Diffusion Flow Research Articles

Development of evaluation method for radiocesium availability in soil by biomimetic approach.

Applicability of biomimetic approach with simulation of plant uptake for assessment of radiocesium availability in soil was investigated. The soil spiked with 137Cs tracer was contacted with wicking material and copper-substituted prussian blue (Cu-PB), which simulate transpirationally induced mass flow and concentration gradient-induced diffusion of radiocesiumin the soil, respectively. Comparison of the removed 137Cs to the wick and the wick + Cu-PB from the soil during the contact period of 12weeks suggested that the diffusion process has larger contribution than the mass flow process in radiocesium dynamics in root zone. The change of the removed rate of 137Cs from the soil was reflected that its availability decreased with the time elapsing and with subjecting repeated wet-dry treatment. The results suggest that the biomimetic approach can be applicable to the realistic evaluation of the availability of radiocesium in soil.

Modeling two-phase gas-solid flow in axisymmetric diffusers using cut cell technique: an Eulerian-Eulerian approach

AbstractAxisymmetric diffusers find wide applications in various industrial processes, including combustion systems, pneumatic conveying, and fluidized bed reactors. Understanding and accurately modeling the behavior of two-phase flows in these devices are critical for optimizing their performance and efficiency. Various aspects of the two-phase gas-solid flow, including particle-particle and particle-wall interactions, interphase momentum transfer, and phase segregation, are investigated. Our currently developed technique, which uses a cut-cell technique, is an established new method to handle the inclined wall of an axisymmetric diffuser. The near-wall cells are treated as five faces for the new grid; one is the inclined wall. This helps treat the boundary condition at the wall in an accurate physical way. The code FORTRAN, developed in-house, was used to solve the axisymmetric diffuser using a cut-cell technique. The results of this study will provide valuable insights into the behavior of gas-solid two-phase flows in axisymmetric diffuser. A parametric study of the impact of particles diameters ($100,\ 200,\ \text{and} \ 300\mu m$ 100 , 200 , and 300 μ m ), the solid volume loading ratios $(0.005,\ 0.008,\ 0.01)$ ( 0.005 , 0.008 , 0.01 ) , and cant angle $(4.5^{\circ},\ 7^{\circ},\ 9.5^{\circ}) $ ( 4.5 ∘ , 7 ∘ , 9.5 ∘ ) effect of axisymmetric diffuser on the local skin friction, pressure, velocity, turbulent kinetic energy, and separation zone.

Numerical investigation for bioconvection flow of radiative viscoelastic nanofluid with triple diffusion and higher-order slip effects in a porous medium

Owing to enhanced thermal features, the study of nanofluids has become motivating research area which include various applications in renewable energy, thermal management devices, heat exchangers, microelectronics cooling, automotive industry etc. Owing such applications in mind, the aim of current investigation is to present a triple diffusion flow of viscoelastic nanofluid in presence of higher order slip effects. The problem investigates the insight of thermo-diffusion phenomenon with visualization of heat transfer, solutal concentration, nanoparticles concentration and bioconvection applications. The radiative impact with nonlinear relations and heat source are incorporated in the heat equation. The numerical simulations of problem have been performed via shooting scheme. Various applications are claimed with variation of flow parameters. It has been observed that the increase of the slip parameter enhances both the temperature nanofluid and concentration profiles. The solutal concentration profile increases due to Dufour Lewis number. Moreover, the change in nano-Lewis number leads to a decrease of concentration profile.

