Porous Media Model Research Articles

SiO2 soot preform sintering process for the synthesis of large-size and high-purity silica glass

The indirect chemical vapor deposition (CVD) method is a promising method to synthesize high-purity silica glass. As a critical procedure of this method, the sintering process is rather complex due to the interactive physical phenomena, and an in-depth understanding is therefore required. In this paper, a comprehensive multiphase porous medium model is developed by introducing the shrinkage mechanism, which agrees well with the experimental results. With this model, the dynamic characteristics during the sintering process, including temperature distribution, volumetric deformation, hydroxyl content evolution, and internal stress development, are well predicted. A parametric sensitivity analysis is conducted then, which shows that the average porosity and hydroxyl concentration are mainly affected by the average particle radius, while the maximum effective stress is sensitive to both Young's modulus and the average particle radius. Finally, the effects of key operating parameters are explored, and it is found that a lower working pressure, an appropriate sintering temperature such as 1450 °C, or a proper initial porosity such as 80.0%, is more desirable for a sintered product with high optical performance. This study can deepen the understanding of the SiO2 soot preform sintering process, and provide a good basis for further optimization.

Numerical Study on Internal and External Flow Fields of the UHMWPE Cage

Ultra-high molecular weight polyethylene (UHMWPE) is a new kind of fishing gear material applied in deep-sea fishing cages, which is becoming a trend. Studies on the internal and external flow fields of cages made of UHMWPE have been scarce previously. Therefore, a three-dimensional numerical model for the UHMWPE cage is established herein, where the cage is modeled by a porous media model. The Darcy–Forchheimer coefficients of the porous media are obtained by physical model experiments and numerical simulations. Then, the cylindrical cage is divided into 16 planar nets circumferentially, along with an additional bottom net, to investigate its internal and external flow fields numerically. For a single cylindrical cage, the degree of deceleration decreases as the flow velocity increases, and this effect becomes less apparent when the flow velocity reaches a certain threshold. Finally, the flow field characteristics of double cages with different spacing and multiple cages with equal spacing are revealed.

Full-scale numerical simulation and experimental validation of the thermal environment under the heating mode in an air-conditioned room

The numerical simulation of the air flow of the room equipped with the crossflow fan air conditioners has been conducted a lot. However, the simulation of the air flow distribution under the heating mode has always been difficult to match the experimental data well, due to the fact that both the air temperature and the flow are affected by the buoyancy. In addition, very few full-scale simulations were conducted. To precisely predict the air flow and temperature distribution of the room with the crossflow fan air conditioner, a full-scale steady numerical simulation was conducted with the Computational Fluid Dynamics (CFD) package. The full-scale physical model was constructed strictly according to the real settings, including the detailed components of the air-conditioner, such as the crossflow fan and the heat exchangers. The Shear-Stress Transport (SST) k-ω turbulent model was selected to simulate the low Reynold flow near the inner surface of the room, and to simulate the high Reynold and transitional flow near the fan blades of the air-conditioner. The porous medium model was used to simulate the heat exchange fins in the air-conditioner. Furthermore, an experiment under the same condition was also conducted to verify the accuracy of the simulation. Results showed that the simulated supply air flow rate was 537.48 m3/h, producing a 4.97% deviation from the experiment result. Meanwhile, the upward movement of the supply air from the outlet of the air-conditioner was observed, leading to higher air temperatures in the upper region of the room. Furthermore, among all 210 measured temperature data points in the room, 149 data points had the deviation within ±5%, and 194 data points had the deviation within ±10%. 9 abnormal data points with greater deviation near the door were detected, which might be resulted from the outdoor cold air leakage through the crack between the door and the floor in the experiment. Conclusively, the full-scale numerical simulation in this study produced an accurate and reliable solution to predict the air temperature distribution in buildings, thus providing a potential method for the design or optimization of the air-conditioners.

