Experimental and CFD study on two-phase flow characteristics in helically coiled tubes with variable curvature

• Novel Variable Curvature Helically Coiled Tube (VCHCT) design investigated for two-phase flow. • Experimental mapping of five flow regimes (bubbly, slug, plug, annular, slug-annular-like) in VCHCT. • CFD reveals unique simultaneous coexistence of different flow regimes along VCHCT axis. • Variable centrifugal force and delayed interface response proposed as mechanism for flow transition. Helically coiled tubes are vital components in various industrial applications, including steam generators in nuclear power plants. To enhance heat and mass transfer efficiency beyond conventional designs, this study investigates the two-phase flow characteristics in a novel Variable Curvature Helically Coiled Tube (VCHCT). An experimental investigation using an air-water loop identified five typical flow regimes: bubbly, slug, plug, annular, and a slug-annular-like flow. High-speed imaging captured the distinct phase distributions and interfacial structures within the VCHCT. Pressure drops measurements revealed characteristic fluctuation patterns and probability density functions (PDF) associated with each flow regime, showing increased fluctuation intensity and broadening PDF distributions with increasing flow complexity from bubbly to slug-annular-like flow. Complementary three-dimensional numerical simulations employing the Volume of Fluid model qualitatively reproduced key flow regimes (bubbly, plug, slug, stratified-wavy). Notably, the simulations unveiled a unique flow regime transition phenomenon where stratified, wavy, and slug flows coexist simultaneously in different axial subsections under specific conditions. Analysis suggests this transition is linked to the delayed response of the gas-liquid interface to the periodically changing centrifugal acceleration field inherent in the VCHCT geometry. The experimental results indicate that while influencing local flow structures, the variable curvature design maintains typical helical coil flow patterns without introducing prohibitive pressure drop penalties compared to conventional enhancement techniques involving complex internal structures. This combined experimental and numerical study provides fundamental insights into the complex two-phase hydrodynamics within VCHCTs, highlighting their potential for manipulating flow patterns and enhancing transport processes.