Bed Dynamics Research Articles

Controlling the Quality of Two-Way Euler/Lagrange Numerical Modeling of Bubbling and Spouted Fluidized Beds Dynamics

We numerically simulate three-dimensional fluidized beds of monodisperse spheres using a two-way Euler/Lagrange method. Particles trajectories are tracked in a Lagrangian way, and particles collisions are computed by a soft sphere model. The fluid conservation equations are written in a classical Eulerian fashion and are locally averaged on cells 1 order of magnitude larger than particles. We detail the equations of the model and their numerical implementation. We study the influence of numerical parameters and simulation domain size on computed results in a biperiodic fluidized bed. We then validate the model against theoretical results for a bubbling fluidized bed and against experimental data for a single- and multi-nozzle spouted bed. Finally, we investigate the influence of the Coulomb friction coefficient magnitude on a single-nozzle spouted bed dynamics in order to emphasize the importance of tangential friction in such processes.

Lack of Impact of Posidonia oceanica Leaf Nutrient Enrichment on Sarpa salpa Herbivory: Additional Evidence for the Generalist Consumer Behavior of This Cornerstone Mediterranean Herbivore.

The fish Sarpa salpa (L.) is one of the main macroherbivores in the western Mediterranean. Through direct and indirect mechanisms, this herbivore can exert significant control on the structure and functional dynamics of seagrass beds and macroalgae. Past research has suggested nutritional quality of their diet influences S. salpa herbivory, with the fish feeding more intensively and exerting greater top down control on macrophytes with higher internal nutrient contents. However recent findings have questioned this notion and shown that herbivores do not preferentially feed on macrophytes with higher nutrient contents, but rather feed on a wide variety of them with no apparent selectivity. To contribute to this debate, we conducted a field fertilization experiment where we enriched leaves of the seagrass Posidonia oceanica, a staple diet for S. salpa, and examined the response by the herbivore. These responses included quantification of leaf consumption in fertilized and non-fertilized/control plots within the bed, and food choice assays where fertilized and non-fertilized/control leaves were simultaneously offered to the herbivore. Despite the duration of leaf exposure to herbivores (30 days) and abundant schools of S. salpa observed around the plots, leaf consumption was generally low in the plots examined. Consumption was not higher on fertilized than on non-fertilized leaves. Food choice experiments did not show strong evidence for selectivity of enriched leaves. These results add to a recent body of work reporting a broad generalist feeding behavior by S. salpa with no clear selectivity for seagrass with higher nutrient content. In concert, this and other studies suggest S. salpa is often generalist consumers not only dictated by diet nutrient content but by complex interactions between other traits of nutritional quality, habitat heterogeneity within their ample foraging area, and responses to predation risk.

Cylindrical‐conical spouted bed dynamics: Laminar and turbulent flow predictions

AbstractThis work numerically studies transient behaviour of particulate flows in a cylindrical‐conical spouted bed in terms of comparative analysis of different models’ predictions. Two approaches were used: a laminar flow model and a turbulent flow model. As a reference case, we consider experimental studies by He et al.,[14,15] where the profiles of vertical particle velocities and the void fraction in the spout and the fountain of a full‐column spout bed were measured. In this work we used the Euler‐Euler unsteady multiphase model with the Syamlal‐O'Brien, Gidaspow, and Wen‐Yu drag models available in commercial CFD software Fluent 14.0. Analysis of results obtained by the use of laminar and turbulent flow models revealed almost identical solid and gas phase velocities and phase fractions within the spouted bed region. We found that the turbulence plays a significant role only in the gas phase above the spouted bed and it does not have any influence on the solid phase. Comparing our simulation results in the form of time‐averaged vertical velocity of the solid phase against experimental data showed acceptable agreement in the spout and very good agreement in the fountain region. Numerical simulations with the Syamlal‐O'Brien drag model gave better agreement with experimental data than results obtained using the Gidaspow and Wen‐Yu drag models. To analyze the transient behaviour of the spouted bed, we use the volume‐averaged particle velocity and gas phase velocity. The analysis of their time histories showed that the start up time is 2–3 s. The developed unsteady regime is reached after 4–5 s. Additionally we studied numerically the influence of different discretization schemes for convective terms on the final results. We found that the use of 1st order upwind scheme gives a steady state solution for both models, laminar and turbulent. Finally, we investigated the influence of the restitution coefficient on the transient characteristics of a cylindrical‐conical spouted bed. A decrease in the value of the restitution coefficient leads to an increase in the period of oscillations of the volume‐averaged velocities of the gas and solid phases.

