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40X ... understanding combustion

Post questions and info about hybrid (compressed gas with fuel) powered cannons here. This includes discussion about fuels, ratios, ignition systems, build types, safety, and anything else relevant.
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40X ... understanding combustion

Unread postAuthor: willarddaniels » Sun Mar 04, 2007 1:43 am

Please note: I am not an expert. If my logic or thinking is flawed, please kindly let me know so we can all improve and continue to live safely while having fun with our shared, somewhat demented passions. On many of these points, it is just elementary info that we live by; however, it is nice to redress some of them. On with the dissertation:

Facts to keep in mind regarding simple combustion:
#1: Complete combustion of a fuel is possible only in the presence of an adequate supply of oxygen.
#2: Most of air (79%) is nitrogen and traces of other elements. The rest is oxygen @ 20.9% Nitrogen reduces combustion efficiency.
#3: The burn rate in the combustion process is very sensitive to temperature. If flame temperature is increased by 10%, the rate of combustion more than doubles. Any safe ideas?
#4: Regulated amounts of fuel and oxygen (air) greatly increase efficiency. ie 4.02% propane to every 20.9% oxygen.
#5: Increasing the % of oxygen or other oxidizers to a combustion chamber/fuel mixture will greatly increase the combustion rate. ie. DDT
#6: Using a chamber fan enhances combustion by thoroughly mixing all fuel molecules with the oxygen. They need each other to combust, one won't do it without the other. This is why the good brazing / welding rods that mix air and fuel are longer, the two have a longer time to mix.
#7: Multiple ignition points provide a more singular combustion throughout the chamber instead of one initial point combusting and pushing the fuel/air mixture while it completes the combustion of all the fuel mixture. This multiple point ignition completes the combustion process faster, thus providing more initial thrust.
#8: Propane (most commonly used) liquifies at 125 psi. When you multiply your ratios, you have to stop at 9X because your fuel will begin to liquify above that point. At about 9X, provided all your components can withstand the combustion / detonation pressures, you max out and beyond this point you actually lose efficiency in the form of space taken up by liquified fuel and potentialy unsafe second combustions.
#9: Since you need oxygen and not nitrogen to combine with your fuel of choice, why not regulate O2 into your chamber instead of compressed air? Benefits: Higher Multipliers with the same initial pressure (40+X @ 125 psi). Consistently accurate and efficient firings. More options. ie. 9X fuel and 18X O2 level for an even faster burn.
#10: MAPP liquifies at 120 psi; however, if you use the fuel / O2 mix, this is not a problem. MAPP has a 26% faster burning velocity than propane. If you are going to spend the money on the O2, you may as well use MAPP instead of propane. You should use 5% MAPP as opposed to 4% propane.

If I were to go above 6X, I would definitely invest in a digital peak pressure gauge in order to stay within some safety parameters. This way you can see what the peak pressures are during combustion using oxidized mixtures and slowly go up to 40+X. Of course this is with all metal housings, fittings, etc.
Call me crazy, but I'll let you know how it goes if I can ever get my wife to allow me to invest over a grand to try this out. "Honey! Don't cancel the life insurance policy yet!"
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Attachments
combus10.gif
see #4. If you remove excess air from your chamber by regulating your fuel and your oxygen percentages, you will increase your temperature which greatly increases your flame speed.
combus10.gif (16.61 KiB) Viewed 1322 times
common2.gif
What you want to happen: use up all the oxygen (O2), thus maximizing your combustion.
common2.gif (4.64 KiB) Viewed 1322 times
common4.gif
Typically what you see when you use the trial and error method of giving a short blast of propane / MAPP / butane
common4.gif (4.58 KiB) Viewed 1322 times
common3.gif
These next 3 pics are representing a hydrogen / carbon gas as fuel on the left and oxygen / nitrogen as air on the right.

Using -sprays or aerosol propelled liquids that do not evaporate completely and are, for practical purposes, impossible to measure.
common3.gif (4.67 KiB) Viewed 1322 times
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Unread postAuthor: boilingleadbath » Sun Mar 04, 2007 3:56 pm

The 10*C -> rate doubles contradicts the power-law expression that Jimmy uses.

