Part 2: Getting a Slow Burn

!!! Big Red Warning Header !!!

I have zero training in flame art apart from what I've been reading on the internet, and having a doctorate in physics.  Don't attempt any of this.  Don't blame me if you get hurt.  Don't build dangerous things in your basement.  It's a bad life choice for 99.9% of the population.

Broadly speaking, the challenges to building the flaming polygon are:

• Create a flame that appears to propagate
• It's not necessary that the flame actually propagate, as long as we can create the appearance of propagation, using, e.g., a series of valves to control flame height along holes in a pipe.
• It's also important that the flame propagate slowly enough, so that the visual effect is of a flame traveling along the axis, not an immediate combustion along the entire length.
• The apparatus must be mobile, so I need to be able to dismantle it.
• At some point, cost will become an issue.  Roughly speaking, $20k in construction costs is probably viable.  $200k is not.  So designs that require, e.g., dozens of valves per polygon segment are probably impractical.

I have a number of possible designs in mind, ranging from the simple to the outlandish.  The easiest design, which is currently in prototype v2, relies on the fact that the propagation speed of propane flame depends on the ratio of fuel to air:

By carefully controlling the fuel to air ratio, I could theoretically get propagation speeds down to 10 cm/sec, or about 10 seconds to traverse a 4 foot pipe.

For testing purposes, my v1 prototype design involved using an acrylic tube as an ignition chamber, and a syringe for measuring propane volume.

I included a computer case fan at the bottom of the chamber to help circulate the propane and air for fuel/air mixing.  However, in a sealed chamber, the flow turned out to be inadequate, and I wasn't able to get reliable ignition.

Trying to improve my fuel mixing design, my searches led me to the intarwebz netherworld of Burnt Latke, a very specialized web site for people who are really into making home-made potato cannons:

I got a lot of really good ideas from Burnt Latke.  I'll present my v2 design and results in the next post.