Numerical investigation of local scour around twin piles under steady current using CFD-DEM coupling method

In recent years, the local scour has developed into a major threat to the safety of offshore group piles. The numerical simulation methods are useful tools in investigating the key mechanisms behind local scour. However, conventional simulation methods often neglect the microscopic sediment behavior, thus impeding a more profound understanding of the scour mechanism. In this research, the CFD-DEM method was applied to investigate the local scour around twin piles. And the coarse grain method and porous media model were incorporated to enhance simulation efficiency. The CFD-DEM model was constructed and validated using experimental data. Based on the validated model, a series of local scour simulations around twin piles were conducted under different arrangements. The time-development curves of scour depth were analyzed, and the variation of the equilibrium scour depth with pile spacing exhibits good agreement with the dates from previous studies. Based on the simulation results, the flow field, scour pit morphology, sediment flux, and microscopic behavior of sediment around twin piles were analyzed under different arrangements, and four key mechanisms of twin piles' local scour were observed and analyzed during this process.

Computational modeling and simulation for endovascular embolization of cerebral arteriovenous malformations with liquid embolic agents

Endovascular embolization of arteriovenous malformations (AVMs) in the brain usually requires injecting liquid embolic agents (LEAs) to reduce blood flow through the malformation. In clinical procedures, the feeding artery into which the LEAs are injected, and the amount of LEAs needs to be carefully planned preoperatively. Computational fluid dynamics can simulate the injecting process of LEAs in nidus and evaluate the therapeutic effects of different procedures preoperatively. Applying a porous media model avoided the difficulties of geometric modeling of AVMs, and the complex vascular network structure within the nidus was reproduced. The multi-phase flow was applied to simulate the interaction between LEAs and blood. The viscosity of LEAs is determined by the concentration of its solute ethylene-vinyl alcohol copolymer (EVOH). The diffusion process of the solvent dimethyl sulfoxide (DMSO) was calculated by solving the species transport equation. The coagulation of LEAs was simulated by constructing the relationship between the concentration of EVOH and viscosity. The numerical simulation method of LEAs for injection and coagulation was tested on two patient-specific AVMs. The calculations predicted the flow direction of the LEAs in the nidus. The morphology of the injected LEAs could be well visualized by 3D rendering. Quantitative analysis was conducted, including flow rate changes at the feeding arteries and draining veins. The embolization process of AVMs with LEAs can be simulated by computational fluid dynamics (CFD) methods to show the therapeutic effects of different embolization procedure planning, the optimal treatment plan can be determined.

Seepage Evolution Law and Deformation Failure Characteristics of Porous Media Based on Smoothed Particle Hydrodynamics

It is difficult to characterize the whole process of seepage movement by traditional numerical methods, which leads to the unclear mechanism of stress-strain change in soil. Therefore, the meshless method smooth particle hydrodynamics method is proposed to study the seepage law and deformation characteristics of soil. Combined with the strain softening model, a nonlinear cohesive force change model of porous media is proposed to accurately describe the evolution characteristics of soil seepage failure. Secondly, the damping coefficient and stress normalization method are used to better eliminate the defects of numerical stress noise generated by soil deformation after boundary interpolation, and improve the accuracy of SPH method in studying the internal stress of soil deformation. It is concluded that the influence of the viscosity coefficient on the unsteady head is significantly higher than that of the constant head. At the same time, the evolution characteristics of soil seepage instability under linear cohesion change are compared and analyzed. It can be seen that the proposed nonlinear cohesion change model is more in line with the actual failure mode of soil seepage. The research results can provide a new method and idea for the study of slope seepage motion range and stability analysis.

Pore-scale numerical simulation of supercritical CO2-brine two-phase flow based on VOF method

