# Nusselt number is defined as

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1. Heat transfer by conduction / heat transfer by convection
2. (Heat transfer by conduction / heat transfer by convection)2
3. (Heat transfer by convection / heat transfer by conduction)2
4. Heat transfer by convection / heat transfer by conduction

Option 4 : Heat transfer by convection / heat transfer by conduction
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## Detailed Solution

Explanation:-

Nusselt number

It is the ratio of heat flow rate by convection process to the heat flow rate by the conduction process.

$$Nu = \frac{{{Q_{conv}}}}{{{Q_{cond}}}} = \frac{{hA{\rm{\Delta }}T}}{{\frac{{kA{\rm{\Delta }}T}}{L}}} = \frac{{hL}}{k}$$

The Nusselt number is in

 Forced Convection Nu = f (Re, Pr ) Free Convection Nu = f (Gr, Pr )

here Nu = Nusselt's Number

Re = Reynold's Number, Pr = Prandtl Number, Gr = Grashoff Number

Other important dimensionless numbers are:

• Biot number  → Ratio of internal thermal resistance to boundary layer thermal resistance
• Grashof number  → Ratio of buoyancy to viscous force
• Prandtl number  → Ratio of momentum to thermal diffusivities
• Reynolds number  → Ratio of inertia force to viscous force
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