Geohistorical records indicate no impact of the Deepwater Horizon oil spill on oyster body size.

Documentation of the near- and long-term effects of the Deepwater Horizon (DWH) oil spill, one of the largest environmental disasters in US history, is still ongoing. We used a novel before-after-control-impact analysis to test the hypothesis that average body size of intertidal populations of the eastern oyster (Crassostrea virginica) inhabiting impacted areas in Louisiana decreased due to increased stress/mortality related to the oil spill. Time-averaged death assemblages of oysters were used to establish a pre-spill baseline of body-size structure for four impacted and four control locations along a 350 km stretch of Louisiana's coastline. Post-spill body sizes were then measured from live oysters at each site in order to evaluate the differences in body size between oiled (i.e. impact) and unoiled (i.e. control) locations before and after the spill. Our results indicate that average body size of oysters remained relatively unchanged after the oil spill. There were also no temporal patterns in temperature, salinity or disease prevalence that could have explained our results. Together, these findings suggest that oysters either recovered rapidly following the immediate impact of the DWH oil spill, or that its impact was not severe enough to influence short-term population dynamics of the oyster beds.

Dynamics of sedimentary Capibaribe River bed after deepening dredging

ABSTRACT The growth of urban centers in the country are increasingly dependent on the adoption of new mobility options, and before this need was developed and initiated a project to build a river corridor passenger in a part of the Capibaribe River in the city of Recife-PE. For the implementation of a navigation channel deepening are needed dredging to define a geometry of the bed of the river compatible with the needs of ships that will traverse the channel. The monitoring of dredging is done with the constant bathymetric surveys where the information volumes of dredged sediments are removed, aiming to also check the temporal behavior of deposition and erosion processes. In this context, the work aimed to evaluate the geometry of the Capibaribe river bed after the deepening dredging, using data from bathymetric surveys at different times for making longitudinal and transverse profiles, and create digital models of depth to extract information about the deposition/aggradation or erosion/removing sediments and their respective volumes. Early Profile Capibaribe river showed mild slope and heterogeneity in the bottom of the form, especially after kilometer five, with trends of deposition/aggradation downstream of the confluence region with tax arising from the weir Apipucos and upstream of the winding sections in local enlargement of the cross section, and the amount of the fork located at kilometer nine. The scenarios after the dredging to deepen indicate places with predominance of erosion and other sedimentation processes. They compared the longitudinal and transverse profiles of two areas located in western shipping channel, one with 350 meters and over 1000 meters, where it was found that the area 1 to erosion/sediment removal exceeded sedimentation/aggradation, indicating the occurrence of dredging the interval between the two surveys linked to the natural processes. In the study area 2 the sedimentation/aggradation exceeded the removal/erosion, with a volume equivalent to 1 cm thickness. The aggradation occur homogeneously in the navigation channel, while erosion was located on the banks, more intensely on the right bank of the river.

Relation of mussel spatfall on natural and artificial substrates: Analysis of ecological implications ensuring long-term success and sustainability for mussel farming

