Question regarding fueling, fan, and ignition.

[speedster]

I am making my third combustion cannon, which is more advanced than the others and I have a few questions. My chamber is 3 inches in diameter and 18 inches long. Where is the best place to put the fan, ignition source (piezo igniter), and fueling entrance? My fuel meter is a pretty standard pipe and pressure gauge, and where it is connected to the chamber is what i mean by the fueling entrance. My initial thought is to the fan as far back as possible and then put the ignition source and fueling entrance right in front of it. I hope this all made sense, and i thank any more knowledgable spudders for advice!

[Moonbogg]

I'd install the propane inlet at the end of the chamber where there is double thickness from the coupling as shown below.
http://www.burntlatke.com/lp.html

I'd put the fan at the far end as shown below.
http://www.burntlatke.com/jpg600/fan-fr ... ounted.jpg

As far as the ignition, I'd install that through the double thickness at the back as well for more strength. Sometimes people put it in the middle of the chamber so the flame front can expand in two directions, but that means putting it through a single layer of thickness (unless you glue a sleeve over the chamber in the middle). Also, consider a spark strip and stun gun ignition. You can have two (or more) spark gaps that evenly divide the chamber for faster combustion speed and more performance as shown below. There are many ways to make a spark strip.
http://www.burntlatke.com/strip.html

Take a look around the burntlatke website and check out the combustion section here for more examples. Good luck!

[speedster]

Thank you for the advice, very helpful!

[jackssmirkingrevenge]

As far as the ignition, I'd install that through the double thickness at the back as well for more strength. Sometimes people put it in the middle of the chamber so the flame front can expand in two directions

It makes sense in a closed system but as the gas begins to burn and pressure rises, the projectile starts to move so it is more of a dynamic situation.

I am reminded of the experiments I had done with a clear combustion cannon that were not very conclusive but a lot of fun

Also, consider a spark strip and stun gun ignition. You can have two (or more) spark gaps that evenly divide the chamber for faster combustion speed and more performance as shown below. There are many ways to make a spark strip.

Sound advice.

[Moonbogg]

I see what you're saying about the spark in the middle not doing much for performance. I saw a smartereveryday video where it actually resulted in some bad combustion effects toward the back, but he wasn't using metered fuel and it wasn't mixed, so it ran out of oxygen toward the back before fresh air got sucked back in, so I'm not sure about a single spark. What about two or more sparks though? Wasn't there some actual data on that somewhere? I'm sure someone did a performance comparison.