According to Stefan Boltzmann law, the total radiation from a black body per second per unit area is proportional to
Stefen boltzman law is given by
Q = σAT^{4}
hence Q ∝ T^{4}
A perfect black body is the one which
Absorptivity of a body is equal to its emissivity
The ratio of total emissive power of body to the total emissive power of a black body at the same temperature is called
Emissivity for any body is given by
Where
E = Emissive power of a body
E_{b} = Emissive power of a black body
Which one of the following is true for black body
Since black body is perfect absorber of radiation hence absorptivity of black body α = 1.
Which one of the following is true for white body?
A surface is called Grey surface. If
The unit of StefanBotlzmann constant is
When metallic surfaces are oxidized, the emissivity
The solar energy falling on the earth’s surface is called
The shape factor F_{12 }in case of a conical cavity having a semi vertex angle a and height h is given by
If the temperature of a solid surface changes from 27°C to 627°C. Its emissive power will increase in the ratio of
Since emissive power = f(T^{4})
The radiative heat transfer rate per unit area (W/m^{2}) between two plane parallel grey surface (emissivity = 0.9) maintained at 400 K and 300 K is (StefanBoltzmann constant s = 5.67 x 10^{8} W/m^{2}K^{4})
Two long parallel surfaces each of emissivity 07 are maintained at different temperature and accordingly have radiation heat exchange between them. It is desirable to reduce 75% of this radiant heat transfer by inserting thin parallel shields of equal emissivity on both sides. The number of shields should be
where N = Number of radiation shield
Heat transfer by radiation between two gre bodies of emissivity ∈ is proportional to (Notations have their usual meanings)
(α = ∈) Kirchoff’s law
q = A_{i } α_{i}G_{i})
The shape factor of a hemispherical body place on a flat surface with respect to it self is
The shape factor of a cylindrical cavity with respect to it self is
A spherical body with surface A_{1} is completely enclosed by another hollow body with inner surface A_{2}. The shape factor of A_{2 }with respect to A_{1} is
For a circular tube of equal length and diameter shown below, the view factor F_{13} = 0.17. The view factor F_{12} in this case will be
f_{11} + f_{12} + f_{13} = 1
f_{11} = 0
f_{12} = 1  f_{13}
f_{12} = 1  0.17 = 0.83
For a hemisphere, the solid angle is measured
Two spheres A and B of same material have radii 1m and 4m and temperature 1000 K and 2000 K respectively. Which one of the following statement is correct.
The energy radiated by sphere A is
A plate having 10 cm^{2} area each side is hanging in the middle of a room of 100 m^{2 }total surface area, the plate temperature and emissivity are respectively 800 K and 0.6. The temperature and emissivity values for the surface of the room are 300 K and 0.3 respectively. Boltzman’ constant σ = 5.67 x 10^{8} W/m^{2}K^{4}. The total heat loss from the two surface of the plate is
A_{1} = 20 cm^{2} = 20 x 10^{4}m^{2}
A_{2 }= 100 m^{2}
T_{1} = 800K
∈_{1 }= 0.6
T_{2 }= 300 K
∈_{2} = 0.3
Match ListI (surface with orientations) with ListlI (equivalent emissivity) and select the correct answer using the codes given below:
ListI
A. Infinite parallel planes
B. Body 1 completely enclosed by body 2 but body 1 is very small
C. Radiation exchange between two small Grey bodies
D. Two concentric cylinders with large lengths
ListII
1. ∈_{1}
4. ∈_{1}∈_{2}
Codes:
A B C D
(a) 3 1 4 2
(b) 2 4 1 3
(c) 2 1 4 3
(d) 3 4 1 2
For the two long concentric cylinders with surface areas A_{1}, and A_{2}, the view factor F_{22} is given by
For a thin steel sheet, total emissive power is given as 32 W/m^{2}, insolation as 93 W/m^{2}. If thin sheet has reflectivity = 0,6, absorbtivity = 0.1 and transmissivity = 0.3. Then radiosity in W/m^{2}
Radiosity = J = E + ρG
J = 32 + 0.6(93)
J = 87.8 W.m^{2}
The thermal radiative flux from a surface of emissivity = 0 .4 is 22.68 kW /m^{2}. The approximate surface temperature (K) is
Stefan Boltzmann's constant = 5.67x10^{8} W/m^{2}K^{4}
We know, for radiation heat transfer
A solid cylinder (surface 2) is located at the centre of a hollow sphere (surface 1). The diameter of the sphere is 1 m, while the cylinder has a diameter and length of 0.5 m each. The radiation configuration factor F_{11 }is
Match ListI (Thermal spectrum) with Listl! (Wavelength mm) and select the correct aswer using the code given below the lists:
ListI
A. Cosmic rays
B. Radio waves
C. Visible light
D. UVrays
ListII
1. λ < 10^{8}
2.^{ }10^{2} < λ < 10^{10}
3. 0.38 < λ< 0.78
4. 10^{2 }< λ < 0.38
Codes:
A B C D
(a) 4 2 3 1
(b) 2 1 3 4
(c) 4 3 2 1
(d) 1 2 3 4
Match ListI with Listll and select the correct answer using the code given below the lists:
ListI
A. Window glass
B. Grey surface
C. Carbon dioxide
D. Radiosity
Listll
1. Emissivity
2. Kinematic viscosity
3. Diffusion coefficient
Codes:
A B C
(a) 2 3 1
(b) 3 2 1
(c) 1 3 2
(d) 1 2 3
The radiation heat flux from a heating element at a temperature of 800°C, in a furnace maintained at 300°C is kW/m^{2}. The flux when the element temperature is increased to 1000°C for the same furnace temperature is
For radiation heat transfer
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