Multicomponent Convection Research Articles

Modeling Multicomponent Diffusion and Convection in Porous Media

Summary Numerical investigations of diffusion and convection in multicomponent hydrocarbon mixtures in two-dimensional (2D) crosssectional (x,z) porous media are performed using the finite-volume method. Spatial discretization is performed by a second-order centered scheme. It is shown that methane, unlike in binary hydrocarbons where it often segregates towards the bottom-hot side of the porous media, may be at higher concentration at the cold-top side in ternary mixtures and in multicomponent reservoir fluids. This behavior, which is consistent with oilfield data, is due to competing diffusion mechanisms which have not been properly accounted for in the past. It is also demonstrated that convection may significantly affect the compositional variation in some hydrocarbon reservoirs. Depending on fluid mixtures, a weak convection may drastically change compositional variation.

Pattern formation and symmetry-breaking bifurcations fueled by dissipation of chemical energy: a possible model for morphogenesis?

Abstract

Fine structure of boundary flows in media with diffusion and heat conduction

The influence of the boundary conditions at the surfaces confining multicomponent stratified media on the formation of flows in problems of multicomponent diffusion and thermoconcentration convection is investigated. Exact solutions of these problems are given. Analysis of these solutions shows that several boundary layers (concentration and velocity layers) are formed in the case of multicomponent diffusion, which leads to decomposition of the physical fields and splitting of the characteristic spatial scales. In the case of thermoconcentration convection, a more complicated dynamic structure is formed, which, besides boundary layers, includes injection fronts. The latter have a significant effect on the flow characteristics at distances far exceeding the thickness of the boundary layers.

Multicomponent diffusion and convection in molten MgO-Al 2O 3-SiO 2

Multicomponent diffusion and convection in molten MgO-Al 2O 3-SiO 2 at 1550°C and 0.5 GPa were examined experimentally in diffusion couples formed around 22.5 (wt%) MgO, 17.5% Al 2O 3, and 60% SiO 2. The diffusion matrix obtained from a simultaneous least squares inversion of convectively stable intersecting chemical diffusion profiles has large off-diagonal terms indicating, for example, very strong coupling of the flux of SiO 2 with gradients in MgO. In contrast to an earlier report on multicomponent diffusion involving the same compositions studied here, we found no features along the stable diffusion profiles that could not be explained in terms of a 2 × 2 diffusion matrix. X-ray concentration maps of the sectioned diffusion charges showed that double-diffusive convection had occurred in a number of experiments along the direction of constant SiO 2, even though the more dense melt had been placed below the less dense composition. The occurrence of such double-diffusive fingering instabilities is shown to be consistent with the predictions of linear stability theory.

Modeling of micro- and macrosegregation and freckle formation in single-crystal nickel-base superalloy directional solidification

The formation of macrosegregation and freckles by multicomponent thermosolutal convection during the directional solidification of single-crystal Ni-base superalloys is numerically simulated. The model links a previously developed thermodynamic phase equilibrium subroutine with an existing code for simultaneously solving the macroscopic mass, momentum, energy, and species conservation equations for solidification of a multicomponent alloy. Simulation results are presented for a variety of casting speeds and imposed thermal gradients and for two alloy compositions. It is found that for a given alloy composition, the onset of convection and freckle formation occurs at a critical primary dendrite arm spacing, which agrees well with previous experimental findings. The predicted number and shape of the freckle chains in the unstable cases also agree qualitatively with experimental observations. Finally, it is demonstrated how the onset and nature of convection and macrosegregation vary with alloy composition. It is concluded that the present model can provide a valuable tool in predicting freckle defects in directional solidification of Ni-base superalloys.

Formation of thermal plumes in an autocatalytic exothermic chemical reaction.

