Moisture–Conductivity Calibration for Electrical Imaging of Horticultural Substrate

Electrical imaging studies of laboratory- and small-scale plant root zones are gaining increasing attention. However, for essential moisture–conductivity calibrations on numerous substrate columns with variability in dry density, the use of one conventional strategy is relatively laborious or complicated. Thus, in this work, a relatively convenient calibration method is presented, and the objective is to identify its feasibility and potential to assess the effects of factors (e.g., volumetric water content (VWC), and dry density) on conductivity and establish necessary moisture–conductivity curves for porous materials (e.g., soils and substrates). In the method, with a specially designed fixture, variable VWCs, dry densities and related complex conductivities of the samples can be easily acquired through static compaction. The results show that the in-phase conductivity (or magnitude of conductivity) increases with the increasing VWC or dry density, primarily owing to the increase in the dominant pore water connectivity. Moreover, the effect of dry density on conductivity is relatively smaller than that of VWC. Thus, for the substrates at dry densities with certain variability, good power law relations (R2 ≥ 0.99) between in-phase conductivity (or magnitude of conductivity) and VWC at different frequencies can be established. Overall, the proposed approach is practical, promising, and relatively time- and labor-saving.

On the role of microgeometry in conductivity substitution

Conductivity substitution is the process of predicting the change in the effective electrical conductivity of a rock upon a change in conductivity of the mineral or fluid phase. Conductivity substitution is nonunique — only a range of conductivities can be predicted from knowledge of the initial effective conductivity, the porosity, and the initial and final compositions. The precise change depends strongly on the rock microstructure, which is seldom adequately known. Rigorous bounds on the change in effective conductivity upon changes in the phase conductivities for two-phase isotropic composites are used to gain insights into the roles of microgeometry and phase conductivity contrast. When the conductivity contrast between phases is high, the conductivity substitution predicted by Archie’s law corresponds approximately to the upper bound on the change of conductivity upon substitution. Inclusion modeling suggests that vuggy, highly tortuous, or partially disconnected pore space could account for conductivity changes smaller than those predicted by Archie’s law. Substitution behavior computed analytically for known microgeometries correlates with measures of microgeometry, including the fraction of connected fluid phase and variance of electric field strength in each phase. Comparison of the conductivity substitution bounds with brine-saturated sandstone data reveals that the position of measured data with respect to the conductivity substitution bounds can be indicative of the effective clay content. The bounds provide a template for better prediction of effective conductivity if we have at least some knowledge of the pore microstructure. Similarly, multiple conductivity measurements on the same rock might be used to extract more information about the rock and pore space properties than is possible with only a single measurement.

About Thermo-Hydraulic Properties of Open Cell Foams: Pore Scale Numerical Analysis of Strut Shapes

The thermo-physical behavior of open-celled metal foams depends on their microscopic structure. Various ideal periodic isotropic structures of tetrakaidecahedron shapes with constant cross section of the ligament having circular, square, diamond, hexagon and star strut shapes with various orientations are studied. We have proposed a generalized analytical model in order to obtain geometrical parameters correctly and various relationships between different geometrical parameters and porosities (60-95%) are presented. We have also studied the flow parameters namely permeability and inertia coefficient for different strut shapes and various Reynolds number (0.00001<Re<3000). The range of solid to fluid phase conductivity ratios (λs/λf) studied is from 10 to 30000 for different porosities in local thermal equilibrium condition and an analytical correlation is proposed comprising geometrical parameters of foam structure.

Real-time process dynamics monitoring in Anammox reactors

Process dynamics in Anammox systems were evaluated through continuous monitoring of pH, oxidation reduction potential (ORP) and conductivity in two separate newly started-up sequencing batch reactors, one seeded with an enriched Anammox sludge and the other seeded with mixed activated sludge. The pH and ORP profiles exhibited characteristic patterns depending on the process dynamics during early start-up, start-up and enrichment phases of the operational period of 410 days. The simultaneously continuing processes of the start-up period showed apparent indicative trend lines in pH and ORP profiles. Conductivity profiles were consistent with the process dynamics in all phases. During the enrichment phase, conductivity decreases could quantitatively be related to process removal efficiencies and all real-time profiles exhibited specific break-points which coincided with the end of Anammox in each cycle. The end of Anammox was observed as an ‘apex’ on pH profiles and a ‘valley’ on ORP profiles. The ‘apex’ and ‘valley’ points exactly coincided with the end point of the linear decrease in the conductivity profiles. The overall findings suggested a great potential in using real-time pH, ORP and conductivity measurements for quick and reliable monitoring of Anammox systems during start-up and enrichment periods.