Computational Fluid Dynamics Simulations of Two-phase Plug Flow in Microfluidics
Creators: Virappane, Santhosh
Publisher: name: University of Alberta Library
Year: 2024

<p>Primarily driven by applications in Carbon Capture and Sequestration (CCS), but significantly relevant in biotechnology, drug delivery, and food processing, this thesis numerically resolved the local flow dynamics and mass exchange mechanisms in segmented immiscible and miscible CO2 emulsions transported by water in a rectangular microchannel.</p>

<p>We elucidated the flow fields of low Capillary number segmented Taylor flows of immiscible emulsions/bubbles in a low-aspect-ratio microchannel (Chapter 4). We conducted high-resolution two- and three-dimensional (2D, 3D) numerical simulations using a volume-of-fluid (VOF) two-phase flow solver and validated its accuracy compared to experimental data. Our results showed that 3D simulations were necessary to accurately capture the dynamics of liquid and supercritical CO2 emulsions. The 3D simulation results also unveiled diverse patterns of spanwise vortices, which are overlooked in 2D simulations. Calculating the Q-criterion in 3D revealed that vortices with relatively higher vorticity magnitudes were adjacent to the sidewalls, and the strongest ones emerged across the microchannel in the third dimension. More specifically, gaseous CO2 bubbles displayed relatively intense vortex patterns near the interfacial region of the bubble body and the cap due to the influence of the surrounding water flow. At higher Ca, liquid and supercritical CO2 emulsions exhibited similar flow dynamics, however, with prominent vortex patterns occurring in the upstream cap region. These findings pinpointed specific areas within the emulsions to pay attention to enhance the stabilization or exchanging mechanisms for low-Ca Taylor flow of emulsions.</p>

<p>We further elucidated the flow fields and transient mass exchange dynamics in the case of miscible gaseous, liquid, and supercritical (SC) state CO2 Taylor emulsions/bubbles transported by water in a low-aspect-ratio rectangular microchannel (Chapter 5). Our results revealed an asymmetry in mass transfer, initiating primarily at the emulsion/bubble&#39;s front cap across all cases investigated. We illustrated different mass transfer mechanism between the gaseous, liquid, and SC cases, with the gaseous case displaying an additional transfer at the tip regions. The study illustrated the flow patterns within the emulsions/bubbles and surrounding water in both streamwise and spanwise directions. At the front water slug region, we denoted a transport of the dissolved species toward the center, while at the rear, the solute move toward the walls. Within the CO2 bubbles, the streamlines displayed characteristic recirculation patterns, likely increasing the relative transfer rates of CO2 in water gaseous state. Our simulations also illustrated a cross-talk mechanism appearing across all cases, often neglected in the current literature. Through a local analysis of the interfacial mass transfer, we elucidated the advective dominance in the emulsion/bubble cap regions comparatively to the thin film for all cases. Our global analysis of the mass transfer revealed the preponderance of convection effect, especially in dense phases. This final result suggested a higher volumetric mass transfer coefficient for denser CO2 phases underlining the significance of CCS applications, where in subsurface conditions, CO2 is in SC state.</p>

<p>In summary, in this thesis we demonstrated the importance of using high-resolved 3D simulations to complement experimental approaches, and retrieve key local insights about the flow dynamics and mass exchange process of CO2 Taylor flow in microchannels.</p>