The Florida Solar Energy Center Logo
 
 
 
    Home > Education > K-12 > Curricula > Building Performance Matters > Lesson 3 >Connvective Heat Transfer Experiments

Lesson 3: Learning Activity -- Convective Heat Transfer Experiments

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?

  1. 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.
Photo of temperature sensor hooked to apparatus
Figure 1: Example of Convection Activity 1 setup.
  1. 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).
  2. Turn on the fan for 5 minutes, again having students take the air temperature measurements every minute.
  3. Turn off the fan and have students take air temperature measurements an additional 5 minutes.

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?

  1. Tape a thermometer or temperature sensor to the arm of a volunteer student.
  2. Have the student stand or sit so the temperature sensor or thermometer is again between 12" and 18" from the fan.
  3. 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).
Photo of temperature sensor hooked to a student
Figure 2: Example of Convection Activity 2 setup.
  1. Have students take temperature measurements every minute for 5 minutes (after waiting at least 5 minutes after taping on the temperature sensor).
  2. Turn on the fan for 5 minutes, again, having students take temperature measurements every minute.
  3. Turn off the fan, and again, take temperature measurements an additional 5 minutes (you’ll need a patient volunteer).

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:
    1. Since the surface of the skin is warmer than the air the fan is blowing on it, it is being cooled.
    2. 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?
    1. We know that ceiling fans only do us any good if there is someone in the room to benefit from the air movement.
    2. 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.
    3. 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.
    4. The key to achieving energy savings from ceiling fans is to also raise setpoints when fans are in use.
    5. 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.
 
Bar graph of energy use versus savings
  
  Figure 3: Simulation plot shows that cooling energy use is increased when ceiling fans are used unless thermostat setpoint is also raised.  

 

Home | Teacher's Guide | Lesson Plans | Learning Objectives and Tests
National Science Standards | Glossary | References | Site Map | Feedback Form


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).