Ph.D. Dissertation Defense by Muhammad K. Akbar
Thursday, November 11, 2004
(Dr. S. Mostafa Ghiaasiaan, Chair)
"Transport Phenomena in Complex Two and Three-Phase Flow Systems "
Two and three-phase flow processes involving gas, liquid and solid, are common
in nature and industry, and include some of the most complex and poorly-understood
transport problems. In this research hydrodynamics, heat and mass transfer
processes in complex two and three-phase flows, in particular flows involving
a particulate solid phase, were investigated.
The interfacial surface area concentration in a short vertical column subject to the through flow of a solid-liquid-gas slurry made by mixing aqueous fibrous paper pulp with a nitrogen-carbon dioxide gas mixture was experimentally measured. The gas absorption technique was applied, using CO2 as the transferred species and sodium hydroxide as the alkaline agent in water. The experimental data were statistically analyzed in order to elucidate the various parametric dependencies of the interfacial surface area concentration, and were empirically correlated.
The absorption of a gaseous species by a slurry droplet with internal circulation and containing reactive and sparingly soluble micro particles was numerically simulated. The problem is relevant to spray flue gas desulfurization systems, and the objective was to elucidate the effect of the reactive solid particles on the parameters that determine the mass transfer processes. It was shown that the reactant micro particles enhance the absorption rate primarily by increasing the gradient of the concentration of the absorbed species beneath the droplet surface. The absorption rate was sensitive to droplet recirculation strength, and shrinkage of reactive particles with time resulted in a declining total absorption rate.
The transport of soot particles, suspended in laminar hot gas flowing in a tube, was numerically modeled and parametrically studied, in order to assess the coupled effects of thermal radiation and thermophoresis on the transport of monodisperse, as well as polydisperse soot particles. It was shown that, as a consequence of strongly coupled thermal radiation and thermophoresis, a radially-nonuniform temperature profile develops, leading to a sharp, non-uniform radial soot concentration profiles. In comparison with the more realistic log-normally-distributed particles, the assumption of monodisperse particles lead to significant over prediction of the overall effect of thermophoresis.
The transport and removal of particles suspended in gaseous bubbles rising in a stagnant liquid pool were modeled based on a hybrid Eulerian – Monte Carlo method. The bubble hydrodynamics were treated in Eulerian frame, using the Volume-of-Fluid (VOF) technique for modeling the motion of the gas-liquid interphase. Equations of motion were numerically solved for a large number of particles in Lagrangian frame, accounting for sedimentation, Brownian, convection, and inertial effects. It was shown that the bubbles of interest undergo shape change, and have complex internal circulation patterns, all of which influence the removal of particles. Model predictions were also compared with available experimental data.
Using an important resemblance between two-phase flow in microchannels, and in large channels at microgravity, a simple Weber number-based two-phase flow regime map was developed for microchannels, and was shown to predict the available experimental data. The stability of gas-liquid stratified flow regime in horizontal annular channels was also investigated. Based on the available air-water experimental data, a criterion for the prediction of conditions that lead to flow regime transition out of the stratified wavy flow pattern was proposed.