°, 75°, and 90° were simulated using the RFPA2D-Flow code. Simulation results exhibit good agreement with the criterion results for a wide range of angles. The investigation showed that HFs tend to penetrate BPs under high confining pressure differences and intersection angles and open BPs under low confining pressure differences and intersection angles. In addition to the above two forms, HFs slip due to shear. The criterion can provide relevant reference about the formation of complex fracture networks in shale layers.

A new permeability model of the compressible tailings thickening bed based on the throat structure parameters

Microstructural and permeability changes in the compressible tailings thickening bed (CTTB) structure during the tailings thickening process are critical for preparing high-concentration filling slurry. Computer tomography (CT) is employed to obtain the pore structure of the bed at different pressures with and without shear. Based on the study of PNM and bed permeability, a new permeability model is proposed. The results indicate that the pore and throat diameters decrease with increasing bed pressure. The pore and throat diameters are smaller with rake shear compared to the situation without shear. Fluid in the CTTB structure is primarily discharged to the top of the thickening bed through the throat structure in the PNM, and the CTTB permeability decreases with increasing Tw value of the throat. The new model proposed in this study, is more accurate in predicting the CTTB permeability than the Xu and Yu, KC, and Civan models.

Mathematical modeling of a 30 MW biomass-fired grate boiler: A reliable baseline model taking fuel-bed structure into account

Grate boilers are widely applied in biomass combustion for heat and energy generation. However, grate-firing systems often suffer from low efficiency and high emission. Computational fluid dynamic (CFD) becomes increasingly popular as a flexible method to obtain the detailed combustion behaviors in the furnace, and to optimize the performance of existing grate boilers. This paper presents efforts toward a reliable baseline CFD model for an industrial biomass-fired grate boiler, where the structure of the fuel-bed was sufficiently considered. The combustion performances in the furnace were investigated experimentally and numerically by coupling the fuel bed and freeboard, to clarify the impact of bed structure on the simulation accuracy. Results show that when the bed structure has been taken into account, the predictions are in better agreement with the various measurements, as the deviations between them are all within 10 %. Moreover, the flame center drops, the total heat transferred to the water-cooled walls increases, and the residence time of flue gas is greatly prolonged from 2.76 s to 3.90 s, resulting in a distinct difference of the temperature and species patterns in the freeboard. The simulations will provide valuable theoretical guidance for the accurate modeling of grate-fired boiler.

Swift’s Scallop (Chlamys swiftii, Bivalvia) Resources and the Structure of Beds in Coastal Waters of Primorsky Krai, Sea of Japan

Swift’s Scallop (Chlamys swiftii, Bivalvia) Resources and the Structure of Beds in Coastal Waters of Primorsky Krai, Sea of Japan

Laser Powder Bed Fusion Process and Structure Data Set for Process Model Validations

Laser Powder Bed Fusion Process and Structure Data Set for Process Model Validations

Bionic leaf-inspired catalyst bed structure for solar membrane reactor aiming at efficient hydrogen production and separation

Membrane reactor technology permits efficient solar thermochemical conversion at low temperatures, but the performance is often limited by concentration polarization. To cope with the hydrogen (H2) concentration polarization in solar membrane reactors, this study proposes a bionic solar membrane reactor (BSMR) inspired by the excellent production-transport management ability of leaves. Designing the structural parameters of the bionic catalyst bed obtains the efficient BSMR, and mechanistically analyzes its reaction and separation performance enhancement. The results show that the BSMR achieves a synergistic reinforcement pattern of ordered H2 separation driving reaction and reaction providing H2 separation pressure at >90° pore block tilt angle, while large porosity differences and small pore block lengths can further amplify this synergy. Currently, the BSMR with pore blocks of 140° tilt angle, 20 mm length and 0.3 low porosity has the optimal performance, which improves methane conversion and hydrogen recovery by 13.4–99.0% and 13.7–99.0%, respectively, relative to the conventional solar membrane reactor under different operating conditions. Also, it is shown that optimal performance enhancement is achieved at high steam-to-methane ratios, low inlet temperatures and high inlet flow rates. In general, the design referring to leaves makes BSMR well inherit the consistently superior performance of leaves, providing new ideas for efficient hydrogen production and separation.

Numerical study on filtration characteristics of high-temperature particulate matter in a three-dimensional randomly arranged multi-granularity particle bed filter

The transformation of internal filter granules into a heterogeneous size distribution through abrasion alters filtration properties. This research constructs an abraded multi-granularity particle bed filter via unidirectional coupling of the discrete element method and computational fluid dynamics. The study examined the filtration characteristics of fine particulate matter at high temperatures. The findings indicate that the abraded multi-granularity beds were higher than the uniform size granular beds in both pressure drop and filtration efficiency. The abraded multi-granularity bed enhances filtration efficiency with a proportionally minor increase in pressure drop relative to the uniform size granular bed. In comparison to a stratified bed structure, the mixed configuration of the multi-granularity bed yields greater filtration efficiencies with less escalation in pressure drop. To maintain filtration efficiency beyond 90%, higher temperatures necessitate an augmented inlet flow rate, requiring an increase by a factor of 2.23 at 1000 K relative to 300 K.

