Quantitative measurements of injections into porous media with contrast based MRI

Porous flow occurs in a wide range of materials and applies to many commercially relevant applications such as oil recovery, chemical reactors and contaminant transport in soils. Typically, breakthrough and pressure curves of column floods are used in the laboratory characterization of these materials. These characterization methods lack the detail to easily and unambiguously resolve flow mechanisms with similar effects at the core scale that can dominate at the aquifer or oil field scale, as well as the effects of geometry that control the flow at interfaces as in a perforated well or the inlet of an improperly designed column. Non-invasive imaging techniques such as MRI have been shown to provide a far more detailed characterization of the properties of the solid matrix and flow, but usually focus on the intrinsic flow properties of porous media or matching a numerical model to a complex flow system. We show that these MRI techniques, utilizing paramagnetic tagging in combination with a carefully controlled and ideal flow system, can quantitatively characterize the effects of geometry and intrinsic flow properties for a point injection into a core. The use of a carefully controlled and 'idealized' system is essential to be able to isolate and match predicted effects from geometry and extract subtle flow processes omitted in the model that would be hidden in a more heterogeneous system. This approach provides not only a tool to understand the behavior of intentional boundary effects, but also one to diagnose the unintentional ones that often degrade the data from routine column flood measurements.     

对流梯形微通道内气液两相流动的数值研究

采用VOF方法,对梯形微通道内不可压缩气液两相流动进行了数值模拟研究,详细分析了气泡形成过程,以及当量直径、截面形状、液体表面张力和粘度等对气泡液柱形成过程和长度的影响,拟合出微通道气泡液柱长度计算公式。结果表明:气泡液柱的长度受表观气速和表观液速的影响较大;表面张力对气泡尺寸的影响较小,当液体粘度增加为水粘度的10倍时,形成的气泡形状不规则。增大表面张力,形成气泡的时间增加;增大粘度,形成气泡的时间减小。     

固体表面熔池内Marangoni流动产生的小扰动分析

本文应用小扰动法研究了熔化液池内流体先稳形成Marangoni流动的条件，同时对初始熔池的特性作了分析．理论结果表明，蒋池内流动产生主要由表面张力变化引起，自然对流影响甚小．分析还导出了流体关键的临界厚度，并与效植模拟结论定性地相一致．     

A composite grid solver for conjugate heat transfer in fluid–structure systems

We describe a numerical method for modeling temperature-dependent fluid flow coupled to heat transfer in solids. This approach to conjugate heat transfer can be used to compute transient and steady state solutions to a wide range of fluid–solid systems in complex two- and three-dimensional geometry. Fluids are modeled with the temperature-dependent incompressible Navier–Stokes equations using the Boussinesq approximation. Solids with heat transfer are modeled with the heat equation. Appropriate interface equations are applied to couple the solutions across different domains. The computational region is divided into a number of sub-domains corresponding to fluid domains and solid domains. There may be multiple fluid domains and multiple solid domains. Each fluid or solid sub-domain is discretized with an overlapping grid. The entire region is associated with a composite grid which is the union of the overlapping grids for the sub-domains. A different physics solver (fluid solver or solid solver) is associated with each sub-domain. A higher-level multi-domain solver manages the entire solution process.     

Pores resolving simulation of Darcy flows

A theoretical formulation and corresponding numerical solutions are presented for microscopic fluid flows in porous media with the domain sufficiently large to reproduce integral Darcy scale effects. Pore space geometry and topology influence flow through media, but the difficulty of observing the configurations of real pore spaces limits understanding of their effects. A rigorous direct numerical simulation (DNS) of percolating flows is a formidable task due to intricacies of internal boundaries of the pore space. Representing the grain size distribution by means of repelling body forces in the equations governing fluid motion greatly simplifies computational efforts. An accurate representation of pore-scale geometry requires that within the solid the repelling forces attenuate flow to stagnation in a short time compared to the characteristic time scale of the pore-scale flow. In the computational model this is achieved by adopting an implicit immersed-boundary method with the attenuation time scale smaller than the time step of an explicit fluid model. A series of numerical simulations of the flow through randomly generated media of different porosities show that computational experiments can be equivalent to physical experiments with the added advantage of nearly complete observability. Besides obtaining macroscopic measures of permeability and tortuosity, numerical experiments can shed light on the effect of the pore space structure on bulk properties of Darcy scale flows.     