CO2 capture and storage technology is favorable for the reduction of CO2 emissions. In recent years, a great number of research achievements have been obtained on CO2 geological storage from nano scale to oil/gas reservoir scale, but most studies only focus on the flow behaviors in single-dimension porous media. Besides, the physical experiment method is influenced by many uncertain factors and consumes a lot of time and cost. In order to deeply understand the flow behaviors in the process of CO2 geological storage in microscopic view and increase the volume of CO2 geological storage, this paper established 2D and 3D models by using VOF (Volume of Fluid) method which can track the dynamic change of two-phase interface, to numerically simulate supercritical CO2-brine two-phase flow. Then, the distribution characteristics of CO2 clusters and the variation laws of CO2 saturation under different wettability, capillary number and viscosity ratio conditions were compared, and the intrinsic mechanisms of CO2 storage at pore scale were revealed. And the following research results were obtained. First, with the increase of rock wettability to CO2, the sweep range of CO2 enlarged, and the disconnection frequency of CO2 clusters deceased, and thus the volume of CO2 storage increased. Second, with the increase of capillary number, the displacement mode transformed from capillary fingering to stable displacement, and thus the volume of CO2 storage increased. Third, as the viscosity of injected supercritical CO2 gradually approached that of brine, the flow resistance between two-phase fluids decreased, promoting the "lubricating effect". As a result, the flow capacity of CO2 phase was improved, and thus the volume of CO2 storage was increased. Fourth, the influence degrees of wettability, capillary number and viscosity ratio on CO2 saturation were different in multi-dimensional porous media models. In conclusion, the CO2-brine two-phase flow simulation based on VOF method revealed the flow mechanisms in the process of CO2 geological storage at pore scale, which is of guiding significance to the development of CCUS technology and provides theoretical guidance and technical support for the study of CO2 geological storage in a larger scale.

A porous-media model for reactive fluid–rock interaction in a dehydrating rock

We study the General Equations of Non-Equilibrium Reversible–Irreversible Coupling (GENERIC) structure of models for reactive two-phase flows and their connection to a porous-media model for a reactive fluid–rock interaction used in geosciences. For this, we discuss the equilibration of fast dissipative processes in the GENERIC framework. Mathematical properties of the porous-media model and first results on its mathematical analysis are provided. The mathematical assumptions imposed for the analysis are critically validated with the thermodynamical rock datasets.

Stochastic dynamics construction of a three-dimensional microstructure of red clay

Constructing a microstructure of red clay is an important part of studying the physical and mechanical properties of red clay. The study of red clay microstructure is generalized. The red clay matrix and pores are regarded as two types of randomly moving particles, and the microstructure model of three-dimensional red clay random porous media is established from the Langevin equation of the phase separation process in stochastic dynamics, using the separation and aggregation of the two particles. The model controls the evolution process of the porous medium by artificially controlling the particle placement. Here, the trends of porosity, average pore length, Euler’s number, and the fractal dimension of the porous medium during the evolution process under different parameter conditions (smooth length Δ and rise and fall term η) are calculated, and a feasible method for surviving the microstructure of red clay is summarized. Due to the consideration of the interaction forces between the solid- and void-phase particles, the porous media generated by this model are more similar to the real porous media in nature, with connected and unconnected pore structures and tortuous pore channels. Finally, the red clay of Bijie, Guizhou, is modeled as an example to prove the feasibility of the method.

Studying the Transmission Behavior of Two-Phase Flow Oil-Water Mixing Using a Laboratory Multiple Porous Media Model

The formation and flow of emulsions in porous media are common to all technologies used to extract or process oil. In most cases, oil and water emulsions are formed in a porous medium due to oil spills near watery areas or soils containing water. This emulsion leads to a reaction between oil and water, which finds its way to the porous medium. The detailed flow mechanisms of emulsions through porous media are not well understood. In this study, the soil’s oil percentage variation was studied when sand was introduced or an emulsion, i.e., mixed with water, and two porous media, different in composition and physical properties, were used to find out their effect on oil transmission. The percentage of oil and water was calculated at several points at different distances and times. It was noted that the results differed in both mediums due to the difference in permeability, porosity, arrangement of soil particles, compaction process, and other physical properties. Liquids’ viscosity, density, and chemical composition clearly and significantly affect the results. The time for the oil to reach the last point in the pipe differed for both soils. If the time period in the experiment of pumping oil only in sandy soil took 6 hours and the washing process took 3 hours. In organic soil, the time period for the pollutant pumping stage took about 7 hours and the washing stage about 4 hours because the oil is transmitted in sandy soil is faster, but in the experiment of pumping oil and water together, the time period in the process of pumping oil and water together in sandy soil took 4 hours and the washing process 3 hours In organic soil, the period of pumping oil and water together took 5 hours, and the washing period 3 hours, because the percentage of oil was less than in the experiment of pumping oil only. In the soil, that is, if the period of pumping the pollutant increased for more than 15 hours, the oil may reach a distance of more than 4 meters in the soil. As for the washing process, when the oil is mixed with water, it gives better results when washing it than if the oil enters alone into the soil because the proportions of oil when it enters the soil together with the water, it is little and does not rise much, so it is easy to wash.

