Void fraction and pressure drop in gas-liquid downflow through narrow vertical conduits-experiments and analysis

•Experiments & flow pattern specific analysis for void fraction and pressure drop.•Droplet-laden gas core surrounded by annular liquid film analysed in annular flow.•Study useful for design of meso-scale devices in downflow orientation.

The present paper investigates void fraction and pressure drop characteristics of vertical air-water downflow through millichannels (0.83 ≤ Eotvos Number ≤ 20.6). Experiments have revealed the hydrodynamics to be a function of tube diameter and phase velocities with the functional form being different for different flow patterns. Accordingly, separate mechanistic models have been proposed to predict the aforementioned parameters for bubbly, slug, falling film and annular flow distributions. The proposed analysis have been validated with experimental results of the present study and those reported in literature.