Two-Phase Flow Patterns in Converging or Diverging Microchannels With CO₂ Bubbles Produced by Chemical Reactions

The present study investigates experimentally the evolution of two-phase flow pattern in converging and diverging, silicon-based microchannels with mean hydraulic diameter of 128 μm with CO₂ bubbles produced by chemical reactions of sulfuric acid (H₂SO₄) and sodium bicarbonate (NaHCO₃). The microchannels are prepared by bulk micromachining and anodic bonding. Three different concentrations of 0.2, 0.5 and 0.8 mol/L of each reactant at the inlet before mixing and 10 different flow rates from 1.67×10⁻⁹ m³/s to 16.7×10⁻⁹ m³/s are studied. Flow visualization is made possible by using a high-speed digital camera. It is found that the acceleration effect in a converging microchannel tends to diminish chemical reactions and no bubbles are formed at low concentrations and high flow rates, while large spherical bubbles are generated in the regions near the inlet and slug flow is formed in the regions near the exit for high concentrations and low flow rates. On the other hand, the deceleration effect in a diverging microchannel tends to promote chemical reactions, especially in the regions near the exit, and many bubbles are produced in those regions. Slug flow with large bubble slugs tends to appear in most parts of the diverging microchannel for low concentrations. Merger of bubbles appears frequently in the diverging microchannel. The results demonstrate the strong effects of concentration and acceleration or deceleration on the evolution of two-phase flow pattern in a converging or diverging microchannel.