Saturday, January 24, 2015

Mechanical valving prototype, v1

!!! 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.

A camp mate of mine suggested a design for a mechanical valving mechanism with an inner and outer pipe.  The inner pipe would have a line of holes to allow gas to escape, and the outer pipe would have holes at the same spacings, but in a spiral, so that as the pipe was rotated different sets of holes would come into register.  He did a 3D rendering, showing two options (spiral holes on the inner and outer pipes):

During some meetings a couple of weeks ago during which my attention was possibly less engaged than it could have been, I drew up some rough mechanical drawings, including valves, servo motor, pilot, and how to joint them at the ends:

Last weekend I had most of the weekend free, and spent some time working on the prototype.  I used a 30" length of 1/2" galvanized pipe for the inner pipe, and chose to make the outer pipe the rotating one, so that I wouldn't need a swivel valve.  I took my pipe over to Bay Steel, which I highly recommend on account of the fact that they're open on Saturdays, they're cheaper than Bayshore Metals (which is not open weekends), and they let me use their cutoff wheel for free to cut the 20' lengths down so I could put them on my truck rack.  (I tried to be pretty careful with the cutoff wheel, but the old Chinese dudes who worked there were having none of that, and noticing that I was taking a long time, just banged it down a few times to finish the cut for me.  They weren't upset, just wanted to show me how it was done.)  Their selection isn't as great as Bayshore Metal, and they don't have much scrap in stock, so you have to buy full length stock or nothing.  I found a tube with a decent fit by trial and error, and bought a length.  I also brought a 1/4" pipe and bought some stock that fit over that as well, thinking I might try both.  But I started with the 1/2".

I headed down to the Tech Shop on Howard to use the milling machine.  It had been a while, and I was pretty rusty, so the registration of the holes isn't fantastic.  But for the purposes of an initial prototype, I hoped it would be good enough.

Finally got around to setting it up and testing it out today, and the results were okay, but needs improvement.

As you can see, it's far from perfect, but it does give the impression of the flame moving from end to end.  Some notes:

  • I wrapped the "free" end in teflon tape because gas was easily escaping creating a huge flame on that end.  That's easily fixable by creating a set of flanges that hold the unit in place on each end, but for the test I didn't have that.
  • The intratube gap is large enough that gas can easily pass through it and to the misaligned holes, so the difference between those and the aligned holes isn't great.  In fact, the fact that they're rotated out of alignment with the vertical is giving us a better effect there.  It's possible that I could fix this with some grease, which would make the whole thing rotate more efficiently and seal off the other holes.  Not sure if the grease would survive the temperatures, would have to find something that retains its viscosity at high temp and isn't flammable.  Improving the tolerance between the tube and the pipe might help too, but that might be difficult.  Remachining the holes with closer tolerance might help as well, and/or smaller holes both internal and external.
  • It's also noticeable that the flame height is different at both ends.  I think this is primarily due to poor registration at the far end (the registration drifted across the pipe, but I know how to fix that this time.)  But if that persists, running gas into both ends could help.
  • Assuming we can get it to work, I'm concerned with how this will perform in the wind on the playa.  I'm thinking about wrapping the whole thing in steel wool to create a pilot effect along the length.  This might help dampen gas escaping the misaligned holes as well.    But I'd also like to create a third concentric tube with windows cut in it, which would hide the nature of the mechanism, and create some wind dampening.  The windows could be different shapes (I saw something like this at Adam Phelp's place the other night, which he was using for pilot nights for a poofer.)  It would be pretty as well as functional.
I think the zeroth order tests will be reducing the hole size (I used 1/8", so maybe 1/16"), improved registration, and grease.

Wednesday, January 14, 2015

Gas mixing prototype v2: Not a success. But not a total failure.

Gas mixing prototype v2 was somewhat more successful than v1, but I think in the end, I am going to abandon it.

Main differences with the v2 prototype:
  • Using a metering pipe and adjustable regulator for measuring propane.  The idea is to use a calculated length of pipe (4" in this case) at a particular pressure to measure out a volume of propane.  Knowing this and the size of the ignition chamber allows you to create a relatively precise fuel/air ratio.  The adjustable regulator allowed me to experiment with different ratios by adjusting the pressure feeding into the pipe.
  • I used an electric pump (got it off of Amazon I think) to circulate the fuel/air after measuring.
  • I substituted the piezo spark gap ignitor (which I didn't think was highly reliable) with an electric one from Sparkfun.  (It appears they've since retired it, probably because it's kind of dangerous.  But it works great.)
Here's the v2 prototype:

Here's the v2 prototype in action:

As you can see, it works.  We get ignition, and you can also see that my makeshift pressure relief valve works exactly as intended: it's just a piece of metal held onto the top of the tube with rubberbands.  The top of the tube is rimmed with silicone caulk to make a good seal.  When the pressure rises in the tube from ignition, it blows the top off releasing the pressure.

