ANS:A - Grashoff

In natural convection heat transfer, the Nusselt number (Nu) is indeed a function of the Prandtl number (Pr) and the Grashof number (Gr). The Grashof number characterizes the ratio of buoyancy forces to viscous forces in a fluid flow due to density differences caused by temperature gradients. It is defined as: ν2g⋅β⋅(ΔT)⋅L3​ Where:

• g is the acceleration due to gravity,
• β is the coefficient of volume expansion,
• ΔT is the temperature difference,
• L is a characteristic length, and
• ν is the kinematic viscosity of the fluid.

The Prandtl number characterizes the relative importance of momentum diffusivity to thermal diffusivity in a fluid. It is defined as: Pr=αν​ Where:

• α is the thermal diffusivity of the fluid.

In natural convection heat transfer, the Nusselt number is influenced by both the Prandtl number and the Grashof number. Therefore, the correct answer is Grashoff.