Scalar mixing in a turbulent stirred tank with pitched blade turbine: Role of impeller speed perturbation

► Large eddy simulation is performed for fixed and perturbed stirred tank flows. ► Effects of macroinstability on passive scalar mixing are observed. ► Unmixed zones at a fixed rpm are identified. ► Perturbation promoted mixing by break-up of unmixed zones. ► Reduction in mixing time is obtained with nominal increase in power input.