Characteristics of Thermo Diffusion and Buoyant Force Effects in MHD Flow of Copper and Titanium Dioxide Nanoparticles Suspended in Water between Infinite Vertical Plates

Characteristics of Thermo Diffusion and Buoyant Force Effects in MHD Flow of Copper and Titanium Dioxide Nanoparticles Suspended in Water between Infinite Vertical Plates

Experimental Evaluation of Gas-Dynamic Conditions of Heat Exchange of Stationary Air Flows in Vertical Conical Diffuser

Conical diffusers are widely used in technical devices (gasifiers, turbines, combustion chambers) and technological processes (ejectors, mixers, renewable energy). The perfection of flow gas dynamics in a conical diffuser affects the intensity of heat and mass transfer processes, the quality of mixing/separation of working media and the flow characteristics of technical devices. These parameters largely determine the efficiency and productivity of the final product. This article presents an analysis of experimental data on the gas-dynamic characteristics of stationary air flows in a vertical, conical, flat diffuser under different initial boundary conditions. An experimental setup was created, measuring instruments were selected, and an automated data collection system was developed. Basic data on the gas dynamics of air flows were obtained using the thermal anemometry method. Experimental data on instantaneous values of air flow velocity in a diffuser for initial velocities from 0.4 m/s to 2.22 m/s are presented. These data were the basis for calculating and obtaining velocity fields and turbulence intensity fields of the air flow in a vertical diffuser. It is shown that the value of the initial flow velocity at the diffuser inlet has a significant effect on the gas-dynamic characteristics. In addition, a spectral analysis of the change in air flow velocity both by height and along the diffuser axis was performed. The obtained data may be useful for refining engineering calculations, verifying mathematical models, searching for technical solutions and deepening knowledge about the features of gas dynamics of air flows in vertical diffusers.

Natural H2 Transfer in Soil: Insights from Soil Gas Measurements at Varying Depths

The exploration of natural H2 is beginning in several countries. One of the most widely used methods for detecting promising areas is to measure the H2 percentage of the air contained in soils. All data show temporal and spatial variabilities. The gradient versus depth is not usually measured since the standard procedure is to drill and quickly install a tube in the soil to pump out the air. Drill bits used do not exceed one meter in length. These limitations have been overcome thanks to the development of a new tool that enables percussion drilling and gas measurements to be carried out with the same tool until 21 feet deep. This article shows the results obtained with this method in the foreland of the Colombian Andes. The variation of the gradient as a function of depth provides a better understanding of H2 leakage in soils. Contrary to widespread belief, this gradient is also highly variable, and, therefore, often negative. The signal is compatible with random and discontinuous H2 bubbles rising, but not with a permanent diffusive flow. Near-surface bacterial consumption should result in a H2 increase with depth; it may be true for the first tens of centimeters, but it is not observed between 1 and 5 m. The results show that, at least in this basin, it is not necessary to measure the H2 content at depths greater than the conventional one-meter depth to obtain a higher signal. However, the distance between the measured H2 peaks versus depth may provide information about the H2 leakage characteristics and, therefore, help quantify the near-surface H2 flow.

Monotonicity, asymptotics and level sets for principal eigenvalues of some elliptic operators with shear flow

We investigate the joint effects of diffusion and advection on principal eigenvalues of some elliptic operators with shear flow. Some monotonicity and asymptotic behaviors of principal eigenvalues, with respect to diffusion rate and flow amplitude, are established. These analyses lead to a classification of topological structures of level sets for principal eigenvalues, as a function of diffusion rate and flow amplitude. Our analytical results provide a unifying viewpoint to understand mixing enhancement and dispersal-induced growth, which are apparently two unrelated phenomena, one in fluid mechanics and the other in population dynamics. In our analysis, some limiting Hamilton-Jacobi equations, blowup argument and limiting generalized principal eigenvalue problems play critical roles.

