Macroporous Media Research Articles

A multiple-pore-region concept to modeling mass transfer in subsurface media

Recent studies in soil science literature have strongly indicated the need to incorporate pore structures in near-surface mass transport modeling. There is increasing evidence suggesting that pore structures, such as fractures and macropores, facilitate the transport of water and solutes along a preferential flow path while water and solutes are moved into micropores and rock matrices concurrently. This study presents a conceptual model, a multiple-pore-region (or multi-region) concept, to account for pore structures as well as the resultant widely distributed pore water velocities in macroporous media. Pore regions can either be physically identified as discrete features, such as fractures and rock matrices, or be experimentally determined by separation of water retention curves according to pore classification schemes. A multi-region mechanism is proposed to account for the effect of local-scale and field-scale heterogeneities on mass transport under variably saturated conditions. Two numerical codes for subsurface fluid flow and solute transport have been developed with the multi-region concept, in which a firstorder mass exchange model is adopted to simulate the redistribution of pressure heads and solute concentrations among pore regions. The computer codes are used to demonstrate the applicability of the concept to fractured porous media, and to test a three-pore-region hypothesis using laboratory soil column tracer injection data. Based upon the parameters obtained from fitting multi-region and mobile-immobile models to these data, we successfully demonstrated that the former model has the advantage of maintaining consistent conceptual models over the latter under variably saturated conditions.

Study of moisture transfer in a deformable porous medium through attenuation of two different energy gamma rays

The hydromechanical behavior of a moist fibrous medium is tested with the help of the principle of attenuation of two collinear gamma rays (cesium and americium). The detector is made up of a single spectrometric acquisition system and of a multichannel analyzer which differentiates the two gamma rays. This differentiation allows the simultaneous discovery of the density and of the moisture at each part of the medium. The measurement is carried out by moving the sample tested in relation to the gamma ray with the help of a very accurate apparatus which functions with a stepping motor engine. A microcomputer carries out the positioning, the measurement findings, and the signal processing. Software programs have been developed which define the parameters of each experiment as well as visualizing the diffusion and deformation properties of the medium tested. Numerical simulations representative of the medium in experimental situation are presented. This technique, believed original, permitted the study of the hydrodynamical behavior of a macroporous medium (containing 98% of air) in a way unused so far.

Movement of water through macroporous media

Movement of water through macroporous media

Pore space models for transport phenomena in porous media review and evaluation with special emphasis on capillary liquid transport

A classification and characterization is given of the more important transport phenomena in macroporous media (molecular diffusion, viscous flow, transport phenomena involving moving menisci: wetting, capillary rise, drainage, drying). Pore space models are used to obtain values for the transport coefficient (effective diffusion coefficient, permeability) and — when applicable — the driving force (capillary potential) in the transport equation. These models and this approach have to be distinguished from (i) models used in simulating particle packings, (ii) models used in determining the so-called pore size distribution (suction technique, mercury porosimetry), (iii) analytic calculation of the transport coefficient, and (iv) overall description of the transport phenomena (in which the microscopic pore space structure is not accounted for). An enumeration of almost all proposed pore space models is presented and classified with reference to the pore space interconnectivity. This interconnectivity may be one-dimensional (tubes in parallel, in series; tubes with constrictions; random adjacent slices models), two-dimensional (network models), three-dimensional (regular sphere packings; tetrahedra networks; tubes and/or junctions randomly in space), and strictly zero-dimensional (simple capillary elements used in ad hoc explanations; independent domain theory). In order to remove too optimistic ideas about the use of pore space models, all models are checked against the phenomena of capillary rise ( e.g. water in sand). The predictions of the models regarding statics and dynamics of capillary rise are compared with the experimental facts and it appears that none of the models can give even a qualitative description of the observed phenomena. The situation is not as bad for other transport phenomena as it is for capillary rise, but it should be realized that even slightly different transport phenomena require a different approach and, further, that improvement of pore space models should be directed by a more careful analysis of the non-macroscopic physical phenomena. It is the ghost of the concept of a pore size (distribution) that plays a dominant role here. This concept is analysed. Finally, a number of seemingly comparable phenomena or concepts are discussed: drainage — imbibition; suction — mercury porosimetry; molecular diffusion — viscous flow; capillary potential and permeability as a function of liquid content. Tacitly this comparison gives an evaluation of the pore space models for the transport phenomena involved. Suggestions for further research are given.

Analysis of diffusion in macroporous media in terms of a porosity, a tortuosity and a constrictivity factor

The ratio between the effective bulk diffusion coefficient in macroporous media and the diffusion coefficient in the absence of the porous medium is analysed in terms of the porosity, the tortuosity and the constrictivity. The necessity of distinguishing these three parameters is supported by the description of diffusion in partly saturated homogeneous isotropic monodisperse sphere packings. The results are generalized to bulk diffusion in all homogeneous isotropic porous media in the porosity range 0–50 per cent.