ANS:D - Reynolds number for the flow involved is < 2100.

The equation for the heat transfer coefficient for forced convection, 𝑁𝑢=0.023𝑅𝑒0.8⋅𝑃𝑟𝑛Nu=0.023Re0.8⋅Prn, is not valid if the value of 𝑛=0.4n=0.4 is used for heating. The Prandtl number (𝑃𝑟Pr) represents the ratio of momentum diffusivity to thermal diffusivity. For heating situations, the thermal boundary layer is typically thicker compared to cooling situations. Therefore, the value of 𝑛n is often adjusted to account for this difference in boundary layer thickness. A common value for 𝑛n in heating situations is 𝑛=0.4n=0.4, while for cooling situations, it is often 𝑛=0.3n=0.3. So, the correct statement is: 𝑛=0.4n=0.4 is used for heating.