Q.14 Which one of the following statements is correct about Reynolds Number (𝑁𝑅𝑒) in
a stirred tank bioreactor?
(A) 𝑁𝑅𝑒 is independent of the viscosity of the medium
(B) In laminar flow, mixing time increases with an increase in 𝑁𝑅𝑒
(C) 𝑁𝑅𝑒 is inversely proportional to the impeller speed
(D) In turbulent flow, mixing time is independent of 𝑁𝑅𝑒

Understanding Reynolds Number in Stirred Tank Bioreactors

Reynolds Number (NRe) determines flow regimes in bioreactors, crucial for efficient mixing in bioprocess engineering.

Correct Answer

Option (D) is correct: In turbulent flow, mixing time is independent of NRe. Once turbulent conditions dominate (typically NRe > 10,000), mixing relies on power input and geometry rather than further NRe increases.​

Reynolds Number Formula

NRe calculates as NRe=ρD²N/μ, where ρ is fluid density, $D$ is impeller diameter, $N$ is rotational speed, and $\mu$ is viscosity. This dimensionless value predicts laminar (low NRe), transitional, or turbulent (high NRe) flow.​

Option Analysis

• (A) Incorrect: NRe depends on viscosity (μ) in the denominator, so higher viscosity lowers NRe.​

• (B) Incorrect: In laminar flow, mixing time decreases as NRe rises, improving circulation before turbulence.​

• (C) Incorrect: NRe increases directly with impeller speed $N$, not inversely.​

• (D) Correct: Turbulent mixing time plateaus, independent of NRe, ensuring consistent homogeneity.​

Bioreactor Implications

Turbulent regimes optimize oxygen transfer and nutrient distribution in microbial cultures. For viscous broths in plant cell bioreactors, operators target high NRe via faster impellers despite $\mu$ challenges.​