The reaction of chlorite and thiourea is bistable and displays a lateral instability that generates a traveling wave of sulfate, acid, and chlorine dioxide. The wave was visualized by the addition of barium chloride, which gave a white precipitate of barium sulfate. The wave propagates with three distinct regions of varying precipitation intensities. One of the regions is made up of a complete convective roll that forms powerful thermal plumes which rise to the surface of the reactant solution. The plumes originate from a coupling of Marangoni convection with multicomponent convection. (c) 1995 The American Physical Society

Formation of macrosegregation by multicomponent thermosolutal convection during the solidification of steel

The formation of macrosegregation by multicomponent thermosolutal convection during the solidification of steel is simulated by simultaneously solving macroscopic mass, momentum, energy, and species conservation equations with full coupling of the temperature and concentrations through thermodynamic equilibrium at the solid/liquid interface. The flow field, solid fraction evolution, and macrosegregation patterns for four cases are presented. The results show both the formation of channel segregates and the formation of islands of mush surrounded by bulk melt. In examining the solidification of a ten-element steel, the global extent of macrosegregation of an element is found to be linearly dependent on its partition coefficient (more severe segregation for small partition coefficient), although such scaling is not possible locally. Results for the solidification of a binary Fe-C alloy (with the same carbon content as the ten-element alloy) are similar to those for the ten-element alloy due solely to the large contribution of carbon to buoyancy driven flow in the ten-element steel chosen for study. While including only those elements that make significant contributions to buoyancy driven flow reproduces the global extent of macrosegregation seen in the ten-element alloy, local differences in the predictions are visible. Finally, comparison of results for the solidification of the same ten-element steel using two different sets of data to describe the partition coefficients and change in liquidus temperature with concentration of the elements shows completely opposite behavior,i.e., upward flow through the mushy zone for one case and downward flow for the other. Thus, the need to have accurate phase-equilibrium data when modeling multicomponent macrosegregation is illustrated. Together, the results give an indication of what areas require more careful examination if accurate modeling of multicomponent solidification is to be accomplished.

Convection in multicomponent silicate melts driven by coupled diffusion

THE simplest convecting systems are driven by buoyancy forces arising from the density variations produced by the diffusion of a single quantity, usually heat. In such systems, the necessary condition for the onset of convection is that density decreases downwards. But the convection process can be very different if there are two or more diffusing quantities that affect the density1–5, a situation often encountered in geological settings. For example, multicomponent convection plays an important role in the convec-tive evolution and differentiation of terrestrial magmas6–8. A typical terrestrial silicate magma is made up of a number of diffusing components, which not only have different diffusion rates but may also exhibit diffusive coupling — that is, the diffusive flux of one component can be strongly dependent on the spatial gradients of other components9–12. We have investigated convection in a geologically relevant fluid — a multicomponent silicate melt in the system CaO–Al2O3–SiO2 — and we find that diffusive coupling plays a key role in promoting convection. Perhaps the most striking feature of these experiments is that convection can occur regardless of whether the density decreases or increases with depth.

Multicomponent convection induced by fronts in the chlorate-sulfite reaction

An application of a new method is presented for the measurement of the temperature profiles of chemical waves propagating through a solution. Using solutions of thermocolor materials, the temperature distribution caused by the heat released in the propagating chlorate oxidation of sulfite was visualized and recorded using digital image processing methods. After calibration, the temperature gradient was calculated from the gray scale value in a digitized image. Extensive multicomponent convection ('fingering') was induced by descending fronts. Only ascending fingers were observed because of the large thermal gradient that suppressed descending ones. The characteristics of the temperature profile were determined as a function of initial sulfite and chlorate concentration, and tube diameter. Unusual behavior was observed when the fronts propagated under conditions of continuously changing diameter in a conical vessel. Fingering occurred periodically in a front descending in a flask with an increasing diameter. However, when a front propagated down in flask whose diameter decreased, no multicomponent convection was observed.

A numerical study of the leading edge of laminar diffusion flames

This study deals with a fundamental numerical investigation of chemically reacting fluid flows through two-dimensional burners. We make use of a detailed set of finite chemical kinetic rate equations to numerically simulate a laminar diffusion flame. The code has been constructed to consider the viscous effects in a mixing layer, heat conduction, the multicomponent diffusion and convection of important species, the finite rate reactions of these species, and the resulting interactions between the fluid mechanics and the chemistry. The numerical model has been used to obtain a detailed description of the leading edge of laminar diffusion flames obtained above two-dimensional methane/air burners. It is shown that the leading edge flame, a flame holding point for the rest of the diffusion flame, is dominated by the kinetic aspects of the fuel/oxidizer species and is mainly responsible for heat transfer to any upstream boundary surface.

