Abstract
The results of an experimental study on droplet impactions in the flow of a gas-sheared liquid film are presented. In contrast to most similar studies, the impacting droplets were entrained from film surface by the gas stream. The measurements provide film thickness data, resolved in both longitudinal and transverse coordinates and in time together with the images of droplets above the interface and images of gas bubbles entrapped by liquid film. The parameters of impacting droplets were measured together with the local liquid film thickness. Two main scenarios of droplet-film interaction, based on type of film perturbation, are identified; the parameter identifying which scenario occurs is identified as the angle of impingement. At large angles an asymmetric crater appears on film surface; at shallow angles a long, narrow furrow appears. The most significant difference between the two scenarios is related to possible impact outcome: craters may lead to creation secondary droplets, whereas furrows are accompanied by entrapment of gas bubbles into the liquid film. In addition, occurrence of partial survival of impacting droplet is reported.
Highlights
In annular gas-liquid flow, the liquid flows as a thin film along channel walls carried downstream by a high-velocity gas stream in the centre of the channel
These were recently confirmed by Pham et al (2014), who used a rod bundle geometry, and Cherdantsev et al (2014), utilizing a horizontal rectangular cross-section channel; in the latter case, the break-up types were related to the three-dimensional shape and layout of the fast ripples
The detection process is much more complex compared to studying impacts of artificially introduced droplets, since the impacts of naturally entrained droplets may occur in any point of the Region of Interest (RoI) and at any moment of time
Summary
In annular gas-liquid flow, the liquid flows as a thin film along channel walls carried downstream by a high-velocity gas stream in the centre of the channel. Disturbance waves are the longest and the highest waves in the wavy structure of liquid film They travel with high speed and carry the major fraction of liquid. They are separated by the areas of thin residual layer often referred to as the base film which is covered by slow ripples. Azzopardi (1983) identified two main types of break-up events in vertical pipes: bag break-up and ligament break-up These were recently confirmed by Pham et al (2014), who used a rod bundle geometry, and Cherdantsev et al (2014), utilizing a horizontal rectangular cross-section channel; in the latter case, the break-up types were related to the three-dimensional shape and layout of the fast ripples. This fraction grows with both gas and liquid flow rates and can be close to unity (Pan and Hanratty, 2002; Sawant et al, 2008; Cioncolini and Thome, 2010)
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