Upward two-phase flow patterns in vertical circular pipe under rolling condition

The experiments on the air-water two-phase flow patterns in a 24 mm diameter vertical circular pipe under both static and rolling conditions were conducted. The superficial velocities were in the range of 0.05–4.07 m/s and 0.02–20.55 m/s for water and air, respectively. Five typical flow pattern morphologies resulting from a rolling motion were observed: dispersed bubbles; bubbles; slug; churn; and annular flow. Flow pattern transitions were identified by analysis of visual images and a bubble chord-length probability distribution function (PDF). The static experimental data was used to validate existing flow pattern transition models that were then modified to fit rolling conditions by considering the additional forces caused by rolling motion. These models also agreed well with the experimental data. Investigation of the influence of the rolling motion on flow pattern transition showed that, as the rolling amplitude increases, the formation of dispersed bubble flow requires a higher superficial liquid velocity. It was also found that, the greater the rolling amplitude, the larger region of churn. The rolling period, however, has little impact on flow pattern transitions. When nuclear-powered ships sail in the sea, the rolling condition caused by wind, waves and ship's motion provoke changes in the thermal hydraulic characteristics of the reactors. The visual experiments on the air-water two-phase flow patterns in a 24 mm diameter vertical circular pipe under both static and rolling conditions were conducted. Flow pattern transition models under rolling conditions were developed. It is found that the bubbles aggregate along the upper wall in dispersed bubble and bubble flows; the Taylor bubbles swing during slug flow under rolling condition. As the rolling amplitude increases, the formation of dispersed bubble flow requires a higher superficial liquid velocity, whilst the transition from bubble to slug flow advances in relation to static conditions. [Display omitted] •Experimental studies of air-water flow patterns under rolling condition were carried out.•The Flow pattern transition models under rolling condition were put forward.•The transition model agreed well with experimental data.•Effects of rolling parameters on flow pattern transition boundary were investigated.