I have a few burners which I use with forced air and that works awesome. But I see some Youtubers (e.g. BigstackD) have a burner without any forced air and uses natural aspiration and yet gets very hot (> 1300 C). So I tried making a similar 'bell' and testing it by holding the 'bell' (yes the brass one really sounds like a bell) at the air input of the burner, but there was no difference at all :-( What am I doing wrong ? https://www.metallab.net/jwplayer/video.php?f=/forums/burnertest.mp4
If it functions anything like a blowtorch you'd have to have air intake holes drilled in the tube wall in front of the gas nozzle to get some venturi effect happening from the gas flow.
I agree with Mark, google "Oliver upwind burner" to see some more examples of what he's talking about. Also, your flame is separating from your burner, you might need a flare to retain the flame when you're testing it out in open air.
It looks like the gas orifice in your burner is down in front of the bowl shaped items. This diagram shows it positioned farther back, aiming into the mouth of the reducer/bowl shaped object. Maybe if you move yours back like that it will draw better? I've only used forced air, so take all this with a grain of salt. Needing a flare on the other end to run it in open air as Rocco suggests does match up with the diagram and what I've read elsewhere though. Jeff
I use a Reil burner similar to the one in Jeff's post, it is a popular and well tested design but I suggested looking at the Oliver upwind burner because it is very similar to what the OP already has so not much of a redesign would be required, just with a few air holes drilled in the burner tube forward of the gas jet.
I (almost) did it ! Here I had another DIY burner made from stainless steel 1" pipe attached on a 2" => 1" reducer as bowl and I tested it in the furnace without forced air. This burner is somewhat similar to the Reil burner mentioned by Rocco. It did run, but I think a bit too little air ? Closing the lid (furnace was still cold) resulted in a pungent unburnt gas smell and irregular burn. Reducing the pressure from 2 bar (30 psi) to 1 bar (15 psi) allowed closing the lid while continue to run normally, but still the pungent unburnt gas smell (to a lower extent) wast still there. An the end of the video I put the camera behind the burner to show what happens inside. Any ideas ? https://www.metallab.net/jwplayer/video.php?f=/forums/burnertest1.mp4
Are my eyes deceiving me or is the flame blasting away at the plinth? You might try changing the angle of the burner.
The hole in your furnace lid is too small so it's restricting the flow of exhaust gases which then prevents the burner from working properly. You can either use forced air or increase the diameter of the hole in the lid.
There's no swirl at the bottom of the furnace. Instead of the flame making two revolutions there is only a half one and flame is lazy with un-burnt gas as it leaves the furnace. Start with 1/2 bar gas and work up from there after you get the flame off the plinth.
That can be an idea, but I adjusted it that the center of the flame ends between the wall and the plinth. With forced ait it work awesome, so maybe with natural aspiration it should be larger ? Is that needed ? Swirling should take place around the object to be heated (the crucible), which it does and that is not the plinth. Or am I wrong ? Thanks for the reactions, I realize now that naturally aspirated burner is a lot more difficult than a forced air design. Probably I should make a new lid with a larger hole for self-aspirating and the existing lid use for forced air only.
But half of the flame is being stopped by the plinth, inhibiting the swirl. I've run my burner both ways with the same vent, no difference. With forced air you would think it should be larger because there is more volume going in. Swirling should take place around the object to be heated (the crucible)
Just as a rough comparison of the hole diameter versus the lid diameter, a naturally aspirated Devil forge lid hole is about 1:4 hole diameter to lid diameter where your furnace has about a 1:7 hole to lid size (0:42 second mark of your second video). 0:06 mark of this video:
What's the size of your gas jet, an oversized jet can also cause a rich, lazy flame. The jet in my burner is a 0.035" mig tip.
