Distribution of energy between convective and turbulent flow for three frequently used impellers

Turbulence energy dissipation is important in the study of turbulent mixing phenomena in stirred tanks. This paper investigates the characteristics of the turbulence energy dissipation and the overall energy distribution in the impeller region of a stirred tank. One radial flow impeller (Rushton turbine (RT)) and two axial flow impellers (the pitched blade turbine (PBT) and a fluidfoil turbine (A310)) were used. The mean and root-mean-square velocity (RMS) profiles close to the three impellers were measured in a cylindrical baffled tank using laser Doppler anemometry (LDA). The average turbulence energy dissipation, <(epsilon)over bar>(i) was calculated using a macroscopic energy balance equation over several control volumes. The local turbulence energy dissipation epsilon was estimated using epsilon = A nu(3)/L with A = 1 and L = D/10. Integration of the local dissipation over a control volume consistently gave results within 6% of the macroscopic energy balance. The bulk of the energy is dissipated in the small volume occupied by the impeller and the impeller discharge stream for all three impellers: in order of increasing percentages 38.1% (A310), 43.5% (RT) and 70.5% (PBT). The dominant characteristics of energy distribution are different for each impeller. The A310 was most efficient at generating convective flow. The RT generated the most turbulence, and the PBT derived a much larger portion of its energy from the return flow.