Slug front gas entrainment in gas-liquid two-phase flow in horizontal and near horizontal pipes
Creators: name: Abdullahi, Mohammed Kabir
Publisher: name: Imperial College London
Year: 2013

description: This thesis presents experimental studies relating to gas-liquid slug flows in horizontal and near-horizontal pipelines. In such flows, liquid-continuous regions (“slugs”) pass along the pipe separated by regions of stratified flow (“elongated bubbles”) (see Chapter 1). Gas may be entrained at the slug fronts and is subsequently discharged into the elongated bubble at the slug tail. The focus of the work described in this thesis is on this gas entrainment process. Specifically, the aim is to make objective measurements of the slug front gas entrainment rate. A review of the earlier work (see Chapter 2) showed a large diversity of results for slug front gas entrainment. This reflects the difficulty of doing objective experiments on this important parameter. In the work described here, two new techniques for determining gas entrainment rate were implemented and investigated, namely the three-way valve method and the bubble shrinkage method. All of the experiments described in this thesis were with air-water flows at near atmospheric pressure, though the techniques developed could be applied to other fluid pairs and to different pressures. The three-way valve method involves diverting a slug front to pass over a stratified liquid layer and to determine the gas pick-up rate at the slug front from the slug front propagation velocity. The experiments were carried out for a 32.8 mm pipe in the LOTUS facility (described in Chapter 3) and for a 78 mm pipe using the WASP facility (also described in Chapter 3). The experimental results are presented in Chapters 4 and 5 respectively and show that the gas entrainment rate per unit film width is similar for the two diameters and depends on the relative velocity between the slug front and the liquid film ahead of it. The results indicate entrainment rates which are within the range previously measured but which are above those calculated from the existing correlations. A new correlation for prediction of gas entrainment in slug flow in horizontal pipes was developed which adequately predicts previous gas entrainment work (repeated here using LOTUS facility). In the bubble shrinkage method, the aim is to measure the shrinkage rate (and hence the rate of loss of gas by entrainment) of an elongated bubble passing along a pipe in a liquid flow. Development work on this method is described in Chapter 6 and (though the test section lengths were not sufficient to measure shrinkage accurately), it could be deduced (from measurements of the slug front (bubble tail) velocity) that the results were consistent with the three-way valve experiments. The actual phenomena accompanying gas entrainment at the slug front in slug flow are extremely complex and this was demonstrated with high speed video photography using an Olympus i-SPEED 3 system (see Chapter 7). Depending on the actual conditions, the advancing slug front could be attached to or detached from the preceding liquid layer. There were significant differences in the phenomena observed depending on the nature of the preceding liquid layer (stratified flow, descending liquid film, and stationary liquid film).