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coca-cola burner


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im building a forge that should be able to get to as high a temperature as possible. so i started experimenting with propane burners. i have built quite a few burners over time for hardening or forging small pieces and most of them are adjustable to the point where the flame goes out on the lean side. i quickly found out than more air is needed in a closed chamber than in free air so the goal is to have an even larger reserve to lean the burner out.

this is the prototype design i came up with:

 

 

im getting a good flame with the secondary (or is it primary?) intake completely closed, so there is hope for success in the forge. i like to run my burners at around 45 psi. the orifice is made with a 0.7 mm drill (0.0276") so i guess its around 0.029". im using 96% propane (the rest being ethane and butane) at 900 feet elevation.

 

what do you think?

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Presuming that your burner's mixing tube is 3/4" pipe, then your gas orifice diameter is right. But you haven't said how long the hole is; at that diameter it should be about 1" to 1/14" length for maximum air induction. However, If I'm seeing past the bottle well enough, air is induced into  roughly elliptical ground openings in the wall of a pipe fitting. If this is correct, and if they are the only air paths, you need to enlarge them, to achieve sufficient air flow. The large diameter of the bottle only increases air spin; not induction.  The gas orifice and flame retention nozzle should create lots of air induction. But tiny air entrances will put the brakes on that process.

On the other hand, if air also is slipping past the gas pipe, and into the end of the pipe fitting, then as near as a can tell, the gas tube is squar; this will create vortices in incoming air; creating drag, which puts the brakes on air induction. You can check for that effect by moving tghe gas pipe further away from the end of the mixing tube. Without drag, air induction would fall off seadily, with drag, there won't be a sharp decline. What to do? taper the end of the tube close to a point.

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The gas orifice on my burners have been less hole than tunnel. In fact, I used to call them "accelerators" to stress the point of why I prefer MIG contact tips to mere holes in end plates. When I started making burners too small for MIG contact tips to work, capillary tube were mounted in the tips, to keep the accelerator effect going. Finally, with !/4" burners, friction losses and the tars and waxes in propane (which create cleaning problems) have made 3D printer nozzles a smarter choice than gas tunnel orifices.

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the idea might not have been completely clear from the picture. the outer bottle neck is opened up to the id of the pipe and is attached too the pipe by the short connector. (actually there is 6° taper and a corresponding chamfer in the pipe with the edges blended in.) the inner bottle neck is closed by the choke. the air enters the pipe between the two parts through the annual opening that is properly adjusted for free air operation.

as soon as the choke is opened ever so slightly, the flame goes out, so im led to believe there will be enough reserve in the forge.

this tip indeed has a short orifice. however i can screw mig tips in there as well and frankly speeking i dont notice any difference with this or other burners due to the longer orifice. if i think about it, a long orifice will flow less and if more gas velocity should be needed i can go up to 60 psi. im sure you explained your idea with the "accelarator" somewhere. where do i look, please? 

 

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as the experiments looked promissing i started building the burner. i made the outside intake out of kitty-litter-epoxy. it  runs on the slightest opening of the choke.

the inside shape looks like a problem because the wall has to be very thin for the secondary air (siphon?) effect to work . im trying to find/think of something out of metal. the best so far is a thermo-bottle i have, but the throat is too big. does anybody have a good idea without me having to take up metal spinning?

 

the shape, btw, with its parallel inlet and outlet walls is inspired by some rocket jets i came accross. i thought if i works well for propulsion why not as an intake?

 

 

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trying to find best position of the nozzle by replacing the choke with a cardboard-choke. i figured the smaller the opening at a constant flame temperatue the more "suction" im getting. well, it turns out the nozzle can be almost anywhere, the difference being a few degrees. i then settled on what seemed slightly the best.

 

 

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On 2/19/2021 at 9:46 AM, dian said:

the shape, btw, with its parallel inlet and outlet walls is inspired by some rocket jets i came accross. i thought if i works well for propulsion why not as an intake?