CFD simulation study of a biomass pellet-fuelled boiler

Biomass energy is an important substitute for fossil energy. Direct combustion of biomass in the boiler has the problem of high emissions. In this paper, a 0.1 t/h biomass atmospheric pressure hot water boiler is used for CFD modeling, aiming at providing a low-cost and high-efficiency way for the study of biomass boilers. In this paper, the biomass pyrolysis process is divided into three processes: water evaporation, devolatilization, and coke combustion. Using the UDF function combined with a porous medium model to write code, the gas-solid combustion process in the boiler is realized, and the grate is used to split the air. The calculation results show that the model can accurately restore the temperature, velocity, and composition changes in the boiler. Compared with the actual results, the temperature difference between the furnace and the outlet is less than 20 K, and the modeling has achieved good results.

Numerical study on dissolution characteristics of solid-phase oxygen control in liquid LBE systems based on the porous media model

For the proper design and operation of lead oxide mass exchanger (PbO MX), the dissolution kinetics of PbO in flowing liquid Lead-bismuth eutectic (LBE) needs to be determined. A computational fluid dynamics (CFD) method of the PbO MX was established. The solid-phase oxygen control part of the MX is PbO spheres packed bed, which is regarded as a porous media in the simulation calculation. The oxygen mass transfer process between PbO spheres packed bed and liquid LBE was simulated by a porous media model with the species transport model. The relationship between the dissolution rate of MX and the flow rate, temperature and initial oxygen concentration is predicted. The dissolution rate of PbO increases exponentially with increasing temperature of LBE. Besides, the dissolution rate of oxygen is positively correlated with the LBE flow velocity. In the specific oxygen concentration range, the MX inlet oxygen concentration has little effect on the dissolution rate. The smaller the porosity of PbO spheres packed bed has, the smaller the dissolution rate of oxygen is. Furthermore, when the porosity is constant, the dissolution rate of oxygen increases with the increase of the height of the packed bed of PbO spheres.

Mass transport in an excavation using a ternary Theory of Porous Media model

AbstractThis paper tries to simulate a hydraulic failure by using a ternary, saturated, quasi‐static Theory of Porous Media (TPM) model. The concept of a hydraulic failure is shortly explained and the derivation of the TPM setup including the limitation of the volume fractions is depicted. A schematic excavation with a sheet piling is used to test the created model and prove the general feasibility of the idea.

Degradation analysis of horizontal steam generator tube bundles through crack growth due to two-phase flow induced vibration

A correct understanding of vibration-based degradation is crucial from the standpoint of maintenance for Steam Generators (SG) as crucial mechanical equipment in nuclear power plants. This study has established a novel approach to developing a model for investigating tube bundle degradation according to crack growth caused by two-phase Flow-Induced Vibration (FIV). An important step in the approach is to calculate the two-phase flow field parameters between the SG tube bundles in various zones using the porous media model to determine the velocity and vapor volume fraction. Afterward, to determine the vibration properties of the tube bundles, the Fluid-Solid Interaction (FSI) analysis is performed in eighteen thermal-hydraulic zones. Tube bundle degradation based on crack growth using the sixteen most probable initial cracks and within each SG thermal-hydraulic zone is performed to calculate useful lifetime. Large Eddy Simulation (LES) model, Paris law, and Wiener process model are considered to model the turbulent crossflow around the tube bundles, simulation of elliptical crack growth due to the vibration characteristics, and estimation of SG tube bundles degradation, respectively. The analysis shows that the tube deforms most noticeably in the zone with the highest velocity. As a result, cracks propagate more quickly in the tube with a higher height. In all simulations based on different initial crack sizes, it was observed that zone 16 experiences the greatest deformation and, subsequently, the fastest degradation, with a velocity and vapor volume fraction of 0.5 m/s and 0.4, respectively.

