Origin and hierarchy of meso-scale structures in two-phase flows. Dispersed multiphase flows, frequently encountered in chemical reactors and separation devices, are almost always unstable, and complex spatio-temporal patterns are observed ubiquitously. One can usually observe structures at different length scales and time scales - micro, meso and macro scales - and they influence the mixing, mass transfer and heat transfer processes. Over the past decade, we have uncovered the origin and hierarchy of instabilities leading to formation of various meso-scale structures in:
- gas-liquid flows in trickle beds,
- fluid-solid flows in gas- and liquid-fluidized beds,
- granular flows, and
- dilute gas-solid flows in vertical pipes.
Coarsened equations of motion for two-phase flows. The closure relations for the averaged equations of motion for dispersed two-phase flows, which are available in the literature, are usually valid for nearly homogeneous suspensions. Analysis of the averaged equations with these closures reveals that, in gas-particle flows and buoyant rise of bubbly suspensions, the dominant instability through which an uniform state breaks into meso-scale structures occurs at a length scale of the order or 10 - 50 particle (or bubble) diameter. This severely limits the value of the averaged equations as a tool for studying flows in large process vessels, as the required spatial resolution renders the calculations far beyond what is feasible today. With this in mind, we are developing coarsened equations of motion, where one has not only averaged over the flow patterns at the level of the individual particles, but also the meso-scale structures. This is necessary as our recent work has demonstrated that the fluctuations associated with the meso-scale structures can dwarf those at the particle level. Through detailed computational experiments, we are gathering information on the statistical and scaling properties of the meso-scale structures. These will be used to devise meso-scale structural models for coarse simulations of two-phase flows.
Contact stresses in granular assemblies. Flow of dense assemblies of granular materials is encountered in many devices used to handle and mix particles, and in fluidized beds. Frictional stresses transmitted through sustained contact of particles with multiple neighbors play an important role in the flow behavior obtained in such devices. For example, a stable operation of a circulating fluidized bed is possible only when the frictional contact between the particle assembly and the walls of the standpipe can provide a stabilizing effect. Through fluidization and defluidization experiments performed in tubes of different diameters and with glass beads of various diameters (63 - 150 µm), we have isolated the effect of wall friction and exposed the role of contact stresses in imparting stability to fluidized suspensions. This study has raised new questions about mechanism controlling the onset of bubbling in fluidized beds which we are currently pursuing. We are also extending this study to finer particles to investigate how cohesive interactions between particles modify the contact stresses.
Role of static electrification on gas-particle flows. Lateral segregation of particles occurs readily in vertical gas-particle flows and its origin has been a subject of much research. We identified a few years ago a hydrodynamic segregation mechanism arising from the density and velocity fluctuations, which are inherent in gas-particle flows. We have recently recognized that a modest level of electrostatic charges on the particles is enough to cause appreciable lateral segregation. We are now engaged in a study of electrostatic charges on particles and their effects on fluidization and pneumatic transport of particles.