Annular flow of an inner gas core and an annular liquid film is one of the most important flow patterns in engineering application. A disturbance wave occurs at interface of liquid film and gas core

which wave frequency is a characteristic parameter for describing the disturbance wave at annular flow interface and for studying mass and heat transfer in annular flow. A near infrared (NIR) equipment was designed to measure liquid film thickness. Non-contact thickness measurement of partial liquid film in annular flow was achieved by vertical optical path and inserted light guide tube to cover one side of liquid film and liquid droplet in gas core. Then frequency of interface disturbance wave was estimated by power spectral density. Quantitative study on annular flow in 50-mm-diametered vertical tube obtained wave frequencies at 70 different flow conditions with pressure ranging from 0.1 to 0.8 MPa

liquid volume flux ranging from 1.2 to 2.5 m3·h−1

and gas volume flux of 90 and 110 m3·h−1. These experimental results were used to verify current models

and a St-Fr model was established to predict disturbance wave frequency from flow parameters

which the Strouhal number was to describe influence of liquid phase and the Froude number was to describe influence of gas phase. The mean absolute error (MAE) of the St-Fr model prediction to experimental results is 11.42%.