Size Effect on Adiabatic Gas-Liquid Two-Phase Flow Map and Void Fraction in Micro-Channels

Adiabatic gas-liquid two-phase flow patterns and void fractions in micro-channels were experimentally investigated. Using nitrogen gas and water, experiments were conducted in near-square micro-channels with hydraulic diameters of 0.209mm, 0.412mm and 0.622 mm, respectively. The main objective was focused on the effects of micro-scale channel sizes. Gas and liquid superficial velocities were varied from 0.06-72.3 m/s and 0.02-7.13 m/s, respectively. Four defined flow patterns including bubbly-slug flow, slug-ring flow, dispersed-churn flow and annular flow were observed in both micro-channels of 0.412 mm and, 0.622 mm in hydraulic diameter. In the micro-channel of 0.209 mm, the bubbly-slug flow turned into the slug-flow and the dispersed-churn flow was not observed. The current flow regime maps showed that the transition lines shifted towards higher gas superficial velocity due to the stronger surface tension effect as the channel size was reduced. The micro-channel flow regime maps were found to be quite different from those of mini-channels. Measured time-averaged void fractions in our micro-channels held a non-linear relationship with the homogeneous void fraction as oppose to the relatively linear trend for the mini-channels. A new correlation was proposed to predict the non-linear relationship that fits most of the experimental data of the current three micro-channels and those of the 0.1 mm diameter tube reported by Kawahara et al. within ± 15%.