Study on Non-Newtonian Behaviors of Lennard-Jones Fluids via Molecular Dynamics Simulations

Using nonequilibrium molecular dynamics simulations, we study the non-Newtonian rhe-ological behaviors of a monoatomic fluid governed by the Lennard-Jones potential. Both steady Couette and oscillatory shear flows are investigated. Shear thinning and normal stress effects are observed in the steady Couette flow simulations. The radial distribution function is calculated at different shear rates to exhibit the change of the microscopic struc- ture of molecules due to shear. We observe that for a larger shear rate the repulsion between molecules is more powerful while the attraction is weaker, and the above phenomena can also be confirmed by the analyses of the potential energy. By applying an oscillatory shear to the system, several findings are worth mentioning here： First, the phase difference between the shear stress and shear rate increases with the frequency. Second, the real part of complex viscosity first increases and then decreases while the imaginary part tends to increase mono- tonically, which results in the increase of the proportion of the imaginary part to the real part with the increasing frequency. Third, the ratio of the elastic modulus to the viscous modulus also increases with the frequency. These phenomena all indicate the appearance of viscoelasticity and the domination of elasticity over viscosity at high oscillation frequency for Lennard-Jones fluids.

Molecular dynamics simulation of the flow mechanism of shear-thinning fluids in a microchannel

Shear-thinning fluids have been widely used in microfluidic systems,but their internal flow mechanism is still unclear.Therefore,in this paper,molecular dynamics simulations are used to study the laminar flow of shear-thinning fluid in a microchannel.We validated the feasibility of our simulation method by evaluating the mean square displacement and Reynolds number of the solution layers.The results show that the change rule of the fluid system's velocity profile and interaction energy can reflect the shear-thinning characteristics of the fluids.The velocity profile resembles a top-hat shape,intensifying as the fluid's power law index decreases.The interaction energy between the wall and the fluid decreases gradually with increasing velocity,and a high concentration of non-Newtonian fluid reaches a plateau sooner.Moreover,the velocity profile of the fluid is related to the molecule number density distribution and their values are inversely proportional.By analyzing the radial distribution function,we found that the hydrogen bonds between solute and water molecules weaken with the increase in velocity.This observation offers an explanation for the shear-thinning phenomenon of the non-Newtonian flow from a micro perspective.

Desulfurization characteristics of slaked lime and regulation optimization of circulating fluidized bed flue gas desulfurization process--A combined experimental and numerical simulation study

Circulating fluidized bed flue gas desulfurization(CFB-FGD) process has been widely applied in recent years. However, high cost caused by the use of high-quality slaked lime and difficult operation due to the complex flow field are two issues which have received great attention. Accordingly, a laboratory-scale fluidized bed reactor was constructed to investigate the effects of physical properties and external conditions on desulfurization performance of slaked lime, and the conclusions were tried out in an industrial-scale CFB-FGD tower. After that, a numerical model of the tower was established based on computational particle fluid dynamics(CPFD) and two-film theory. After comparison and validation with actual operation data, the effects of operating parameters on gas-solid distribution and desulfurization characteristics were investigated. The results of experiments and industrial trials showed that the use of slaked lime with a calcium hydroxide content of approximately 80% and particle size greater than 40 μm could significantly reduce the cost of desulfurizer. Simulation results showed that the flow field in the desulfurization tower was skewed under the influence of circulating ash. We obtained optimal operating conditions of 7.5 kg·s^(-1)for the atomized water flow, 70 kg·s^(-1)for circulating ash flow, and 0.56 kg·s^(-1)for slaked lime flow, with desulfurization efficiency reaching 98.19% and the exit flue gas meeting the ultraclean emission and safety requirements. All parameters selected in the simulation were based on engineering examples and had certain application reference significance.

