Linear stability of horizontal and inclined stratified channel flows ofNewtonian/non-Newtonian shear-thinning fluids is investigated with respect toall wavelength perturbations. The Carreau model has been chosen for themodeling of the rheology of a shear-thinning fluid, owing to its capability todescribe properly the constant viscosity limits (Newtonian behavior) at low andhigh shear rates. The results are presented in the form of stability boundarieson flow pattern maps (with the phases' superficial velocities as coordinates)for several practically important gas-liquid and liquid-liquid systems. Thestability maps are accompanied by spatial profiles of the criticalperturbations, along with the distributions of the effective and tangentviscosities in the non-Newtonian layer, to show the influence of the complexrheological behavior of shear-thinning liquids on the mechanisms responsiblefor triggering instability. Due to the complexity of the considered problem, aworking methodology is proposed to alleviate the search for the stabilityboundary. Implementation of the proposed methodology helps to reveal that inmany cases the investigation of the simpler Newtonian problem is sufficient forthe prediction of the exact (non-Newtonian) stability boundary of smoothstratified flow (i.e., in case of horizontal gas-liquid flow). Therefore, theknowledge gained from the stability analysis of Newtonian fluids is applicableto those (usually highly viscous) non-Newtonian systems. Since the stability ofstratified flow involving highly viscous Newtonian liquids has not beenresearched in the literature, interesting findings on the viscosity effects arealso obtained.
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