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Abstract
Evolution of thin liquid films sheared by co-current gas stream in a vertical 11.7 mm pipe is studied experimentally using BBLIF technique. The main goal is to investigate the transition of wave patterns due to increase in the gas stream velocity, VG, from 0 to 24 m/s. Apart from relatively weak quantitative changes in the characteristics of the primary waves, replacement of capillary precursor by slow secondary waves is found. The transition is indentified in the range of VG = 8 - 16 m/s for all liquid flow rates. It is observed that the appearing secondary waves may be the main reason of the decay of the capillary precursor. The experimental results are compared to prediction of evolutionary theoretical model, showing qualitative agreement on secondary waves generation, but with no agreement on precursor's disappearance. Introducing artificial perturbations mimicking the action of turbulent pulsations in the gas phase is recommended to improve the model.
Highlights
Joint flow of liquid film along pipe walls and high-speed gas stream is referred to as annular flow
At large liquid flow rates this flow is strongly complicated by formation of large-scale disturbance waves and entrainment of liquid droplets from the film surface
Even at low liquid flow rates, in absence of entrainment, two kinds of waves appear on film surface: fast long-living primary waves with unstable rear slopes generating slow shortliving secondary waves [1]
Summary
Joint flow of liquid film along pipe walls and high-speed gas stream is referred to as annular flow. Even at low liquid flow rates, in absence of entrainment, two kinds of waves appear on film surface: fast long-living primary waves with unstable rear slopes generating slow shortliving secondary waves [1] This picture is different to the case of liquid films flowing under action of gravity in absence of gas stream: in this latter case short-length capillary precursors exist in front of the nonlinear waves, whilst no secondary waves are observed. Further stages of downstream development of non-linear waves are usually modeled in frames of pseudostationary approach in the reference system of a moving nonlinear wave, which prevents generation of smaller wavelets with different propagation speed This issue can be solved by application of evolutionary approach, such as in a recent paper [3], which showed that secondary waves may appear in the modeled film thickness traces. The present paper is a continuation of this work, aimed at systematic study of the wave pattern transition with gradual increase in the gas velocity
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