Air convection through a tube

Kenna007 · Post by **Kenna007** » Sun Dec 15, 2013 1:39 pm

I am helping my 6th grade daughter with her science fair project but we need some additional help. She is trying to prove convection of air through a vertical tube (as part of the project). By heating the top part of the tube with a heat lamp or blow dryer (or any other ideas?), she is hoping that, by hot air rising, air will be pulled through the bottom of the tube. She needs a way to prove/show this, and if possible, to measure the air flow. To show air movement, which will probably be minimal, I don't believe a light streamer will work and thought smoke might work but don't know of a way to produce a light stream of smoke (and not smoke-out the class room). Another possibility might be some sort of wind measuring device but don't know if something like this can be inexpensively acquired, especially one that will register low air speeds. Or are there any other ideas?

We thank anyone with some helpful insight!

Kenna007 · Post by **Kenna007** » Sun Dec 15, 2013 2:16 pm

I am helping my 6th grade daughter with her science fair project but we need some additional help. She is trying to prove convection of air through a vertical tube (as part of the project). By heating the top part of the tube with a heat lamp or blow dryer (or any other ideas?), she is hoping that, by hot air rising, air will be pulled through the bottom of the tube. She needs a way to prove/show this, and if possible, to measure the air flow. To show air movement, which will probably be minimal, I don't believe a light streamer will work and thought smoke might work but don't know of a way to produce a light stream of smoke (and not smoke-out the class room). Another possibility might be some sort of wind measuring device but don't know if something like this can be inexpensively acquired, especially one that will register low air speeds. Or are there any other ideas? Indicating air movement is most important, measuring air flow is secondary.

We thank anyone with some helpful insight!

rmarz · Post by **rmarz** » Sun Dec 15, 2013 3:28 pm

Kenna007 - This sounds like a very nice project, and you seem to understand, very well, the difficulty in measuring a very slight movement of air with some precision. These are very low levels of wind current or pressure differentials. A couple approaches come to mind. I don't know what the measurements of the tube are or what it is constructed of. It would seem to me that a longer tube will enable you to keep the top, or hot zone, at a significant temperature differential to the lower part of the tube. If the tube is metal, like a length of metal conduit, perhaps a foot or so would be fine. As far as a sensor goes, here is one possible approach. A small, very low mass fan blade placed near the bottom of the tube. Perhaps made of paper with a low friction bearing, like metal foil, slightly and carefully deformed to form a dimple at the top, that could rest on a sewing needle or pin. This might give good response to low velocity air currents. Clearly the use of a longer tube is important if you are using a heat gun or hair dryer. You don't want to disturb the air near the bottom where you are making measurements. The longer tube might also enable an increase in velocity that would be helpful. Attached is a link to some commercial sensors that suggest the ability to resolve to 0.01 meters of air flow per second. Note that the sensor is a very small 6 bladed fan. That might be something like the design I'm suggesting. Good luck.

http://www.pce-instruments.com/english/ ... 1347_1.htm

Rick Marz

Kenna007 · Post by **Kenna007** » Sun Dec 15, 2013 5:56 pm

Sounds like you are right on track. We will take a look at the link you posted shortly. We are using1/2" copper pipe rather than steel conduit because copper conduces heat quicker. The pipe will be several feet long for the reasons you mentioned - to avoid the heating "element" (hair dryer or other device) interfering with registering the air entering the pipe and to prevent the heat applied at the top traversing all the way to the bottom of the pipe. I think we can come up with a paper-finned fan blade suspended by a needle. That ought to be ideal (especially for the price) and sensitive enough to register low volumes of air.

The second part of the project involves a bend in the pipe at the bottom that leads to a section of pipe buried in dirt and ending outside of the dirt. The entirety of the project is to use convection properties to draw air through a geothermal heat sink.

Thank you for your quick response. Any further suggestions by anyone would still be appreciated.