Wavefield simulation in porous media saturated with two immiscible fluids

Wavefields in porous media saturated by two immiscible fluids are simulated in this paper.Based on the sealed system theory,the medium model considers both the relative motion between the fluids and the solid skeleton and the relaxation mechanisms of porosity and saturation（capillary pressure）.So it accurately simulates the numerical attenuation property of the wavefields and is much closer to actual earth media in exploration than the equivalent liquid model and the unsaturated porous medium model on the basis of open system theory.The velocity and attenuation for different wave modes in this medium have been discussed in previous literature but studies of the complete wave-field have not been reported.In our work,wave equations with the relaxation mechanisms of capillary pressure and the porosity are derived.Furthermore,the wavefield and its characteristics are studied using the numerical finite element method.The results show that the slow P3-wave in the non-wetting phase can be observed clearly in the seismic band.The relaxation of capillary pressure and the porosity greatly affect the displacement of the non-wetting phase.More specifically,the displacement decreases with increasing relaxation coefficient.

Dynamic simulation of differential accumulation history of deep marine oil and gas in superimposed basin:A case study of Lower Paleozoic petroleum system of Tahe Oilfield,Tarim Basin,NW China

According to the complex differential accumulation history of deep marine oil and gas in superimposed basins,the Lower Paleozoic petroleum system in Tahe Oilfield of Tarim Basin is selected as a typical case,and the process of hydrocarbon generation and expulsion,migration and accumulation,adjustment and transformation of deep oil and gas is restored by means of reservoine-forming dynamics simulation.The thermal evolution history of the Lower Cambrian source rocks in Tahe Oilfield reflects the obvious differences in hydrocarbon generation and expulsion process and intensity in different tectonic zones,which is the main reason controlling the differences in deep oil and gas phases.The complex transport system composed of strike-slip fault and unconformity,etc.controlled early migration and accumulation and late adjustment of deep oil and gas,while the Middle Cambrian gypsum-salt rock in inner carbonate platform prevented vertical migration and accumulation of deep oil and gas,resulting in an obvious"fault-controlled"feature of deep oil and gas,in which the low potential area superimposed by the NE-strike-slip fault zone and deep oil and gas migration was conducive to accumulation,and it is mainly beaded along the strike-slip fault zone in the northeast direction.The dynamic simulation of reservoir formation reveals that the spatio-temporal configuration of"source-fault-fracture-gypsum-preservation"controls the differential accumulation of deep oil and gas in Tahe Oilfield.The Ordovician has experienced the accumulation history of multiple periods of charging,vertical migration and accumulation,and lateral adjustment and transformation,and deep oil and gas have always been in the dynamic equilibrium of migration,accumulation and escape.The statistics of residual oil and gas show that the deep stratum of Tahe Oilfield still has exploration and development potential in the Ordovician Yingshan Formation and Penglaiba Formation,and the Middle and Upper Cambrian ultra-deep stratum has a certain oil and gas resource prospect.This study provides a reference for the dynamic quantitative evaluation of deep oil and gas in the Tarim Basin,and also provides a reference for the study of reservoir formation and evolution in carbonate reservoir of paleo-craton basin.

Inhomogeneous 2D Lennard-Jones Fluid: Theory and Computer Simulation

In this work, thermodynamical properties of a two-dimensional （21）） Lennard-Jones （L J） fluid are studied. Here, to increase the accuracy of our theoretical calculations, the correlation functions in three-particle level （triplet） are applied. To obtain the triplet correlation functions, the Attard＇s source particle method is extended to 21） systems. In the Attard＇s procedure, the inhomogeneous Ornstein Zernike （OZ） equation is solved using the Treizenberg Zwanzwig （TZ） expression and a closure relation like the hy2ernetted-chain （HNC） approximation. In the present work, we also have performed the Monte Carlo （MC） simulation. The theoretical results are in fairly agreement with the MC simulation. Also, our results show that the approach proposed here is suitable to study the 2D LJ fluid.