The problem with the v2 prototype is that, no matter how far down I pushed the fuel/air ratio, I wasn't able to get substantially slower flame propagation.  I titrated pressure from 80 PSI (which should have been the optimal fuel/air ratio) down to around 30 PSI (which should have been below the fuel/air ratio required for combustion.)  At the bottom end, I titrated in 1 - 2 PSI increments, and basically found that below about 33 PSI, I got no ignition at all.  Above 33 PSI, I would get ignition, but the flame front propagation is essentially instantaneous to the naked eye.  At very low fuel/air ratios, the pressure produced by the ignition was definitely lower: instead of popping the top off, it would simply escape from the tube with a hiss.  But the flame front propagation never got anywhere close to 10 cm/sec.

My hypothesis here is that 10 cm/sec is the limiting rate for deflagration in free space at atmospheric pressure.  In my experiments, the ignition chamber is highly anisotropic, and my hypothesis is that the driving factor for flame front propagation isn't heat diffusion (as it would be with deflagration in free space) but is being pressure driven by the space constraints.  Essentially, the flame front is getting pushed upwards rapidly by the pressure.  (In a sense, this is similar to the physics of detonation, as opposed to combustion, in which the pressure wave drives the reaction).

As it turns out, computer modeling combustion is really hard, and there are no open source tools for doing it, because presumably if you've developed a tool to do it, it's worth a lot.  There are a bunch of proprietary ones on the market.  It's not outside of the realm of possibility that I could write one myself, at least a basic one, but it requires simultaneous modeling of chemical reaction, chemical diffusion, and heat diffusion rates and dynamics.  Suffice it to say, this would be ambitious, and it's not clear that it would work well enough to help me design something that works.  I may try some further experiments with some configurations of air/fuel mixing in various ways (adding holes to the tube in various ways, etc.)  But it would be mostly to help my intuition about combustion.  I doubt I'm going to come up with a clever way to get this to work just by kind of cogitating on it.

Fortunately, posting the blog to my burning man group resulted in some really great feedback from my campmates about ideas for a mechanical solution.  I had been thinking about mechanical solutions before, but the ideas I had weren't very good.  The new design that I'm working on based on the ideas that I got from my campmates are much better, and I think this is going to be the primary direction of my new attempts.  I'll post some schematics and thoughts soon.

Wednesday, January 7, 2015

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.

Tuesday, January 6, 2015

Building a Giant Flaming Polygon: Is It Art? Who Cares?

Since I've been working for about the past six months on a conceptual flame art project, I thought I'd start collating some of my notes and results here for public consumption.  I use Evernote for keeping most of my notes, but wanted to make some of them more publicly available, as I start to hone in on a design.

!!! 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.

The overall design goal of the as-yet-unnamed artwork will be to create a polygon in which the edges are a propagating flame.  The flame will propagate along the edges, from vertex to vertex, highlighting each vertex as it goes, and eventually existing in a column of flame from the top of the polygon.  My eventual goal is to build a 4-foot-per-side regular icosahedron:
30 edges, 12 vertices, 20 sides.  Basically, your regular Dungeons and Dragons D20.

In the short term, the goal will be to create something smaller, possibly with longer edges, such as a square pyramid (eight edges.)

My inspiration for this project came from Burning Man 2014, but not from any particular art project.  Rather, it came from talking with artists, and experiencing the art there.  In past years, I've worked on relatively small scale projects, such as my solar powered waffle maker (The Waffle House of the Rising Sun):

I thought that, as my thoughts about art evolve, I would make for myself a different goal: instead of building something that I can create in under a year with the express intention of taking it to the Burn, I would set out an ambitious project for myself, something that had not yet been attempted, simply for the sake of creating something.  I am the more neural half of a set of fraternal twins; my twin sister has always been The Artist™ of the family.  So I have only reluctantly come to think of myself as capable of engaging in art in any serious way.  And I still am reluctant to think of it as a serious pursuit, rather than a weekend hobby.  But, as weekend hobbies go, it's a fun way to spend my hours, doing what I love: tinkering, for the sake of tinkering.  Whether I call it art or not is, in that sense, irrelevant (until I apply for a grant, that is.)