Slug formation and frequency of slugging in gas-liquid flows

When gas and liquid flow concurrently in a pipe, a common flow pattern is slug flow, characterized by the presence of intermittent, aerated slugs of liquid that translate downstream at the local gas velocity. The prediction of flow conditions at which slugs flow are observed and the prediction of the frequency of slugging are a critical technological problems which have not been fully resolved. This thesis examines this problem by documenting how slugs are formed in horizontal and near--horizontal pipes. Experiments were conducted with air-water flow in a 0.0763 m pipeline at atmospheric pressure. Measurements of the variation of the interfacial displacement were made simultaneously at a number of locations in a 23 m pipeline. These yield information about how slugs evolve and how the liquid layer rebuilds after the formation of a slug. Results are obtained for horizontal flows ({dollar}theta = 90.0spcirc{dollar}) and for downwardly inclined flows ({dollar}theta = 89.8spcirc , 89.5spcirc ,{dollar} and {dollar}89.2spcirc{dollar}).; At low gas velocities, slugs evolve from small wavelength (16 to 20 cm), large amplitude waves in horizontal flows. The growth of this wave evolves into a slug when it reaches the top of the pipe. These waves obtain their energy from smaller wavelength waves (8 to 10 cm) through a non-linear growth mechanism. At high gas velocities, the liquid height is not thick enough for these small wavelength waves to reach the top of the pipe. Rather, the wave crests grow until they break, forming roll waves. Slugs are observed to evolve from the coalescence of roll waves at these flow conditions.; Surprisingly, the large amplitude small wavelength waves observed at the transition to slug flow at low gas velocities for horizontal flows are damped for inclined flows. The transition to slug flow for slightly inclined pipes is associated with long wavelength, small amplitude waves. A viscous long wavelength linear stability analysis predicts the appearance of these wave. A local Kelvin Helmholtz instability at the crest of a growing long wavelength wave is observed when a slug forms. At high gas velocities, the conditions at the onset to slug flow for inclined flows are similar to the horizontal case.; When a slug forms, the liquid level in a large length of pipe drops as the slug propagates downstream. Thus two time scales enter the slug frequency analysis: one characterizing wave evolution and one involving liquid buildup. When the Froude number of the stratified flow in a horizontal pipe at the onset of slug flow is less than unity, the frequency is related to the motion of upstream propagating gravity waves associated with the formation of slugs. A stochastic model is used to describe the frequency for supercritical flow conditions for which slugs form from roll wave coalescence far from the inlet, and also for conditions for which slugs are forming very close to the inlet. For stratified downflows in inclined pipes, the flow is supercritical. However, the frequency of slugging is observed to be periodic and equal to the frequency of long wavelength waves that are predicted by linear stability.