Experimental Research on Wet Steam Flow With Shock Wave

The article presents research on a wet steam transonic flow in a Laval half-nozzle with a shock wave. The motivation for this research was to investigate the shock wave/liquid phase interaction in the transonic wet steam flow. This phenomenon is responsible for the lack of good correspondence between experimental data and computational fluid dynamics results. For the tests, the geometry of the half-arc nozzle was used. The shock wave formation at the divergent section of the nozzle is caused by too high back-pressure. The observed instabilities in the flow are mainly initiated by the shock wave/liquid film (boundary layer) interaction. The numerical calculations were compared with experimental results with respect to the static pressure distribution along the nozzle.     

On calculating the oscillations of a viscous liquid layer over a solid spherical core in terms of the boundary layer theory

The theory of a boundary layer near the periodically oscillating free surface of a spherical viscous liquid layer over a solid core (bottom) is modified. Two boundary layers are considered to adequately describe a liquid viscous flow in the system: one at the free surface of the liquid and the other at the solid bottom. The thicknesses of the boundary layers are estimated, which provide any given discrepancy between an exact solution to the model problem and a solution obtained in the small viscosity approximation. Taking into account the boundary layer near the solid bottom is shown to be significant only for lower oscillation modes. For higher modes, the flow near the core can be considered potential. In the case of lower modes and shallow liquid, the surface and bottom boundary layers overlap and an eddy flow occupies the entire volume of the liquid.     

The role of the “Casimir force analogue” at the microscopic processes of crystallization and melting

Melting (crystallization), a phase transition from a crystalline solid to a liquid state, is a common phenomenon in nature. We suggest a new factor, “the Casimir force analogue”, to describe mechanisms of melting and crystallization. The Casimir force analogue is a force occurring between the surfaces of solid and liquid phases of metals caused by different energy density of phonons of these phases. It explains abrupt changes in geometry and thermodynamic parameters at a melting point. “The Casimir force analogue” helps to estimate latent melting heat and to gain an insight into a solid–liquid transition problem.     

Modeling of Ice-Water Phase Change in Horizontal Annulus Using Modified Enthalpy Method

Phase change in ice-water systems in the geometry of horizontal cylindrical annulus with constant inner wall temperature and adiabatic outer wall is modeled with an enthalpy-based mixture model. Solidification and melting phenomena under different temperature conditions are analyzed through a sequence of numerical calculations. In the case of freezing of water, the importance of convection and conduction as well as the influence of cold pipe temperature on time for the complete solidification is examined. As for the case of melting of ice, the influence of the inner pipe wall temperature on the shape of the ice-water interface, the flow and temperature fields in the liquid, the heat transfer coefficients and the rate of melting are analyzed. The results of numerical calculations point to good qualitative agreement with the available experimental and other numerical results.     

Acoustic resonance and atomization for gas-liquid systems in microreactors

It is shown that a liquid slug in gas–liquid segmented flow in microchannels can act as an acoustic resonator to disperse large amounts of small liquid droplets, commonly referred to as atomization, into the gas phase. We investigate the principles of acoustic resonance within a liquid slug through experimental analysis and numerical simulation. A mechanism of atomization in the confined channels and a hypothesis based on high-speed image analysis that links acoustic resonance within a liquid slug with the observed atomization is proposed. The observed phenomenon provides a novel source of confined micro sprays and could be an avenue, amongst others, to overcome mass transfer limitations for gas–liquid processes in flow.     

