Radiation
Exam Duration: 45 Mins Total Questions : 20
What is the basic equation of thermal radiation from which all other equations of radiation can be derived?
- (a)
Stefan-Boltzmann equation
- (b)
Planck's equation
- (c)
Wien's equation
- (d)
Rayleigh-Jeans formula
A large spherical enclosure has a small opening. The rate of emission of radiative flux through this opening is 7.35 kW/m2. The temperature at the inner surface of the sphere will be about (assume Stefan-Boltzmann constant a = 5.67 x W/m2-K4)
- (a)
6000C
- (b)
3270C
- (c)
373 K
- (d)
1000 K
Heat transfer by radiation between two grey bodies of emissivity e is proportional to (notations have their usual meanings)
- (a)
\(\frac { ({ E }_{ b }-J) }{ (1-\varepsilon ) } \)
- (b)
\(\frac { ({ E }_{ b }-J) }{ (1-\varepsilon )/\epsilon } \)
- (c)
\(\frac { ({ E }_{ b }-J) }{ (1-\varepsilon )^{ 2 } } \)
- (d)
\(\frac { ({ E }_{ b }-J) }{ (1-\varepsilon ^{ 2 }) } \)
Solar radiation of 1200 W/m2 falls perpendicularly on a gray opaque surface of emissivity 0.5. If the surface temperature is 50°C and surface emissive power is 600 W/m2, the radiosity of that surface will be
- (a)
600 W/m2
- (b)
1000 W/m2
- (c)
1200 W/m2
- (d)
1800 W/m2
Sun's surface at 5800 K emits radiation at a wavelength of 0.5 μ, A furnace at 300°C will emit through a small opening, radiation at a wavelength of nearly
- (a)
10 μ
- (b)
5 μ
- (c)
0.25 μ
- (d)
0.025 μ
The spectral emissive power E⋋ for a diffusely emitting surface is
1. E⋋ =0 for ⋋≤3 μm
2. E⋋=150 W/m2-μm for 3 < ⋋ < 12 μm
3. E⋋=300 W/m2-μm for 12 < ⋋ < 25 μm
4. E⋋=0 for ⋋ < 25 μm
The total emissive power of the surface over the entire spectrum is
- (a)
1250 W/m2
- (b)
2500 W/m2
- (c)
4000 W/m2
- (d)
5250 W/m2
The shape factor of a hemispherical body placed on a flat surface with respect to itself is
- (a)
zero
- (b)
0.25
- (c)
0.5
- (d)
1.0
For an opaque plane surface, the irradiation radiosity and emissive power are respectively 20, 12 and 10 W/m2. What is the emissivity of the surface?
- (a)
0.2
- (b)
0.4
- (c)
0.8
- (d)
1.0
A plate having 10 cm2 area each side is hanging in the middle of a room of 100 m2 total surface area. The plate temperature and emissivity are respectively 800 K and 0.6. The temperature and emissivity values for the surfaces of the room are 300 K and 0.3 respectively. Boltzmann constant σ = 5.67 x 10-8 W/m2-K4. The total heat loss from the two surfaces of the plate is
- (a)
13.66 W
- (b)
27.32 W
- (c)
27.87 W
- (d)
13.66 MW
Two long parallel plates of same emissivity 0.5 are maintained at different temperatures and have radiation heat exchange between them. The radiation shield of emissivity 0.25 placed in the middle will reduce radiation heat exchange to
- (a)
1/2
- (b)
1/4
- (c)
3/10
- (d)
3/5
The net radiation from the surfaces of two parallel plates maintained at T1 and T2 is to be reduced 99%.
(I) Number of shields which should be placed between two surfaces to achieve this reduction is (દshield=0.05), દsurface = 0.8)
- (a)
3
- (b)
4
- (c)
5
- (d)
6
The net radiation from the surfaces of two parallel plates maintained at T1 and T2 is to be reduced 99%.
If દshield = દsurface, then what will be the number of shields?
- (a)
100
- (b)
3
- (c)
99
- (d)
4
There is given two concentric spheres, the inner sphere contains liquid nitrogen at Ta = -1830C and outer surface is exposed to surrounding with T∞= = 40°C (દ = 0.03).
What is the rate of heat transfer?
- (a)
4.2 W
- (b)
2.96 W
- (c)
2.18 W
- (d)
2.7 W
One side of metallic plate is insulated while the other side absorbs a radiation flux of 900 W/m2. The convective heat transfer coefficient between the plate and ambient air is 10 W/m2- K . Given, દplate = 0.8, Tair = 270C.
At steady state condition
- (a)
qabsorbed = qradiation
- (b)
qabsorbed + qconveciton = qradiation
- (c)
qconvection = qradiation
- (d)
qabsorbed = qradiation + qconvection
One side of metallic plate is insulated while the other side absorbs a radiation flux of 900 W/m2. The convective heat transfer coefficient between the plate and ambient air is 10 W/m2- K . Given, દplate = 0.8, Tair = 270C.
Temperature of plate under steady state is
- (a)
89°C
- (b)
410 K
- (c)
355 K
- (d)
780C
Air is flowing inside a cylindrical tube in which a thermocouple is kept. Convective heat transfer coefficient between air and thermocouple is 60 W/m2 0- દthermocouple = 0.5, દwall = 0.6. If Ta = 870° and Tc = 8000C then, Tw is
- (a)
1041.70C
- (b)
10000C
- (c)
768.70C
- (d)
5090C
Three thin walled infinitely long hollow cylinders of radii r1 = 1cm, r2= 2 cm and r3 = 4 cm are arranged concentrically as shown in figure. If T1 = 1000 K, T3 = 500 K. Assume vacuum in the spaces between the cylinders.
If દ = 0.5, then at steady state
- (a)
T2 = 7620C
- (b)
T2 = 10240C
- (c)
T2 = 782 K
- (d)
T2 = 1024 K
Three thin walled infinitely long hollow cylinders of radii r1 = 1cm, r2= 2 cm and r3 = 4 cm are arranged concentrically as shown in figure . If T1 = 1000 K, T3 = 500 K. Assume vacuum in the spaces between the cylinders.
Heat flow per m2 area of cylinder 1 is
- (a)
14.198 kW
- (b)
15.033 W
- (c)
14198 kW
- (d)
150.3 W
Radiative heat transfer is intended between the inner surfaces of two very large isothermal parallel metal plates. While the upper plate (designated as plate 1) is a black surface and is the warmer one being maintained at 7270C the lower plate (plate 2) is a diffuse and gray surface with an emissivity of 0.7 and is kept at 2270C. Assume that the surface are sufficiently large to form a two-surface enclosure and steady state conditions to exist. Stefan-Boltzmann constant is given as 5.67 x 10-8 W/m2-K4.
The irradiation (in kW/m2) for the plate (plate 1) is
- (a)
2.5
- (b)
3.6
- (c)
17.0
- (d)
19.5
Radiative heat transfer is intended between the inner surfaces of two very large isothermal parallel metal plates. While the upper plate (designated as plate 1) is a black surface and is the warmer one being maintained at 7270C the lower plate (plate 2) is a diffuse and gray surface with an emissivity of 0.7 and is kept at 2270C. Assume that the surface are sufficiently large to form a two-surface enclosure and steady state conditions to exist. Stefan-Boltzmann constant is given as 5.67 x 10-8 W/m2-K4.
If plate 1 is also a diffuse and gray surface with an emissivity value of 0.8, the net radiation heat exchange (in kW/m2) between plate 1 and plate 2 is
- (a)
17.0
- (b)
19.5
- (c)
23.0
- (d)
31.7