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Which of the following non-dimensional terms is an estimate of Nusselt number?

1. Ratio of internal thermal resistance of a solid to the boundary layer thermal resistance
2. Ratio of the rate at which internal energy is advected to the rate of conduction heat transfer
3. Non-dimensional temperature gradient
4. Non-dimensional velocity gradient multiplied by Prandtl number

Correct answer: Non-dimensional temperature gradient

Solution

Nusselt number is the non-dimensional temperature gradient at the surface (Nu = hL/k). The stored option (internal to boundary-layer thermal resistance ratio) describes the Biot number, not Nusselt.

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