World's smallest pulsejet?

[Stephen H]

mark you mean these kinda things http://www.erowid.org/chemicals/show_im ... llage2.jpg ?
they are real small.. but i found one in my room (not from me using it for laughing gas) about 20 mins ago! what size hole it have ?

[steve]

are those the same size as co2 cartredges? I have a bunch of those too.

I bet it work better if you cut the bottom off and welded a flat bottom onto it. I think it is stainless though, and i cant torch-weld that very well.

[Stephen H]

the rounded bottem wouldnt help, while back a guy paplo who used to post here tried making a jam jar out of a old propane cycilinder and came to the conclusion the rounded bottem didnt help!

[sam]

Steve,
I work out the Helmholtz frequency to be about 1100Hz when cold and probably about 2kHz when hot. Clearly the combustion occurs only every [i]n[/i]th cycle but I bet n is an integer around 10.
If you assume the flame speed is about 10m/s (probably about right) and assume that burning occurs for half the cycle (past experience suggests this is roughly correct) then the frequency of flame oscillation in the longitudinal mode is 160Hz { (2*0.061/(2*10))^-1 } which is much closer to what you observe.
Has anyone got access to a high speed camera? It would be really cool to film a jam jar at about 1000 fps.
Sam

[Bruno Ogorelec]

So, it's another example of pulsating combustion outside the acoustical parameters.

I really wonder about the efefcts of trying to introduce acoustics to the thing.

Can we assume that -- since the flame speed dictates the maximum frequency of about 160Hz -- a resonant exhaust tract would not improve matters much if it pushed the frequency beyond that? logic says teh engine should die.

If we used a resonant exhaust that would dictate the frequency of, say, something between 140 and 160, would it increase the amplitude? I mean, something would have to be amplified, right? OK, the frequency would probably increase a little because of the higher temperature, but probably not by much. That leaves the amplitude. Bigger bangs but at the same frequency.

Still bigger bangs at a lower frequency as the exhaust length goes up, within a limited range of longer lengths.

After a certain length, a gradual lowering of both frequency and amplitude because of the rising impedance of the exhaust. Finally, the engien dying when the exhaust becomes too long.

[Mark]

mark you mean these kinda things http://www.erowid.org/chemicals/show_im ... llage2.jpg ?
they are real small.. but i found one in my room (not from me using it for laughing gas) about 20 mins ago! what size hole it have ?

Yes, that's the size I am referring to. I drilled it out a bit where the puncture mark is, the soft metal is almost all drilled away, leaving the rim of the cartridge and perhaps a tiny bit of the soft center metal remaining. Typically, ( I just now ran it again) it revs for about 3 seconds on a few drops of methanol. On a good day maybe 5 seconds.
I was at Walmart and noticed they have some CO2 cartridges of the same size and some longer and perhaps a little wider, I didn't measure them.
There's some even larger, for inflating life vests at sea I saw on eBay or the net.
I don't think my hole is optimized, it could be a tiny bit smaller I think. It's often hard to ignite a hole much smaller, to get the flame to go inside the cartridge. It helps too if you have a syringe or something to inject the fuel in the tiny hole, often the fuel beads on the lip of the rim when I use a "fat" eye dropper tip to squirt fuel into the cartridge.
Mark

[Mark]

http://cgi.aol.ebay.com/ws/eBayISAPI.dl ... 28129&rd=1

I kind of think the threaded kind could be useful too for a quickly made tiny pulsejet.

http://cgi.aol.ebay.com/ws/eBayISAPI.dl ... otohosting

http://cgi.aol.ebay.com/ws/eBayISAPI.dl ... 27064&rd=1

Mark

[Mark]

Here's something funny that happened to me with laughing gas. The other day, I put a thick black sheet of rubber in my vise so as to clamp and hold a N2O cartridge without marring the metal. Then I went to puncture it with my ISI canister; I lost the cartridge holder that screws into the needle long ago. So carfully I pressed the thick needle, (on the head of the canister), into the cartridge.
The N20 cartridge slipped free of the rubber sheet and took to spinning wildly about my garage floor for some time. I guess I just poked the tiniest of holes in the soft metal before it escaped.
Mark

[Mark]

