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# Important Questions and Answers - Heat and Mass Transfer - Radiation

Mechanical - Heat and Mass Transfer - Radiation

The heat transfer from one body to another without any transmitting medium is known as radiation. It is an electromagnetic wave phenomenon.

2. What is black body?

Black body is an ideal surface having the following properties.

1.A black body absorbs all incident radiation, regardless of wave length and direction.

2.For a prescribed temperature and wave length, no surface can emit more energy than black body.

3. State   Wienâ€™s   displacement   law.

The Wienâ€™s relationshiplawgivesbetweentemperaturethe and wave length corresponding to the maximum spectral emissive power of the black body at that temperature. 4. What is meant by gray body?

If a body absorbs a definite percentage of incident radiation irrespective of their wave length, the body is known as gray body. The emissive power of a gray body is always less than that of the black body.

This law states that the ratio of total emissive power to the absorptive is constant for all surfaces which are in thermal equilibrium with the surroundings. This can be written as It also states that the emissivity of the body is always equal to its absorptive when the body remains in thermal equilibrium with its surroundings.

a1 = E1; a2 = E2 and so on.

It is defined as the total radiation incident upon a surface per unit time per unit area. It is expressed in W/m2.

7. What is the purpose of radiation shield?

Radiation shields constructed from low emissivity (high reflective) materials. It is used to reduce the net radiation transfer between two surfaces.

8. Define intensity of radiation (Ib)?

It is defined as the rate of energy leaving a space in a given direction per unit solid angle per unit area of the emitting surface normal to the mean direction in space. It is used to indicate the total radiation leaving a surface per unit time per unit area. It is expressed in W/m2.

10. What is meant by shape factor?

The shape factor is defined as the fraction of the radioactive energy that is diffused from on surface element and strikes the other surface directly with no intervening reflections. It is represented by Fij. Other names for radiation shape factor are view factor, angle factor and configuration factor.

11. State Stefan â€“Boltzmann law.

The emissive power of a black body is proportional to the fourth power of absolute temperature. Where    E b =       Emissive power, w/m2

s       =        Stefan. Boltzmann constant

=       5.67 x 10-8  W/m2  K4

T                 =       Temperature, K

12.What are the assumptions made to calculate radiation exchange between the surfaces?

1.      All surfaces are considered to be either black or gray

2.      Radiation and reflection process are assumed to be diffuse.

3.      The absorptive of a surface is taken equal to its emissivity and independent of temperature of the source of the incident radiation.

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