The characteristic that enables mussels to settle on filamentous type substrates is exploited to supply seeds for mussel farming via spatfall on ropes. This produces a strong socio-economic impact, as it allows independent and micro-producers to participate directly in an activity that produces >3milliontonnes annually worldwide. Despite the importance of this ecosystem service, there is no record of an association between the dynamics of natural beds and spatfall on ropes in literature published to date. This paper presents the case of Mytilus chilensis a species that dominates the inter-tidal and shallow sub-tidal communities in estuaries on the Chilean coast and part of the Argentine coast. Cultivation of this species in Chile produces about 240,000tonnes annually. Our aim is to study the dynamics of natural M. chilensis beds in Reloncaví fjord, one of the most important areas for mussel spatfalls in Chile, and investigate their relationship with spatfall on collectors (ropes) installed by local producers. Veliger competent larvae were present in relative abundance during most of the breeding season, which was reflected in a high spatfall on ropes. However, this larval abundance resulted in low recruitment into the natural beds, suggesting a mismatch between larval supply and recruitment. Oversupply of artificial substrate can promote recruitment failures in natural beds, which can be a threat to the mussel farming industry. There is a risk that the search for a possible alternative to natural seed supply (e.g. seed production in the laboratory) to satisfy mussel farming requirements, may come into conflict with the principles of the Ecosystem Approach to Aquaculture; thus, management measures should be implemented, in order to ensure the long-term sustainability of this activity.

Spatiotemporal variability of sedimentology and morphology in the East Frisian barrier island system

The highly dynamic East Frisian barrier island system (southern North Sea) is characterized by a complex morphology of tidal inlets, ebb-tidal deltas and foreshore beaches that reacts to storms and fair-weather conditions with characteristic patterns of sediment grain-size distributions. The morphological and sedimentological response to varying hydrodynamic conditions yet occurs in short time spans that are not covered by common monitoring strategies with measuring intervals typically of years. This study applies process-based numerical modelling with multiple sediment fractions to interpolate morphological states in time between bathymetrical surveys conducted in the summer months of 2004 and 2006. Morphodynamic simulations driven by real-time boundary conditions of tides, wind and waves are carried out for a representative period of 2 years. The spatiotemporal variability of the nearshore sedimentology and morphology is assessed by graded ranges of bed dynamics (i.e. bed elevation range) and the definition of sediment grain-size variability (i.e. mean diameter range). The effect of storm events and timescales of the sedimentological adaptation after storms to typical fair-weather conditions are exemplified at an ebb-tidal delta lobe where the morphological and sedimentological variability is found to be largest in the study area. The proposed method may serve to identify areas of high sedimentological and morphological activity for system understanding or in the framework of coastal monitoring strategies.

Effects of flow regulation on river bed dynamics and invertebrate communities in a Mediterranean river

Mediterranean rivers are hotspots for biodiversity, and riverine species are adapted to regular physical perturbations that affect channel morphology during flashy rainfall-runoff events. Dams alter flow regimes, changing flood magnitude and frequency; they also interrupt the continuity of sediment transport. Changes in both flood and sediment transport regimes affect downstream channel dynamics and the ecological functioning of fluvial systems. This paper examines the effects of flow regulation on bed disturbance, invertebrate assemblages and their biological traits in a Mediterranean river (the Siurana, NE Iberian Peninsula). Results are put in the broader context of the whole Ebro river catchment. The Siurana Reservoir causes a complete inversion of the seasonal flow regime and reduces flood magnitude notably. Upstream from the reservoir, torrential floods mobilize surface and subsurface bed materials, regularly disturbing the physical habitat; downstream, geomorphological activity in the channel is almost nonexistent. Altogether, damming causes significant differences in taxonomic composition of the benthic invertebrate communities; density and biomass increase notably below the dam although diversity decreases. At the broader scale taxa with active aerial dispersal traits dominate unregulated reaches in areas with marked Mediterranean hydro-climatic regimes, whereas reaches with more stable regimes are characterized by fully aquatic detritivore species.

The cycle of mussels: long-term dynamics of mussel beds on intertidal soft bottoms at the White Sea

Dense blue mussel assemblages are unstable, their structure changing from year to year. Three types of models can be used to describe this instability: (1) “exogenous” model based on regional temperature fluctuations, (2) “endogenous” deterministic model associated with negative impact of adult mussels on juveniles and (3) “density-linked stochasticity” model based on positive feedbacks resulting in overcrowding and destabilizing the settlement. We compared predictions deduced from these models with a time series based on the results of long-term (18 years) monitoring of abundance and demographic structure of three mussel beds at the White Sea. Most of our findings agreed well with the predictions deduced from the endogenous model. In particular, (1) long-term changes in mussel abundance and demographic structure were strictly cyclic, with non-matching periods (5–9 years) at different sites; (2) stages with the dominance of old mussels alternated with those where juveniles dominated and (3) some signals of delayed density dependence were revealed. However, the time series also contained elements of long-term trends, which may testify to the involvement of some exogenous factors (probably long-term climate changes).

