Nanocatalytic chemohydrodynamic instability: Deposition effects.

Due to the high surface area to volume ratio of nanoparticles, nanocatalytic reactive flows are widely utilized in various applications, such as water purification, fuel cell, energy storage, and biodiesel production. The implementation of nanocatalysts in porous media flow, such as oil recovery and contaminant transport in soil, can trigger or modify the interfacial instabilities called viscous fingering. These instabilities grow at the interface of the fluids when a less viscous fluid displaces a high viscous one in porous media. Here the flow dynamics and the total amount of chemical product are investigated when two reactive miscible fluids meet in a porous medium while undergoing A+B+n → C+n reaction. Nanocatalysts (n) are dispersed in the displacing fluid and deposited gradually with time. Four generic regimes are observed over time as a result of the particle deposition: (1) the initial diffusive regime, where the flow is stable with decreasing production rate, (2) the mixing-dominant fingering regime, where the flow is unstable and the production rate generally increases, (3) the transition regime, where the production rate generally decreases regardless of whether the system is stable or unstable, and (4) the final zero-production regime, where the product diffuses and fades away in the channel. Although the general trend shows a decreasing reaction rate with nanocatalysts deposition, there is a period in which the production rate increases due to the moderate deposition rates. Such an increase of production, however, is not observed in two groups: first, those systems in which the nanocatalysts do not change the viscosity of the base fluid and, second, a subgroup of the systems that are stable before and after the reaction in the absence of deposition.

Scaling and unified characterization of flow instabilities in layered heterogeneous porous media.

The physics of miscible flow displacements with unfavorable mobility ratios through horizontal layered heterogeneous media is investigated. The flow model is solved numerically, and the effects of various physical parameters such as the injection velocity, diffusion, viscosity, and the heterogeneity length scale and variance are examined. The flow instability is characterized qualitatively through concentration contours as well as quantitatively through the mixing zone length and the breakthrough time. This characterization allowed us to identify four distinct regimes that govern the flow displacement. Furthermore, a scaling of the model resulted in generalized curves of the mixing zone length for any flow scenario in which the first three regimes of diffusion, channeling, and lateral dispersion superpose into a single unifying curve and allowed us to clearly identify the onset of the fourth regime. A critical effective Péclet number w_{c} based on the layers' width is proposed to identify flows where heterogeneity effects are expected to be important and those where the flow can be safely treated as homogeneous. A similar scaling of the breakthrough time was obtained and allowed us to identify two optimal effective Péclet numbers w_{opt} that result in the longest and shortest breakthrough times for any flow displacement.

Stability of double-diffusive double-convective miscible displacements in porous media.

The present study offers a paradigm on the stability of two-component miscible displacements in a homogeneous porous medium. The components have, in general, different mobility ratios, may diffuse at different rates, and are convected at different speeds. As a result, one of the components may lag behind the other. For the adopted transport models, it is found that the differences in the rates of diffusion and the lag between the two component fronts resulting from the differences in the speeds of convection can modify radically the instability characteristics. In particular, an unstable single-component displacement is always made more unstable by the presence of a second unfavorable component. The instability of the same flow is, on the other hand, attenuated by the presence of a favorable, less diffusive lagging component. However, this same flow instability can actually be enhanced when the favorable lagging second component is more diffusive. Furthermore, the larger the lag between the two components is, the more unstable the flow is when the lagging component is favorable to the displacement. An opposite trend is found when the the lagging component is unfavorable to the displacement. Finally, changes due to the lag between the two components fronts are stronger for a less diffusive lagging component and weaker for a more diffusive one. For illustration, these results are discussed in the special context of a thermal displacement where mass and heat are transported in the porous media.

Hydrodynamic instability in the transport of miscible reactive slices through porous media.

Miscible displacement of a slice of a pollutant solution by a carrier solution in homogeneous porous media is examined. The carrier solution reacts with the slice solution to generate a chemical product, and as a result of differences in viscosities of the three species, a hydrodynamic instability known as viscous fingering is observed. The dynamics of the instability and the rate of consumption as well as spread of the pollutant are examined through numerical simulations. The study shows that the rate of consumption of the pollutant is the highest when the chemical product is the most or the least viscous solution in the system. It was also found that displacements in which the pollutant viscosity is the smallest or the largest of all three species lead to the widest spread of the pollutant in the porous media. In addition, the most complex finger structures are observed when the carrier solution has the smallest or largest viscosity in the flow. Furthermore, a mechanism of channeling whereby the carrier is able to break through the slice, therefore bypassing the pollutant, is found in cases where the chemical product is more viscous than the carrier solution. The dynamics of the displacement are analyzed and physical interpretations of their development are presented.