This
post covers the unit wise 2 marks questions with answers of Unit IV Radiation, repeatedly
asking from Anna University, Chennai, for the subject ME6502- Heat and Mass
Transfer which is the one of the important subjects of Mechanical Engineering
students whom were studying V semester.
Click: Unit II Convection 2 Marks with Answers
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Click: Unit IV Radiation 2 Marks with Answers
Click: Unit V Mass Transfer 2 Marks with AnswersClick:Unit III Phase change Heat Transfer and Heat exchangers 2 Marks with Answers
Click: Unit IV Radiation 2 Marks with Answers
Unit
IV Radiation
Part- A (2 Marks with Answers)
1. Define emissive power.
It is defined as the total amount of
radiation emitted by a body per unit time per unit area.
It is denoted as Eb and it is
expressed in W/m2.
2. Define monochromatic emissive
power.
The energy emitted by the surface at a
given length per unit time per unit area in all directions is known as
monochromatic emissive power.
3. What is meant by absorptivity, reflectivity
and transmissivity?
Absorptivity:
It
is the ratio between radiations absorbed to the incident radiation.
Reflectivity:
It
is the ratio between radiations reflected to the incident radiation.
Transmissivity:
It is the ratio between
radiations transmitted to the incident radiation.
4. What is black body?
If
a body absorbs all incident radiation, regardless of wavelength and direction,
then it is said to be black body.
For a prescribed temperature and
wavelength, no surface can emit more energy than black body.
5. State Planck’s Distribution law.
It states that the relationship between
the monochromatic emissive power of a black body and wavelength of a radiation
at a particular temperature.
6. Wien’s Displacement law.
It states that the relationship between
temperature and wavelength corresponding to the maximum spectral emissive power
of the black body at that temperature.
7. State Stefan-Boltzmann law.
It
states that the emissive power of a black body is proportional to the fourth
power of absolute temperature.
Eb ∞ T4
Eb = σ T4
Where
Eb - Emissive power in W/m2
σ – Stefan Boltzmann-constant=5.67*10-8
W/m2K4
T – Temperature in K
8.
Define emissivity.
It is
defined as the ability of the surface of a body to radiate heat.
It is
also defined as the ratio of emissive power of a body to the emissive power of
a black body at equal temperature.
9.
What is meant by gray body?
The
emissive power of a gray body is always less than that of black body.
10.
State Kirchhoff’s law of radiation.
It
states that the ratio of total emissive power to the absorptivity is constant
for all surfaces which are in thermal equilibrium with the surroundings.
E1/α1 = E2/α2
= E3/α3
11.
Define intensity of radiation.
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.
In
= Eb / π
12.
State Lambert’s cosine law.
It
states that the total emissive power from a radiating plane surface in any
direction proportional to the cosine of the angle of emission.
Eb
∞ cos θ
13.
What is the purpose of radiation shields?
Radiation
shields are constructed from low emissivity (high reflectivity) materials.
It is
used to reduce the net radiation transfer between two surfaces.
14.
Differentiate irradiation and radiosity.
Irradiation:
It is
defined as the total radiation incident upon a surface per unit time per unit
area.
It is
expressed in W/m2
Radiosity:
It is used to indicate the total
radiation leaving a surface per unit time per unit area.
It is expressed in W/m2
15.
What are the assumptions made to calculate radiation exchange between the
surfaces?
(i)
All surfaces are considered to be either black or gray.
(ii) Radiation and reflection process are
assumed to be diffuse.
(iii)
The absorptivity of a surface is taken equal to its emissivity and independent of
temperature of the source of the incident radiation.
16. Discuss the radiation characteristics
of carbon dioxide and water vapor.
The CO2 and H2O
both absorb and emit radiation over certain wavelength regions called
absorption bands.
The radiation in these gases is in a
volume phenomenon.
The emissivity of CO2 and the
emissivity of H2O at a particular temperature increases with partial
pressure and mean beam length.
17. What is meant by shape factor
and mention its significance.
It
is defined as the fraction of the radiate energy that is diffused from one
surface element and strikes the other surface directly with no intervention
reflections.
It is used to analyze the radiative heat
exchange between two surfaces.
