PIMS/AMI Seminar: Stable multi-layer flows at large Re

Parallel multi-layer flow configurations occur in transport, co-extrusion and coating applications. In the industrial setting throughput (i.e. flow rate) is often limited by the onset of interfacial instabilities. Here we focus on a core annular flow, which is easy to establish in the lab setting. If the outer lubricating fluid has a yield stress highly stable multi-layer configurations can be achieved. The outer fluid preserves an unyielded ring about the interface with the core fluid, preventing the growth of interfacial instabilities; see Frigaard (2001). This flow has been demonstrated experimentally by Huen et al (2007), and is one of relatively few multi-fluid flows known to be nonlinearly stable; see Moyers Gonzalez et al. (2004). We present the results of our ongoing and recent work on these flows. First we characterise the entry/start-up flow of the core-annular configuration and a plane channel analogue. Here a Newtonian central fluid is surrounded by a Bingham lubricating fluid. Both fluids are miscible. We show that these flows are achievable with both contraction and expansion inlet geometries. Secondly, we study the temporal stability of the fully developed flow numerically. We show that even for initial perturbations of 50% amplitude and Re = O(100), the flow remains stable. For larger perturbations we show that Further increases in amplitude and Re result in secondary flow regimes which still have stable unyielded interfaces, but for which mixing has occurred at the interface. Apart from finding temporally stable regimes, we show that the same process can be adapted to produce interesting streamwise-spatially periodic structures. Finally, we present preliminary analytical and experimental results in which we include the effects of visco-elasticity into the core fluid.