Droplet model of an atomic cluster at a solid surface

A method for calculating the characteristics of the stability, energy, and geometry of an atomic cluster at a solid surface is proposed, which is based on a droplet model that takes into account the cluster-solid interaction. As an example, the interaction of a neutral argon cluster with a (001) surface of graphite is considered. The results of calculations performed within the framework of the droplet model show good agreement with the results of numerical simulation based on a dynamic search for the most stable isomers in the course of cluster growth. It is shown that the droplet model can be used for simple evaluation of the geometry, stability, and energy characteristics of clusters at solid surfaces.     

润湿性及磁场对液态金属自由表面膜流流动状态的影响

通过数值模拟及实验研究了润湿性及磁场对液态金属膜流流动状态的影响。首先,通过数值模拟研究了润湿性对膜流流动状态的影响。结果表明,当润湿性不好时,液态金属膜流容易发展为溪状流而不能完全覆盖底壁,入口膜厚较薄时更易发展为溪状流;在入口膜厚及其它情况相同时,密度越小越易发展为溪状流。其次,研究了磁场对膜流流动状态的影响。结果表明,槽道与流体润湿性不好时,有磁场情况下液态金属膜流覆盖底壁的区域较无磁场时增加,强磁场对膜流的湍流有抑制作用。最后,液态金属膜流实验结果表明,润湿性不好时,镓铟锡合金膜流容易收缩发展为溪状流,这与数值模拟的结果是一致的。上述研究结果对磁约束聚变堆液态第一壁的设计具有指导意义。     

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通过数值模拟及实验研究了润湿性及磁场对液态金属膜流流动状态的影响.首先,通过数值模拟研究了润湿性对膜流流动状态的影响.结果表明,当润湿性不好时,液态金属膜流容易发展为溪状流而不能完全覆盖底壁,入口膜厚较薄时更易发展为溪状流;在入口膜厚及其它情况相同时,密度越小越易发展为溪状流.其次,研究了磁场对膜流流动状态的影响.结果表明,槽道与流体润湿性不好时,有磁场情况下液态金属膜流覆盖底壁的区域较无磁场时增加,强磁场对膜流的湍流有抑制作用.最后,液态金属膜流实验结果表明,润湿性不好时,镓铟锡合金膜流容易收缩发展为溪状流,这与数值模拟的结果是一致的.上述研究结果对磁约束聚变堆液态第一壁的设计具有指导意义.     

Two-step melting of the vortex solid in layered superconductors with random columnar pins

We consider the melting of the vortex solid in highly anisotropic layered superconductors with a small concentration of random columnar pinning centers. Using large-scale numerical minimization of a free-energy functional, we find that melting of the low-temperature, nearly crystalline vortex solid (Bragg glass) into a vortex liquid occurs in two steps as the temperature increases: the Bragg glass and liquid phases are separated by an intermediate Bose glass phase. A suitably defined local melting temperature exhibits spatial variation similar to that observed in experiments.     

二维弹塑性磁流体力学数值模拟

为了研究物质弹塑性对磁驱动实验运动过程、不稳定性发展等的影响,在MDSC2程序的基础上,增加了弹塑性模块,研制了包括弹塑性的磁流体力学程序,并进行了弹塑性项影响的数值模拟和分析。数值模拟表明：没有初始扰动时,弹塑性项几乎不影响套筒内外半径的运动轨迹;有初始扰动时,弹塑性项对磁驱动固体套筒的Rayleigh-Tayor不稳定性有明显的抑制作用。     

Numerical simulation of non-viscous liquid pinch-off using a coupled level set-boundary integral method

Simulations of the pinch-off of an inviscid fluid column are carried out based upon a potential flow model with capillary forces. The interface location and the time evolution of the free surface boundary condition are both approximated by means of level set techniques on a fixed domain. The interface velocity is obtained via a Galerkin boundary integral solution of the 3D axisymmetric Laplace equation. A short-time analytical solution of the Raleigh–Taylor instability in a liquid column is available, and this result is compared with our numerical experiments to validate the algorithm. The method is capable of handling pinch-off and after pinch-off events, and simulations showing the time evolution of the fluid tube are presented.     

