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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}
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?
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
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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