The influence of the rheology of concentrated, non-Brownian
suspensions on solute mass transfer characteristics
Arun Ramchandran,
University
of Toronto, Canada
Abstract
The transport of
solutes in sheared non-Brownian, particulate suspensions is a problem of great
interest in industrial operations such drilling and hydraulic fracturing for
oil and gas production. But, while there are several studies in the literature
of the particle distribution in the pressure-driven flow of suspensions, there
are relatively few studies of the mechanisms and rates by which solutes are
transported in such flows. In this work, we characterize the mass transfer
properties of a simple, model suspension - spherical, rigid particles dispersed
in a Newtonian suspending medium at moderate volume fractions (30% to 50%)
sheared at high Brownian Peclet numbers, flowing through a straight channel
with a rectangular cross-section.
In the absence of flow,
the mixing of a passive solute in the presence of a concentration gradient can
be attributed only to molecular diffusion. On imposing shear, flow-induced
inter-particle collisions lead to particulate self-diffusion, which in turn,
leads to solute shear-induced self-diffusion and can enhance the mixing rate.
In this work, we will discuss a third mechanism of shear-induced solute mass
transfer enhancement - the secondary convection accompanying the flow of a
concentrated suspension through a non-axisymmetric geometry, arising from the
second-normal stress differences exhibited by suspensions. For small particle
sizes, the enhancement of solute diffusivity by shear-induced self-diffusion is
weak. However, the magnitude of the secondary currents is unaffected by
particle size. Thus, for suspensions with particles much smaller than the
conduit size, secondary convection, and not shear-induced self-diffusion, can
be the dominant mechanism for shear-induced enhancement of mass transfer.
To further investigate
the secondary convection mechanism, we carried out experiments in a
silicon-glass microchannel designed in the shape of a Y-Junction, and mounted
on a laser scanning confocal microscope.
A suspension containing a fluorescently-labeled dye in the medium was introduced
through one branch of the Y-junction, while an identical suspension without the
dye was introduced through the other. The mixing of the dye as the two
suspensions flowed through the straight, downstream channel was monitored using
confocal microscopy. The experimental
results clearly demonstrate secondary convection of the dye. However, two
observations surprised us. First, the
mixing length was much shorter than the numerical predictions. Second, in some
cases, the solute travelled in a direction opposite to the predictions of
secondary current profile! The reasons for these unexpected results will be
discussed in the talk.
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