Flow transitions in triple-helical microchannel involving novel parallel flow patterns

Abstract

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 a 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≥1. The transition is theoretically explained based on different interfacial and instability phenomena, and the 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 Dean number in individual phases. Results establish 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 Dean number. Overall, the analysis elucidates two-phase parallel flow operation in THM, unfolding novel phenomena and physics.