Droplet Size and Velocity Measurement in Annular Flow by Fiber Optical Reflectometer

The characteristics of droplet in annular flow play an important role in the study of heat and momentum transfer between gas and liquid phases. A novel fiber optical reflectometer (FOR) technique is developed for the measurement of droplet size and velocity in horizontal annular flow. The experiment is conducted under 32 conditions in a duct with an inner diameter of 50 mm, with the superficial gas velocity ranging from 15 to 24 m/s, and the superficial liquid velocity ranging from 0.04 to 0.35 m/s, which expands the range of liquid velocities compared to previous reports. First, the differential method and wavelet synchrosqueezed transform are proposed to ensure accurate identification of droplet parameters. Subsequently, the local droplet frequency, droplet fraction, droplet size, and velocity in annular flow are measured by FOR technique. The results indicate that the entrainment of liquid droplets increases significantly with the increase of gas and liquid flow rates. For small droplets in annular flow, they tend to move at higher velocities and have a wide range of velocity distribution, while larger droplets always have narrower and lower velocity distribution. Simultaneously, the distributions of droplet size and velocity depend on the gas and liquid velocities. Moreover, four typical empirical correlations are used to evaluate the measured droplet Sauter mean diameters, and a new correlation is proposed to predict droplet size in the center of annular flow.