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Burnign speed of hybrid gas mix ?

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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Burnign speed of hybrid gas mix ?

Unread postAuthor: matti » Thu May 03, 2012 1:37 pm

Hi, is there a way to calculate the "explosion" speed of hybrid gas mixtures, for a given mix number (10x or like 4X mix).. I hope it changes depending on the X mix otherwise ill be very Embarrassed.
I dont remember seeing any information about it here or wiki.

I never used Hgdt, can it help with this ?(Im more like pneumatic guy, i use ggdt lol )



btw I just got my new very high pressure chamber for my hybrid, its made from hydraulic cylinder. The pressure its designed to take is at least 300bar. chamber ID 63mm and about 250mm long (has wall thickness of ~20mm :D )
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Unread postAuthor: jimmy101 » Fri May 04, 2012 4:27 am

IIRC the flame speed decreases slightly as the pressure (or density) of the mixture goes up. That is somewhat counterintuitive.

But, as the pressure (or density) goes up the energy released per unit of flame front movement goes up. So about the same burn time but more energy released per unit time for the higher pressure.

Fiddling with HGDT indicates that the burn speed is basically constant from 1x to 10x. Though I don't know how careful Dave was with that part of the calculation.
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Unread postAuthor: DYI » Fri May 04, 2012 7:14 am

I'll admit that I'm far from having a strong background in the field, but what I know about the subject indicates that effective flame propagation rate increases with increasing pressure. "Burning Rate Studies of Fuel Air Mixtures at High Pressures" (Knapton, Stobie, Krier) seems to contradict this conclusion with their study of the combustion of JP4 in high pressure air - they found the approximate relation (mass burn rate)∝P<sub>initial</sub><sup>2/3</sup>. This indicates a lower flame propagation rate at higher pressure due to the density going up roughly linearly with pressure (actually, considering the pressure they used and how compressible air is at that pressure, the flame propagation speed may actually be nearly constant...).

The more recent "Laminar burning speeds of ethanol/air/diluent mixtures" (Eisazadeh-Far et al.) was better instrumented and came to a conclusion which is more interesting for our purposes - that laminar flame propagation speed decreases with increasing initial pressure, but that greater initial pressure promotes cell formation and instabilities. These effects increase effective burn rate, but Jimmy appears to be correct in that the initial laminar flame speed does decrease.

It's also worth noting that second study covered much lower initial pressures and was more interested in the effects of fuel ratio than in those of initial pressure. This brings me to my final, rather tangential point - in the ethanol/air/diluent system, rich mixes tended to promote cell formation just like increased initial pressure. This ran counter to my previous understanding, that rich mixes in general burned more slowly than lean mixes.
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Unread postAuthor: jimmy101 » Fri May 04, 2012 2:06 pm

One more thing to add, extracted from DYI's post is that the pressure (or density) exponent is 2/3. I seem to recal an exponent more like 0.9 but close enough. There is also the temperature effects on the flame speed and that is much greater. So it doesn't matter all that much that the flame rate is small to start with since the heat induced acceleration is huge.

If you properly fueled and mixed a gymnasium size room with propane you could light a match at the center of the room, turn, and at a walking speed "out run" the flame front. The flame front speed is only about 1.5 foot/second. We'll, you could "out run" if for a few seconds, once the unburned mix starts to heat up the flame speed goes way up. Flames speeds of perhaps 300 feet/second are probably reached in a typical 1x spudgun.
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Unread postAuthor: matti » Sat May 05, 2012 4:45 am

hmmm ok, I was trying to do some work with figuring out what kind of shock waves could form from a hybrid gas mix burning.
I was comparing hybrid gas explosion to LE and HE explosions, and trying to find out if i can get supersonic shock waves from hybrid of if they are only subsonic. Overpressure in shock wave is only created from supersonic shocks I think.

All this have something to do with my urge to build a hybrid shock wave generator :D

Im tying to get an idea of what would be best, low pressure low speed burning gas mix and huge chamber or something like small chamber high mix hybrid with long cone shaped barrel to create a torus shape vortex ring or only use cylindrical barrel and try to get shock from there
:roll:
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Unread postAuthor: Zeus » Sat May 05, 2012 4:57 am

matti wrote:hmmm ok, I was trying to do some work with figuring out what kind of shock waves could form from a hybrid gas mix burning.
I was comparing hybrid gas explosion to LE and HE explosions, and trying to find out if i can get supersonic shock waves from hybrid of if they are only subsonic. Overpressure in shock wave is only created from supersonic shocks I think.

All this have something to do with my urge to build a hybrid shock wave generator :D

Im tying to get an idea of what would be best, low pressure low speed burning gas mix and huge chamber or something like small chamber high mix hybrid with long cone shaped barrel to create a torus shape vortex ring or only use cylindrical barrel and try to get shock from there
:roll:


Do a site search for DDT. Deflagration-detonation transition. We've been trying to avoid it for the last 7-8 years.

The ideal conditions are a long chamber and high mixes, though if DYI corrects me, he's right.
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Unread postAuthor: matti » Sat May 05, 2012 5:51 am

Hey I know we been trying to avoid DDT and I know what that means but, has it really even happened to anyone here (with air/propane).
DYI maybe can help me with this, thats true.. but Im not sure if I want/need a DDT to get what i need (shock wave cannon), thats the problem here :wink:
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Unread postAuthor: jackssmirkingrevenge » Sat May 05, 2012 7:07 am

matti wrote:Hey I know we been trying to avoid DDT and I know what that means but, has it really even happened to anyone here (with air/propane).


That was my first thought, in spite of all the worry about DDT has it actually ever been documented in the spudgun community?
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Unread postAuthor: DYI » Sat May 05, 2012 8:03 am

None of us really have the instrumentation to study it properly. Larda's shot with a 40X oxygen/propane mix with absolutely no buffer gas is what I'd call the best evidence of DDT occurring in a launcher posted on SF. Embedding fragments of the brass burst disk in the back of the steel projectile with such a short distance for acceleration leads me to suspect a strong shockwave in the chamber gas.

Honestly, you'll find out more from Google Scholar than you will from me on the subject of DDT - like most people here, I just know the basic guidelines for avoiding it. I could tell you a few surefire ways to initiate it, but those aren't anything you're unlikely to come up with for yourself.

Looks like you're going to have to do some actual research here :shock:

Perish the thought...

EDIT: never thought I'd see the day when I wrote "your" as a contraction of "you are"...
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Last edited by DYI on Sat May 05, 2012 8:41 am, edited 1 time in total.

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Unread postAuthor: mark.f » Sat May 05, 2012 8:19 am

DYI wrote:Looks like your going to have to do some actual research here :shock:

Perish the thought...


Good to see you're back at things DYI. :wink:
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