Heat Transfer
Fluid motion in the natural convection heat transfer between a solid surface and a fluid in contact with it, results from the

Buoyancy of the bubbles produced at active nucleation site
Existence of thermal boundary layer
None of these
Temperature gradient produced due to density difference

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Heat Transfer
In sub-cooled boiling,

Bubbles from heating surface are absorbed by the mass of the liquid
Temperature of the heating surface is less than the boiling point of the liquid
Temperature of the heating surface is more than the boiling point of the liquid
Very large vapour space is necessary

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