Features of the liquid film in downward vertical air-water annular flow

Abstract

The two-phase gas-liquid annular flow pattern is characterized by a gaseous core flowing at high rates through the central region of a pipe and a thin, wavy liquid film wetting the pipe wall. The liquid film surface exhibits wave activity, with small amplitude waves known as ripples and large ones called disturbance waves. These structures form intermittent arrangements called unit waves, which repeat over time and define the liquid film thickness profile. Understanding and characterizing this flow is crucial for studying phase interactions, for developing theoretical models, and for improving existing ones. Additionally, such characterization informs equipment design, quality control and advancements in the remote monitoring process. While upward vertical annular flows have been extensively studied, research on downward vertical annular flows is relatively scarce. Many experiments use rigs with limited length-to-diameter ratios (L/D) and coaxial-type inlets that favor annular pattern formation. The type of phase inlet can significantly influence flow pattern structures and their evolution along the axial direction. In this context, experiments on downward vertical air-water annular flow were conducted in pipes with 26-mm and 50-mm Ids, 14-meters long each. The objective was to analyze and understand the main characteristics of the liquid film thickness, considering phase superficial velocities, pipe diameter, axial flow evolution, and phase inlet device type. Measurement tools included a non-intrusive conductance sensor, a wire-mesh sensor, and a high-speed camera. For each diameter, the downstream flow evolution was evaluated at two test sections, and 35 combinations of air and water superficial velocities ranging from 0 m/s to 20 m/s and from 0.05 m/s to 0.25 m/s, respectively, were examined. Analyses of the liquid film thickness time series provided quantitative features such as the average film thickness, liquid film roughness, velocity, frequency and geometrical characteristics of the disturbance waves. Individual identification of disturbance waves allowed the investigation of their velocity, frequency, distribution, and interaction. New correlations were proposed to estimate the unit wave parameters in downward vertical annular flow. These correlations, derived through data-driven methods, facilitated the generation of time series for the synthetic liquid film thickness. Comparative analyses between the experimental and synthetic data demonstrated satisfactory agreement, thence validating the proposed approach. Based on the measurements from the WMS, flow reconstructions were performed, confirming the analyses of the liquid film. Additionally, droplets flowing with the gas phase in thecentral region of the flow were identified.