Analysis of air–water flow pattern in parallel microchannels: A visualization study

• For the same time step, single-phase regimes were simultaneously obtained together with two-phase flow patterns at each microchannel.
• The variety of single-phase and two-phase flow configurations was also observed for different time steps at the same microchannel.
• For gas quality over 0.08 and mass velocity over 100 kg/m2 s, there is a clear predominance of the annular flow pattern.

Air–water adiabatic upward flows were measured in seven parallel microchannels. The total mass velocity was adjusted to 50, 100, 200 and 300 kg/m2 s. Superficial air velocities ranging from 0.16 to 41.70 m/s were applied for each mass velocity. The superficial water velocity ranged from 0.005 to 0.30 m/s in order to maintain the mass velocity constant. A gas quality of up to 0.90 was obtained. The visualizations showed different flow configurations in each microchannel, although approximately homogeneous conditions were verified in the inlet manifold for single-phase flows. For the same time step, single-phase regimes (only liquid or only gas) were simultaneously obtained together with two-phase flow patterns (annular, bubbly, slug and churn, and their transitions) at each microchannel. The variety of single-phase and two-phase flow configurations was also observed for different time steps at the same microchannel. The Mishima and Ishii (1984) flow pattern map for single microchannels was reassessed with parallel flows. For quality over 0.08 and mass velocity over 100 kg/m2 s annular flow patterns are predominant. This corresponds approximately to superficial gas and liquid velocities over 10 and 0.1 m/s, respectively, at the inlet manifold. For other inlet conditions there are miscellaneous flow configurations. Similarities to results of Niño (2002) indicate that the current analysis can be extended to air–water horizontal flows or to microchannels with other cross-sectional geometries if the hydraulic diameter is kept at around 1 mm.