A laboratory acoustic emission experiment and numerical simulation of rock fracture driven by a high-pressure fluid source

In order to improve our understanding of rock fracture and fault instability driven by high-pressure fluid sources, the authors carried out rock fracture tests using granite under a confining pressure of 80 MPa with fluid injection in the laboratory. Furthermore, we tested a number of numerical models using the FLAC;modeling software to find the best model to represent the experimental results. The high-speed multichannel acoustic emission（AE） waveform recording system used in this study made it possible to examine the total fracture process through detailed monitoring of AE hypocenters and seismic velocity.The experimental results show that injecting high-pressure oil into the rock sample can induce AE activity at very low stress levels and can dramatically reduce the strength of the rock. The results of the numerical simulations show that major experimental results, including the strength, the temporal and spatial patterns of the AE events, and the role of the fluid can be represented fairly well by a model involving（1） randomly distributed defect elements to model pre-existing cracks,（2） random modification of rock properties to represent inhomogeneity introduced by different mineral grains, and（3）macroscopic inhomogeneity. Our study, which incorporates laboratory experiments and numerical simulations, indicates that such an approach is helpful in finding a better model not only for simulating experimental results but also for upscaling purposes.

Numerical Simulation of Fluid Migration during Ore Formation of Carboniferous Exhalation-Sedimentary Massive Sulfide Deposits in the Tongling District, Anhui Province

Numerical simulation of fluid migration during the ore-forming process of the Carboniferous exhalation- sedimentary (Sedex) massive sulfide deposits in the Tongling district shows that fluid and thermal activities in lying-wall rocks were limited to a small area around the main draining passage, which led to weak mineralization and alteration in the lying-wall rock. Temperature and fluid fields indicate that mineralization and alteration in the lying-wall rock of the Sedex-type deposits are usually weaker than those of volcano-hosted massive sulfide deposits. Fluid migration involves the following processes: seawater penetrating and leaching the lying-wall rocks, then mixing with ascending hydrothermal fluids in the main draining passage, and finally jetting into seafloor. Although fluid activity-influenced area is rather small, the content of metals leached out from the lying-wall rocks is high enough to form large-scale ore deposits. Tensional contemporaneous faults accompanied with strong heat flows controlled the formation and distribution of Sedex deposits. The tensional tectonic regime on the northern margin of the Yangtze block during the Hercynian provided Sedex deposits with a prerequisite geodynamic condition.

Progress in molecular-simulation-based research on the effects of interface-induced fluid microstructures on flow resistance

In modern chemical engineering processes, solid interface involvement is the most important component of process intensification techniques, such as nanoporous membrane separation and heterogeneous catalysis. The fundamental mechanism underlying interfacial transport remains incompletely understood given the complexity of heterogeneous interfacial molecular interactions and the high nonideality of the fluid involved. Thus, understanding the effects of interface-induced fluid microstructures on flow resistance is the first step in further understanding interfacial transport. Molecular simulation has become an indispensable method for the investigation of fluid microstructure and flow resistance. Here, we reviewed the recent research progress of our group and the latest relevant works to elucidate the contribution of interface-induced fluid microstructures to flow resistance.We specifically focused on water, ionic aqueous solutions, and alcohol–water mixtures given the ubiquity of these fluid systems in modern chemical engineering processes. We discussed the effects of the interfaceinduced hydrogen bond networks of water molecules, the ionic hydration of ionic aqueous solutions, and the spatial distributions of alcohol and alcohol–water mixtures on flow resistance on the basis of the distinctive characteristics of different fluid systems.

COMPUTATIONAL FLUID DYNAMICS(CFD) SIMULATIONS OF DRAG REDUCTION WITH PERIODIC MICRO-STRUCTURED WALL

Computational fluid dynamics（CFD） simulations are adopted to investigate rectangular microchannel flows with various periodic micro-structured wall by introducing velocity slip boundary condition at low Reynolds number. The purpose of the current study is to numerically find out the effects of periodic micro-structured wall on the flow resistance in rectangular microchannel with the different spacings between microridges ranging from 15 to 60 pm. The simulative results indicate that pressure drop with different spacing between microridges increases linearly with flow velocity and decreases monotonically with slip velocity; Pressure drop reduction also increases with the spacing between microridges at the same condition of slip velocity and flow velocity. The results of numerical simulation are compared with theoretical predictions and experimental results in the literatures. It is found that there is qualitative agreement between them.

