Duhok Technical Institute Petrochemical Department
Heat Transfer Lab. 2nd Year Majeed.Abdulrazaq@uoz.edu.krd
Experiment No. 5
Free and forced convection heat transfer
from Heated Surfaces
1- Objective:
To demonstrate the use of extended surfaces to improve heat transfer
from heated surfaces
2- Introduction:
Convection is mechanism of heat transfer through a fluid in the
presence of bulk fluid motion, or heat transfer between a fluid and a solid
surface takes place by the movement of fluid particles relative to the
surface. convection is classified as natural (or free) and forced convection
depending on how the fluid motion is initiated. The intensity of the
mixing motion is generally high in forced convection and consequently
the heat transfer coefficients are higher than free convection.
If the movement of fluid particles is caused by means of external
agency such as pump or blower that forces fluid over the surface, then the
process of heat transfer is called forced convection.
There are certain situations in which the fluid motion is produced
due to change in density resulting from temperature gradients, then the
process of heat transfer in this case called free convection. The movement
of fluid in free convection is due to the fact that the fluid particles in the
immediate vicinity of the hot object become warmer than the surrounding
fluid resulting in a local change of density (become lesser). The warmer
fluid rises and replaced by the colder fluid creating convection currents.
The force which induces these convection currents is called a buoyancy
force which is due to the presence of a density gradient with in the fluid.
(1)
Duhok Technical Institute Petrochemical Department
Heat Transfer Lab. 2nd Year Majeed.Abdulrazaq@uoz.edu.krd
Figure (1) force and free convection heat transfer
3- The apparatus :
The apparatus, consists from a vertical rectangular duct with
internal fan, the ambient air enters the duct at the bottom and heated air
leaves the duct at the top , the air velocity in the duct is indicated on a
portable anemometer and the anemometer sensor is inserted through the
wall of the duct , a three heated surfaces ( a flat plate , pinned, or finned)
each surface incorporates an electrical heating element with thermostatic
protection against overheating , the temperature at the base of each
surface is monitored by a thermistor sensor. The heated surfaces may be
viewed through an acrylic window in the wall of the duct.
The display and control unit incorporates power regulator with
digital reader (wattmeter) to control and indicate power supply to heated
surfaces, and also contains on the variable DC voltage power supply to
control on the fan speed. and digital reader to indicate temperature of the
heated surfaces.
heated
surface
Figure (2) free and forced convection apparatus
(2)
Duhok Technical Institute Petrochemical Department
Heat Transfer Lab. 2nd Year Majeed.Abdulrazaq@uoz.edu.krd
4- Procedure:
1- Clamp the flat plate surface into the duct by using the two toggle clamps.
2- Connect the heater power supply lead to the socket at the front side of the
display and control unit.
3- Connect the fan power supply lead to the socket at the front side of the
display and control unit.
4- Connect the temperature probe lead to the socket at the front side of the
display and control unit. Then connect the temperature probe to the socket
at the base of flat plate surface.
5- Place the portable anemometer into the bracket on the duct wall, then
insert the anemometer sensor into wall of the duct and ensure that the
sensor hole is aligned with the direction of the airflow.
6- Switch on the main power supply.
7- Set the heater power supplied to the flat plate surface by rotating the
power control knob clockwise.
8- Set the fan speed by rotating speed control knob clockwise.
9- After the device reached steady state condition, record all temperatures,
heater power, and air velocity.
10- Repeat the above procedures at different air velocities and for the finned
plate and pinned plate.
Ambient air temperature Ta=24 °C Input power = 20 W
Flat plate surface
Run No. Air Velocity Plate Temperature (TH-TA) °C
(m/s) (TH) °C
1 0 90
2 1 84
3 2 78
4 2.5 75
Ambient air temperature Ta=24 °C Input power = 20 W
Finned plate surface
Run No. Air Velocity Plate Temperature (TH-TA) °C
(m/s) (TH) °C
1 0 37
2 1 34
3 2 32
4 2.5 30
(3)
Duhok Technical Institute Petrochemical Department
Heat Transfer Lab. 2nd Year Majeed.Abdulrazaq@uoz.edu.krd
Ambient air temperature Ta=24 °C Input power = 20 W
Pinned plate surface
Run No. Air Velocity Plate Temperature (TH-TA) °C
(m/s) (TH) °C
1 0 54
2 1 46
3 2 39
4 2.5 35
5- Discussion:
1- Plot a graph of air velocity against temperature difference (TH-TA)
for the heated surfaces (flat, finned, and pinned) .
2- Indicate which one is best for dissipating heat. and why?
3- What are the sources of the error? How can be it minimized?
4- Discuss all results.
(4)
