Background
These
activities are designed to demonstrate convective heat transfer while
also looking at some common convection experiences. There is a common
misconception, for example, that a ceiling fan will cool air in the room
that it’s in. This activity will show that non-wetted surfaces will
not be cooled by moving air if the surface and air are the same temperature,
but that convection can cool the surface of objects (or people) that are
warmer than the air.
Materials
- Lab
stand (1 total)
-
Clamps and clamp holders (as needed to hold thermometer)
-
Thermometer or electronic temperature sensor with displays (1 total)
-
Small box or circular fan (1 total)
Part 1- Do fans cool the air?
- Set
up test stands as shown in
Figure 1.
- Make
sure the temperature sensor or thermometer is between 12"
to 18" from the fan
- Make
sure the fan will not be drawing air from an area in the room
that is not generally representative of the room temperature
(e.g. near a radiator or window).
- Since
temperature differences will be small, as much as possible,
keep back a few feet from the set-up to minimize undesired variables.
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Figure
1: Example of Convection Activity 1 setup.
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- Have
students take air temperature measurements every minute for 5
minutes (after waiting at least 5 minutes after setting up the
experiment to allow the thermometer or sensor to show ambient
conditions).
- Turn
on the fan for 5 minutes, again having students take
the air temperature measurements every minute.
- Turn
off the fan and have students take air temperature measurements
an additional 5 minutes.
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Data
Analysis and Discussion
- Plot and
average the data.
- Ask the
questions:
- What
happened when the fan was turned on?
- What
happened when it was turned off again?
- What
can we conclude?
- Why?
- Even if
your results didn’t clearly show this, include in discussion that
fans will actually slightly heat the air in a room because of the heat
given off by the motor.
Convection
Activity 1 Sample Results |
Fan
Status |
Time
(min) |
Tair
(oF) |
Off
|
0 |
73.2 |
On |
1 |
73.2 |
On |
2 |
73.2 |
On |
3 |
73.2 |
On |
4 |
73.4 |
On |
5 |
73.4 |
Off |
6 |
73.4 |
Off |
7 |
73.6 |
Off |
8 |
73.4 |
Off |
9 |
73.2 |
Off |
10 |
73.4 |
Part
2- Do fans cool people?
- Tape
a thermometer or temperature sensor to the arm of a volunteer
student.
- Have
the student stand or sit so the temperature sensor or thermometer
is again between 12" and 18" from the fan.
- Make
sure the fan will not be drawing air from an area in the room
that is not generally representative of the room temperature (e.g.
near a radiator or window).
|
Figure
2: Example of Convection Activity 2 setup.
|
- Have
students take temperature measurements every minute for 5 minutes
(after waiting at least 5 minutes after taping on the temperature
sensor).
- Turn
on the fan for 5 minutes, again, having students take temperature
measurements every minute.
- Turn
off the fan, and again, take temperature measurements an additional
5 minutes (you’ll need a patient volunteer).
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Data
Analysis and Discussion
[From the
last experiment some students may have gotten the idea that the student
will not be cooled by the fan.]
- Plot the
data.
- What
happened when the fan was turned on?
- What
happened when it was turned off again?
- What
can we conclude?
- Why?
- Note that
the convection is causing two effects:
- Since
the surface of the skin is warmer than the air the fan is blowing
on it, it is being cooled.
- Cooling
that is actually produced by moisture on the skin is evaporating.
- If you
live in a colder climate or it’s winter, also discuss wind chill,
which is the same cooling effect of moving air on exposed skin, but
is caused by wind and is generally an undesired cooling effect!
- So based
on what we’ve learned, how can we improve how we use ceiling fans?
- We
know that ceiling fans only do us any good if there is someone in
the room to benefit from the air movement.
- Research
has shown that because of the cooling we saw in the Part 2 experiment
above, effective use of ceiling fans can allow thermostat setpoints
to be raised 2-3oF and occupants be just as comfortable
as they were at a lower temperature without the fans.
- Figure
3 shows simulation results indicating increased home cooling energy
use though from ceiling fans (because of motor uses electricity
and produces heat) unless the thermostat setpoint is increased.
- The
key to achieving energy savings from ceiling fans is to also raise
setpoints when fans are in use.
- Using
ceiling fans properly can also save energy on days when turning
on a fan will allow a family not to turn on the air conditioner
at all when they would have chosen to if the fans were not available.
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Figure
3: Simulation plot shows that cooling energy use is increased when
ceiling fans are used unless thermostat setpoint is also raised. |
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Individual pages from this web site may be printed
and duplicated for
student classroom use
provided that proper credit is given to
the Florida Solar
Energy Center (FSEC).