Three-dimensional visualization of viscous fingering for non-Newtonian fluids with chemical reactions that change viscosity

Viscous fingering (VF) is a hydrodynamic flow instability that occurs when a less-viscous liquid (LVL) displaces a more-viscous liquid (MVL) in porous media and takes place in either miscible or immiscible systems. In this study, we investigated the three-dimensional (3D) characteristics of VF for miscible non-Newtonian fluids with and without chemical reactions using a microfocus x-ray computed tomography scanner. When the viscosity of a system increased with a chemical reaction, the VF was suppressed, and pistonlike displacement occurred. The cylindrical domain of displacing LVL was surrounded by a high-viscosity filmlike layer because of the chemical reaction that impeded the mixing of the LVL and MVL. Consequently, the high LVL concentration inside this layer was retained without mixing with the MVL. A clear local peak in viscosity appeared in the viscosity profiles at the interface between the LVL and MVL. Behind the peak, an unfavorable viscosity profile of increasing viscosity along the flow direction appeared, but the VF was not induced because of the smooth viscosity gradients. After the peak, the viscosity profiles were in favorable conditions. In the nonreactive case, the tip of the fingers further advanced compared with that in the reactive case and went further with the decrease in Pe, which reflected the property of shear-thinning fluids. By contrast, in a decreasing-viscosity system with a chemical reaction, the tips of the fingers were located at a larger distance compared with that without reaction. The concentration in the finger intensively decreased from the point of injection because of the intensive mixing of the LVL and MVL. As a result, the area fraction of injected LVL in the reactive cases was lower than that in the nonreactive cases. The tip of the finger advanced with time, whereas LVL concentration in the finger hardly recovered above 0.4 in the reactive case, which reflected intensive fingering and mixing.