Terrestrial Systems > Soil Physics > Research > Porous Media and Soils
Experimental Study of Fingering Flow in Porous Hele-Shaw Cells
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Fereidoun Rezanezhad, Hans-Jörg Vogel, Kurt Roth
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Experimental Setup
and Method: Fundamental studies on multiphase flow and transport
processes of fingering phenomena within porous media require
experimental techniques as light transmission system to measure
state-variable at high resolution because the key to understanding
fingered flow are rapid high-resolution measurements of the water
saturation. This technique involves placing a thin, translucent and
vertical two-dimensional experimental Hele-Shaw cell, two parallel
glass plates separated by a few millimeters, in front of a uniform
and stable light source.
We developed a unique, high
resolution, 2D transmitted light imaging system to use for
quantitative imaging of transient and steady state flows in porous
media. The Light Transmission Method (LTM) is a nondestructive and
simple tool that permits visualization and measurement of water
saturation in Hele-Shaw cells with a high spatial (millimeters) and
temporal (seconds) resolution. This technique also opens promising
perspectives for investigation of multiphase phenomenon. This
technique was used to visualize and analyze, qualitatively and
quantitatively, fingering phenomena. This research presents the
development and application of the LTM for two-phase flow, aimed at
investigating unstable fingered flow, in a sand-air-water system. We
established a Hele-Shaw cell where a layer of fine-textured sand was
placed on top of a coarse-textured sand and studied experimentally
the flow paths and instabilities of gravity driven fingers through an
initially dry porous medium.
Infiltration experiments are
conducted in a large two-dimensional transparent Hele-Shaw cells (160
× 60 × 0.3 cm) with multi layers of dry fine and
coarse sands with grain size between 0.025-1.25 mm (Figure 1). A
microscopic view of the grain size distribution as well as particle
shapes of the fine and coarse sands used in the experiment is
presented in Figure 2. Homogenized transmitted light of the entire
surface of the cell from a uniform light source was recorded by a
digital camera. The experimental procedure consists of several steps
such as sand preparation, cell cleaning, filling and packing of the
cell with sand, injection of water into the cell and recording light
intensity. Infiltration experiments were carried out with an
initially dry sand in the Hele-Shaw cell under varied conditions for
different flow rates. A set of many experiments was attempted with
constant and different flow rates infiltration. After reaching
stationary water flow to visualize the velocity field inside the
cell, additional infiltration experiments were performed by using a
dye tracer (0.5 gr/lit Brilliant Blue) which was applied to the
almost stationary flow field behind the water infiltration front.
Figure 1: Sketch (side and front view) and lab photo of the experimental setup. A transparent Hele-Shaw cell with four layers of sand was placed in front of a uniform light source. Transmitted light was recorded by a digital camera. The front view shows the highly localized flow paths that originate from the flow instability in the uniform part of the medium.
Figure 2: Photographs of the grain size distribution and particle shapes for the fine (top) and the coarse (bottom) sands used in the experiment. The pore interface acts as a water flow gate between two less permeable fine toplayer and more permeable coarse sublayer. The length scale for images are given below of images.
This
technique is based on the fact that the intensity (I=1/3 (R+G+B)
where R, G, and B are the Red, Green, and Blue components of the RGB
captured image) of transmitted light increases with water saturation.
The transmitted light intensity was calibrated to get the absolute
water saturation by simultaneous measurements of X-ray transmission
which is accurate but too slow and also too expensive for routine
measurements (Bayer
et al. 2004 (pdf 0.67MB)) .
Thereby, we used intensity of transmitted light through the cell as a
proxy for water content and hence, the changing water content within
flow fingers could be measured in great detail.
The
experimental aspect of this study requires rapid point measurements
of phase pressure during the passage of fingers. The monitoring of
internal dynamics of water pressure change was not possible with
optical methods, hence we employed the traditional instruments e.g.
pressure sensors installed into access ports. Many mini-tensiometers
were installed at different locations in the designated holes over
the back wall of the cell to measure the potential energy of the
water (Figure 3). The calibration of tensiometers was carried out by
using sequence defined heights of water in a calibration tube.
Figure 3: Sketch and Photo of the tensiometer installed over the cell and a Sketched cross section of a mounted tensiometer. The porous ceramic plate (light gray) connects directly to the pressure transducer via a firm plastic tube so that the ceramic plate was in contact with the sand. It was necessary to avoid gaps between the glass and the tube for install the tensiometer in the cell.
