Experimental investigation of transverse mixing in porous media under helical flow conditions

Plume dilution and transverse mixing can be considerably enhanced by helical flow occurring in three-dimensional heterogeneous anisotropic porous media. In this study, we perform tracer experiments in a fully three-dimensional flow-through chamber to investigate the effects of helical flow on plume spiraling and deformation, as well as on its dilution. Porous media were packed in angled stripes of materials with different grain sizes to create blocks with macroscopically anisotropic hydraulic conductivity, which caused helical flows. Steady-state transport experiments were carried out by continuously injecting dye tracers at different inlet ports. High-resolution measurements of concentration and flow rates were performed at 49 outlet ports. These measurements allowed quantifying the spreading and dilution of the solute plumes at the outlet cross section. Direct evidence of plume spiraling and visual proof of helical flow was obtained by freezing and slicing the porous media at different cross sections and observing the dye-tracer distribution. We simulated flow and transport to interpret our experimental observations and investigate the effects of helical flow on mixing-controlled reactive transport. The simulation results were evaluated using metrics of reactive mixing such as the critical dilution index and the length of continuously injected steady-state plumes. The results show considerable reaction enhancement, quantified by the remarkable decrease of reactive plume lengths (up to four times) in helical flows compared to analogous scenarios in uniform flows.

Figure 1

(a) Photograph of the experimental setup with marked layers. (b) Inner heterogeneous anisotropic structure of the porous medium, built with high and low hydraulic conductivity angled stripes, and streamlines traced from the central injection port.

Figure 2

Effect of the complex flow field on plume deformation at different longitudinal distances of the domain. Simulated patterns of streamlines injected from (a) a centered injection and (b) a noncentered injection, corresponding to the experiment using New Coccine as tracer (Fig. 3).

Figure 3

Plume distribution at different cross sections. (a) Enhanced photographs showing the location and distribution of the New Coccine plume. Contour lines of simulated normalized concentrations are added to the photographs for direct comparison. (b) Concentration maps at the given longitudinal distances computed with the numerical model.

Figure 4

Experimental and modeling results showing the normalized concentration distributions at the outlet of the heterogeneous anisotropic porous media packed with fine (grain size of 0.4–0.6 mm and 1.5–2.0 mm) and coarse (grain size of 0.6–0.9 mm and 2.4–2.9 mm) materials.

Figure 5

Experimental and model results of the plume's second central moments in the vertical and horizontal transverse directions at the outlet of the flow-through setup packed with fine (F) and coarse (C) media and using the central inlet port (3-3; Fig. 4) and the neighboring one (3-2) as the tracer-injection ports.

Figure 6

Experimental and modeling results of the flux-related dilution index at the outlet of the flow-through system packed with fine (F) and coarse (C) materials using the central inlet port (3-3) and the neighboring one (3-2) as the tracer-injection ports.

Figure 7

Mixing-controlled reactive plumes ($X_{\mathrm{crit}}=0.5$) computed in a heterogeneous anisotropic porous medium and in a homogenous setup. Identical mass flux of reactant A was injected at the inlet in both setups.