Computational and experimental studies on bed dynamics of a gas–solid fluidized bed using Geldart-A particle: A comparison

ABSTRACTThe bed dynamics of a two-dimensional gas–solid fluidized bed is studied experimentally and computationally using Geldart-A particles. Commercial software ANSYS FLUENT 13 is used for computational studies. Unsteady behavior of gas–solid fluidized bed is simulated by using the Eulerian–Eulerian model coupled with the kinetic theory of granular flow. The two-equation standard k−ϵ model is used to describe the turbulent quantities. The simulation predictions are compared with experimentally observed data on volume fraction, bed pressure drop and bed expansion ratio. The results of simulations are found to be in close agreement with the experimental observations, implying that computational fluid dynamics (CFD) can be used for the design of an efficient bench-scale catalytic fluidized bed reactor.

Influence of Air on the Bouncing Dynamics of Shallow Vibrated Granular Beds: Kroll’s Model Predictions

In the present work, we explore the modification of the periodicity of the motion of shallow granular columns in the framework of Kroll’s one-dimensional model for the motion of a vibrated bed. Within the model, bed dynamics depend on two parameters, the dimensionless maximum acceleration, Γ, and a dissipative parameter, α, depending on air viscosity, grain density, bed static porosity, oscillation frequency and grain diameter. We show how the bifurcation diagram for the flight time of the bed as a function of Γ changes with α. For α = 0, Kroll’s prediction equals that of the inelastic bouncing ball model. When α is increased, bifurcations shift to higher Γ up to a point where not even a single bifurcation is predicted in a range of Γ where an inelastic bouncing ball displays several bifurcations. We also illustrate how the flight time reduces nonlinearly with increasing α in a monotonic way. We introduce isoperiodic maps to illustrate regions of single, double, or more periodicities in the phase space. W...

Effect of kinetics on the thermal performance of a sorption heat storage reactor

To reach high solar fractions for solar thermal energy in the built environment, long-term heat storage is required to overcome the seasonal mismatch. A promising method for long term heat storage is to use thermochemical materials, TCMs. In this research, a lab-scale test thermochemical heat storage system is tested experimentally and modeled numerically. Water-zeolite 13X is used as the working pair in an open packed bed reactor. The purpose of this study is to understand the effects of the kinetic parameters for the adsorption of water vapor on zeolite 13X (2mm spherical beads), on the thermal performance of a sorption heat storage packed bed reactor. A mathematical model is developed incorporating the kinetics model and the isotherm curves and including the heat losses from the side wall of the reactor, and is validated by comparing the calculated temperature profiles with experimental ones from a lab-scale test setup. The numerical and experimental results are used to calculate the heat fluxes in the reactor and are compared to evaluate the thermal performance of the reactor. With the validated model, a parameter study is carried out into the effect of the reaction kinetics and the gas flow rate on thermal performance of a thermochemical heat storage reactor under full scale normal operating conditions. From this work, predictions of the thermal dynamics of an adsorption bed on reactor scale can be achieved, which can and will be used for further studies on the design and optimization of a thermochemical heat storage system.

