Experimental investigation on the distribution characteristics of droplets in annular flow with wide liquid viscosity range

The existence of droplets in an annular flow has a non-negligible influence on the mass transfer and heat transfer between phases, and a phase property, such as liquid viscosity, can dominate the droplet characteristics. However, the effects of liquid viscosity on droplets have not yet received sufficient attention. In this study, the distribution characteristics of droplets in a vertical annular flow with a liquid viscosity of 1-30 mPa.s are experimentally investigated using a particle dynamic analyzer. The results reveal that the diameter and momentum of the droplet undergo two different changes during the increase of liquid viscosity, mu L. In the low-liquid viscosity range (mu L < 10 mPa.s), the diameter and momentum of the droplet increase significantly with an increase in viscosity, whereas in the high-liquid-viscosity range (mu L >= 10 mPa.s), their dependence on viscosity is considerably low. Owing to the different degrees of aggregation and fragmentation of droplets under different liquid viscosities, the radial evolution of droplets also show obvious distinctions. In addition, the slip ratio of the droplet velocity under different liquid viscosities is determined. When the liquid viscosity increases from 1 to 30 mPa.s, the droplet velocity decreases by 20.3% on average in the gas velocity range of 25-58 m/s, and small droplets have a relatively higher velocity and velocity range. Further, on the experiments show that the droplet deformation increases sharply with an increase in liquid viscosity because of the change in droplet generation mechanisms. The minimum proportion of the spherical droplet reduces to 9.2% at mu L= 30 mPa.s, indicating that the droplet cannot be simply regarded as spherical when calculating the mass and heat transfer with high liquid viscosity. Finally, the present results can assist in multiphase flow modeling under high-viscosity conditions.