INTRODUCTION
Heat transfer by simultaneous conduction and convection, whether free or forced, forms
the basis of most industrial heat exchangers and related equipment. The measurement and
prediction of heat transfer coefficients for such circumstances is achieved in the Armfield
apparatus by studying the temperature profiles and heat flux in an air duct with associated
flat and extended transfer surfaces. The vertical duct is so constructed that the air
temperature and velocity can be readily measured, and a variety of plug-in modules of
heated solid surfaces of known dimensions can be presented to the air stream for detailed
study. A fan situated at the top of the duct provides the air stream for forced convection
experiments.
An independent bench mounted console contains temperature measurement, power
control, and fan speed control circuits with appropriate instrumentation. Temperature
measurement, to a resolution of 0.1 deg C is effected using thermistor sensors with direct
digital read-out in deg C.
Air velocity is measured with a portable anemometer mounted on the duct.
The power control circuit provides a continuously variable, electrical output of 0-100
watts with a direct read-out in watts.
Using the instrumentation provided, free and forced convective heat transfer
characteristics may be determined for:
1. A flat surface
2. An array of cylinders
3. An array of fins
Each module may be used independently on the bench, to demonstrate free convection in
a horizontal mode.
The apparatus is fully self-contained
For more information on convection visit:
Free Convection:
http://www.efunda.com/formulae/heat_transfer/convection_free/overview_free.cfm
Forced Convection
http://www.efunda.com/formulae/heat_transfer/convection_forced/overview_forced.cfm
Notes:
1. Don’t let the temperature of the heating plate exceed 100 degrees C. The heater
will automatically shut off if this happens and you won’t be able to restart the
experiment until it has cooled down sufficiently.
2. Keep hands, papers, ect away from and off of the top of the duct. There are live
exposed electrical wires and this is also the exit for the air from the fan.
3. The wattmeter is calibrated to read in the range 0 to 99.9 watts. Depending on the
supply voltage it may be possible to exceed the maximum reading on the
wattmeter if the power control knob is turned fully clockwise. This will not cause
damage to the wattmeter but readings in excess of 99.9 watts will commence
again at 0 watts. As the suggested experiments do not exceed 80 watts this
characteristic will not affect the operation of the equipment.
4. If the “Battery Low” light is on when the anemometer is set to “Velocity”, ask the
person on duty in the checkout office to change the batteries (needs 4 AA alkaline
batteries).
5. If the anemometer doesn’t appear to be working properly, ensure that the arrow
on the tip of the probe is aligned with the direction of the airflow (upwards) when
inserting the probe through the wall of the duct.
6. To set the anemometer to the zero position, slide the protective cover forward
over the probe head to isolate it from any air movement. Set the switch to
'Velocity' and adjust the knurled wheel marked 'Zero' until the pointer is aligned
with the zero on the scale.
INDEX OF EXPERIMENTS
EXPERIMENT A
To demonstrate the relationship between power input and surface temperature in free
convection.
EXPERIMENT B
To demonstrate the relationship between power input and surface temperature in forced
convection.
EXPERIMENT C
To demonstrate the use of extended surfaces to improve heat transfer from the surface.
EXPERIMENT D
To determine the temperature distribution along an extended surface.
EXPERIEMNT E
Comparison of a horizontal and vertical flat plate in free convection.
EXPERIMENT A
OBJECT OF EXPERIMENT:
To demonstrate the relationship between power input and surface temperature in
free convection.
EQUIPMENT SET-UP:
SUMMARY OF THEORY:
A heated surface dissipates heat primarily through a process called convection. Heat is
also dissipated by conduction and radiation, however these effects are not considered in
this experiment. Air in contact with the hot surface is heated by the surface and rises due
to a reduction in density. The heated air is replaced by cooler air which is in turn heated
by the surface and rises. This process is called free convection.
The hotter the temperature of the surface, the greater the convective currents and the
more heat (power) will be dissipated.
If more power is supplied to a surface, the temperature of the surface must rise to
dissipate this power.
READINGS TO BE TAKEN:
Place the finned heat exchanger into the test duct. Record the ambient air temperature
(tA). Set the heater power control to 20 Watts. Allow sufficient time to achieve steady
state conditions before noting the heated plate temperature (tH). Repeat this procedure at
40, 60 and 80 Watts.
RESULTS:
Ambient air temperature (tA) =
deg C
Plot a graph of power against surface temperature (tH - tA).
Input power (W)
20
40
60
80
Heater Temperature (tH deg C)
tH-tA deg C
EXPERIMENT B
OBJECT OF EXPERIMENT:
To demonstrate the relationship between power input and surface temperature
in forced convection.
EQUIPMENT SET-UP:
SUMMARY OF THEORY:
In free convection the heat transfer rate from the surface is limited by the small
movements of air generated by this heat. More heat is transferred if the air velocity is
increased over the heated surface. This process of assisting the movement of air over the
heated surface is called Forced Convection. Therefore a heated surface experiencing
forced convection will have a lower surface temperature than that of the same surface in
free convection, for the same power input.
READINGS TO BE TAKEN:
Place the finned heat exchanger into the duct. Note the ambient (tH). Set the heater power
control to 50 Watts. Allow sufficient time to achieve steady state conditions before noting
the heated plate temperature (tH). Set the fan speed control to give a reading of 0.5m/s on
the thermal anemometer, allow sufficient time to achieve steady state conditions. Record
heated plate temperature. Repeat this procedure at 1.0m/s and 1.5m/s.
RESULTS:
Ambient air temperature (tH)
deg C
Power input
Air Velocity m/s
0
1
2
3
Watts
Heater Temperature (tH deg C)
Plot a graph of air velocity against temperature.
tH-tA deg C
EXPERIMENT C
OBJECT OF EXPERIMENT:
To demonstrate the use of extended surfaces to improve heat transfer from the
surface.
