Gas-liquid two-phase flow in microchannels - Part II: void fraction and pressure drop

Void fraction and two-phase frictional pressure drop in microchannels were experimentally investigated. Using air and water, experiments were conducted in transparent circular microchannels with 1.1 and 1.45 mm inner diameters and in microchannels with semi-triangular (triangular with one corner smoothed) cross-sections with hydraulic diameters 1.09 and 1.49 mm. Gas and liquid superficial velocities were varied in the 0.02-80 m/s and 0.02-8 m/s ranges, respectively, and void fractions were calculated by analyzing photographs taken from the test sections with circular cross-section. Measured void fractions were compared with several correlations. The homogeneous flow model provided the best prediction of the experimental void fractions in bubbly and slug flow patterns. The homogeneous flow model and all other tested empirical correlations significantly over predicted the void fractions in annular flow pattern, however. A one-dimensional model, based on the numerical solution of mass and momentum conservation equations was applied for the calculation of test section pressure drops, using various two-phase friction models. For bubbly and slug flow patterns, the two-phase friction factor based on homogeneous mixture assumption provided the best agreement with experimental data. For annular flow the homogeneous mixture model and other widely used correlations significantly over predicted the frictional pressure drop. (C) 1999 Elsevier Science Ltd. All rights reserved.