Theoretical one-dimensional porous media model for microbial growth on pore plugging and permeability evolution and its verification

ABSTRACT The growth, reproduction, and metabolic activities of microorganisms can lead to blockages within porous media, a phenomenon commonly observed in landfill engineering. Termed as microbial plugging, this phenomenon is significantly influenced by the inherent permeability characteristics of the system. In this study, we propose a simulation model based on the Monod equation to elucidate the clogging process caused by microorganisms in one-dimensional pore channels. Our primary focus is on the application of this model in landfill bioreactor systems. We demonstrate that microbial clogging in these systems is predominantly affected by factors such as the maximum environmental carrying capacity and pore size. These factors are directly influenced by the presence of solid waste within the landfill. By offering a theoretical foundation for mitigating microbial clogging in pore channels of landfill bioreactor systems, this research has the potential to contribute to the development of more efficient and effective waste management practices.

Towards Prediction of Blood Flow in Coiled Aneurysms Before Treatment: A Porous Media Approach.

Modeling blood flow in aneurysms treated with coils could be used to understand the complete embolization of the aneurysm, through thrombus formation that fills the entire sac. Modeling of the endovascular coil mass as a porous medium is a technique that allows for study of aneurysm hemodynamics, efficiently for patient-specific treatment outcome predictions. Models in the literature use mean porosity of coils in the aneurysmal volume, proving inadequate for outcome prediction. However, models that consider heterogeneous porosity distribution have shown more accurate hemodynamics. We recently published the porous crown model, considering the heterogeneous coil mass distribution, validated on two patients. This study aims (i) to validate the porous crown model for a larger cohort (eight patients), and (ii) to propose a porous medium model translatable to clinical practice in treatment planning. We analyzed the porosity distribution of the endovascular coils deployed inside the cerebral aneurysm phantoms of eight patients using 3D x-ray synchrotron images. The permeability and inertial factor of the porous crown model are calculated using previously published methodology. We propose a new "bilinear" porous model, that uses the same hypothesis, but the permeability and inertial factor can be defined from just basic information available in the neuro-suite, i.e., the aneurysmal sac volume and the coil volume fraction targeted by the neurosurgeon. These two models are compared to the coil-resolved simulations, considered as the gold standard. The results show that both the porous crown model and the bilinear model produce similarly accurate hemodynamics in the aneurysm. The error in the standard (mean porosity) porous model is 66%, whereas the error of the bilinear model is 26%, compared to the coil-resolved. The bilinear model is promising as a means of treatment outcome prediction at time of intervention.

CFD-DEM study of impacts of the porous distributor medium on fluidization characteristics of a 2D-fluidized bed

Gas–solid bubbling fluidized beds are widely used in chemical, energy, construction, and other industrial fields due to their excellent fluidization performance. 2D-fluidized beds with planar rectangular columns of finite thickness are widely used in fluidized bed studies, and it is essential for understanding the hydrodynamics of gas-particle systems. Moreover, the distributor (porous medium) at the bottom of the bed has a crucial influence on the fluidization performance of the 2D-fluidized bed. In this work, the fluidization mechanism and gas–solid dynamic characteristics of the fluidized bed under three porous media are studied by CFD–DEM coupled with the porous medium model, and the accuracy of numerical simulation is verified by a high-speed photography experiment. Results show that with the increase in the flow resistance of the porous medium, the average and standard deviation of the bubble diameter decrease, and the time and position of particles in the dead zone to participate in the core–annular flow advance. At a low fluidization velocity, the dead zone in the bed can be considerably reduced by increasing the flow resistance.

