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Question

A spherical steel ball of 12 mm diameter is initially at 1000 K. It is slowly cooled in a surrounding of 300 K. The heat transfer coefficient between the steel ball and the surroundings is 5 W/m2K. The thermal conductivity of steel 20 W/mK. The temperature difference between the centre and the surface of the steel ball is

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Solution

The correct option is **D**

small because conduction resistance is far less than the convective resistance

Biot number,

Bi=Conduction resistance:lkAConvection resistance:1hA=hlk

where l=VA=π6d3πd2=d6

∴ Bi=hkd6=5×0.01220×6=0.0005

It means that the conduction resistance is far less than convective resistance

small because conduction resistance is far less than the convective resistance

Biot number,

Bi=Conduction resistance:lkAConvection resistance:1hA=hlk

where l=VA=π6d3πd2=d6

∴ Bi=hkd6=5×0.01220×6=0.0005

It means that the conduction resistance is far less than convective resistance

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