Supermacman is in the right path, but you have to remember that what
you have is a NON-PREMIXED FLAME...this is very important, because if
you go about looking for equations on the web, most will be for
pre-mixed flames and do not apply to your situation.
The difference is this: In a premixed flame, the fuel and oxygen are
mixed at the molecular level. Think like inside your gasoline
engine's cylinders. The fuel gets injected into the cylinder along
with air. Gets mixed (relatively evenly), and ignition.
However, in the case of a candle or campfire or even a bic lighter,
your fuel and oxygen are not mixed pre ignition. You have a definite
boundary between your fuel and oxygen, and thus we have a non-premixed
flame. A Diesel engine is another example of this.
(Some nice things to mention to the judges - they'll be impressed)
Now, what you are looking for is the adiabatic flame speed, which is a
indicative factor in how fast a flame propogates through a medium, and
thus how fast a fuel burns. However, this equation only applies to
pre-mixed flames, so it does not do you any good.
But to answer your question, supermacman has the right idea. The more
carbons in an atom, the slower it burns in both premixed and
non-premixed flames; no source required; here's the reason:
In a combustion reaction, you need three things: 1) fuel 2) oxygen 3)
spark/ignition. And, all three of these are necessary before
combustion can take place. Well, technically, if you throw O2 and
methane into a sealed box with no spark and let them sit forever and a
day, they will react. But in terms of what you and I think of
combustion (to provide light/heat/horsepower), you need all three.
Most important, though, is that you need them in the proper
proportions, preferably stoichiometric. In heavier carbon chains, you
need significantly more oxygen atoms than you do for light carbon
chains, which they usually have a difficult time obtaining since they
can only get O2 at the flame, and it is based on the surface area of
your fuel. The process of getting oxygen is far more important than
the type of fuel. So it's not that the fuel necessarily burns slower,
it's that the fuel cannot obtain enough oxygen to burn at the
appropriate rate. Increase the surface area of your fuel and you will
get faster burn times.
Don't believe me? Try this. Get a narrow (1-inch) candle made of
paraffin wax. Next, get a block of paraffin wax and place it so the
largest area side is flat on a table. So, the height here should be
about an inch and the width/length 6x4 or whatever they come in. Put
20 wicks in it and light them simultaneously. (um....maybe you should
have a fire extinguisher handy for this one). The candles are of the
same material, but the 2nd candle can obtain more oxygen and thus
burns way way faster than the first. NOTE: THIS CANDLE WILL BURN
REALLY FAST AND BIG!!
So, that's the reason your lower carbon count chains burn faster...has
not much to do with the actual fuel; has more to do with oxygen.
Therefore, now that you know the "magic formula", for your experiment,
you have to make sure all your candles are the same size. Your wicks
have to be the same size, and so on. That way, one of your (biggest)
variables is eliminated.
Your son can also increase surface area to volume ratios and determine
burn rates thereof. (it would be a good idea to do both experiments --
when he presents, tell him to say something like I did some research
and found surface area is very important, so I did more
experiments...blah blah)
A final option is to perform the experiment in an oxygen-rich
environment. Contact your local university for this one - usually
some sort of sealed steel box which they pump full of O2 and perform
experiments in...very dangerous, don't try to build one
yourself...standard disclaimer.
Good luck - if you have any other q's, you can re-post or email me at
f es en joon [a t] yahoo [d.o.t.] co m.
By the way, my username has no spaces in it, and sorry for the cryptic
language - just trying to prevent stupid bots from spamming me. |
