Heat Transfer
A steel sphere of radius 0.1 m at 400°K is immersed in an oil at 300°K. If the centre of the sphere reaches 350°K in 20 minutes, how long will it take for a 0.05 m radius steel sphere to reach the same temperature (at the centre) under identical conditions ? Assume that the conductive heat transfer co-efficient is infinitely large.

5 minutes
20 minutes
40 minutes
10 minutes

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Heat Transfer
Prandtl number is the ratio of

Thermal diffusivity to momentum diffusivity
Thermal diffusivity to mass diffusivity
Momentum diffusivity to mass diffusivity
Momentum diffusivity to thermal diffusivity

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