Flow of a liquid inside a helical tube is composed of axial and circumferential components, the latter arising because of its specific geometry. For a gas–liquid two-phase flow inside a helical tube, the coupled effect of these two flow components leads to a variety of flow patterns, for example, slug, bubble, and stratified flow. We present here a novel triple-helical microchannel, in which, the two-phase flow is found to engender several additional flow patterns not observed with the conventional geometries, for example, the parallel and oscillating annular flow and even simultaneous occurrence of several such patterns. We show that the transition between these patterns depends not only on the fluid rates of the two liquids but also on the helix angle. We have presented detailed phase diagrams to elaborate these effects. We have examined also the effect of channel geometry on the specific features of these flow patterns.

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Triple-helical microchannel (THM) constitutes a unique configuration for implementing parallel flows with enhanced interfacial mass transfer through torsion-induced advection in the bulk phases. Key operational aspects include identifying stable regimes of parallel flow with respect to phase flow rates and characterizing the flow intensification achieved through secondary flow. The current work investigates two-phase flow, typical in liquid-liquid extraction operation, inside THM over wide range of flow rates. Flow visualization and allied image analysis revealed a sequential flow transition with increasing ratio of organic to aqueous flow rates (qO/qA): from one stable 'arc' helical parallel flow regime for qO/qA<1 to an intermittent slug flow regime and finally to another 'clip' helical parallel flow regime for qO/qA{greater than or equal to}1. The transition is theoretically explained based on different interfacial and instability phenomena, and effects of centrifugal forces. The parallel flow regimes were exclusively assessed by evaluating the flow fields based on phase contours obtained experimentally and quantifying the secondary flow intensification based on Deans number in individual phases. Results establish a greater stability of the clip parallel flow regime based on the higher organic phase capillary number and also greater flow intensification in this regime based on Deans number. Overall, the analysis elucidates two-phase parallel flow operation in THM, unfolding novel phenomena and physics.

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