MECH 352: HEAT TRANSFER 1
EXPERIMENT 3: Combined Convection and Radiation
Performed on 07 October 2019
Submitted on 21 October 2019

OBJECTIVE
This experiment is to show the effects of natural and forced convection on the rate of heat
transfer and how it differs to generate each type of convections on a simple cylinder.
INTRODUCTION
Exercise A
: NATURAL CONVECTION
Heat is transferred by means of two mechanisms, natural convection and forced convection,
keeping in mind that radiation occurs simultaneously in both cases. Natural convection occurs
when the surrounding air is relatively stagnant and there exists a temperature difference with
the surface area of the exposed body (cylinder) and the air. The natural convection then
transfers heat from the hot body to the surrounding air, which results in the rise of heated air
due to decreased density, also heat is transferred via radiation.
Fig 1. Apparatus that natural and forced convections will occur
Natural convection makes the rate of heat transfer limited by slight movements of air, because
of the air that gained thermal energy and the replacement of colder air at the hot surface, and
so on.
1

Some important equations that will be used in this Exercise are the followings:
Total heat loss:
Q
tot
=
Q
c
+
Q
r
The average heat transfer coefficient h
cm
can be obtained using the below relationships:
T
film
=
T
s
+
T
a
2
β
=
1
T
film
Gr
D
=
gβ
(
T
s
−
T
a
)
D
3
ν
2
Since Ra
D
= Gr
D
* Pr →
Ra
D
=
gβ
(
T
s
−
T
a
)
D
3
ν
2
∗
Pr
Mean Nusselt number:
Nu
m
=
c
∗
(
Ra
D
)
n
Average heat transfer coefficient for natural convection:
h
cm
=
k Nu
m
D
Average heat transfer coefficient for radiation:
T
(
¿¿
s
4
−
T
a
4
)
T
s
−
T
a
¿
h
rm
=
σ
∗
ξ
∗
F
∗
¿
Exercise B
: FORCED CONVECTION
In forced convection condition, the air movement can be greatly increased by an external means
such as a fan, compressor, etc. Now, air will be moving at a much faster rate, which results in a
higher rate of heat transfer. Thus, the exposed body to forced convection will cool the body
quicker and more efficient than natural convection. As the air flows faster, the quicker the hot
body cools off, thanks to the increased rate of heat transfer.
Important equations for forced convection:
Average heat transfer coefficient for forced convection:
h
fm
=
k Nu
m
D
Mean Nusselt number:
Nu
m
=
0.3
+
0.62
ℜ
0.5
Pr
0.33
[
1
+
(
0.4
Pr
)
0.56
]
0.25
∗
[
1
+
(
ℜ
282000
)
0.5
]
Reynolds number:
ℜ=
U
c
D
ν
2