Some modifications in the Scaba 6SRGT impeller to enhance the mixing characteristics of Hershel–Bulkley fluids

Abstract

Mixing in unbaffled cylindrical tanks by open turbines rotating at low Reynolds numbers requires usually a longer mixing time, especially for shear thinning fluids possessing yields stress, where the mixing operation is characterized by the formation of caverns. To overcome this issue, some modifications in the impeller design are introduced. We are interested in mixing of shear thinning fluids with yield stress by curved bladed impellers (known as Scaba 6SRGT) in the laminar and transitional flow regimes. Two types of cuts have been introduced in the impeller blade: one at the middle of blade (referred as Design 2) and the other from the middle until the tip of blade (referred as Design 3). Lengths of cuts in Design 2 and 3 are: l/d2=0.3 and 0.6, respectively, where d2 is the length of blade for the classical impeller. The cuts introduced in the blades allowed a reduction in power consumption by 17.5% and 33% for Design 2 and 3 compared to the classical blade. Also, other modifications are introduced in order to enhance the mixing characteristics. It concerns the impeller eccentricity (e/D=0.12 and 0.25) and blade number (nb=2, 3 and 6) for the case which has given the most reduced power input (i.e. Design 3). The off-centred impeller (case e/D=0.25) with 6-blades has yielded the widest cavern size.