Pressure Distributions and Flow Regimes: An Integrated Approach For CO2 Sequestration Project Design and Evaluation in Hydraulically Fractured Shale Reservoirs

This paper introduces an integrated approach for estimating the key parameters of CO2 storage projects in depleted hydraulically fractured shale reservoirs. The approach focuses deep insights into evaluating these projects based on the pressure distribution and flow regimes. The objective is reducing the uncertainties in the design and evaluation criteria of CO2 storage by better understanding the flow mechanisms in the porous media. This understanding helps in perfectly modeling CO2 distribution and characterizing the expected flow regimes.Several analytical models are developed for the pressure behavior of CO2 multi-size scale flow mechanisms. Diffusion flow, slip flow, adsorption flow, and Darcy and non-Darcy flow are all considered. The flow in the nano-size organic particles, the micro-size kerogen particles, and the macro-size matrix is analytically described. The flow inside natural fractures in the stimulated and unstimulated reservoir volume as well as the flow of CO2 in the hydraulic fractures are modeled. Several rectangular reservoir configurations, hydraulic fracture characteristics, and petrophysical properties of the matrix, kerogen, and organic matter are examined. An estimation model for the total volume of CO2 that could be stored in the depleted fractured reservoirs is presented. The contribution of the unstimulated and stimulated porous media as well as the hydraulic fractures to the total volume is determined by characterizing the flow regimes. Different models are proposed for the time at which CO2 reaches the hydraulic fracture tips, the borders between stimulated and unstimulated porous media, and the reservoir boundary. Profound explanations are introduced for the impact of the constrained pressure on the total capacity of the reservoirs for CO2 storage.The study has reached several conclusions. The characteristics of the organic matter, kerogen, and the micro-size scale matrix do not significantly impact the pressure distribution and flow regimes. Conversely, the pressure distribution and flow regimes are significantly impacted by the characteristics of the hydraulic and natural fractures. The hydraulic fractures may offer a reasonable capacity for CO2 storage, meanwhile, the capacity of stimulated and unstimulated reservoir volume is controlled by reservoir configuration and fracture spacing. The constrained pressure may strictly reduce the total capacity of CO2 storage in the reservoir. The total volume of the injected CO2 can be determined from the pressure point when the injection pulse has reached to reservoir boundary. Beyond this point, it is not recommended to inject CO2 as it could increase reservoir pressure more than initial pressure. The pressure of the depleted reservoir before injection is a key parameter in the design and evaluation of the CO2 storage in shale reservoirs.This study introduces a novel approach for the design and evaluation criteria of CO2 storage in depleted shale reservoirs. The approach uses the pressure distribution and flow regimes that could be observed in the multi-size scale components of the reservoirs. The study proposes several models for estimating CO2 geosequestration capacity in the hydraulic fractures, stimulated and unstimulated reservoir volume as well as the total storage capacity of the reservoirs. The approach confidently addresses the concerns regarding the applicability of unconventional resources for CO2 storage and the total capacity offered by these reservoirs for this purpose.

Fluctuating hydrodynamics of an autophoretic particle near a permeable interface

We study the autophoretic motion of a spherical active particle interacting chemically and hydrodynamically with its fluctuating environment in the limit of rapid diffusion and slow viscous flow. Then, the chemical and hydrodynamic fields can be expressed in terms of integrals. The resulting boundary-domain integral equations provide a direct way of obtaining the traction on the particle, requiring the solution of linear integral equations. An exact solution for the chemical and hydrodynamic problems is obtained for a particle in an unbounded domain. For motion near boundaries, we provide corrections to the unbounded solutions in terms of chemical and hydrodynamic Green's functions, preserving the dissipative nature of autophoresis in a viscous fluid for all physical configurations. Using this, we give the fully stochastic update equations for the Brownian trajectory of an autophoretic particle in a complex environment. First, we analyse the Brownian dynamics of particles capable of complex motion in the bulk. We then introduce a chemically permeable planar surface of two immiscible liquids in the vicinity of the particle and provide explicit solutions to the chemo-hydrodynamics of this system. Finally, we study the case of an isotropically phoretic particle hovering above an interface as a function of interfacial solute permeability and viscosity contrast.

SIMS and Numerical Analysis of Asymmetrical Out-Diffusion of Hydrogen and Carbon in CdxZn1-xO:Eu Multilayer.

