Tut 11
Radiation Heat Transfer
MEL242, Heat and Mass Transfer
1. A glass plate 30 cm square is used to
view radiation from a furnace. The
transmissivity of the glass is 0.5 from 0.2
to 3.5 μm. The emissivity may be
assumed to be 0.3 up to 3.5 μm and 0.9
above that. The transmissivity of the glass
is zero, except in the range from 0.2 to
3.5 μm. Assuming that the furnace is a
blackbody at 2000 °C, calculate the
energy absorbed in the glass and the
energy transmitted.
2. A certain surface has the following
absorption properties:
αλ = 0.05
0<λ<1.2 μm
αλ = 0.5
1.2<λ<3 μm
αλ = 0.4
3<λ<6 μm
αλ = 0.2
6<λ<20 μm
αλ = 0
20<λ<∞μm
Calculate the total absorptivity of the
surface if it is irradiated with blackbody
radiation
at (a) 300 K, (b) 500 K, (c) 1000 K, (d)
2000 K, (e) 5000 K.
3. A surface has the emissive characteristics
shown in Figure P8-13. Calculate the
emissive power for the surface
maintained at 2000 K.
4. Consider a small surface of area A1 = 10-4
m2, which emits diffusively with a total,
hemispherical emissive power of E1 = 5 x
104 W/m2. At what rate is this emission
intercepted by a small of area A2 = 5 x
10-4 m2, which is oriented as shown.
What is the irradiation G2 on A2?
5. An enclosure has an inside area of 100
m2, and its inside surface is black and is
maintained at a constant temperature. A
small opening in the enclosure has an
area of 0.02 m2. The radiant power
emitted from this opening is 70 W. What
is the temperature of the interior
enclosure wall? If the interior surface is
maintained at this temperature, but is now
polished, what will be the value of the
radiant power emitted from the opening?
6. An opaque surface with the prescribed
spectral,
hemispherical
reflectivity
distribution is subjected to the spectral
radiation shown.
(A) Sketch the spectral, hemispherical
absorptivity distribution.
(B) Determine the total irradiation on the
surface.
(C) Determine the radiant flux that is
absorbed by the surface.
(D) What is the total hemispherical
absorptivity of this surface?
7. Find the radiation shape factors F1-2 for
the situations shown below.
8. Two parallel plates 0.5 by 1.0 m are
spaced 0.5 m apart, as shown in Figure.
One plate is maintained at 1000 °C and
the other at 500 °C. The emissivities of
the plates are 0.2 and 0.5, respectively.
The plates are located in a very large
room, the walls of which are maintained
at 27 °C. The plates exchange heat with
each other and with the room, but only
the plate surfaces facing each other are to
be considered in the analysis. Find the net
transfer to each plate and to the room.
9. Two rectangles 50 by 50 cm are placed
perpendicularly with a common edge.
One surface has T1 =1000 K, 1=0.6,
while the other surface is insulated and in
radiant balance with a large surrounding
room at 300 K. Determine the
temperature of the insulated surface and
the heat lost by the surface at 1000 K.
10. Two large parallel planes having
emissivities of 0.3 and 0.5 are maintained
at temperatures of 900 K and 400 K,
respectively. A radiation shield having an
emissivity of 0.05 on both sides is placed
between the two planes. Calculate (a) the
heat-transfer rate per unit area if the
shield were not present, (b) the heattransfer rate per unit area with the shield
present, and (c) the temperature of the
shield.
11. Find the radiation shape factors for the
following five figures as mentioned
alongside them.