Dean Instability in Non-Newtonian Fluids

Abstract

We present a numerical study of Dean instability in non-Newtonian fluids in a laminar 180° curved-channel flow of rectangular cross section. A methodology based on the Papanastasiou model [1] was developed to take into account Bingham-type rheological behavior. After validation of the numerical methodology, simulations were carried out (using Fluent CFD code) for Newtonian and non-Newtonian fluids in curved channels of square and rectangular cross section and of large aspect and curvature ratios. A criterion based on the axial velocity gradient was defined to detect the instability threshold. This criterion is used to optimize the grid geometry. The effects of curvature and aspect ratios on the instability are studied for all fluids, Newtonian and non-Newtonian. In particular, we show that the critical value of the Dean number decreases with increasing duct curvature ratio. The variation of the critical Dean number with duct aspect ratio is less regular. The results are compared with those for Newtonian fluids to emphasize the effect of the power-law index and the Bingham number. The onset of Dean instability is delayed with increasing power-law index. The same delay is observed in Bingham fluids when the Bingham number is increased.