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

Pattern formation in pulsed gas-solid fluidized beds - The role of granular solid mechanics

Under certain conditions, gas-solid fluidized beds are known to develop a structured flow of bubbles when exposed to periodically pulsating air flows. In quasi-two-dimensional beds, periodically rising bubbles form a triangular tessellation in the vertical plane. Bubble nucleation sites at the distributor plate alternate during each cycle. This pattern sets an excellent benchmark for fundamental studies of fluidization. Notably, most common Eulerian descriptions of granular flow do not yet capture this interplay between solid mechanics and fluid-solid momentum exchange, which we show to be instrumental to the dynamic rearrangement of bubbles in a pulsed bed. We report the first successful CFD simulations of structured bubble flows in a deep, quasi-2D geometry using a Eulerian-Lagrangian CFD-DEM framework. Numerical results are in quantitative agreement with experiments. The simulated dynamics reveal that the patterns emerge from the transition of the granular collective behavior between solid-like and fluid-like, which is an outcome of dynamical coupling between gas and particles. The simulated results point out the essential role of solid frictional stresses on inducing and maintaining the formation of bubble patterns. This underscores the value of investigating pulsation-induced patterns as a prime manifestation of the mesoscopic physics underpinning fluidization, and highlights the direction for improving current practices

Chapitre 3 • Acoustic logging

Approaches that are typically applied in deep exploration geophysics, combining different seismic and logging methods, can be technically adapted for certain geotechnical or hydrogeological surveys or some site characterizations in the framework of seismic hazard studies. Currently it is entirely feasible to implement this type of geophysical surveying if the situation requires. After reviewing the current state of knowledge regarding borehole measurements of subsurface shear velocities applied to the geotechnical field, this book illustrates the feasibility of carrying out vertical seismic profiles (VSPs) and logs in this field. This approach also illustrates the value of combining velocity measurements of formations provided by borehole seismic tools (VSP) and acoustic (sonic) tools. An innovative example of the application of borehole seismic and logging methods is then presented in the case study of a relatively near-surface (from 20 to 130 m) karst carbonate aquifer. It shows how a multi-scale description of the reservoir can be carried out by integrating the information provided by different 3D-THR surface seismic methods, full waveform acoustic logging, VSP with hydrophones, borehole optical televiewer and flow measurements. In this book the authors provide readers with guidelines to carry out these operations, in terms of acquisitions as well as processing and interpretation. Thus, users will be able to draw inspiration to continue transferring petroleum techniques and other innovative methods for use in near-surface studies

Chapitre 2 • Well seismic surveying

Approaches that are typically applied in deep exploration geophysics, combining different seismic and logging methods, can be technically adapted for certain geotechnical or hydrogeological surveys or some site characterizations in the framework of seismic hazard studies. Currently it is entirely feasible to implement this type of geophysical surveying if the situation requires. After reviewing the current state of knowledge regarding borehole measurements of subsurface shear velocities applied to the geotechnical field, this book illustrates the feasibility of carrying out vertical seismic profiles (VSPs) and logs in this field. This approach also illustrates the value of combining velocity measurements of formations provided by borehole seismic tools (VSP) and acoustic (sonic) tools. An innovative example of the application of borehole seismic and logging methods is then presented in the case study of a relatively near-surface (from 20 to 130 m) karst carbonate aquifer. It shows how a multi-scale description of the reservoir can be carried out by integrating the information provided by different 3D-THR surface seismic methods, full waveform acoustic logging, VSP with hydrophones, borehole optical televiewer and flow measurements. In this book the authors provide readers with guidelines to carry out these operations, in terms of acquisitions as well as processing and interpretation. Thus, users will be able to draw inspiration to continue transferring petroleum techniques and other innovative methods for use in near-surface studies

Hydrodynamics and Local Turbulent Mixing of Submerged, Parallel Liquid Jets: Experiments and CFD Simulations

The hydrodynamics and local turbulent mixing of parallel multiple liquid jets, submerged in liquid, were investigated by means of experiments and computational fluid dynamics (CFD). A renormalization group (RNG) k-ε turbulence model was used to simulate the flow field. The model was validated experimentally by particle image velocimetry (PIV) measurements. In the converging region adjacent to the nozzle exits, the recirculation region disappears, and there is only ambient fluid entrainment. Different jet arrays were compared to evaluate the effects of the jet spatial arrangement on the hydrodynamics and mixing performance. A shorter mixing length in the merging region suggests that mixing is more efficient in the triple-jet system than in other jet systems. Compared with the jet Reynolds number, the jet spacing plays a more significant role in determining the critical mixing regions, while the linear relationship between them is more sensitive than that for multiple parallel plane jets

Bioinspired supramolecular macrocycle hybrid membranes with enhanced proton conductivity