Pattern formation in fluidized beds as a tool for model validation: A two-fluid model based study

Computational fluid dynamics (CFD) models have been broadly used during the last twenty years to engineer and understand fluidized beds. Nevertheless, there is some controversy about the rigor of their current validation methodologies (Powder Technol. 139 (2004), 99). A robust tool to determine whether or not a model reproduces—let alone, can predict—the dynamics of a fluidized bed is still missing. This is especially relevant for the validation of the fluid-particle closures that are emerging with the help of direct numerical simulation.More than a decade ago, it was demonstrated experimentally that regular patterns emerge in pulsed fluidized beds under certain experimental conditions. These patterns are not a singular feature of the dynamics, such as average bubble size or bed expansion, but form as a result of a precise coupling between multi-scale physical phenomena. Remarkably, CFD has not been able, so far, to reproduce the experimental bubble patterns convincingly.In this work, we want to bring to the attention of the fluidization community the power of pattern formation in fluidized beds as a tool for model validation. As a proof of concept, we apply this validation test to two-fluid models. Our two-fluid simulations reproduce bubble properties reasonably well, but fail to reproduce the experimentally witnessed patterns, suggesting that the physics of the fluidized state are not correctly captured by this approach, under any of its common implementations.

Dynamics of a slowly-varying sand bed in a circular pipe

The long wave-length dynamics and stability of a bed of sand occupying the lower segment of a circular pipe are studied analytically up to first-order in the small parameter characterizing the slope of the bed. The bed is assumed to be at rest, with at most a thin sand layer (the bedload) moving at the sheared interface. When the sand bed is plane, with depth independent of position z along the axis of the pipe, the velocity of the liquid is known from previous studies of stratified laminar flow of two Newtonian liquids (the lower one with infinite viscosity representing the sand bed). When the depth of the sand bed varies with z, secondary flows develop in the cross-sectional (x, y) plane, and these are computed numerically, assuming that the sand bed remains a straight horizontal line in the cross-sectional plane. The mean shear stress acting on the perturbed sand bed is then determined both from the computed secondary flows and by means of the averaged equations of Luchini and Charru. The latter approach requires knowledge only of the flow over the unperturbed, flat sand bed, combined with an accurate approximation of the distribution of the perturbed stresses between the pipe wall and the sand bed. The perturbed stresses determined by the two methods agree well with each other. Using these stresses, it is then possible to apply standard theories of bed stability to determine the balance between the destabilizing effect of inertial (out-of-phase) stresses and the stabilizing effects of gravity and relaxation of the particle flux, and various examples are considered.

Experimental study on the axial mass transport of minced biomass (rape straw) into a pyrolysis rotating reactor working in the slipping regime

The paper presents an experimental study of the behavior of a minced biomass bed advancing into an inclined rotating drum to be used for biomass pyrolysis in the slipping regime. Minced rape straw was fed into the reactor with different feed rates, and its axial motion was examined at different slopes and cylinder rotating rates. By collecting and analysing images of the bed during the experimental runs, axial flow rates were determined; a mathematical model derived from mass and momentum balances was then applied, and the main parameter, the bed-wall friction factor, was obtained by regression of experimental data. Finally, in order to generalize bed dynamics, residence times and flow rates (mean values) were related to the experimental conditions by parametric semi-empirical and dimensionless correlations. Apart from its essential role in the future development of a whole chemical process model (including heat exchange, gas fluid dynamics, pyrolysis and gas phase reaction kinetics) to determine optimal working conditions, the dynamic model proposed here could also be of more general interest and be applied with other kinds of material and conditions on both an experimental and industrial scale.

Developed Hybrid Model for Propylene Polymerisation at Optimum Reaction Conditions.

A statistical model combined with CFD (computational fluid dynamic) method was used to explain the detailed phenomena of the process parameters, and a series of experiments were carried out for propylene polymerisation by varying the feed gas composition, reaction initiation temperature, and system pressure, in a fluidised bed catalytic reactor. The propylene polymerisation rate per pass was considered the response to the analysis. Response surface methodology (RSM), with a full factorial central composite experimental design, was applied to develop the model. In this study, analysis of variance (ANOVA) indicated an acceptable value for the coefficient of determination and a suitable estimation of a second-order regression model. For better justification, results were also described through a three-dimensional (3D) response surface and a related two-dimensional (2D) contour plot. These 3D and 2D response analyses provided significant and easy to understand findings on the effect of all the considered process variables on expected findings. To diagnose the model adequacy, the mathematical relationship between the process variables and the extent of polymer conversion was established through the combination of CFD with statistical tools. All the tests showed that the model is an excellent fit with the experimental validation. The maximum extent of polymer conversion per pass was 5.98% at the set time period and with consistent catalyst and co-catalyst feed rates. The optimum conditions for maximum polymerisation was found at reaction temperature (RT) 75 °C, system pressure (SP) 25 bar, and 75% monomer concentration (MC). The hydrogen percentage was kept fixed at all times. The coefficient of correlation for reaction temperature, system pressure, and monomer concentration ratio, was found to be 0.932. Thus, the experimental results and model predicted values were a reliable fit at optimum process conditions. Detailed and adaptable CFD results were capable of giving a clear idea of the bed dynamics at optimum process conditions.

