Transition from turbulent to transitional flow in the top half of a stirred tank

• Flow is transitional at Re=20 000 in a substantial part of the bench scale tank.
• Regions farther from the impeller are more likely to be in the transitional regime.
• Different fluids may have different limits of fully turbulent Re.
• Fully turbulent flow penetrates to a higher z as Re increases.
• The addition of more impellers provides a more uniform distribution of turbulence.

The limits of fully developed turbulence are defined in the wall jet and in the recirculation flow for three different impeller geometries: A310, HE3 and PBTD. At two different tank scales, mean velocity profiles in both the wall jet and the recirculation flow were measured to determine whether the condition of fully turbulent self-similarity is satisfied. Higher impeller Reynolds numbers are able to keep the flow fully turbulent higher in the tank. In addition to the expected effect of impeller Reynolds number, the fluid also affected the flow regime. Measurements in a pilot scale stirred tank gave similar results. Although the power number is often constant for Re > 20 000, an impeller Re ≥ 300 000 is required to keep the flow in the fully turbulent regime in the top third of the tank and in the recirculation zone.

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