Simulation and Analysis on the Two-Phase Flow Fields in a Rotating-Stream-Tray Absorber by Using Computational Fluid Dynamics

The flow field of gas and liquid in a φ150mm rotating-stream-tray (RST) scrubber is simulated by using computational fluid dynamic (CFD) method. The sismulation is based on the two-equation RNG κ-ε turbulence model, Eulerian multiphase model, and a real-shape 3D model with a huge number of meshes. The simulation results include detailed information about velocity, pressure, volume fraction and so on. Some features of the flow field are obtained: liquid is atomized in a thin annular zone; a high velocity air zone prevents water drops at the bottom from flying towards the wall; the pressure varies sharply at the end of blades and so on. The results will be helpful for structure optimization and engineering design.

Fast and Stable Surface Feature Simulation for Particle-Based Fluids

In order to efficiently and realistically capture microscopic features of fluid surface,a fast and stable surface feature simulation approach for particle-based fluids is presented in this paper.This method employs a steady tension and adhesion model to construct surface features with the consideration of the adsorption effect of fluid to solid.Molecular cohesion and surface area minimization are appended for surface tension,and adhesion is added to better show the microscopic characteristics of fluid surface.Besides,the model is integrated to an implicit incompressible smoothed particle hydrodynamics(SPH)method to improve the efficiency and stability of simulation.The experimental results demonstrate that the method can better simulates surface features in a variety of scenarios stably and efficiently.

Computational fluid dynamics simulations of respiratory airflow in human nasal cavity and its characteristic dimension study

To study the airflow distribution in human nasal cavity during respiration and the characteristic parameters of nasal structure, three-dimensional, anatomically accurate representations of 30 adult nasal cavity models were recons- tructed based on processed tomography images collected from normal people. The airflow fields in nasal cavities were simulated by fluid dynamics with finite element software ANSYS. The results showed that the difference of human nasal cavity structure led to different airflow distribution in the nasal cavities and variation of the main airstream passing through the common nasal meatus. The nasal resistance in the regions of nasal valve and nasal vestibule accounted for more than half of the overall resistance. The characteristic model of nasal cavity was extracted on the basis of characteristic points and dimensions deduced from the original models. It showed that either the geometric structure or the airflow field of the two kinds of models was similar. The characteristic dimensions were the characteristic parameters of nasal cavity that could properly represent the original model in model studies on nasal cavity.

Physical simulation of fluid flow and production performance in extra-low permeability porous media

For extra-low permeability reservoirs, with a permeability of about 0.3×10?3 μm2, fluid flow and production performance in cores were studied. A long core holder with a multi-location piezometric measurement was specially designed. An artificial long core, about 700 mm long and with a cross section of 45mm×45mm, was used. In the experiment, pressure distribution along the core can be measured in real time. Single phase flow in the core was investigated. Different modes of production in long cores were also simulated including natural depletion, water flooding, and advanced water flooding. Through physical simulation, flow parameters were collected and production characteristics in extra-low permeability cores were studied. From experimental results, it can be seen that fluid flow in extra-low permeability cores is different from that in high permeability cores. Transmission of pressure in extra-low permeability cores is very slow, and it needs a long time for the pressure to become stable. The distribution curve of pressure along the core is nonlinear and the production rate in extra-low permeability reservoirs decreases sharply. The development effects of different production modes in extra-low permeability cores were compared with one another. Among the production modes, advanced water flooding has much potential for effective development of extra-low permeability reservoirs. Natural depletion and conventional water flooding can also be used in early production periods. In addition, the countermeasures and some ideas especially for the potential development of extra-low permeability reservoirs are suggested.

Validation of the numerical simulation of fluid flow in the continuous casting tundish

The 3-dimensional fluid flow in a water model of the continuous casting tundish is simulated with the k-ε two-equation turbulence model. The methods to decide the size of wall-adjacent grids and the effects of residuals and mesh size on the simulation accuracy are discussed. The current investigation concludes that the following condition should be satisfied to get accurate enough simulation results: (1) If the dimension of the domain is in the order of cubic meters, the average size of cells in the mesh system should be at least smaller than 30 mm; (2) The normalized non-scaled residual should be reached at least smaller than 10-4.