This study employs secondary ion mass spectrometry (SIMS) to investigate the diffusion behavior of hydrogen and carbon in a CdxZn1-xO:Eu multilayer at different annealing temperatures (500-900 °C). The SIMS results reveal a significant out-diffusion of these elements toward the surface and diffusion to the interface region. The diffusion flow rates are asymmetric and favor the interface direction. The depth profiles of diffused elements are fitted using the forward timecentered space (FTCS) iteration method. The activation energies are determined to be 0.35 ± 0.06 eV for hydrogen and 0.33 ± 0.09 eV for carbon, suggesting an interstitial mechanism in CdxZn1-xO. The results indicate that increasing the annealing temperatures leads to a significant decrease in impurity concentrations.

Binary Nature of Collisions Facilitates Runaway Electron Generation in Weakly Ionized Plasmas.

Dreicer generation is one of the main mechanisms of runaway electron generation in weakly ionized plasmas. It is often described as a diffusive flow from the Maxwellian core into high energies under the effect of the electric field. In this Letter we demonstrate a critical role of the binary nature of inelastic collisions in weakly ionized plasma during tokamak startup, where some electrons experience virtually no collisions during acceleration to the critical energy. We show that using the Fokker-Planck collisional operator can underestimate the Dreicer generation rate by several orders of magnitude.

Structural stability for double-diffusion Brinkman fluid with Soret effect

This paper considers the double diffusive Brinkman flow in a semi-infinite pipe. By establishing a priori estimates of the solutions and setting an appropriate “energy” function, we not only obtain the continuous dependence and convergence of the solution on the Soret coefficient but also prove that the solutions decay exponentially with the distance from the finite end of the cylinder.

Numerical Simulations of Impact River Morphology Evolution Mechanism Under the Influence of Floodplain Vegetation

Shallow floodplains play a crucial role in river basins by providing essential ecological, hydrological, and geomorphic functions. During floods, intricate hydrodynamic conditions arise as flow exits and re-enters the river channel, interacting with the shallow vegetation. The influence and mechanism of shoal vegetation on channel hydrodynamics, bed topography, and sediment transport remain poorly understood. This study employs numerical simulations to address this gap, focusing on the Xiaolangdi–Taochengpu river section downstream of the Yellow River. Sinusoidal-derived curves are applied to represent the meandering river channel to simulate the river’s evolutionary process at a true scale. The study simulated the conditions of bare and vegetated shallow areas using rigid water-supported vegetation with the same diameter but varying spacing. The riverbed substrate was composed of non-cohesive sand and gravel. The analysis examined alterations in in-channel sediments, bed morphology, and bed heterogeneity in relation to variations in vegetation density. Findings indicated a positive correlation between vegetation density and bed heterogeneity, implying that the ecological complexity of river habitats can be enhanced under natural hydrological conditions in shallow plain vegetation and riparian diffuse flow. Therefore, for biological river restoration, vegetation planting in shallow plain regions can provide greater effectiveness.

Improving approximation accuracy in Godunov-type smoothed particle hydrodynamics methods

The study examines the origin of errors resulting from the approximation of the right hand sides of the Euler equations using the Godunov type contact method of smoothed particle hydrodynamics (CSPH). The analytical expression for the numerical shear viscosity in CSPH method is obtained. In our recent study the numerical viscosity was determined by comparing the numerical solution of momentum diffusion in the shear flow with theoretical one. In this study we deduce the analytical expression for the numerical viscosity which is found to be similar to numerical one, confirming the obtained results. To reduce numerical diffusion, diffusion limiters are typically applied to expressions for contact values of velocity and pressure, as well as higher-order reconstruction schemes. Based on the performed theoretical analysis, we propose a new method for correcting quantities at interparticle contacts in CSPH method, which can be easily extended to the MUSCL-type (Monotonic Upstream-centered Scheme for Conservation Laws) method. Original CSPH and MUSCL-SPH approaches and ones with aforementioned correction are compared.

