The objective of this work is to gain information that could be used to design full scale mixing systems, and also could develop a design guide that can provide a reliable prediction of the power draw of different types of impellers. To achieve this goal, the power number behavior, including three operation regimes, the limits of the operation regimes, and the effect of baffling on power number, was compared across the full Reynolds number spectrum for Newtonian fluids in a laboratory-scale agitator. Six industrially significant impellers were tested, including three radial flow impellers: D-6, CD-6, and S-4, and also three axial flow impellers: P-4, SC-3, and HE-3.
Results in laminar regime indicate that baffling has no effect on power number in this operation regime. There is an inversely proportional relationship between power number and Reynolds number. The upper limit for this operation regime should be lower than 10, the limit commonly noted in the literature. The product of power number and Reynolds number in this particular regime is approximately proportional to the number of blades for these six impellers; however, other shape factors that were not included in this study also contribute to it.
In turbulent operation, baffling has a significant effect on power number: the power number for most impellers remains relatively constant in the baffled configuration while that for unbaffled configuration decreases with increasing Reynolds number. The impeller blade number is not the dominant factor that affects power number in this regime. Two hydrofoil impellers, SC-3 and HE-3, exhibit much lower power numbers when compared with the other impellers. Additionally, the impellers with higher power numbers in baffled tank tend to have lower ratios between unbaffled power number and average baffled power number when comparing at same Reynolds number.
No difference between two configurations, baffled and unbaffled, exists at low Reynolds number end of transitional regime, and the difference starts at intermediate Reynolds number and increases with an increase of Reynolds number. In the baffled configuration, four out of six impellers exhibit a minimum power number. In the unbaffled configuration, power numbers drop with increasing Reynolds number throughout the entire transitional regime for all impellers. The ratio of unbaffled to turbulent average power number for the six impellers retains a consistent order through the entire Reynolds number range with two high efficiency impellers having the highest ratio, then pitched blade impeller, and three radial flow impellers having the lowest ratios.