Visualization and pressure drop measurements of water–air two-phase flow in circular microchannels (d = 486 μm) was conducted. In order to investigate effects of the flow rates and T-junction size on the pressure drop of the two-phase slug flow, three test channels containing various T-junctions (136, 194, and 252 μm) were prepared. The measured pressure drop was compared with the results from semi-empirical model and theoretical model, and it was found that the experimental data generally agreed with the theoretical model. However, the pressure drop increased as the T-junction size decreased. In order to detect the causes of this increase in pressure drop, effects of the velocity fluctuation on the pressure drop were investigated. The velocity fluctuations were measured from the recorded images that were obtained by a high-speed camera. Although it was found that the instantaneous velocity fluctuated in large amplitudes and its cycle was synchronized with a period of the bubble pinch-off, the effects of the velocity fluctuation were negligible on the pressure drop. Finally, from a scaling analysis, it was suggested that the bubble overpressure was the cause of the increase in pressure drop.

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