propane and oxygen

[willarddaniels]

Meanwhile, gexcon shows propane/air transitioning in about 11 meters in a 4" pipe.

That is an interesting read...
Note that these numbers are from experimental data.
A 4" ID chamber would need to be nearly 36' long in order to propagate a transition from deflagration to detonation (DDT) in a 4.2% propane/air mix at 1 atm. In other words: A simple, yet massively long combustion cannon with metered propane, no chamber fan and one point of ignition at the furthest point from the barrel.
Smaller diameter pipes reach DDT in shorter distances.
At the point of DDT, pressure spikes of 50 times the initial pressure have been measured.
This being said, if you want that huge boost of pressure, make your combustion cannon to the above specs and make sure you make the DDT site of your canon extra strong.

"The run-up distance, i.e. the distance from ignition to transition to detonation in pipes is an experimental value giving some indication of the likelihood of transition to detonation."
Factors influencing location of DDT:
initial pressure: increase this and the run-up distance decreases
initial temp: decrease this and the run-up distance decreases
turbulance: increase this and the run-up distance decreases.

Therefore, what to do so your cannon isn't 1/4 the length of a football field:
Use a chamber fan to create more turbulance.
Do not increase the starting temp of your fuel/air mix. (This is counter-intuitive to me)
Increase the intital pressure. ie. hybrids.
Decrease the diameter of the chamber to reach DDT pressures in a shorter distance ie. 16" ID chamber needs to be 104' long... not a significant factor.

This article does not take into account the use of an oxidizer, which will also significantly decrease the run-up distance.

[jimmy101]

Woah...
So, I was looking for hard numbers to, you know, shut these people up...

<a href="http://thermal.mech.saitama-u.ac.jp/fil ... F">This</a> doesn't confirm my former statements; rather, it shows that DDT occurs within 7" in a 4" tube!

Meanwhile, <a href="http://www.gexcon.com/index.php?src=han ... 2b">gexcon </a>shows propane/air transitioning in about 11 meters in a 4" pipe.

This massive decrease in required distance is backed up by <a href="http://www.galcit.caltech.edu/EDL/publi ... f">This</a> paper too...

Cool papers bolingleadbath.

Is it possible that a typical spud gun using propane+air could reach DDT? Looking at the first and third papers in your post it doesn't really seem out of the question. Heck, both papers are talking about combustion in essentially an open chamber. How can they possible be getting high enough pressures in the uncombusted fuel for it to self ignite?

I've always figured that a typical combustion spud gun was pretty much self-regulating. In order to get to DDT you need pretty high pressure and temperature in the chamber. Before you can get to high enough pressure the spud starts to move and the pressure drops (or, at least it isn't rising as fast).

Thanks for the links, they will make interesting reading.

[heavymetalsarge]

thanks for clarifying that for me, I should have thought about what I said.
thanks once again

[AmYisroelChai]

and i am prepared for like 5 people to answer "you are an idiot to use propane and pure oxygen" but oh well. i don't care as long as one person answers my question

Did five people call you an idiot yet?
LoL
Just kidding...

And no questions are stupid. There are plenty of people who are happy to help you not kil yourself. Have fun and be safe.

Post the final cannon when your done.
Or better yet post a thread while you are building it for feedback.[/quote]

[boilingleadbath]

Correct me if I'm wrong here, but my understanding is that detonation is initiated by little pockets of air being compressed - quickly - to temperatures hot enough to diesel.
As such, it should be fairly independent of the actual pressure of the gas at the time that the flame front is near it.

Anyway, if you WANT to get a DDT, blockages are the way to do it. Early on, they create turbulence and increase the acceleration of the flame front, and later they reflect the pressure waves backwards, creating 'hot spots'.
I'm not speculating on the 'hot spot' buisness - a paper I read had REALLY cool pictures in which they pointed them out (DDT in a schelkin spiral, btw).
Perhaps a scalloped ring would be better for the creation of the hot-spot detonation centers?

[rebalgray]

Go here for a electronic ignition. It's legal as it isn't a stun gun.
rebalgray

http://www.launchpotatoes.com/

[spudzinger]

Words of Warning! Although this topic deals with propane and oxygen and youd have t be very stupid to do this, if you use hydrogen as a fuel never use steel as the chamber or barrel as the process of the hydrogen burning will make methane from the carbon in the steel and the hydrogen. This, over time, will weaken your chamber until the point that it may explode (if it doesnt to begin with). On the rest of the topic I'd say just follow others ideas about sch80 or above or metal chambers and barrels as well as remote ignition.Just my two cents. Spudzinger

[super spuder]

and i am prepared for like 5 people to answer "you are an idiot to use propane and pure oxygen" but oh well. i don't care as long as one person answers my question

Did five people call you an idiot yet?
LoL
Just kidding...

And no questions are stupid. There are plenty of people who are happy to help you not kil yourself. Have fun and be safe.

Post the final cannon when your done.
Or better yet post a thread while you are building it for feedback.

[/quote]
no not yet. edit:i am not sure if the above post is referring to my idea or another one but if it is referring to me. that is one
i will post some pics of the combustion chamber and stuff once i get them, unfortunately this project got put off for my pneumatic golf ball cannon because the junkyard i want to get all the metal from was covered in snow. but i will be starting it in the next month or so. sorry i have not posted here in a while. but i completely did not understand what anyone was saying with all the formulas. but someone asked where bouts i am from, to answer your question, i am from ontario canada.