ANS:C - heat flux at the wall to that across the entire pipe.

The Nusselt number (Nu) is the ratio of the heat flux at the wall to that across the entire pipe. Explanation:

• The Nusselt number (Nu) is a dimensionless quantity used in heat transfer to characterize the convective heat transfer efficiency of a fluid flow.
• It represents the ratio of convective heat transfer to conductive heat transfer across a fluid boundary layer.
• Mathematically, the Nusselt number (Nu) is defined as the ratio of the convective heat transfer coefficient (h) multiplied by a characteristic length scale (L) to the thermal conductivity of the fluid (k). It is often expressed as: Nu=ℎ⋅Nu=kh⋅L​
• The heat flux at the wall represents the rate of heat transfer per unit area at the surface (wall) where heat exchange occurs.
• The Nusselt number indicates how effectively heat is transferred through convection relative to conduction. A higher Nusselt number signifies more efficient convective heat transfer.
• Therefore, the correct statement is that the Nusselt number is the ratio of the heat flux at the wall to that across the entire pipe.