You don't mention the size gas orifice you are using but it may be too large for your furnace and burner tube combination, but for sure, the gas orifice is too close to the burner tube opening. For the most efficient ejector, you want the gas delivered into the burner tube opening at the highest velocity. If the gas orifice is too large, you will get lower velocities and/or too much gas, and never entrain/eject enough air for a good or near stoichiometric burn. What is the size of the gas orifice? Most have had the best performance with .023"-.035" orifice with up to 30 psig of gas pressure. People typically use MIG welding tips as gas orifices. They are inexpensive, and threaded so easily changed. The ejector works on Bernoulli principle and the increase in velocity of the gas creates a reduction in pressure and this low pressure at the burner tube causes the entrained flow of air. But the pressure difference created in these burners can be small so naturally aspirated burners can be very sensitive to down stream restrictions because it reduces pressure drop across the tube and air ejection and then as others have mentioned, the flame impinging on the plinth and size of vent create additional pressure drop which means a lower pressure drop from the entrance to exit of the burner tube and that's why the burner goes rich in the furnace. If you want to optimize your burner design and performance: Make the distance of the orifice tip to the burner tube opening adjustable, but still accurately centered in the burner tube opening. Drill a hole for a small sensing line in the side of the burner tube right at the entrance of the burner tube. Connect the sensing line to the topside of an inclined water manometer. This can simply be an 1/8" hose taped to a board at an angle with say 5-6" of water height and the bottom side connected to a water reservoir. You can add food coloring to the water for better visibility. Then use compressed air at different pressures and observe the position that creates the lowest pressure on the water column. With a high velocity jet of gas, you may be surprised how far from the opening is optimal. Or, if that's too much bother, you can hang strings around the opening to try to observe the increase in entrained air. Using a gradual transition (not abrupt step) in the bell opening like shown in the Reil drawing, will also help efficiency. The bell should be about 2x the burner tube diameters in diameter and length. Also, as I was viewing your video, it looked like you had another tube inside the length of your burner tube. If that is so, remove it. It will inhibit proper mixing of the gas and air prior to ignition. As ejector efficiency increases, so will potential burner performance and you will almost certainly need a flare to test the burner outside the furnace. You can also add an adjustable disc to the bell opening to make the burner tunable at different power levels. It's some fussing around but I know you like to experiment and as you increase the efficiency of the ejector, it will become less sensitive to the down stream obstructions and produce a hotter more stable furnace. Best, Kelly
@Alumina: Thanks for your extensive help. I measured the orifice and it indeed too large (1mm == .040"). I just ordered MIG welding tips. I also saw that on some Youtube videos. Here a photo of the burner with the flare attached which I remove when I put it in the furnace. I have tested it outside the furnace with flare attached and it did burn well. The bell is 2" and the tube 1". There is indeed another tube inside it as otherwise the out-of-furnace test blows the flame out, but did not test it inside. That I should do.
Most recommendations for propane orifice size that I've read is a #60 drill, which is 0.040". Any MIG tip will have a bore several thousandths over the advertised (stamped) size for wire clearance. Example: 0.035" wire requires 0.035" stamped tip, in which the bore is several thou over. Same with 0.023", 0.030", 0.040", 0.045" etc. , same with metric sizes. I've drilled a 0.030" tip with a #60 drill out to a true 0.040" orifice (as recommended). The 0.030" tip bore was probably 0.036" to 0.037" off the shelf. 0.035" tip was over 0.040" bore off the shelf. All MIG tips I use are genuine Miller. I've used the 0.023" tips for low fire with decreased pressure for starting refractory curing as well as coatings (ITC 100 HT) to bring temps up slowly. YMMV.
UPDATE: Thanks, @Alumina, I removed the inner tube (inside the 1" burner tube) and it did work ! Despite the too small exhaust hole, but OTOH, that wastes less heat ? Almost no unburnt gas stench now. So the tube was the main limiting factor. https://www.metallab.net/jwplayer/video.php?f=/forums/burnertest2.mp4 Next time I test really melting brass or copper with this burner.
The general rule of thumb for burner tube length for good mixing is 8x-10x tube diameter. Looks to me like you may get further improvement if the gas orifice is a little farther from the burner tube opening. Best, Kelly
Now this burner goes rather well, it heats up the (empty) crucible in 15 minutes. No unburnt gas smell occurs and the flame correctly swirls around the crucible. Even with the (too small ?) hole in the lid. But sometimes it flashes back. In this video even when turning on shortly after turning off (when furnace is hot). When it runs normally at 30 psi and I lower it to 15 psi, the dreaded flashback occurs. Mostly this is fixed by turning the gas off and on, but some time later it starts again. Any ideas ? https://www.metallab.net/jwplayer/video.php?f=/forums/burnertest3.mp4
