Critical analysis of simplified physical models predicting rate of liquid entrainment in annular flow

In annular gas-liquid flow, liquid droplets are entrained from liquid film surface into the core of gas stream, which strongly affects the integral parameters of the flow. A popular physical model aimed at predicting the rate of entrainment was proposed by Holowach and coauthors in 2002 and reproduced in a number of later papers. Here we analyze the main assumptions of the model by comparing their outcome to the available experimental data on qualitative and quantitative levels. It is found that the model underestimates the wavelength of disturbance waves and typical time of entrainment by over 2 orders of magnitude. Smaller but still significant discrepancies are found for disturbance wave velocity and amplitude. The assumptions on longitudinal and transverse shape of disturbance waves as well as on entrainment mechanism do not agree with experimental observations on qualitative level. In particular, they ignore the fast ripples which travel over disturbance wave crests and get scattered into droplets. An alternative set of assumptions based on experimental studies of three-dimensional wave structure and entrainment process is proposed. The resulting formula for entrainment rate includes several parameters; predicting them requires novel systematic experiments and advanced theoretical analysis.