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Flame speeds

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Flame speeds

Unread postAuthor: WOW!! » Sat Mar 03, 2007 7:34 pm

Does any know the flame speed of a 2x/2atm mix?
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Last edited by WOW!! on Sat Mar 03, 2007 7:51 pm, edited 1 time in total.
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Unread postAuthor: Pete Zaria » Sat Mar 03, 2007 7:45 pm

2x mix of what? Propane and air?

Flame speed depends on lots of factors; dimensions of the combustion chamber, temperature, turbulence (for instance, from a chamber fan), etc...

DDT occurs when the flame speed approaches or exceeds the local speed of sound. Note, I said "local speed of sound" because the speed of sound is different in different gases under different pressures.

BLB, am I right or way off?

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Unread postAuthor: WOW!! » Sat Mar 03, 2007 7:48 pm

A propane air mix. For a Hybrid. I have to right a paper for school. And I forgot what DDT was for a sec :oops:
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Unread postAuthor: LGM » Sat Mar 03, 2007 8:52 pm

A school paper on hybrid cannons? You have a cool teacher.
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Unread postAuthor: Pete Zaria » Sat Mar 03, 2007 9:54 pm

@WOW!!,

Again, it depends on chamber dimensions and conditions.

I was going to tell you to search, but I tried searches under:
"flame front speed propane" "flame front speed" "flame front velocity" etc... and found very little in the way of free information. Lots of sites that want to sell you a PDF file or book for $30.

Here are two useful links I did find:
http://www.anvilfire.com/FAQs/gasfacts.htm
http://in3.dem.ist.utl.pt/LXLASER2006/d ... s/35.6.pdf

Try to get ahold of BoilingLeadBath, he'll know where you can find the info you're looking for. Damn teenage physics geniuses... :)

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Unread postAuthor: WOW!! » Sat Mar 03, 2007 10:16 pm

And use I did search before I posted. lol

But thanks for the links, and the chamber is 24'' of Sch80 PVC with no chamber fan and a NGK spark plug. With 18 layers of foil for the burst disk. I tried to get a hold of BLB.

Thanks for the help
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Unread postAuthor: willarddaniels » Sat Mar 03, 2007 11:30 pm

I had posted this a few weeks ago in another thread when someone wanted to know the differences between butane and propane... I hope this helps.

Liquification pressures: @ 70*F
Butane: 30 psi ***be sure to meter with a gauge, don't use with hybrids
Propane: 125 psi ***BEST
MAPP: 120 psi

Explosive limits in air:
Butane: 1.9-8.5% ***BEST
Propane: 2.2-9.5% ***4% is optimal
MAPP: 3.4-10.8%

Flame Temp in Air & Oxygen: (*F)
Butane: 3200 & 4925
Propane: 3450 & 4579 ***BEST for the money
MAPP: 3650 & 5301

Burning Velocity (ft/sec) - THIS IS A CRITICAL NUMBER
Butane: 11.0
Propane: 12.2
MAPP: 15.4 ***QUICKEST combustion time, fastest acceleration

Heating values, primary combustion: (BTU/Ft^3)
Butane: 315
Propane: 255 ***BEST for your chamber fan and PVC housing
MAPP: 517

You can easily find the numbers for acetylene as well, but use the stuff with extreme caution in your cannons- you can die, it is very unstable.

Looks like MAPP is the best performer, but with propane's lower cost and lower heating properties, it seems obvious why it is the most popular (did I mention that refills are cheap?)

But ... draw your own conclusions.

Furthermore:
Oxygen and other oxidizing gases do not burn, but will support the combustion of flammable materials. Increasing the concentration of an oxidizer accelerates the rate of combustion.

I don't think compressing just the propane will cause it to burn any faster, it will just add that extra psi to the thrust. I don't believe it is an exponential (as you'd see with using an oxidizer), but an additional modifier.

"An explosive substance that undergoes decomposition or combustion with great rapidity, evolving much heat and producing a large volume of gas. The reaction products fill a much greater volume than that occupied by the original material and exert an enormous pressure, which can be used for blasting and for propelling." -encyclopedia

Another edit:
"The burn rate in combustion process is very sensitive to temperature. If flame temperature is increased by 10%, the rate of combustion more than doubles." -web site regarding furnaces and combustion. This leads me to ask, what if you had some type of heat source inside a chamber?

However, I am not considering the 2x,3x,4x concept of this ...yet.
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Unread postAuthor: boilingleadbath » Sun Mar 04, 2007 3:49 pm

Jimmy found us the power-law equation for propane-air laminar flame front speed a while back...

.43m/s * (temp/298K)^2.13 * (pressure/1atm)^-.17

solving (.43*1^2.13*2^-.17) gives us .38 m/s

...but that totaly ignores:
1) increase in temperature during the combustion sequence
2) increase in pressure during the combustion sequence
3) turbulence (both r-t instabilities and that caused by obstructions)

So it's really only the velocity in the moment imediatly after the ignition.
(which, of course, is significantly before the disc ruptures - and thus before we really care about how long it takes to burn)
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Unread postAuthor: WOW!! » Sun Mar 04, 2007 4:32 pm

boilingleadbath wrote:Jimmy found us the power-law equation for propane-air laminar flame front speed a while back...

.43m/s * (temp/298K)^2.13 * (pressure/1atm)^-.17

solving (.43*1^2.13*2^-.17) gives us .38 m/s

...but that totaly ignores:
1) increase in temperature during the combustion sequence
2) increase in pressure during the combustion sequence
3) turbulence (both r-t instabilities and that caused by obstructions)

So it's really only the velocity in the moment imediatly after the ignition.
(which, of course, is significantly before the disc ruptures - and thus before we really care about how long it takes to burn)


Dam your good.
Thanks
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Unread postAuthor: willarddaniels » Mon Mar 05, 2007 10:58 am

Upon further reflection and some discussion with others ...I believe the flame speed of a 2X mixture is the same as a 1X mixture. It has twice the expansion, but without an oxidizer, the flame speed won't change. The additional pressure would make a difference in displacing the expansion, a perceived faster flame speed, but only in an additive nature.
...which is why using an oxidizer is so important.

This point leads me to ask: For each magnitude / atmosphere do you need to increase the volume of your barrel by the same ratio? The thinking behind this is to effectively use all force of the expanding gasses.
Hmm... I am guessing this is flawed somewhere- can anyone point it out? If this were the case, a hybrid would only be a way to save on the size of the combustion chamber and you would continue to have a combustion, just a very large one.
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Unread postAuthor: boilingleadbath » Mon Mar 05, 2007 5:21 pm

If the power law is to be believed, willard, the flame speed decreases slightly... which might be an effect of the increased viscosity or something.

...and as relating to hybrids, my analysis with evbec shows that increased pressure doesn't make as much of a difference on the optimum ratio as I was expecting (that being: a 3x mix requires a barrel 3 times as big)
...but the data:
1) isn't all that good
2) involves effects I don't account for

So take my statement as a vauge guess.

That said, a 2x hybrid with a chamber half as big (assuming that we retain fairly short barrels, say .7:1 C:B on the combustion) is significantly more powerful than a combustion.
Evbec (I trust it in this region) gives me roughly 600 fps with the hybrid - compared to ~500 fps with the combustion.

It's simply because the average pressure for the hybrid chamber is higher - because the pressure starts higher and returns to (roughly) the same value.
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