Modeling of flow nonuniformities in fissured porous media

Abstract

A review is performed to.trace how fissured porous media arise in nature. It is fourid that packed beds resemble fissured porous media. The range of multiple flow time-scales inherent in such media becomes larger as the particle size range becomes broader, resulting in flow nonuniformities. The status of experimental measurements of voidage and flow nonuniformities is assessed. Some of the difficulties associated with modeling of the multiple flow time-scales are discussed, and a tentative model is proposed.acked beds a r e frequently used a s energy con-P verters in fossil and nuclear related industries.They are used in the catalytic reforming of petroleum, in the gasification of coal, in high-temperature gascooled pebble-bed nuclear reactors and in the retorting of oil shale. Considering the demand f o r fossil fuels, the efficient and safe operation of these energy converters is important to maximize recovery. Channeling and by-passing caused by nonuniform flow prodwe hot spots indicative of runaway reactions and combustion of valuable products. This paper shows that real packed beds resemble fissured porous media since even so-called random packing results in non-random voidage variations and flow structures. Even small voidage variations cause considerable flow nonuniformities" that can produce a highly dispersive system."' Later sections discuss the basic difficulty associated with modeling flow in fissured porous media having a multiplicity of time-scales. A tentative flow model that uses multi-phase flow concepts is suggested. How fissured porous media and flow nonuniformities arise in natureFigure 1 illustrates the basic concept of a fissured porous m e d i~m . '~' The figure, which appeared in a n article by Bear and Braester who attributed it to the Soviet Barenblatt,'4,'J shows a network of interconnecting gaps or fissures, in between which a r e regions called porous blocks containing characteristically smaller gaps. Such a situation might arise from fracturing of tight sands for t h e recovery of unconventional natural gas, f o r example. Flows through t h e smaller gaps are called porous-media flows, while flows through the larger gaps are called fissure flows. Hence the flow field has overlapping continua with exchange of fluid between the fissures and the porous block. This fissured porous medium On a fait une revue de littirature sur l'origine naturelle de la fissuration des milieux poreux; on a trouvt que les lits garnis prtsentent une certaine ressemblance avec ces milieux. La gamme des temps d'hcoulements multiples inhtrents aux milieux en question s'hlargit A mesure que celle des dimensions des particules s'accroft et il en rksulte un manque d'uniformitk dans les kcoulements. On a fait le point de ]'&at de la question des mesures expkrimentales du degrt de vide et des manques d'uniformitt des hcoulements. On discute quel- ques-unes des difficultbs de modblisation des temps mis en jeu dans les icoulements multiples et on propose un modhle a vtrifier.representation is a simplification of much ...

Investigation of dispersion in porous media using fiber-optic technology

“…The inside diameter was chosen to reduce wall effects to a level consistent with the objectives of the experiment Wall effects have been investilgated by Schwartz and Smith (1953), Lyczkowski (1982), and Dybbs and Edwards (1986). Their inve.3tigations indicate that unless the column to bead diameter ratio J,sgreater than 30, there will be significant variations in flow velocities across the column.…”

Investigation of dispersion in porous media using fiber-optic technology

“…The experimental apparatus (see The length of the column was selected to red.uce apparatus induced dispersion since this effect is most significant in short columns 0ames and Rubin, 1972). The inside diameter was chosen to reduce wall effects to a level consistent with the objectives of the experiment Wall effects have been investilgated by Schwartz and Smith (1953), Lyczkowski (1982), and Dybbs and Edwards (1986). Their inve.3tigations indicate that unless the column to bead diameter ratio J,sgreater than 30, there will be significant variations in flow velocities across the column.…”