Numerical Simulations of Structural Deformation and Fluid Flow in Xiangshan Deposit

The Xiangshan deposit in Jiangxi province is one of the most important uranium deposits in China. The aim of this study is to achieve a better understanding of mineralization in the Xiangshan deposit through numerical simulation. In order to find the most favorable locations of mineralization and to help further mineral exploration, a coupling deforma- tion and fluid flow model has been established to describe the mineralization process. In this model, the simulation re- constructs the strata deformations under fields of compressive stress and thrust structure on the hanging wall of the Zou-Shi fault. Compared with practical information, the simulation results are consistent with the No. 51 exploration section of the western Xiangshan. In addition, on the basis of geological information provided by previous investigators, the model simulates the flow process of fluids under compressive stress fields. The result suggests that many tensional areas are formed, which can help the fluid flowing upward from deeper parts. The fluid is easy to concentrate on the breccia fractured zone between two volcanic layers, especially on the intersection parts with faults, resulting in the for- mation of favourable locations of mineralization. In addition, the model is significant in guiding the exploration of ura- nium deposits in the western Xiangshan and provides clues for further exploration of deposits.

Numerical Simulation of Fluid Flow and Heat Transfer in Funnel Shaped Mold of Thin Slab Continuous Caster

Thin slab casting is used widely in the world. The control of molten steel flow and solidification in the mold is difficult due to the high casting speed and complicated configuration of the mold. Numerical simulation was carried out to study the fluid flow and heat transfer in the funnel shaped mold. The influence of nozzle design, casting speed and nozzle submersion depth on the flow and temperature fields in the mold was investigated, and guidance for selecting configurations of submerged nozzle was obtained.

Numerical simulation of single bubble rising in shear-thinning fluids by level set method

The behavior of single bubble rising in quiescent shear-thinning tlmds was lnvestlgateO numerically by level set metnoa. number of bubbles in a large range of Reynolds number and Eotvos number were investigated including spherical, oblate and spherical. The bubble shape and drag coefficient were compared with experimental results. It is observed that the simulated results show good conformity to experimental results over a wide range of Reynolds number. In addition, the detailed flow field based on the reference coordinate system moving with the bubble is obtained, and the relationship among flow field, bubble shape and velocity is discussed.

Simulation of pore scale fluid flow of granular ore media in heap leaching based on realistic model

Two-dimensional images of the granular ore media with different grain sizes were obtained from the X-ray computed tomography.Combined with the digital image processing and finite element techniques,the original grayscale images were transformed into the finite element models directly.By using these models,the simulations of pore scale fluid flow among particles were conducted with the COMSOL Multiphysics,and the distribution characteristics of fluid flow velocity and pressure were analyzed.The simulation results show that there exist obvious preferential flow and leaching blind zone in each granular medium.The flow velocity at pore throat is larger than that of pore body and the largest velocity reaches 0.22 m/s.The velocity decreases gradually from the center of pore throat and body to the surface of particles.The flow paths of granular media with larger grain size distribute equally,while the fluid flow velocities in most of areas of granular media with smaller grain size are lower,and some of them approach to zero,so the permeability is very low.There exist some pore clusters with different pressures,which is the basic reason for the uneven flow velocity distribution.

Computational fluid dynamic simulation of gas-liquid flow in rotating packed bed:A review

The rotating packed bed(RPB)has been widely used in gas-liquid flow systems as a process intensification device,exhibiting excellent mass transfer enhancement characteristics.However,the complex internal structure and the high-speed rotation of the rotor in RPB bring significant challenges to study the intensification mechanism by experiment methods.In the past two decades,Computational fluid dynamics(CFD)has been gradually applied to simulate the hydrodynamics and mass transfer characteristics in RPB and instruct the reactor design.This article covers the development of the CFD simulation of gasliquid flow in RPB.Firstly,the improvement of the simulation method in the aspect of mathematical models,geometric models,and solving methods is introduced.Secondly,new progress of CFD simulation about hydrodynamic and mass transfer characteristics in RPB is reviewed,including pressure drop,velocity distribution,flow pattern,and concentration distribution,etc.Some new phenomena such as the end effect area with the maximum turbulent have been revealed by this works.In addition,the exploration of developing new reactor structures by CFD simulation is introduced and it is proved that such new structures are competitive to different applications.The defects of current research and future development directions are also discussed at last.