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

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

Three

One

Four

Two

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Heat and Mass Transfer
When heat is transferred by molecular collision, it is referred to as heat transfer by

Convection

Conduction

Radiation

Scattering

Convection

Conduction

Radiation

Scattering

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Heat and Mass Transfer
Which of the following has least value of conductivity?

Air

Water

Glass

Plastic

Air

Water

Glass

Plastic

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Heat and Mass Transfer
According of Kirchhoff's law

Radiant heat is proportional to fourth power of absolute temperature

Emissive power depends on temperature

Emissive power and absorptivity are constant for all bodies

Ratio of emissive power to absorptive power for all bodies is same and is equal to the emissive power of a perfectly black body

Radiant heat is proportional to fourth power of absolute temperature

Emissive power depends on temperature

Emissive power and absorptivity are constant for all bodies

Ratio of emissive power to absorptive power for all bodies is same and is equal to the emissive power of a perfectly black body

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Heat and Mass Transfer
Heat conducted through per unit area and unit thick face per unit time when temperature difference between opposite faces is unity, is called

Thermal conductivity

Thermal resistance

Temperature gradient

Thermal coefficient

Thermal conductivity

Thermal resistance

Temperature gradient

Thermal coefficient

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Heat and Mass Transfer
Metals are good conductors of heat because

They have high density

Their atoms are relatively far apart

They contain free electrons

Their atoms collide frequently

They have high density

Their atoms are relatively far apart

They contain free electrons

Their atoms collide frequently

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Heat and Mass Transfer

Heat and Mass Transfer When heat is transferred by molecular collision, it is referred to as heat transfer by

Heat and Mass Transfer Which of the following has least value of conductivity?

Heat and Mass Transfer According of Kirchhoff's law

Heat and Mass Transfer Heat conducted through per unit area and unit thick face per unit time when temperature difference between opposite faces is unity, is called

Heat and Mass Transfer Metals are good conductors of heat because

Heat and Mass Transfer
When heat is transferred by molecular collision, it is referred to as heat transfer by

Heat and Mass Transfer
Which of the following has least value of conductivity?

Heat and Mass Transfer
According of Kirchhoff's law

Heat and Mass Transfer
Heat conducted through per unit area and unit thick face per unit time when temperature difference between opposite faces is unity, is called

Heat and Mass Transfer
Metals are good conductors of heat because