Detonation or Deflagration?

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Bent
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Detonation or Deflagration?

Post by Bent » Wed Aug 08, 2007 8:19 pm

Well Gentlemen, I've finished up my current contract and have a few days to work on my PDE.
I filmed a few minutes of it running and would like some input as to whether or not a detonation is actually occurring.
I must say that when using my golf ball cannon which has a larger combustion chamber than this PDE and I am using oxy/acetylene, also the barrel is plugged off with a golf ball that has to be beat down the barrel with a steel bar, so it takes quite a bit of force to launch the ball out of the cannon. Now the golf ball cannon is very loud and I have no doubt that detonation is taking place but when compared to the propane air Detonation (?) the g/b cannon isn't half as loud. Are my baffles/redirectors responsible for focusing the initial Deflagration back on itself and causing detonation? Or is it just a loud deflagration?
I must also add that most of my welds are usually hydro tested to 3350 psi, but on occasion (2500 ansi fittings) we are required to test them at 9275 psi. So, my line of thinking is like this, any pipe with an open end like what we are testing would never see pressures that high. 100 atmospheres comes to mind which would equal 1470 psi +/-
Multiplied up to possibly 100 x per second. Hence the problem of metal fatigue comes into play. My welds are zero defect from bead to cap and I am using oilfield quality materials which will withstand the pressures heat and stresses ...for a while anyway:)

What I’m trying to say is … Detonation can kill, so be sure of what your doing! unless your a little "Bent" like me :wink:
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Eric, you mentioned that a concentric reducer at the back end of the combustion chamber would give a harder kick for the other Combustion bank. When you conc down though, does this not add turbulence and restrict flow? I don't have any simulation proggys to verify any of this other than what I have learned through the years I spent on cannons. Bruno mentioned that the combustor tube connecting both banks was to short, it needed to be 1/2 a cycle long. When calculating 1 cycle, do I measure the volume from end of chamber to centreline of CC? or where it concs down to meet the barrel?

EDIT: I had to take a different video as the other one was to large. I asked most of my questions in the larger video, so I will have to ask them again in the forum when I have time.
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Bent
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Post by Bent » Wed Aug 08, 2007 9:17 pm

OK, I purchased the apple quick time 7 proggy and split the 1st video in half
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pezman
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Pressures in detonation tube

Post by pezman » Wed Aug 08, 2007 9:26 pm

I used to know most of this stuff off the top of my head, but I think that pressures behind the detonation front in a detonation tube will generally reach about 5 ATM. At the deflagration to detonation transition point, there is often a transient pulse that is considerably higher than that (a figure like 15 to 18 ATM comes to mind, but I'm not sure of that). If the DDT point is consistent, I suppose that you could reinforce structure to deal with the stress.

There may be some handy links in older posts about PDEs.

pezman
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Off by less than an order or magnitude

Post by pezman » Wed Aug 08, 2007 9:30 pm

Hmm -- looks like a nominal pressure of 50 bar at the DDT point might be more like it.

http://www.gexcon.com/index.php?src=han ... Bchap6.htm

I guess that my original guess wouldn't look so bad if plotted against the real figure on a log scale ...

Bent
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Post by Bent » Wed Aug 08, 2007 11:49 pm

OK, pezman...so it's around 50 atmospheres then.
Now, can someone answer this for me?
( I am assuming that 1 cycle= the volume of the combustion chamber.) upon detonation, there is the main shockwave traveling down the barrel but also one traveling through the compression tube to the other bank.
The wall at the end of the chamber is 8" diameter and the compression tube is 4" diameter, so we get 1/4 of the overall pressure traveling through the compression tube and the remaining 3/4 out the barrel.
Q:
when trying to calculate exactly what length the compression tube must be, do I have to take into consideration the % of flow traveling through it? Or am I making it more complicated than it needs to be. Also, do I have to consider the pressure leakage from the Intakes?
Last edited by Bent on Mon Aug 13, 2007 6:27 pm, edited 1 time in total.

Anders Troberg
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Post by Anders Troberg » Thu Aug 09, 2007 6:13 am

OK, I'm just guessing here and may be wrong, but two things that springs to my mind are:

1. A detonation pressure wave moves faster than the speed of sound. Could it be that a fair amount of the sound energy generated is on frequencies above what the human ear can ear? There may be times when you just can't trust your own ears.

2. I don't think it's specifically mentioned in the rules of golf, but I think that using a cannon is somewhat contrary to the spirit of the sport.

Bent
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Post by Bent » Fri Aug 10, 2007 3:08 am

Anders, If I could find a way to pack this launcher with me at the course...I would! if I get a chance tomorrow I'll tape it launching a ball through an old dodge tailgate.
I'm an avid hunter and load my own 300 Weatherby magnum loads. We usually go for elk, so I use a 220 grain slug (3100fps) for coyote I use a 110 grain slug (4100 fps) both are super sonic and aren't as loud as this propane/air reaction. The damn video phone doesn't do it justice so I will refrain from filming any more footage with it . When I go to the city to get my '06 dodge 1 ton back (the tranny went already...lol) I will grab the proper gear.
Now, I capped off the 2" Intakes and ran the fuel lines through the 3" Intakes now the engine won't even run in pulse mode with compressed air! It did fire up faithfully without air prior to this mod. I seem to remember Larry Cottrill mentioning that when you enlarge the Intake Diameter you need to lengthen the Intaketube, so I will try this tomorrow

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