Experimental study on entrained droplets in vertical two-phase churn and annular flows

Abstract

In this paper, the air-water vertical two-phase pipe flow experiment was carried out at room temperature and pressure to study the characteristics of entrained droplets in churn and annular flows with the high-speed camera and phase Doppler anemometry. The results show that there are five kinds of droplet entrainment modes in churn and annular flows, namely shearing-off of waves, bag break-up of waves, ligament break-up of waves, droplet impacting liquid film, and burst of bubbles. With the flow pattern transition from churn flow to annular flow, the dominant droplet entrainment changes from shearing-off of waves to ligament break-up of waves. The gas superficial velocity plays a decisive role in determining the characteristics of entrained droplets, while the effect of the liquid superficial velocity can be neglected. In churn flow, the shearing-off of waves produces a small amount of entrained droplets with low sphericity and initial velocity. Large droplets tend to deposit, and small droplets undergo a certain degree of fragmentation and coalescence during the accelerated migrating in the gas core. In annular flow, the ligament break-up of waves produces a large amount of entrained droplets with high sphericity and initial velocity. Droplet size presents continuous distribution. The size of droplets increases slightly due to the coalescence of small droplets during the accelerated migrating in the gas core. Compared with churn flow, the momentum exchange between the gas phase and droplets is greater in annular flow, which leads to a significant increase in droplet momentum. The size distribution of the entrained droplets determines the momentum exchange between the gas core and the entrained droplets to a certain extent.