Here's a shape I wish some of you would try. It's a foot long flask, the exhaust tube is 7.5 inches long, the bulb about 4.5 inches long. Putting in an eyedropper full of ethanol or methanol, and swirling it around to coat the insides, I light the tip with a lighter. Talk about hurting your ears, this shape is inspiring. It goes off with a sharp crack bang with the slightest of rev up before the wolfy big bang.
It hurt my ears and you can really feel the sudden push it exerts as you hold it. I was thinking if this approximate shape was reconstructed with valves, you'd have a really powerful foot long pulsejet. It makes me edgy that the glass may crack, it is so loud when it goes off.
The last report I used some ethanol and there was some methanol in it from the previous bang shot. The terrific noise got me to wondering if a blend of methanol and ethanol would have some synergistic quality. I can still feel a little pain in my right ear!
Anyway, pictured next to the flask is a N20 cartridge that I have been using to make short jam jar runs with. You can see the proportion of the hole, I think the hole could be a tiny bit smaller, but as is it gets hot and dies on the times it runs for 5 seconds.
Mark

[Mark]

The inside diameter of the glass exhaust tube is ~950ths. The exterior of the bulb is a little over 3 inches wide. It's just a very peppy shape if it were to run constant, it would be a screamer.
Mark

[Mark]

I just remembered I have a 35mm black plastic film canister an ordinary roll of film comes in. I was making one of those poppers that fire off with Binaca breath spray and a spark, or methanol for those that like methanol. Anyway, I put a little methanol in it and lit the round hole in the lid I drilled, basically it's a small plastic jam jar. It runs for a few seconds and of course softens a little from the heat, but it runs.
One time I took a yellow plastic lemon that "Real Lemon" lemon juice came in and I took out the little nozzle insert. Then I primed it with methanol after letting it dry out and lit the opening, it too revs up jam jar style and weathers the heat alright because it only runs for a few seconds, but the thing is it runs and it's kind of a neat demonstration too.

http://www.scitoys.com/scitoys/scitoys/ ... ermo2.html

Mark

[Mark]

One time I read an account of these poppers at a party where our favorite stock trader Martha Stewart was there. After dinner they all had a fun time firing these film can cannons as an after dinner game. Seems a little jam jar device would be as marketable for people who like simple but unique combustion toys. Here's your chance to enlighten the world with a jam jar toy of a size of your choosing.
You could add color to flame with chemicals or use a glass that takes the heat and color the glass, I remember Larry? had a nice cobalt blue bottle that lit up with a bright blue color. Maybe install a whistle or pinwheel to spin in the breeze of the exhaust, the sky is the limit. And lest we forget, there's that all important aroma therapy dispensing capability.
Mark

[Mike Everman]

Steve,
I work out the Helmholtz frequency to be about 1100Hz when cold and probably about 2kHz when hot. Clearly the combustion occurs only every nth cycle but I bet n is an integer around 10.
If you assume the flame speed is about 10m/s (probably about right) and assume that burning occurs for half the cycle (past experience suggests this is roughly correct) then the frequency of flame oscillation in the longitudinal mode is 160Hz { (2*0.061/(2*10))^-1 } which is much closer to what you observe.
Has anyone got access to a high speed camera? It would be really cool to film a jam jar at about 1000 fps.
Sam

Sam, I get 620 at 25C and 1667 at 1000C. I agree that the flamespeed seems to be all it's about. It couldn't hurt if this frequency were an integer fraction of the resonant frequency, as you say, but it's unlikely Steve stumbled upon it at random. Still, one would want to optimize.

I wonder if the cool (intake phase) and hot (exhaust phase) frequencies would preferentially also be a factor of 2 or more of each other, so when the pot switches frequency mid-cycle, some of the nodes and antinodes are already coincedent. (Anyone know if that's actually the least damping case?..)

[steve]

so what you are saying is that even if I was to make a smaller version, the frequency wouldn't increase much because the flame speed remains constant?

several of you have used water jackets to cool reynst combustors. Will exess heat cause the engine to die if run too long without it? I have seen pictures of Mark's snorkeler glowing from heat and apparently still running so I don't know what to think.

[steve]

If I ever do get this to run with lip feed I have I pretty good idea for what I will do with it:

a fuel tank made from part of a co2 cartredge will be supported by two small pieces of copper tubing (or steel brake line). these will supply methanol to the lip feed and provide pressure to the tank by venting it from the engine. the vent will be severly restricted to prevent pumping too much fuel and killing the engine.

It looks like it might work to me, what do you guys think?