Entrained droplets in two-phase churn flow

The presence of droplets exerts a strong influence on liquid film flow rate, pressure drop, heat transfer and many other flow characteristics. Due to the complexity of churn flow, profound knowledge on the droplets in this flow pattern is not well documented in the existing literature. In the present paper, we study a gas-liquid two-phase churn flow in a 19-mm-inner-diameter pipe under atmospheric pressure and carefully design a shadow detection technique to capture the high-resolution images of the entrained droplets in churn flow. The images are subsequently binarized using an adaptive threshold. The results indicate that the amount of the entrained droplets is high in the chaotic churn flow regime and gradually decreases during the transition from churn flow to annular flow and finally reaches a minimum around the churn-annular flow transition. The detailed process of the droplet entrainment is discussed. Based on our experimental observations, it is concluded that the large droplets (chunks) are related to the breakdown of slugs and bag breakup mechanism, whereas the smaller droplets can be ascribed to the breakup of chunks, ligament breakup and impingement. A normalized size probability density function is implemented to analyze the effect of the entrainment mechanism on the range of droplet size. The Sauter diameter of the droplets in churn flow is compared with the existing empirical correlations in annular flow and a formula for the entrained droplet size in churn flow is proposed.