A stochastic model approach for optimisation of lowland river restoration works

Over the course of centuries, river systems have been heavily trained for the purpose of safe discharge of water, sediment and ice, and improves navigation. Traditionally, dikes are used to be reinforced and heightened to protect countries from ever higher flood levels. Other types of solutions than technical engineering solutions, such as measures to increase the flood conveyance capacity (e.g., lowering of groynes and floodplains, setting back dikes) become more popular. These solutions may however increase the river bed dynamics and thus impact negatively navigation, maintenance dredging and flood safety. A variety of numerical models are available to predict the impact of river restoration works on river processes. Often little attention is paid to the assessment of uncertainties. In this paper, we show how we can make uncertainty explicit using a stochastic approach. This approach helps identifying uncertainty sources and assessing their contribution to the overall uncertainty in river processes. The approach gives engineers a better understanding of system behaviour and enables them to intervene with the river system, so as to avoid undesired situations. We illustrate the merits of this stochastic approach for optimising lowland river restoration works in the Rhine in the Netherlands.

A numerical study exploring the effect of particle properties on the fluidization of adhesive particles

The effects of varying the elastic modulus, coefficient of restitution, and coefficient of friction of adhesive particles on fluidized bed dynamics have been investigated via numerical simulations. It is found that lower values of the elastic modulus and coefficient of restitution lead to a greater degree of particle clustering, and the formation of smaller bubbles. Coordination numbers are found to initially increase, and then fall, with increasing coefficient of friction, while bubble velocities follow the opposite trend. It is concluded that artificially reducing the elastic modulus of adhesive particles has a significant impact on the fluidization behavior. The change in dynamics of the fluidized bed due to varying the coefficient of friction is more complex: particle clustering increases up to a point, beyond which clusters become increasingly rigid. © 2016 American Institute of Chemical Engineers AIChE J, 62: 1467–1477, 2016

Field driven charging dynamics of a fluidized granular bed.

A simplified model has previously described the inductive charging of colliding identical grains in the presence of an external electric field. Here we extend that model by including heterogeneous surface charge distributions, grain rotations and electrostatic interactions between grains. We find from this more realistic model that strong heterogeneities in charging can occur in agitated granular beds, and we predict that shielding due to these heterogeneities can dramatically alter the charging rate in such beds.

Effect of Frequency on Pulsed Fluidized Beds of Ultrafine Powders

Deagglomeration of ultrafine powders poses an important challenge towards their efficient and effective utilization. In the present study, we investigate the effect of frequency on the hydrodynamics of pulsed fluidized beds of ultrafine powders that show strong agglomeration behavior. We have carefully selected square waves of three different frequencies: 0.05 Hz, 0.10 Hz, and 0.25 Hz. The lowest frequency used here allowed the fluidized bed to settle completely before another pulse was introduced whilst the highest frequency ensured that the bed remained in a state of continuous turbulence between occurrences of consecutive pulses. On the other hand, the intermediate frequency pulse was just sufficient to complete the process of bed collapse before the start of the next pulse. Both local and global bed dynamics in all the three cases were rigorously monitored using fast response pressure transducers. The pressure transient data during the bed collapse were processed using the bed collapse model reported in the literature to compute the effective hydrodynamic diameter of agglomerates. Though there was substantial decrease in the agglomerate size, the effect of the frequency appeared to be rather insignificant as the global pressure transients remained rather insensitive to the change of the fluidization velocity.