The entrainment mechanism and droplet size distribution in gas-liquid churn flow

Abstract

Knowledge of entrained liquid is important in calculation of pressure drop and in the determination of the film flow rate and thence its dryout and consequent deterioration in heat transfer in heating systems. Note that the excessive liquid entrainment may cause the liquid film completely removed from the wall (dryout), which could lead to disastrous accident. As one of the least understood gas-liquid flow regimes, churn flow appears a highly-disturbed flow of gas and liquid. According to the observation, churn flow is characterized by huge waves with flow reversal between the waves and the highly oscillatory liquid film, which is accompanied a continuous gas core with considered amount of entrained liquid. Although there have been enduring efforts to show that the entrained fraction is high in churn flow and reaches the minimum around the churn/annular transition, the underlying mechanisms of droplet entrainment in churn flow are still not well explored. Additionally, there is a dearth of data about entrained droplets in churn flow due to its chaotic nature. The measurement techniques which are suitable to determine entrained droplets in annular flow have limitations when applied to churn flow because they might not be able to distinguish between liquid carried as droplets and that transported in huge waves. In this study, we focused on the entrainment mechanism, liquid distribution and droplet size distribution under churn flow condition. The high-speed camera and the high-resolution camera were employed to capture a more detailed description of the huge wave breakup and the liquid distribution in the cross-section of the pipe with the inner diameter of 19 mm. In order to obtain the high-resolution images of entrained droplets, a delicate shadow detecting method was developed in the present paper, which provides a reliable approach to capture the entrained droplets with various sizes. Subsequently, Matlab was employed to analyze the complete series of pictures and recognize each of the blobs. The blobs were interpreted as droplets after the sharpness was satisfied. Every accepted edge datum is considered to be a droplet and thus its diameter can be obtained by Image J. The results indicate that there are three mechanisms for droplet generation in churn flow: liquid bridge breakup dominants at slug/churn transition, bag breakup plays a dominant role at low gas superficial velocities, but ligament breakup comes to gain greater importance with the increase of gas flow rate. Accordingly, the amount of liquid entrained 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 transition boundary. Detailed process of droplet entrainment was also provided. Based on observation, large droplets (chunks) are related to the breakdown of slugs and bag breakup mechanism, whereas smaller droplets can be ascribed to the breakup of chunks, ligament breakup. No doubt, the present paper helps understand the mechanism of the entrainment in churn flow, which is essential for the development of mechanistic models to predict the pressure drop and dryout condition.