To keep the power input constant for a stirred vessel operating under fully developed turbulent flow conditions (constant power number), if the impeller diameter is increased by 20%, the impeller speed should be decreased by a factor of_________________?
Correct answer: D. (1.2)5/3
- A. (1.2)3/2
- B. (1.2)3/5
- C. (1.2)2/3
- D. (1.2)5/3
Explanation
In fully turbulent mixing, power varies as N³D⁵ when the power number is constant. Keeping power fixed therefore requires N to vary as D⁻⁵ᐟ³, so a 20% diameter increase requires a speed reduction by (1.2)⁵ᐟ³.
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