Experimental study on bubble and droplet entrainment in vertical churn and annular flows and their relationship

Abstract

Bubble and droplet entrainment plays an important role in the mass transfer between the liquid film and the gas core in churn and annular flow. In this work, bubble and droplet entrainments in vertical churn and annular flows are systematically studied in an air-water two-phase vertical flow experiment using a high-speed camera. The results show that there are three kinds of bubble entrainment and five kinds of droplet entrainment in churn and annular flows. The entrainment can be categorized as primary entrainment and secondary entrainment. The primary entrainment includes one kind of bubble entrainment, which is a large wave roll and overturn, and three kinds of droplet entrainment, which are shearing-off of waves containing bubbles, bag break-up of waves, and ligament break-up of waves. The secondary entrainment includes two kinds of bubble entrainment, which are synchronized bubble entrainment along with droplet entrainment and droplets from the gas core impacting the liquid film, and two kinds of droplet entrainment, which are droplets impacting the liquid film and the burst of bubbles. With the flow pattern transition from churn to annular flow, the dominant bubble entrainment changes from large wave roll and overturn to the synchronized bubble entrainment along with droplet entrainment accompanied by a decreasing intensity. The dominant droplet entrainment changes from shearing-off of waves containing bubbles to ligament break-up of waves with the intensity first decreasing and then increasing. There is a close relationship between bubble entrainment and droplet entrainment. The entrained bubbles reduce the strength of large-scale waves and facilitate the shearing-off of waves. A small number of droplets are entrained due to the burst of residual bubbles in the liquid film. Droplets that are generated by bag or ligament break-up of waves can directly impact the nearby liquid film, leading to synchronous bubble entrainment. A small number of bubbles can be entrapped by droplets from the gas core impacting the liquid film. The primary entrainment intensity is significantly greater than that of the secondary entrainment. With the transition from churn to annular flow, the change in the primary entrainment intensity determines the variety of the secondary entrainment intensity.