In the present study, water-oil flow patterns and slug hydrodynamics were experimentally studied in square glass microchannels with various hydraulic diameters (Dh = 600 μm, 400 μm, 200 μm). The aqueous phase is the continuous phase while the organic phase is the dispersed phase. The ranges of flow rates of the continuous phase and the dispersed phase are 0–200 ml/h and 0–12 ml/h, 0–120 ml/h and 0–6 ml/h, and 0–60 ml/h and 0–2 ml/h in the microchannels with Dh = 600 μm, 400 μm and 200 μm, respectively. The results show that the hydraulic diameter has significant effects on flow patterns and three main flow patterns are observed, i.e., annular flow, slug flow and droplet flow. Generally, annular flow appeared at high flow rates of the dispersed phase and low flow rates of the continuous phase, while droplet flow appeared at low flow rates of the dispersed phase and high flow rates of the continuous phase. However, slug flow existed at comparable flow rates of the continuous and dispersed phases. A dimensionless analysis is carried out and a new dimensionless group including Weber number and Reynolds number is derived. The new defined dimensionless group performs well to develop a general flow pattern map. In addition, slug flow hydrodynamics are investigated as well in the present study, considering the slug length and slug velocity. Based on the present experimental results, a new scaling law is proposed to predict the slug length and it shows a good agreement with the experimental results. It has been widely reported that slug velocities depend linearly on the total flow rates of the two phases, which is consistent with the present study. The linear law provides a good prediction of the experimental slug velocities but different slopes are suggested in microchannels with different hydraulic diameters.

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