Q&A
1. Define the following:
a. Adsorptivity
b. Reflectivity
c. Transmissitivity
d. Emissivity
2. What is the view factor F21 of a sphere (1) of diameter D inside a cubical box (2) of
lngth L=D?
3. A space satellite in the shape of a sphere is travelling in outer space, where its
surface temperature is held at 283.2 K. the sphere sees only outer space, which can
be considered as a black body with a temperature of 0K. the polished surface of the
sphere has an emissivity of 0.1. Calculate the heat loss per m2by radiation?
4. A plane surface having an area of 1.0 m2 is insulated on the bottom side and is
placed on the ground exposed to air at 290 K, and the convective heat transfer
coefficient from the air is 12 W/m2-K. The plane radiates to clear sky. The effective
radiation temperature of the sky can be assumed as 80K. If the plane is a black body,
calculate, the temperature of the plane at equilibrium.
5. A horizontal oxidized steel pipe carrying steam and having an OD of 0.1683m has a
surface temperature of 374.9K and is exposed to air at 297.1 K in a large enclosure.
Calculate the heat loss for 0.305m of pipe from natural convecton plus radiation. For
the steel pipe, use an ε of 0.79.
1. A. Absorptivity- the degree to which something absorbs energy, a liquid or
another substance.
B. Reflectivity- the property of reflecting light or radiation, especially reflectance as
measured independently of the thickness of a material.
C. Transmissitivity- the degree to which a medium allows something in particular
electromagnetic radiation.
D. Emissivity- the relative power of a surface to emit heat by radiation.
2. F21 = 0
3. T=283.2 K
T=0K
e=0.1
Q=?
Q=ɑE(Ts4-TA4)
Q=(5.67x10-8 W/m2K4) (0.1)(283.24-04)
Q= 36.4717 W/m2
4. A=1.0m2
12=
T=290K
T=80K Teq=?
h=12 W/mK
T 2  250
1
T2 = 266K
5. OD of 0.1683m
Ts=374.9K
Tair=297.1K Q=? L=0.305m
e=0.79
Q=ɑE(Ts4-TA4)
A=2πrh+2πr2 = 2π(0.1683/2)(0.305)+2π(0.1683/2)2=0.2058
Q=0.2058(0.79)(5.67x10-8)(374.944-297.14)
Q=163.3 W