(the relevent portion being (temperature/298k)<sup><b>2.13</b></sup>)
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Unread postAuthor: willarddaniels » Sun Mar 04, 2007 4:28 pm

I was quetioning that myself, but I found that particualr tidbit on a reputable business site regarding various gases and their combustion rates, so I included it. I figure that personal research will best determine ...wait, let me bring up excell... okay, according to "Jimmy's power-law expression", every time you increase the flame temperature by 10%, you increase the flame velocity by 23%. Still, a 23% or 46% or 69% boost is sweet.

The essential point is to increase the flame temp- the only way to do that is by adding an oxidizer, which is covered in point #5. I guess in my tired state at midnight and haste to post this, I covered the same aspect twice.
I suppose if you heated the mixture prior to ignition you may get faster burns, but... ahh, more research is needed.
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Unread postAuthor: drac » Sun Mar 04, 2007 8:39 pm

I'm not a chemistry whiz, so i didn't understand much of that, but did you take into account partial pressure? I've heard about that a lot when reading about the pressurization of propane.
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Unread postAuthor: boilingleadbath » Sun Mar 04, 2007 10:56 pm

The .43 constant is for perfect propane-air mixes, so for the purposes of the calculation, the partial pressure of the propane is directly proportional to the total pressure of the mix.

Ah, a buisness site.
My geuss? They called up one of their buddies who took AP chem (because, you know, he knows chemistry stuff) and asked him about the relationship.
And their buddy told 'em that it doubles if you increase the temperature 10*C.

This describes the rate of many reactions fairly well - but I don't think it really applies to combustion.
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Unread postAuthor: willarddaniels » Mon Mar 05, 2007 10:37 am

About that web site: a business's site about what they train HVAC guys who come to your house to install/fix your gas funace. It is from a school / training center. ...apparently they seem to dramatize this point, but the rest of their info is great.
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Unread postAuthor: joannaardway » Mon Mar 05, 2007 2:37 pm

Actually, you've given me a thought about this kind of stuff! I should have thought that temperature was doubly important - both for the pressure, and the rate of reaction.

You may not be an expert, but you've definately done an excellent job of summing up the facts.

I think I'm going to have to design an uber-combustion with all of these facts in mind, and then try and get it built.

I expect good things from it.
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Novacastrian: How about use whatever the heck you can get your hands on?
frankrede: Well then I guess it won't matter when you decide to drink bleach because your out of kool-aid.
...I'm sorry, but that made my year.
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Unread postAuthor: Fnord » Mon Mar 05, 2007 4:15 pm

Idea for heating the mix:
Put 1 inch peice of pvc pipe on where your gas inlet is and glue it, so that th inlet is at the bottom of sort of a "cup", then put layers of copper mesh(can be found at most hardware stores) in the cup. As the gun fires, the copper will store the wasted heat energy and warm the propane as it enters.
You could also line the chamber with copper to heat up the O2 you put in.
someone should try this to see if it helps any.
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Unread postAuthor: willarddaniels » Mon Mar 05, 2007 10:44 pm

I have collected all the pieces (exception: copper 1/2" union and piezo) from my shop to make a simple, mini hybrid from copper. The problem: it isn't so mini once you add all the T's, gauges and valves for metering. The whole thing, barrel included, will be 1/2" pipe and about 30" long.
I'm making this mini to use O2 as well.

Unfortunately, no digital pressure gauge yet... The cheapest one I have found so far is $252.00 - and it would need to be ordered. I will have to try and order it through work to get a better deal.
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Unread postAuthor: pyrogeek » Mon Mar 05, 2007 10:53 pm

Warmer air is less dense, and would contain less oxygen. For this reason, they run intercoolers on vehicles. A colder incoming air supply is more dense, and has more oxygen. If you heat it after sealing the chamber, I guess it could improve performance since it would increase the pressure it starts at. Something like a heating element out of a toaster or something would work, I guess.
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Unread postAuthor: willarddaniels » Mon Mar 05, 2007 10:59 pm

<grins> now we need yet another gauge on our hybrids: temp
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