MHD double diffusive convective squeezing ternary nanofluid flow between parallel plates with activation energy and viscous dissipation

Purpose This study aims to present the consequences of activation energy and the chemical reactions on the unsteady MHD squeezing flow of an incompressible ternary hybrid nanofluid (THN) comprising magnetite (FE3O4), multiwalled carbon nano-tubes (MWCNT) and copper (Cu) along with water (H2O) as the base fluid. This investigation is performed within the framework of two moving parallel plates under the influence of magnetic field and viscous dissipation. Design/methodology/approach Due to the complementary benefits of nanoparticles, THN is used to augment the heat transmit fluid’s efficacy. The flow situation is expressed as a system of dimensionless, nonlinear partial differential equations, which are reduced to a set of nonlinear ordinary differential equations (ODEs) by suitable similarity substitutions. These transformed ODEs are then solved through a semianalytical technique called differential transform method (DTM). The effects of several changing physical parameters on the flow, temperature, concentration and the substantial measures of interest have been deliberated through graphs. This study verifies the reliability of the results by performing a comparison analysis with prior researches. Findings The enhanced activation energy results in improved concentration distribution and declined Sherwood number. Enhancement in chemical reaction parameter causes disparities in concentration of the ternary nanofluid. When the Hartmann number is zero, value of skin friction is high, but Nusselt and Sherwood numbers values are small. Rising nanoparticles concentrations correspond to a boost in overall thermal conductivity, causing reduced temperature profile. Research limitations/implications Due to its firm and simple nature, its implications are in various fields like chemical industry and medical industry for designing practical problems into mathematical models and experimental analysis. Practical implications Deployment of the squeezed flow of ternary nanofluid with activation energy has significant consideration in nuclear reactors, vehicles, manufacturing facilities and engineering environments. Social implications This study would be contributing significantly in the field of medical technology for treating cancer through hyperthermia treatment, and in industrial processes like water desalination and purification. Originality/value In this problem, a semianalytical approach called DTM is adopted to explore the consequences of activation energy and chemical reactions on the squeezing flow of ternary nanofluid.

Pore-Scale Simulation of Deep Shale Gas Flow Considering the Dual-Site Langmuir Adsorption Model Based on the Pore Network Model.

As a key resource in the oil and gas sector, shale gas development profoundly influences the advancement of the global energy industry. Deep shale gas reservoirs, typically found at depths exceeding 3500 m, represent a significant portion of total shale gas reserves. Currently, pore network models primarily simulate middle and shallow shale gas, insufficiently addressing the unique challenges posed by high-temperature and high-pressure conditions in deep shale gas formations. There is an urgent need to explore the microscale flow dynamics of deep shale gas. In this work, we use the pore network model to simulate the flow dynamics of deep shale gas, particularly under nanoconfinement conditions. The simulation integrates adsorption, slip flow, Knudsen diffusion, and bulk flow phenomena. Utilizing the dual-site Langmuir adsorption model tailored for high-temperature and high-pressure conditions enhances the accuracy of gas conductivity calculations for the adsorbed phase, thereby reflecting the specific characteristics of deep shale gas reservoirs. Moreover, this research investigates the flow characteristics of deep shale gas under varying sensitivity parameters, such as water saturation, temperature, and pressure. It explores methane flow dynamics, focusing on effective diffusion coefficients and permeability. The modified approach to calculating adsorption phase conductivity using the dual-site Langmuir adsorption model accurately captures the adsorption behaviors and characteristics of deep shale gas reservoirs.

Beyond the Surface: Interconnection of Viscosity, Crystal Growth, and Diffusion in Ge25Se75 Glass-Former.

