INTERFACIAL MOMENTUM AND MASS TRANSFER IN TWO-PHASE FLOWS
Johns Hopkins University
The process by which a mixture of fluids of different phases transfer mass and momentum across a deformable interface constitutes one of the most basic multi-phase flow problems that occur in many natural and industrial applications. However, a lack of consistent experimental framework to resolve this complex interplay between two fluid phases at different length and time scales severely limits our understanding of this problem. This is in part due to the inadequacy of applying classical experimental facilities designed for single-phase flows directly to two-phase flows as well as the limit of many existing diagnostic systems. Therefore, the goal of this thesis is to provide an experimental framework that consists of two key components: experimental apparatus that can isolate the momentum and mass transfer between two phases and diagnostic systems that can probe these parameters. In addition, to cover different flow regimes, two types of multi-phase flows will be introduced and each one will come with its own apparatus and diagnostic system. The first part of this thesis focuses on the momentum transfer between gas bubbles and surrounding turbulence, which is an unclosed term in the two-fluid model. Most previous work assumes spherical bubbles with idealized drag, lift, and added mass forces, even though gas bubbles could be deformed by surrounding turbulence, exhibiting different momentum transfer between the two phases and ultimately modulating the macro-scale spatial distribution of bubbles and their mean rise velocity. By leveraging this inherent connection between the interfacial and macro-scale quantities, we develop a method to determine the drag and lift forces of bubbles in turbulence by measuring the bubble rise velocity, which shows a dramatic change in turbulence that was thought to be impossible. This dramatic change is later connected to the changes in both lift and drag forced modulated by turbulence-induced deformation. In addition, after determining the lift and drag coefficients of deformable bubbles, we have also successfully measured the added-mass force, which is an unsteady force that contributes significantly to the random motion of bubbles in turbulence. It is well known that this force is sensitive to the geometrical information, such as the shape and orientation of an object, in turbulence. Given the complexity of these two quantities in turbulence, this coefficient is often assumed to be impractical to measure. By following the same framework and constraining it using macro-scale bubble acceleration variance, we showed that the added mass coefficient should gradually drop as the bubble aspect ratio increases due to the preferential alignment of the slip acceleration between the two phases and the bubble major axis. The second multi-phase flow problem is in the opposite limit where the liquid-liquid two-phase flows mix together through Rayleigh-Darcy convective instability in a Hele-Shaw cell, which is motivated primarily by the geological sequestration of anthropogenic CO2. In particular, we use a surrogate system to investigate how the layered heterogeneity and anisotropy found in underground saline aquifers affect the mass transfer rate between the two phases, CO2 and brine. Due to the density and refractive-index mismatch between the two phases, the shadowgraph technique was used to obtain 2D quantitative measurements of the mixing efficiency. Surprisingly, a 10% reduction in bulk permeability resulted in as much as 80% reduction in the mixing efficiency. Based on the experimental results, a model was developed to predict this behavior of two-phase mass transfer on the properties of heterogeneity.
Two-phase flows, bubble dynamics, isotropic turbulence, carbon sequestration