This work numerically simulates immiscible liquids flow in co-axial microtubes and investigates the effects of changing the injection velocities and physical properties of the liquids and the diameters of the co-axial microtubes on the prevailing flow regimes for forming disperse droplets. These regimes are dripping, transition and jetting. The solution is validated by comparing the present results with those of reported numerical and experimental investigations in all three flow regimes. The generated motion picture movies of the computation domain for up to 18 cycles of forming disperse droplets, help determine not only the breakup mechanisms of the droplets but also the conditions for shifting among the three regimes. A dimensionless correlation is developed based on the present numerical results for predicting the boundary between the transition and the jetting regimes. The correlation is good agreement, to within ±10%, with numerical results and within 20% of the reported experimental measurements for ionized water and PDMS (Polydimethylsiloxane) mineral oil. A flow regime map of the dripping, transition and jetting regimes and of the condition for shifting among them is developed.

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