Study on dryout heat flux of axial stratified debris bed under top-flooding

The coolability of the debris bed with a simulant of solidified corium is experimentally studied, focusing on the effects of the structure of the axial stratified debris bed on the dryout heat flux (DHF). DHF was obtained for the four structures with different particle sizes for the axial stratified debris bed under top flooding. The experimental results show that the dryout position of the axial stratified debris bed is formed at the stratified interface indicated by the temperature rise, and the DHF of the axial stratified bed is much lower than that of the homogeneous bed packed with the upper small particles. To predict the dryout heat flux of the stratified debris beds, by considering the properties of the mixed area, a one-dimensional dryout heat flux model of the porous medium is derived from a water and vapor momentum equation for porous medium, two-phase permeability modifications, interfacial drag, and the correlation between capillary pressure and liquid saturation and verified with the experimental data. The modified model can give reasonable results under different structures.

Numerical and experimental study of the flow distribution inside slender packed beds of spherocylindrical particles

In this paper, Magnetic Resonance Imaging (MRI) and a Pore Network Model (PNM) are used to characterize the flow in packed beds of spherocylindrical particles. PNM is chosen as it is a relatively fast numerical approach, which provides local information on the bed flow pattern. MRI scans of packed bed reactors provide detailed information on the bed structure and the flow. In this study, the packed beds are reconstructed from the MRI images. The impact of the image quality on the PNM’s flow field prediction is assessed. It is shown that improved image quality significantly enhances prediction accuracy. With a sufficient image quality, PNM is able to closely match MRI in predicting the flow fields and capture important characteristics such as wall channeling.

Study on the Stiffness and Dynamic Characteristics of a Bridge Approach Zone: Tests and Numerical Analyses

This study focuses on the stiffness and dynamic characteristic rules of a bridge approach zone in a high-speed railway (HSR). Indoor and in situ tests were performed to explore the stiffness and dynamic characteristics of the roadbed filling. Based on the test results, an effective track-subgrade finite element model (FEM) of a high-speed train (HST) was established. The FEM simulated the train load and model boundaries based on the obtained loads and viscoelastic artificial boundaries. Suitable elements were then selected to simulate the various components of the system and the constraint equations were established and solved using multi-point constraints. The model was verified by comparing the time–history curve characteristics, the frequency-domain characteristics and the results obtained from different modeling methods with the measured results. The influence of stiffness on the dynamic characteristics of the bridge approach zone were subsequently analyzed based on the aforementioned tests and simulations. The results indicate that (i) the model produced reliable results using the proposed approach; (ii) the influence of train load on the embankment was generally reflected in the upper part of the structure, and thus, bed structures are recommended to be strengthened; and (iii) under stationarity, the stiffness ratio between the bridge and normal subgrade is recommended as 1:6, with a transition length of 25 m.

Effect of interlayer mixed zone and effective stress on permeability anisotropy of NGH turbidite reservoir

Resulting from the bedding structure of turbidite reservoir, permeability anisotropy is an essential parameter for natural gas hydrate (NGH) exploitation. Fundamentally, the evolution of permeability anisotropy under increasing effective stress are investigated in this study. Taking the turbidite sediments in northern Cascadia as an example, a series of experimental tests and co-simulations of discrete element method (DEM) and computational fluid dynamics method (CFD) are conducted. For layered turbidite sediments, an interlayer mixed zone will be formed in the interface between fine-grained layer and coarse-grained layer after loading. Fine particles will migrate to the coarse layer, blocking the horizontal preponderance flow paths and further reduce the permeability anisotropy. Moreover, the development of interlayer mixed zone mainly occurs in initial compaction stage and slows down in higher stress level, for which the permeability anisotropy shows a staged reduction with increasing effective stress. Considering the effect of interlayer mixed zone, a predictive model of permeability anisotropy is proposed and verified by the experimental results, which has higher accuracy than traditional “layered cake” model. The finding of this research also confirms the mixing effect in deep sediments, which may affect the geological history records in target sedimentary sequence.

Cracking Behavior and Stress Field Evolution in Coal Specimens Containing Bedding under Uniaxial Compression.

During the underground mining process, various coal seams with different bedding structures are often encountered. The presence of bedding structures is one of the primary factors that influence the strength and deformation characteristics of the coal seam and then affect gas extraction and gas disaster prevention. However, there is still a lack of mechanical properties of coal rock with structural anisotropy influenced by bedding structures. In this study, numerical models were established by using the particle flow code method to simulate coal specimens containing bedding with varying inclination angles. The results demonstrate the impact of the bedding inclination angle on the mechanical properties, crack propagation patterns, and the temporal and spatial evolution of the stress field in coal specimens with bedding during the loading process. Furthermore, three crack initiation patterns were investigated for coal specimens with different bedding angles. Additionally, the quantitative relationship between the mechanical properties and the fractal dimension was analyzed. The numerical simulation results were effectively validated through laboratory tests.

