ANS:B - 2

The Nusselt number (𝑁𝑢Nu) for heat transfer from a sphere (droplet or spherical particle) to a surrounding stagnant film by conduction can be calculated using the formula: 𝑁𝑢=ℎ𝐷𝑘Nu=khD​ Where:

• ℎh is the heat transfer coefficient,
• 𝐷D is the diameter of the sphere, and
• 𝑘k is the thermal conductivity of the fluid.

In the case of heat transfer by conduction from a sphere to a surrounding stagnant film, ℎh represents the conductive heat transfer from the sphere to the fluid. For a sphere, the Nusselt number 𝑁𝑢Nu is typically considered to be constant at a value of 2. This is a theoretical result derived from the heat transfer analysis of a sphere with a surrounding stagnant fluid film. Therefore, the correct value of the Nusselt number for this scenario is 2.