Enhancing the proton conductivity of proton exchange membranes (PEMs) is essential to expand the applications of proton exchange membrane fuel cells (PEMFCs). Inspired by the proton conduction mechanism of bacteriorhodopsin, cucurbit[n]urils (CB[n], where n is the number of glycoluril units, n = 6, 7, or 8) are introduced into sulfonated poly(ether ether ketone) (SPEEK) matrix to fabricate hybrid PEMs, employing a nature-inspired chemical engineering (NICE) methodology. The carbonyl groups of CB[n] act as proton-conducting sites, while the host–guest interaction between CB[n] and water molecules offers extra proton-conducting pathways. Additionally, the molecular size of CB[n] aids in their dispersion within the SPEEK matrix, effectively bridging the unconnected proton-conducting sulfonic group domains within the SPEEK membrane. Consequently, all hybrid membranes exhibit significantly enhanced proton conductivity. Notably, the SPEEK membrane incorporating 1 wt.% CB[8] (CB[8]/SPEEK-1%) demonstrates the highest proton conductivity of 198.0 mS·cm−1 at 60 °C and 100% relative humidity (RH), which is 228% greater than that of the pure SPEEK membrane under the same conditions. Moreover, hybrid membranes exhibit superior fuel cell performance. The CB[8]/SPEEK-1% membrane achieves a maximum power density of 214 mW·cm−2, representing a 140% improvement over the pure SPEEK membrane (89 mW·cm−2) at 50 °C and 100% RH. These findings serve as a foundation for constructing continuous proton-conducting pathways within membranes by utilizing supramolecular macrocycles as fuel cell electrolytes and in other applications. [Figure not available: see fulltext.]

Universal stability curve for pattern formation in pulsed gas-solid fluidized beds of sandlike particles

A granular layer can form regular patterns, such as squares, stripes, and hexagons, when it is fluidized with a pulsating gas flow. These structures are reminiscent of the well-known patterns found in granular layers excited through vibration, but, contrarily to them, they have been hardly explored since they were first discovered. In this work, we investigate experimentally the conditions leading to pattern formation in pulsed fluidized beds and the dimensionless numbers governing the phenomenon. We show that the onset to the instability is universal for Geldart B (sandlike) particles and governed by the hydrodynamical parameters = ua/(utφ¯) and f/fn, where ua and f are the amplitude and frequency of the gas velocity, respectively, ut is the terminal velocity of the particles, φ¯ is the average solids fraction, and fn is the natural frequency of the bed. These findings suggest that patterns emerge as a result of a parametric resonance between the kinematic waves originating from the oscillating gas flow and the bulk dynamics. Particle friction plays virtually no role in the onset to pattern formation, but it is fundamental for pattern selection and stabilization

Steady-State Properties of Single-File Systems with Conversion

We have used Monte-Carlo methods and analytical techniques to investigate the influence of the characteristic parameters, such as pipe length, diffusion, adsorption, desorption and reaction rate constants on the steady-state properties of Single-File Systems with a reaction. We looked at cases when all the sites are reactive and when only some of them are reactive. Comparisons between Mean-Field predictions and Monte-Carlo simulations for the occupancy profiles and reactivity are made. Substantial differences between Mean-Field and the simulations are found when rates of diffusion are high. Mean-Field results only include Single-File behavior by changing the diffusion rate constant, but it effectively allows passing of particles. Reactivity converges to a limit value if more reactive sites are added: sites in the middle of the system have little or no effect on the kinetics. Occupancy profiles show approximately exponential behavior from the ends to the middle of the system.Comment: 15 pages, 20 figure

Effect of extended short-circuiting in proton exchange membrane fuel cells

Short-circuiting is regularly utilized in Proton Exchange Membrane Fuel Cells (PEMFCs) to reverse short-term reversible catalyst degradation. However, do these improvements in fuel cell performance and durability still exist after extended operation? We provide an answer to this question by comparing the performance and durability of a PEMFC under open-circuit voltage (OCV) and a commercial short-circuiting protocol, against a PEMFC under OCV without short-circuiting for the same extended period (∼144 h). The experimental results demonstrate the detrimental effect of extended short-circuiting on the durability of the catalyst and the performance of the fuel cell. Electrochemically active surface area losses reach ∼46% for the short-circuiting case, compared to only ∼18% losses for the OCV without short-circuiting. TEM and XPS measurements are employed to monitor the morphological changes of the catalyst layer, revealing that Ostwald ripening, carbon corrosion, and Pt migration and precipitation into the polymer membrane are the main degradation mechanisms of the cathode catalyst layer. At the end of PEMFC operation, XPS measurements reveal that only ∼0.1% (atomic) of Pt remains on the surface of the cathode catalyst layer after OCV with short-circuiting, compared to the initial ∼0.4% Pt of the unused cathode MEA and ∼0.3% Pt for the cathode MEA after OCV without short-circuiting. These results show that short-circuiting can cause facile degradation of the catalyst layer and significant decrease in fuel cell performance, rendering this technique non-beneficial for extended operation