Effects of T-junction size on bubble generation and flow instability for two-phase flows in circular microchannels

A visualization experiment was conducted to detect the effects of the T-junction size on bubble generation and air–water two-phase flow instability in circular microchannels. Four test channels were prepared by connecting two glass capillaries (I.D.: 0.486 mm) in a connecting chamber, and the gaps between the capillaries (0.14–0.25 mm) were used as air injectors. The bubble generation and two-phase flow downstream of the T-junctions were visualized using a high-speed camera. The bubble and slug lengths and bubble generation frequencies were measured from the obtained images. We confirmed that the bubble length can be predicted by a correlation derived from a scaling analysis. However, the correlation was invalid when the flow rate ratio of the two phases was either extremely high or extremely low, or when the bubbles were generated from the smallest T-junction. These flows were defined as unstable or quasi-stable flow, and the transition mechanism to these flows were investigated by analyzing the fluctuations in the Laplace pressure and water phase during the bubble generation process. The time lag in the pressure changes between the Laplace pressure and the water phase destabilizes the flow, and the destabilization increases for small T-junctions and high water velocities.