Convective Heat Transfer & Dimensionless Numbers

In the Dittus-Boelter equation $Nu = 0.023 Re^{0.8} Pr^n$ for fully developed turbulent flow in smooth pipes, the exponent $n$ takes the value:

Options:

• (A) $n = 0.4$ for heating of the fluid and $n = 0.3$ for cooling of the fluid
• (B) $n = 0.3$ for heating and $n = 0.4$ for cooling
• (C) $n = 0.5$ for both heating and cooling
• (D) $n = 0.33$ regardless of heat flux direction