Numerical and Experimental Study of the Squeezing-to-Dripping Transition in a T-Junction

In this work, we study the transition from squeezing to dripping during the formation of bubbles in a capillary T-junction in conditions relevant to microgravity. The junction is formed by two perpendicular cylinders of equal section (1 mm of internal diameter). The capillary number Ca (based on the continuous phase) is used as the key parameter of the study. For the range of Ca covered in this paper, the same two common bubble formation mechanisms as the ones described in the related literature have been observed: squeezing regime at low Ca and dripping regime for higher Ca. This paper provides a new value of the critical Ca for the transition from squeezing to dripping. This value has been obtained with two independent approaches, experimentally and numerically. Experimental photographs have been used to determine the value of Ca at which a gap appears between the forming bubble and the capillary's wall, as an evidence of the activation of the shearing mechanism related to the dripping regime. Additionally, the dependence of the bubble volume on the capillary number and the gas/liquid flow rate ratio has been analysed. In this work, we also propose a new numerical approach, complementary to the experimental one, carried out with the Computational Fluid Dynamics solver ANSYS Fluent v15.0.7. Numerical simulations have been performed to study the geometry and the behaviour of the gas-liquid interface during the cycle of bubble formation. Upstream the T-junction, as the fluctuation in pressure decreases, the vertical movement of the rear meniscus (gas-liquid interface in contact with the solid vertical capillary) also decreases, and the shear stresses begins to play an active role until overcoming the squeezing mechanism. Numerical simulations presented in this paper support the experimental observations, confirming that Computational Fluid Dynamics studies can be a useful tool to improve the experimental knowledge.