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Berlin Brandenburg

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  • 1
    Language: English
    In: Transport in Porous Media, 2016, Vol.112(1), pp.207-227
    Description: According to experimental observations, capillary trapping is strongly dependent on the roughness of the pore–solid interface. We performed imbibition experiments in the range of capillary numbers ( Ca ) from $$10^{-6}$$ 10 - 6 to $$5\times 10^{-5}$$ 5 × 10 - 5 using 2D-micromodels, which exhibit a rough surface. The microstructure comprises a double-porosity structure with pronounced macropores. The dynamics of precursor thin-film flow and its importance for capillary trapping are studied. The experimental data for thin-film flow advancement show a square-root time dependence. Based on the experimental data, we conducted inverse modeling to investigate the influence of surface roughness on the dynamic contact angle of precursor thin-film flow. Our experimental results show that trapped gas saturation decreases logarithmically with an increasing capillary number. Cluster analysis shows that the morphology and number of trapped clusters change with capillary number. We demonstrate that capillary trapping shows significant differences for vertical flow and horizontal flow. We found that our experimental results agree with theoretical results of percolation theory for $$Ca =10^{-6}$$ C a = 10 - 6 : (i) a universal power-like cluster size distribution, (ii) the linear surface–volume relationship of trapped clusters, and (iii) the existence of the cutoff correlation length for the maximal cluster height. The good agreement is a strong argument that the experimental cluster size distribution is caused by a percolation-like trapping process (ordinary percolation). For the first time, it is demonstrated experimentally that the transition zone model proposed by Wilkinson (Phys Rev A 30:520–531, 1984) can be applied to 2D-micromodels, if bicontinuity is generalized such that it holds for the thin-film water phase and the bulk gas phase.
    Keywords: 2D-micromodel with rough surface ; Precursor thin-film flow ; Snap-off trapping ; Universal power law ; Ordinary bond percolation
    ISSN: 0169-3913
    E-ISSN: 1573-1634
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  • 2
    In: Water Resources Research, May 2014, Vol.50(5), pp.4514-4529
    Description: A major difficulty in modeling multiphase flow in porous media is the emergence of trapped phases. Our experiments demonstrate that gas can be trapped in either single‐pores, multipores, or in large connected networks. These large connected clusters can comprise up to eight grain volumes and can contain up to 50% of the whole trapped gas volume. About 85% of the gas volume is trapped by gas clusters. This variety of possible trapped gas clusters of different shape and volume will lead to a better process understanding of bubble‐mediated mass transfer. Since multipore gas bubbles are in contact with the solid surface through ultrathin adsorbed water films the interfacial area between trapped gas clusters and intergranular capillary water is only about 80% of the total gas surface. We could derive a significant (R = 0.98) linear relationship between the gas‐water‐interface and gas saturation. We found no systematic dependency of the front velocity of the invading water phase in the velocity range from 0.1 to 0.6 cm/min corresponding to capillary numbers from 2 × 10 to 10. Our experimental results indicate that the capillary trapping mechanism is controlled by the local pore structure and local connectivity and not by thermodynamics, i.e., by the minimum of the , at least in the considered velocity range. Consistent with this physical picture is our finding that the trapping frequency (= bubble‐size distribution) reflects the pore size distribution for the whole range of pore radii, i.e., the capillary trapping process is determined by statistics and not by thermodynamics. No systematic dependency of trapping efficiency on capillary number Majority of trapped gas bubbles (85%) are multipore trapped Trapping of gas clusters is determined by statistics and not by thermodynamics
    Keywords: Gas Clusters ; Capillary Trapping ; Interfacial Area
    ISSN: 0043-1397
    E-ISSN: 1944-7973
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  • 3
    Language: English
    In: Vadose Zone Journal, 2015, Vol.14(5), p.0
    Description: We used X-ray computed microtomography to study gas trapping in a fluctuating water table. Our results show that capillary forces control trapping and phase distribution in dynamic capillary fringes. In porous media, the nonwetting phase is trapped on water saturation due to capillary forces acting in a heterogeneous porous structure. Within the capillary fringe, the gas phase is trapped and released along with the fluctuation of the water table, creating a highly active zone for biological transformations and mass transport. We conducted column experiments to observe and quantify the magnitude and structure of the trapped gas phase at the pore scale using computed microtomography. Different grain size distributions of glass beads were used to study the effect of the pore structure on trapping at various capillary numbers. Viscous forces were found to have negligible impact on phase trapping compared with capillary and buoyancy forces. Residual gas saturations ranged from 0.5 to 10%, while residual saturation increased with decreasing grain size. The gas phase was trapped by snap-off in single pores but also in pore clusters, while this single-pore trapping was dominant for grains larger than 1 mm in diameter. Gas surface area was found to increase linearly with increasing gas volume and with decreasing grain size.
