Interfacial stability in the swirling core–annular flow of two immiscible fluids

The stability of the interface in a core–annular flow (CAF) of two immiscible Newtonian fluids with contrasting densities has been investigated, emphasising the role of strong circumferential rotation for the first time. The aim of the investigation is to give insight into the physical mechanisms underlying interfacial disruption. We examine the combined effects of gravity, interfacial tension, axial and azimuthal shear stresses, and centrifugal force on interface stability. The Rayleigh–Taylor instability, induced by gravity, appears as a spiral mode with a azimuthal wavenumber of one. As gravitational effects decrease, the most unstable mode number increases sharply before decreasing with increasing rotation. This non-monotonic behaviour is attributed to the interplay between azimuthal shear and centrifugal acceleration. We demonstrate that this velocity ratio fundamentally governs the onset of spiral modes by varying the ratio of the axial velocities of the core and annular fluids. Higher Reynolds numbers in the annular phase promote the emergence of higher-order spiral modes concomitant with amplified azimuthal shear at the interface. In a parametric study of the gap between the core and pipe wall, we identified a suppressive effect of reduced annular thickness on the growth of higher azimuthal wavenumbers. An energy budget analysis further delineated distinct mechanisms underpinning each instability regime and clarified transitions between them. These findings extend our understanding of interfacial stability in swirling CAFs and provide a predictive framework to control spiral-mode selection.