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Flamethrowers - reliable ignition
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chunkulator



Joined: 27 Jul 2016
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Flamethrowers - reliable ignition

Anyone got any good advice on getting my flamethrower to ignite reliably? I have a nitrous solenoid and a 100g camping propane canister and a spark generator. I put a nozzle with a very small hole (about 1.5mm) on the outlet of the solenoid valve which squirts the gas an impressive distance, but the point at which the gas/air mixture is right for ignition is a long way out in front of the bot. This would require having the spark generator way out front on some flimsy arm.

I have tried squirting the gas through various off-the-shelf burners, plumbing parts and even trying to make a Venturi burner but haven't really found the magic formula that makes a big impressive flame while igniting reliably.

I guess the next step is some kind of arrangement to divert a bit of gas off the side of the main jet at low pressure for ignition or maybe using some kind of blower to force more air into the mix, but anyone got any good practical advice from the trenches?

Post Tue Nov 22, 2016 5:37 pm 
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Nick
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That 1.5mm nozzle is huge; the largest burner I could find with a 50mm diameter has around a .75 to 1mm nozzle. Gas won't ignite until it has mixed with enough oxygen, so that's why its not catching until the jet has travelled quite a distance. You need to slow the gas down and let it mix with air as soon as its out of the nozzle.
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Post Wed Nov 23, 2016 8:02 am 
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DumHed
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I find a pilot flame system works well - have one nozzle setup for a small flame with good ignition, and use that to ignite the main nozzle.
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Post Wed Nov 23, 2016 10:47 am 
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Valen
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Try adding a flame holder, put a wire in front of your gas stream so the jet hits it and breaks up, ignite it there.
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Post Wed Nov 23, 2016 11:16 am 
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chunkulator



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All good suggestions. I will give them a go. Thanks!

Post Wed Nov 23, 2016 6:01 pm 
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chunkulator



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Just thinking about your suggestion dumhead. How do you go about getting a second nozzle with low enough gas speed to get a pilot flame? If my main jet nozzle is too big at 1.5mm and should be more like 0.75-1mm that presumably means I need an orifice for the pilot flame which is on the order of 0.1mm. I don't know how to drill a hole that small. How do you usually do it?

Post Wed Nov 23, 2016 6:45 pm 
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Nick
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Perhaps you can borrow some bits from a BIC lighter or a BBQ gas lighter?
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Post Wed Nov 23, 2016 7:20 pm 
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chunkulator



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Last night I managed to make a 0.8mm nozzle and things are in a much happier state now. Thanks for spotting that Nick - i was spewing too much gas for it to mix with the air in the space available. I have a Venturi burner made from brass reducer/expander couplings with a bunch of air holes near the gas inlet and a sparker inside the output cowling. I can get it to ignite pretty consistently (sometimes takes a second) and burn with a hot blue flame that extends about 25cm. I am now, as they say, cooking with gas.

One effect I have noticed quite strongly over the last few evenings of experimentation is that the canister pressure drops pretty significantly over a few minutes of use even though you can hear there is plenty of liquid fuel still in the can. Of course the canister temp drops substantially as expanding gas leaves it, so this probably makes it harder for the liquid to boil into gas. My instinct is not to go messing with trying to regulate the temperature of the fuel tank - putting the fuel tank near a heat source sounds pretty dangerous - but has anyone tried that sort of thing?

Post Thu Nov 24, 2016 6:01 am 
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DumHed
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for the pilot nozzle I re-used parts from a BBQ lighter
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Post Thu Nov 24, 2016 8:40 am 
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Daniel
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I've used a silicon nitride ignitor with no problems. I got mine from Sparkfun, but they don't seem to have them in stock anymore. https://www.sparkfun.com/products/retired/11694

Post Thu Nov 24, 2016 9:17 pm 
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chunkulator



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Hey Daniel, All the silicon nitride igniters I've seen only have a single electrode. The other terminal is usually the metal body of the BBQ. I'm not sure you want the chassis of the bot connected to the sparker. That sounds bad from an RF point of view. How did you handle that?

Post Sat Nov 26, 2016 5:45 pm 
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Nick
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Its not ideal to have either side of your battery connected to the bot's frame, but the silicone nitride ignitors don't generate RF with a spark, they work by heating up a resistance element. Looking at the ignitors on this site: https://crystaltechnica.com/buy-online/index.php?route=product/category&path=65 , Many of them have two leads, so the mounting tabs are probably not grounded.
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Post Sat Nov 26, 2016 6:24 pm 
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Valen
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I think he means having the bot's power connected to the shell could be bad for spraying motor noise everywhere, it's also bad because it means you are only one dodgy wire away from smoke town (rather than 2)
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Post Sun Nov 27, 2016 9:33 am 
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chunkulator



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Yes, I had assumed a silicon nitride igniter was the standard sparky thing hooked up to the piezo in your BBQ. Sounds like it's a different widget altogether. I was worried that if I had one side of my high voltage generating transformer (for making the sparks) connected to the case I would have built quite an effective spark gap transmitter (aka Radio Jammer).

Post Sun Nov 27, 2016 2:13 pm 
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Daniel
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Well before I used the silicon nitride ignitor, I used a USB plasma globe to create sparks. That used the chassis as a ground, but the chassis did not need to be connected to the ground of the circuit, the steel chassis just held the (very small) charge. Much like static electricity. It didn't cause any radio issues, but the spark wasn't reliable.

Post Sun Nov 27, 2016 2:48 pm 
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