Experimental study of void fraction and its distribution characteristics in vertical narrow rectangular channels

Two-phase flow in narrow rectangular channels is of continued interest in multiphase flow research because of its importance in compact heat transfer applications. Gas-liquid flow in these geometries exhibits complex interfacial behavior and flow-regime transitions driven by the interactions of inertial, viscous, and capillary forces, which strongly affect heat and mass transfer performance. In this study, visualization experiments using high-speed photography were conducted to investigate vertical upward gas–liquid two-phase flow in narrow rectangular channels with cross-sectional dimensions of 15 × 2 mm 2 and 50 × 2 mm 2 . A MATLAB-based image-processing procedure was used to identify flow patterns and extract two-phase flow parameters. Four distinct flow patterns were identified, and the corresponding local void fraction distributions and probability density functions (PDFs) of instantaneous cross-sectional void fraction were obtained. The results showed that the void fraction increased consistently from the wall toward the channel center. At a fixed liquid flow velocity, increasing the gas flow velocity shifted the PDF from a sharp peak at low void fraction to a broader distribution at higher void fraction, with the dominant peak gradually evolving into a bimodal distribution. A modified drift-flux model was developed to predict the time-averaged cross-sectional void fraction, and the predicted values agreed well with the experimental data, with errors within 10 % for both channels.