Heat and Mass Transfer
Two long parallel surfaces each of emissivity 0.7 are maintained at different temperatures and accordingly have radiation heat exchange between them. It is desired to reduce 75% of the radiant heat transfer by inserting thin parallel shields of emissivity 1 on both sides. The number of shields should be

Three
One
Four
Two

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Heat and Mass Transfer
Thermal conductivity of a material may be defined as the

Heat conducted in unit time across unit area through unit thickness when a temperature difference of unity is maintained between opposite faces
Quantity of heat flowing in one second through one cm cube of material when opposite faces are maintained at a temperature difference of 1°C
Quantity of heat flowing in one second through a slab of the material of area one cm square, thickness 1 cm when its faces differ in temperature by 1°C
All of these

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Heat and Mass Transfer
Which of the following would lead to a reduction in thermal resistance?

In conduction, reduction in the thickness of the material and an increase in thermal conductivity.
In radiation, increasing the temperature and reducing the emissivity.
In convection, stirring of the fluid and cleaning the heating surface.
All of these

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