    Keywords: Grain Size ; Water Table ; Mass Transport ; Buoyancy ; Pores ; Porous Media ; Particle Size ; Water Table ; Saturation ; Vadose Water ; Fluctuations ; Trapping ; Buoyancy ; Methods and Instruments ; General;
    ISSN: Vadose Zone Journal
    E-ISSN: 1539-1663
    Source: CrossRef
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  • 4
    In: Water Resources Research, November 2015, Vol.51(11), pp.9094-9111
    Description: We study the impact of pore structure and surface roughness on capillary trapping of nonwetting gas phase during imbibition with water for capillary numbers between 10 and 5 × 10, within glass beads, natural sands, glass beads monolayers, and 2‐D micromodels. The materials exhibit different roughness of the pore‐solid interface. We found that glass beads and natural sands, which exhibit nearly the same grain size distribution, pore size distribution, and connectivity, showed a significant difference of the trapped gas phase of about 15%. This difference can be explained by the microstructure of the pore‐solid interface. Based on the visualization of the trapping dynamics within glass beads monolayers and 2‐D micromodels, we could show that bypass trapping controls the trapping process in glass beads monolayers, while snap‐off trapping controls the trapping process in 2‐D micromodels. We conclude that these different trapping processes are the reason for the different trapping efficiency, when comparing glass beads packs with natural sand packs. Moreover, for small capillary numbers of 10, we found that the cluster size distribution of trapped gas clusters of all 2‐D and 3‐D porous media can be described by a universal power law behavior predicted from percolation theory. This cannot be expected a priori for 2‐D porous media, because bicontinuity of the two bulk phases is violated. Obviously, bicontinuity holds for the thin‐film water phase and the bulk gas phase. The snap‐off trapping process leads to ordinary bond percolation in front of the advancing bulk water phase and is the reason for the observed universal power law behavior in 2‐D micromodels with rough surfaces. Surface roughness controls capillary trapping efficiency The transition‐zone model can be applied to 2‐D micromodels with rough surfaces The 2‐D and 3‐D porous media belong all to the same universality class
    Keywords: Surface Roughness ; Precursor Thin‐Film Flow ; Snap‐Off Trapping ; Universal Power Law ; Ordinary Bond Percolation
    ISSN: 0043-1397
    E-ISSN: 1944-7973
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  • 5
    In: Water Resources Research, April 2009, Vol.45(4), pp.n/a-n/a
    Description: High‐resolution optical bench‐scale experiments were conducted in order to investigate local gas flow pattern and integral flow properties caused by point‐like gas injection into water‐saturated glass beads. The main goal of this study was to test the validity of the continuum approach for two‐fluid flow in macroscopic homogeneous media. Analyzing the steady state experimental gas flow pattern that satisfies the necessary coherence condition by image processing and calibrating the optical gas distribution by the gravimetrical gas saturation, it was found that a pulse‐like function yields the best fit for the lateral gas saturation profile. This strange behavior of a relatively sharp saturation transition is in contradiction to the widely anticipated picture of a smooth Gaussian‐like transition, which is obtained by the continuum approach. This transition is caused by the channelized flow structure, and it turns out that only a narrow range of capillary pressure is realized by the system, whereas the continuum approach assumes that within the representative elementary volume the whole spectrum of capillary pressures can be realized. It was found that the stochastical hypothesis proposed by Selker et al. (2007) that bridges pore scale and continuum scale is supported by the experiments. In order to study channelized gas flow on the pore scale, a variational treatment, which minimizes the free energy of an undulating capillary, was carried out. On the basis of thermodynamical arguments the geometric form of a microcapillary, macrochannel formation and a length‐scale‐dependent transition in gas flow pattern from coherent to incoherent flow are discussed.
    Keywords: Air Sparging ; Continuum Modeling ; Pore‐Scale Modeling ; Gas Flow Pattern ; Instability Analysis ; Image Processing
    ISSN: 0043-1397
    E-ISSN: 1944-7973
    Source: John Wiley & Sons, Inc.
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  • 6
    In: Water Resources Research, August 2019, Vol.55(8), pp.6653-6672
    Description: —a key process for water exchange between soil and atmosphere—is controlled by internal water fluxes and surface vapor fluxes. Recent studies demonstrated that the dynamics of the water flow in corners determine the time behavior of the evaporation rate. The internal water flux of the porous media is often described by capillary flow assuming . Particularly, the crucial influence of , that is, the nonlinear dependency of the capillary flow has been neglected so far. The focus of the paper is to demonstrate that SiO‐surfaces can exhibit contact angles of about 40°. This reduces the internal capillary flow by 1 order of magnitude compared to complete wetting. First, we derived the contact angle by inverse modeling. We conducted a series of evaporation experiments in a 2‐D square lattice microstructure connected by lognormal distributed throats. We used an explicit analytical power series solution of the single square capillary model. A contact angle of 38° ± 1° was derived. Second, we directly measured the contact angle of the Si‐SiO wafer using the Drop Shape Analyzer Krüss 100 and obtained an averaged contact angle of 42° ± 2°. The results support the single square capillary model as an appropriate model for the description of the evaporation process in an ideal square capillary. Evaporation rate dependence on contact angle and temperature: Influence of capillary, viscous, and gravitational forces Visualization micromodel experiments of corner flow: Micromodels produced by a new interval‐based ICP‐DRIE technology Analytical solution for 1‐D corner flow and analysis of the fluid‐fluid patterns and geometric characteristics of the evaporation process
    Keywords: Water Flow ; Silicon Dioxide ; Water Exchange ; Surfaces ; Internal Water ; Power Series ; Water Exchange ; Soil Water ; Evaporation ; Contact Angle ; Dependence ; Evaporation Rate ; Water Flow ; Water Exchange ; Microstructure ; Evaporation ; Porous Media ; Porous Media ; Silica ; Evaporation ; Water Flow ; Capillary Flow ; Atmospheric Models ; Wetting ; Silicon Dioxide ; Fluxes ; Soils ; Evaporation ; Evaporation Rate ; Water Exchange ; Contact Angle;
    ISSN: 0043-1397
    E-ISSN: 1944-7973
    Source: John Wiley & Sons, Inc.
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