The knowledge of viscosity behavior, crystal growth phenomenon, and diffusion is important in producing, processing, and practical applications of amorphous solids prepared in different forms (bulk glasses and thin films). This work uses microscopy to study volume crystal growth in Ge25Se75 bulk glasses and thermally evaporated thin films. The collected growth data measured over a wide temperature range show a significant increase in crystal growth rates in thin films. The crystal growth is analyzed using near-surface viscosities obtained in bulks and thin films using nanoindentation and melt viscosities measured by a pressure-assisted melt filling technique. The crystal growth analysis provides information on the size of the structural units incorporated into the growing crystals, essential for estimating the diffusion coefficients and explaining the difference in crystal growth rates in bulk and thin films. The crystal growth analysis also reveals the decoupling between diffusion and viscous flow described by the Stokes-Einstein-Eyring relation. Moreover, to the authors' best knowledge, the manuscript provides the first evaluation estimation of the effective self-diffusion coefficient directly from growth data in chalcogenide glass-formers. The present data show a similar relation between diffusion coefficients (D) and crystal growth rates (u): u ≈ D0.87, which is found in several molecular glasses.

Enhancing diffuser performance using transverse grooves to delay flow separation

A flow-control method is applied to enhance the efficiency and flow homogeneity of three-dimensional diffusers used in open-jet wind tunnels. Suitably shaped grooves are introduced in the diffuser diverging walls. The grooves promote the relaxation of the non-slip condition along the streamlines bounding the small recirculation regions forming passively inside the grooves. That reduces momentum losses and results in a downstream boundary layer with higher momentum, which is more separation-resistant. The proposed flow-control device has been successfully validated for plane diffusers [Mariotti et al., “Separation control and efficiency improvement in a 2D diffuser by means of contoured cavities,” Eur. J. Mech.-B 41, 138–149 (2013); and Mariotti et al., “Control of the turbulent flow in a plane diffuser through optimized contoured cavities,” Eur. J. Mech.-B 48, 254–265 (2014)]. In this study, we examined circular and square-section diffusers with different degrees of flow separation. Given that the investigated diffusers are part of open-jet wind tunnels, the entire wind tunnel geometry was included in the numerical simulation. The grooves significantly enhanced performance in circular diffusers by reducing the extent of separation and promoting an axisymmetric and spatially uniform flow. However, negligible benefits were observed for square-section diffusers. In these cases, since flow separation originates from one of the four inclined edges of the diffuser, placing grooves along the diverging walls does not effectively reduce the separation extent. Nonetheless, the grooves become effective again in diffusers with rectangular cross sections of high aspect ratio.

Diagnosis Method of Failure in PEMFC Using Magnetic Fields

Proton exchange membrane fuel cells (PEMFCs) are currently used in various applications because they are compact, lightweight, and easy to start up. One of the challenges in PEMFCs is the improvement of gas diffusion. To overcome this challenge, appropriate water management in the gas diffusion layer and flow channels is necessary. Therefore, PEMFC diagnostic techniques are needed to detect flooding and plugging, which affect performance and efficiency. In this study, the magnetic field inside a PEMFC was analyzed to evaluate the changes caused by the internally generated water. Three cell types were used in the experiments: normal, channel-modified, and membrane–electrode assembly-modified cells. The results showed that the current was diverted because of the channel modification and water content, which resulted in a significant difference in the vertical magnetic field. This indicates that power generation inhibition due to flooding can be detected using vertical magnetic fields.

Diagnosis Method of Failure in PEMFC Using Magnetic Fields

Proton exchange membrane fuel cells (PEMFCs) are currently used in various applications because they are compact, lightweight, and easy to start up. One of the challenges in PEMFCs is the improvement of gas diffusion. To overcome this challenge, appropriate water management in the gas diffusion layer and flow channels is necessary. Therefore, PEMFC diagnostic techniques are needed to detect flooding and plugging, which affect performance and efficiency. In this study, the magnetic field inside a PEMFC was analyzed to evaluate the changes caused by the internally generated water. Three cell types were used in the experiments: normal, channel-modified, and membrane–electrode assembly-modified cells. The results showed that the current was diverted because of the channel modification and water content, which resulted in a significant difference in the vertical magnetic field. This indicates that power generation inhibition due to flooding can be detected using vertical magnetic fields.