| The regimes of two-phase cocurrent downflow through a single vertical millimeter-scale constricted passages are surrogates for the pore-level regimes in packed beds, as demonstrated in the experiments reported in this thesis. Flow regimes arise from the contention between liquid and gas (water and air in the experiments) for the constriction space. Flow regimes, and associated pressure losses, depend upon fluid flow rates, properties of the fluids, and the shape and size of the constricted passage. At low liquid flow rates the liquid descends as an annular film--in a well-wetted passage--and the gas occupies the core. At high liquid flow rates the annular film thickens, oscillates, then bridges and clogs the constriction. Gas flow inhibits bridging, then induces pulsating flow and bubbling. The position of the gas-liquid interface(s) identifies the different flow regimes which were visualized and recorded. Flow regimes and the transitions between them were mapped in flux coordinates.;Flow of both air and water through constricted passages is governed by the Navier-Stokes system which is complicated by the free surfaces present and the converging-diverging shape of the passage. Theoretical predictions of steady and transient two-phase axisymmetric annular flow were obtained by means of the Galerkin method with finite element basis functions. Predictions of interfacial positions matched those measured in the experiments. The influence of gas and liquid flow rates on the interfacial shape and on the total pressure loss along the passage was assessed.;Experiments in an 'almost two-dimensional' transparent-walled packed bed demonstrated that the bed-scale cocurrent downflow regimes, which are widely reported in the literature, are outcomes of local competitions for the pore space. Useful signatures of certain bed-scale regimes and the transitions between them were found in the power spectra of sound detected at the outlet of the model packed bed. |