The Effect of the Aeration Condition on the Liquid–Solid Material Mixing in a Stirred Tank with a Single-Layer Impeller

In order to increase industrial production quality and efficiency, it is essential to understand how the aeration and no-aeration condition affects liquid and solid material mixing in the stirred tank. Due to complicated shear flows, the related mass-transfer mechanism confronts numerous difficulties. This paper put forward an improved computational fluid dynamics and discrete element method (CFD–DEM) modeling approach to explore the effect mechanism of aeration conditions on liquid–solid material mixing. Firstly, a mass-transfer dynamic model is set up with a volume of fluid and piecewise linear interface construction (VOF–PLIC) coupling strategy to explore flow modes and vorticity evolution trends under aeration control. Then, a self-developed interphase coupling interface is utilized to modify the coupling force and porosity of the porous media model in the DEM module, and random dispersion properties of the particle phase under non-aeration and aeration are obtained. Results show that the aeration and flow-blocking components transform fluid tangential speeds into axial and radial speeds, which can improve the material mixing quality and efficiency. The mixed flow field can reach a greater turbulent process under the impeller rotation, making the particles have an intensive disorder and complex flow patterns. The enhanced motion efficiency of the vortex clusters encourages their nesting courses and improves cross-scale mixed transport. It can serve as some reference for the three-phase flow mixing mechanism, vorticity distribution law, and particle motion solution and has a general significance for battery homogeneous mixing, biopharmaceutical processes, and chemical process extraction.

Sheet and void porous media models for brain interstitial space.

The interstitial space (ISS) component of brain extracellular space resembles an unconsolidated porous medium. Previous analysis of the diffusion of small molecules in this domain shows that the typical porosity is 0.2 and typical tortuosity 1.6. An ensemble of cubic cells separated by uniform sheets of ISS cannot generate the measured tortuosity, even if some of the tortuosity value is attributed to interstitial viscosity, so more complex models are needed. Here two models are analysed: the corner cubic void (CCV) and the edge tunnel void (ETV). Both models incorporate dead spaces formed from local expansions of the ISS to increase geometrical tortuosity. Using Monte Carlo simulation of diffusion it is found that in the range of normal porosities, the square of the tortuosity is a linear function of the ratio of void to sheet volumes for the CCV model and this model can generate the experimentally observed tortuosities. For abnormally high porosities, however, the linear relation fails. The ETV model shows a quartic functional relation and can only generate the observed tortuosity if interstitial viscosity is present. The CCV model is used to analyse the recently described changes in porosity between asleep and awake brain states.

Multiscale Thermal-Hydraulic Analysis of the ATHENA Core Simulator

In the framework of the ALFRED research and development program, the ATHENA facility will be constructed for thermal-hydraulic analysis of full-scale ALFRED components and systems. The source system of the facility is the core simulator, which aims to be representative of an ALFRED average fuel assembly. Computational fluid dynamics (CFD) codes are gaining attention for the analysis of complex systems in pool-type reactors since they are able to reproduce three-dimensional phenomena. In this paper, a multiscale approach based on porous media is proposed to reduce the computational cost of the core simulator CFD model. The multiscale approach starts with the detailed simulation of the infinite lattice domain of the fuel assembly to characterize the porous media hydraulic behavior. Then the porous media are applied in the system model. Three different approaches are investigated: (1) adopting a single porous media for the entire fuel assembly, (2) representing the bundle with two porous domains, and (3) adopting the so-called hybrid medium. The results have been compared with the reference detailed CFD simulation for performance evaluation. The first step of the analysis is the application of the multiscale approach on the CIRCE fuel pin simulator to carry out a turbulence model validation against experimental data and a comparison of the three approaches with a proven CFD model. Then the approach is applied on the ATHENA core simulator exploiting the CIRCE results. The results obtained with the porous media models are compared with a detailed CFD simulation of the core simulator to evaluate the performance of the three approaches. Eventually, the best solution is applied on a model of the entire ATHENA core simulator integrated with the feeding region. The model is tested also in transient conditions. The numerical experiment demonstrates the effectiveness of the multiscale approach in reducing the computational cost while maintaining high accuracy in representing the quantities of interest.