Investigation of instability in viscosity-stratified flow: Experiments and simulations.

摘要

Static or motionless mixers are widely used in mixing operations. A typical inlet geometry to the static mixer is the centerline injector. An undesirable flow instability can occur under certain conditions downstream of the injector between the injected core-flow and the outer co-flow due to viscosity stratification at the interface. Experiments were performed to investigate a viscosity-stratified flow in a circular geometry using Laser Induced Fluorescence (LIF) and Particle Image Velocimetry (PIV). Two new flow patterns were observed for the first time. Moreover, the unstable interfacial waves displayed a spiral development along the centerline. The experimental results for stable and unstable regions are compared against analytical results. Time-averaged scalar dispersion and velocity profiles for the unstable region indicate a broadening of the core-flow as expected.; A front-tracking/finite difference method was used to investigate interfacial instabilities for a two-layer density-matched viscosity-stratified Poiseuille flow. The effect of Reynolds number, thickness ratio, viscosity ratio, and wavenumber on the growth rate of the instability was investigated. The simulation replicates experimental features such as the sharp curvature of the interface and the fingers penetrating from the more viscous fluid into the less-viscous layer. Results of linear and nonlinear instabilities are presented for different parameters. Next, we studied three-layer Poiseuille flow using the same method and concentrated on the large amplitude nonlinear instability. Results are presented for varying viscosity ratio, Weber number, and phase difference. Interfacial tension has a very strong effect on the evolution of the interface. The sinuous instability appears to evolve faster than the varicose one. The intruding high-viscosity finger exhibits a bulbous tip for certain parameters. Drop formation is precluded by the two-dimensional nature of the calculations.