EQUIPMENT SET-UP:
SUMMARY OF THEORY:
Heat transfer from an object can be improved by increasing the surface area in contact
with the air. In practice it may be difficult to increase the size of the body to suit. In these
circumstances the surface area in contact with the air may be increased by adding fins or
pins normal to the surface. These features are called extended surfaces. A typical
example is the use of fins on the cylinder and head of an air-cooled petrol engine. The
effect of extended surfaces can be demonstrated by comparing finned and pinned surfaces
with a flat plate under the same conditions of power input and airflow.
READINGS TO BE TAKEN:
Place the flat plate heat exchanger into the duct. Record the ambient air temperature (tH).
Set the heater power control to 75 Watts. Allow the temperature to rise to 80 deg C, then
adjust the heater power control to 20
Watts until a steady reading is obtained. Record heated plate temperature (tH). Set the fan
speed control to give 1m/s using the thermal anemometer. Repeat this procedure at 1.5
and 2.0m/s for the flat plate. Replace the flat plate with the finned plate and repeat
experiment. Replace the finned plate with the pinned plate and repeat experiment.
RESULTS:
Ambient air temperature =
Input power
= 20
Air Velocity m/s
0
1
2
3
deg C
Watts
Heater Temperature (tH deg C)
tH-tA deg C
Plot graphs of velocity against temperature for each of the plates.
Comment on the correlation between total surface area of the heat exchanger and the
temperature achieved.
FURTHER EXPERIMENTS:
Increase power input and repeat experiments.
EXPERIMENT D
OBJECT OF EXPEPIMENT:
To determine the temperature distribution along an extended surface
EQUIPMENT SET-UP:
SUMMARY OF THEORY:
For a heat exchanger to be 100% efficient the complete extended surface temperature
must be the same as the back plate. In practice however this does not occur because heat
must flow along a pin or fin by conduction, which causes a temperature gradient to occur.
The steeper the gradient the less efficient the heat exchanger. The efficiency however
must not be confused with surface area e.g. comparing pinned and finned. In reality the
pin is more efficient, but in this particular case the fin is marginally more efficient
because of its surface area.
READINGS TO BE TAKEN:
Measure from the back plate the distances of the three access holes on the pinned and
finned heat exchangers. Place the pinned heat exchanger into the duct. Note the ambient
air temperature (tA). Set the heater power control to 60 Watts. Allow sufficient time to
achieve steady state conditions before noting the heated plate temperature (tH). Insert the
anemometer probe into the duct through the hole nearest the heated plate ensuring that
the tip of the probe is in contact with the pin. A small amount of heat conducting
compound on the tip will ensure a good thermal contact. Note this temperature (tl).
Record the pin temperatures (t2 and t3) using the remaining two holes. Repeat this
procedure at 1.0 m/s and 2 m/s.
Remove the pinned heat exchanger and replace with the finned heat exchanger. Repeat
complete experiment.
RESULTS:
Ambient air temperature (tH)
Power input
Distance of nearest hole
Distance of middle hole
Distance of farthest hole
Air velocity m/s
T1
deg C
Watts
mm
mm
mm
Temperature deg C
T2
T3
T4
0
1
2
Plot graphs of surface temperature against distance from back plate for the two heat
exchangers at various air velocities.
For a heat exchanger to be 100% efficient, the whole of the extended surface must be at
the same temperature as the back plane.
In practice, this cannot occur because the flow of heat along the pins or fins by
conduction causes a temperature gradient to occur. The greater this gradient, the less
efficient the heat exchanger will be.
The efficiency of the heat exchanger must not be confused with the effect of a change in
surface area. For example, if the pinned and finned heat exchangers supplied with the
equipment are compared, the pin is more efficient than the fin (slightly smaller
temperature gradient) but the finned exchanger has a significantly larger surface area than
the pinned exchanger and can dissipate more heat for the same surface temperature.
EXPERIMENT E
OBJECT OF EXPERIMENT:
Comparison of a horizontal and vertical flat plate in free convection.
EQUIPMENT SET-UP:
SUMMARY OF THEORY:
When a temperature difference is established between a wall and a stationary fluid, the
fluid adjacent to the wall will move upward if the wall temperature is higher than that of
the fluid, and downward if the wall temperature is lower. Density gradients are set up in
the fluid resulting in buoyancy forces and free convective currents. The rate of heat
transfer depends mainly on the fluid motion. The orientation of the plate affects this
movement of air. A horizontal plate restricts the movement of air and reduces the heat
transfer. The same plate mounted vertically will give improved heat transfer.
READINGS TO BE TAKEN:
NOTE: The wattmeter is calibrated to read in the range 0 to 99.9 watts. Depending on the
supply voltage it may be possible to exceed the maximum reading on the wattmeter if the
power control knob is turned fully clockwise. This will not cause damage to the
wattmeter but readings in excess of 99.9 watts will commence again at 0 watts.
Place the flat plate exchanger horizontally onto a suitable surface. Record the ambient air
temperature (tA). Set the heater power control to 15 Watts. The surface temperature of
the flat plate (tH) can be recorded using the temperature probe, the tip of the probe being
held in contact with the surface by hand. To ensure good contact the thermal compound
should b used. Allow sufficient time to achieve steady state conditions before noting the
heated plate temperature. The plate temperature should be taken at the centre of the plate.
Place the flat plate vertically taking care not to touch the heated surface with your hands.
Repeat the experiment.
RESULTS:
Ambient air temperature (tA)
Input power
deg C
= 15
Heater Temp (tH) deg C
Watts
tH-tA
Horizontal
Vertical
Comment on the effects of plate orientation on surface temperature.