Effects of Prescribed Burning on Surface Dead Fuel and Potential Fire Behavior in Pinus yunnanensis in Central Yunnan Province, China

Prescribed burning is a widely used fuel management employed technique to mitigate the risk of forest fires. The Pinus yunnanensis Franch. forest, which is frequently prone to forest fires in southwestern China, serves as a prime example for investigating the effects of prescribed burning on the flammability of surface dead fuel. This research aims to establish a scientific foundation for managing dead fuel in forests, as well as fire prevention and control strategies. Field data was collected from P. yunnanensis forests located in central Yunnan Province in 2021 and 2022. The study implemented a randomized complete block design with two blocks and three treatments: an unburned control (UB), one year after the prescribed burning (PB1a), and three years after the prescribed burning (PB3a). These treatments were evaluated based on three indices: surface dead-bed structure, physicochemical properties, and potential fire behavior parameters. To analyze the stand characteristics of the sample plots, a paired t-test was conducted. The results indicated no significant differences in the stand characteristics of P. yunnanensis following prescribed burning (p &gt; 0.05). Prescribed burning led to a significant decrease in the average surface dead fuel load from 10.24 t/ha to 3.70 t/ha, representing a reduction of 63.87%. Additionally, the average fire−line intensity decreased from 454 kw/m to 190 kw/m, indicating a decrease of 58.15%. Despite prescribed burning, there were no significant changes observed in the physical and chemical properties of dead fuels (p &gt; 0.05). However, the bed structure of dead fuels and fire behavior parameters exhibited a significant reduction compared with the control sample site. The findings of this study provide essential theoretical support for the scientific implementation of prescribed burning programs and the accurate evaluation of ecological and environmental effects post burning.

Bubble behavior in a vacuum fluidized bed

Bubble behavior in a vacuum fluidized bed was investigated in this work. Experimental results showed that bubble diameter and rise velocity increased with declining the pressure, whereas bubble density decreased. The evolution of bubble density with bed height could be divided into three stages on the basis of the corresponding net-coalescence rates. The decrease in bubble density in the bottom region accelerated as the pressure decreased, whereas the increase in bubble density in the top region was gentle. Increasing the vacuum degree enlarged the variation in bubble size, resulting in the decline of operating stability in the fluidized bed. A new correlation that considered the effect of operating pressure on bubble behaviors exhibited accurate prediction in the vacuum fluidized bed. Bubble velocity was proportional to bubble diameter for small bubbles, and bed structure obviously affected the rise velocity of large bubbles. The distribution of the bubble aspect ratio was positively skewed and many bubbles had a tendency to become slender as the operating pressure decreased.

Spatial Patterns and Determinants of Bed and Breakfasts in the All-for-One Tourism Demonstration Area of China: A Perspective on Urban–Rural Differences

The spatial structure of Bed and Breakfast (B&amp;B) development plays a crucial role in promoting integrated urban–rural development. However, existing B&amp;B research has predominantly focused on single large cities, neglecting to explore the spatial patterns of B&amp;B development and their influencing factors from the perspective of urban–rural differences. To address this gap, we conducted a comprehensive case study in an all-for-one tourism demonstration area in Hainan Province, China. We adopt geospatial analysis methods and ridge regression models to investigate the characteristics of urban–rural disparities in B&amp;B distribution and to identify the primary factors influencing their spatial arrangement. The research findings reveal valuable insights: (1) B&amp;B establishments in the tourism demonstration area exhibit clustering with notable variations in clustering intensity between urban and rural regions; (2) essential factors affecting the spatial distribution of B&amp;Bs include transportation accessibility, reliance on tourism attractions, B&amp;B development infrastructure, and the availability of living services; (3) tourism resource dependence emerges as the most significant driving force behind B&amp;B agglomerations in the tourism demonstration area; and (4) road network density, hotel service availability, and neighborhood residential density are three additional critical factors affecting B&amp;B distribution, with their influence varying between urban and rural B&amp;Bs. Based on these key findings, we propose development strategies for optimizing B&amp;Bs’ spatial structure in the tourism demonstration area and outline a blueprint for fostering integrated urban–rural development.

Thermal pyrolysis-induced shale micro-cracks: 3D characterization and implication for reservoir stimulation

The geometrical characteristics of thermal cracks at the micro-scale contain valuable information for subsurface enhancing the production of shale oil and gas via in-situ pyrolysis. In this study, six organic-rich shale samples from the Junggar Basin, western China, were thermally pyrolyzed to 250–500 °C, respectively, which were then imaged using X-ray micro-CT. A novel image-processing technique was employed to separate and identify individual cracks that intersect and form 3D networks. Unprecedentedly, over thousands of cracks in each of the volumes were individually characterized by their length, width, thickness, and orientation and then statistically analyzed. It is shown that crack orientations are predominantly parallel to the bedding structures of the shale, different crack networks of the samples represent the heterogeneity of the shale, and the fractal dimension of the crack length ranges from 1.50 to 1.85. All parameters, including porosity, specific surface area, percolation, and the fractal dimension of crack length, show a distinct increase from 350 °C, which is consistent with the threshold temperature for gas generation during thermal pyrolysis. The analysis quantifies the efficacy of pyrolytic stimulation methods and paves the way for developing pyrolysis stimulations for producing shale oil and gas.