Enhanced Polymer Transport Due to the Presence of Salts in Multicomponent Convection

Abstract Transport rate of poly(vinylpyrrolidone) in aqueous dextran matrix was enhanced by the addition of CsCl or NaCl but not by LiCl. This enhancement can be partly attributed to a decrease in hydrodynamic resistance.

Convective effects on chemical waves. 3. Multicomponent convection in the iron(II)-nitric acid system

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTConvective effects on chemical waves. 3. Multicomponent convection in the iron(II)-nitric acid systemJohn A. Pojman, Istvan P. Nagy, and Irving R. EpsteinCite this: J. Phys. Chem. 1991, 95, 3, 1306–1311Publication Date (Print):February 1, 1991Publication History Published online1 May 2002Published inissue 1 February 1991https://doi.org/10.1021/j100156a050RIGHTS & PERMISSIONSArticle Views139Altmetric-Citations69LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (2 MB) Get e-Alerts

Convective effects on chemical waves. 1. Mechanisms and stability criteria

We consider the effects of convection on chemical waves in which density gradients result from the exothermicity as well as from the isothermal volume change of the reaction. If the signs of the enthalpy (ΔH) and volume (ΔV) of the reaction are opposite, then simple convection will occur, and the increase in the front propagation velocity will be equal to the velocity of the convective fluid flow. If the signs are the same, then simple convection will not occur. Instead multicomponent convection may be present, even though the overall density gradient may appear to be stable. The stability conditions for both mechanisms as functions of ΔH and ΔV and of the direction of front propagation (ascending, descending or horizontal) are analyzed

Multicomponent diffusion and convection in capillary structures

AbstractA previously developed method (Sotirchos and Burganos, 1988) for studying isobaric multicomponent diffusion in a class of capillary networks is extended for application to the problem of convection and diffusion of gaseous mixtures in porous media. The method is used to develop flux models for nonisobaric transport in capillary structures, which are then compared with corresponding “three‐parameter” dustygas models. Results for discrete and continuous pore size distributions show that the predictions of the developed flux models depend strongly, both qualitatively, and quantitatively, on the procedure used to average the dusty‐gas model equations for a single pore over the pore‐size distribution.

A numerical model for supersonic reacting mixing layers

A current research effort is underway at the NASA Langley Research Center to achieve a detailed understanding of important phenomena present when a supersonic flow undergoes a chemical reaction. A computer program has been developed to study the details of such flows. The program has been constructed to consider the multicomponent diffusion and convection of important species, the finite-rate reaction of these species, and the resulting interaction between the fluid mechanics and chemistry. Code results from the analysis of a spatially developing and reacting mixing layer are presented, and conclusions are drawn regarding the structure of the evolving layer and its associated flame.

Multicomponent crystallization and convection beneath volcanoes

Some recent experiments and theoretical calculations of the effects of cooling and crystallization multicomponent systems are briefly reviewed. The research has been motivated mainly by a desire to understand phenomena which occur in magma chambers beneath volcanoes. However, the results can be applied to a large variety of other natural and industrial situations.

Convection in multicomponent systems

It should now be clear that multicomponent convection processes can be important in many disciplines. In particular, it has been shown that coupling between vertical motions driven by gravity, and diffusion acting on small scales, produces much more rapid transports of properties than is possible with molecular diffusion alone. If such effects are ignored, many qualitatively different phenomena can be missed entirely. There are no doubt many more potential applications in fields where analogous processes have not yet been discussed in these terms.

Multisolute mass transport with chemical interaction kinetics

An algorithm, based on the finite-element method, for solving the multicomponent convection—dispersion equation with solid-phase chemical interaction is presented. The main feature of the method is the modularization of the chemical calculations. This unique feature makes the algorithm ideal for situations where several different chemical systems are to be explored. Example numerical results are compared with both analytical and experimental results.

Multicomponent convection: Turbulence in Earth, Sun and sea

Multicomponent convection: Turbulence in Earth, Sun and sea