Macro- and micromixing studies in an unbaffled vessel agitated by a Rushton turbine

Abstract

Macromixing characteristics, power number and visual observation of the vortex behaviour and micromixing in an unbaffled tank agitated with a Rushton turbine are reported. The latter has also been compared in detail with earlier results from an identical tank containing baffles. The maximum mean specific energy dissipation rate, ε ¯ T , in the unbaffled tank that can be utilised without severe air incorporation is ∼ 0.18 W / kg compared to ∼ 1.2 W / kg with baffles. However, at this lower ε ¯ T , the micromixing efficiency is always significantly greater without baffles except when addition is made onto the top of the liquid or into the trailing vortex very close to the impeller. In these latter cases, they are approximately the same but even a small submergence of the feed tube below the liquid surface greatly enhances micromixing in the unbaffled case whilst it is still very poor with baffles. This good micromixing performance of the unbaffled vessel was very unexpected. Furthermore, an established method of estimating the local ε T gave values of ε T / ε ¯ T = φ ⪢ 1 at every feed position where measurement was undertaken. Since the spatially averaged value of φ = 1 , this result suggests the possibility that the accepted concept of micromixing being totally controlled by the local ε T at the feed point may not be valid for such swirling flows.