Contaminant Flow and Transport Simulation in Cracked Porous Media Using Locally Conservative Schemes

The purpose of this paper is to analyze some features of contaminant flow passing through cracked porous medium, such as the influence of fracture network on the advection and diffusion of contaminant species, the impact of adsorption on the overall transport of contaminant wastes. In order to precisely describe the whole process, we firstly build the mathematical model to simulate this problem numerically. Taking into consideration of the characteristics of contaminant flow, we employ two partial differential equations to formulate the whole problem. One is flow equation; the other is reactive transport equation. The first equation is used to describe the total flow of contaminant wastes, which is based on Darcy law. The second one will characterize the adsorption, diffusion and convection behavior of contaminant species, which describes most features of contaminant flow we are interested in. After the construction of numerical model, we apply locally conservative and compatible algorithms to solve this mathematical model. Specifically, we apply Mixed Finite Element (MFE) method to the flow equation and Discontinuous Galerkin (DG) method for the transport equation. MFE has a good convergence rate and numerical accuracy for Darcy velocity. DG is more flexible and can be used to deal with irregular meshes, as well as little numerical diffusion. With these two numerical means, we investigate the sensitivity analysis of different features of contaminant flow in our model, such as diffusion, permeability and fracture density. In particular, we study $K_d$ values which represent the distribution of contaminant wastes between the solid and liquid phases. We also make comparisons of two different schemes and discuss the advantages of both methods.     

3D Analysis of Crystal/Melt Interface Shape and Marangoni Flow Instability in Solidifying Liquid Bridges

Solidification of gallium (Pr=0.02) in liquid bridges in zero-gravity conditions is investigated by numerical solutions of the three-dimensional and time-dependent flow-field equations. A single region (continuum) formulation based on the enthalpy method is adopted to model the phase-change problem. This paper analyzes the influence of the azimuthally asymmetric and steady first bifurcation of the Marangoni flow on the shape of the solid/melt interface during the crystal growth process. The numerical results show that this interface is distorted in the azimuthal direction. The distortion is related to the sinusoidal three-dimensional temperature disturbances due to the instability of the Marangoni flow. The three-dimensional flow field organization, related to the wave number, changes during the solidification process; this behavior is explained according to the variation of the aspect ratio of the solidifying liquid bridge. A correlation law is found for the azimuthal wave number of the instability as function of the melt zone aspect ratio.     

II. Recirculation flow induced by a bubble stream rising in a viscous liquid

The recirculation flow induced by the rising motion of a bubble stream in a viscous fluid within an open-top rectangular enclosure is studied. The three-dimensional volume averaged conservation equations are solved by a control-volume method using a hybrid finite differencing scheme to describe the liquid phase hydrodynamics. The momentum exhange between the bubbles and the liquid phase is modeled with a source term equals to the volumetric buoyancy force acting on the gas in the bubble stream. The volumetric buoyancy force accounts for in line interactions between bubbles through the average gas volume fraction in the gas liquid column which depends on the size and the rising velocity of bubbles. The fluid flow within an open-top rectangular enclosure is further investigated by particle image velocimetry for a bubble stream rising in a water-glycerol solution. The measured fluid velocities in a vertical plane are compared with the predictions of the numerical model over a wide range of fluid viscosity (43 mPa s-800 mPa s) and gas flow rates. Finally, the recirculation flows resulting from the interaction of two neighbouring vertical bubble streams are studied. Received: 23 July 1997 / Revised: 19 December 1997 / Accepted: 11 May 1998     

固相萃取-高效液相色谱测定污水中的地塞米松

建立固相萃取-高效液相色谱法测定污水中地塞米松含量的方法。水样经固相萃取柱萃取富集后进行HPLC分析,以甲醇:水=65:35(V/V)为流动相,色谱柱为Agilent C18柱(5μm,4.6mm×150mm),柱温30℃;流速为1mL/min;紫外检测波长为240nm。地塞米松在25—300ng/mL范围内线性关系良好(n=5,R2=0.9992),最低检测浓度为25ng/mL(S/N>10),回收率为98.36%,地塞米松的低、中、高的日内精密度与日间精密度RSD均小于3%。本法准确、灵敏,能较好的应用于污水中地塞米松的测定。