The gasses are going opposite directions? 

If you look at commercially made burner intakes they are typically trumpet shape. The intake air passing over a convex airfoil curve accelerates it, lowers the pressure and induces a stronger spin. Just like water down a drain.

I just don't know where to find something to use as a mold to cast one and I've looked at plastic soda bottles more than once thinking about doing the experiments you are. 

I'm just mostly following along. Great job so far.

Frosty The Lucky.

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propulsion gasses exit on the big end. i think my coke funnel has the shape you are talking about. weirdly  enough they are the only ones among all the other plastic bottles, the problem being that i dont drink the stuff. the convex shape is in the throat and the rest is there to stabilize the air (and prevent wind messing with it, as i hope).

it bugged me that there was not much difference in the position of my nozzle, so i figured it might need to go deeper into the throat, leading to the tapered variety (same size orifice). this one reaches into the steel pipe itself but results were the same, i ended up with the same position as before. what i found, however, was that now i could fully open the cardboad-choke, so the suction was less and i got a lower temperature as well (10000f less).

as the nozzle is step drilled it occured to me, that it might not be flowing as much as the "stubby-nozzle". but going from 45 to 60 psi didnt make any difference.

any thoughts on that?

 

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The bell on a rocket is internally a concave surface and is designed to direct the thrust and resembles your intake bell. If you lay a straight edge on the inside, a pencil is good it touches two points and there is open space in the middle. Right?

A trumpet bell is convex, if you lay a straight edge on it it will only contact it in the center or rock like a teeter totter. Yes?

This type bell is used on horns to disperse sound waves, not focus them. 

When the intake bell is convex air coming in if forced to the center compressing making a higher pressure zone. There is greater friction as well, air flowing into it is in increasingly hard contact with the walls of the bell. Any bell beats no bell but there are better.

Air flowing down a trumpet bell is constantly trying to flow AWAY from contact with the bell and must follow it because there is a low pressure layer of air in contact with the curve. This is exactly the same behavior that makes airplane wings generate lift. Air flowing over the curved upper surface must travel a longer distance in the same amount of time and is constantly trying to move away from it causing a low pressure boundary layer. Higher pressure on the bottom of  the wing pushes the wing towards the low pressure.

The Bernoulli principle explains it with math and everything. 

Anyway, as air is drawn into a trumpet bell intake it flows faster and with less resistance because of the low pressure boundary layer contacting the bell itself. Conservation of angular momentum causes the air to form a vortex as it flows to the center and out the mixing tube. A vortex generates low pressure in it's center which causes more air to be drawn in.

I couldn't find an image I really liked to illustrate a trumpet bell. Most trumpets get polished before pics are taken and it's really hard to see details like curved surfaces in polished metal. Pic 1 shows the shape pretty well from the outside. Pic 2 is made from carbon fiber and isn't shiny so might show what makes a proper intake bell shape.

Please note a trumpet isn't likely to be the right trumpet bell to make a 3/4" burner but it's the shape you see in commercially made propane burners that aren't made from plumbing parts. 

Frosty The Lucky.

1430468179_trumpetbell1.jpg.f095411d07ed002e58313a24b1ca64d3.jpg       431379063_trumpetbell2.thumb.jpg.70f39a43a0e9b4e398dc2a1e099d19c9.jpg

 

 

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one could also copy a carburetor or injection stack, but how they work with gas induction would have to be seen.

 

anyway, the basis is the classic pipe-fitting-burner that produces a flame temperature of 2165/2206°f (45/60 psi). the position of the probe is optimised for each measurement and choke adjusted to max. temperature. as the flame tends to get hotter with time and there are some "inexplicable" fluctuations there is some scatter to the data and i took the highes achieved. my setup consists of an inexpensive instrument and probe, combined with a professional compensating cable (that cost me more than the rest, because i had to get it locally). therefore the absolute values are a bit questionable and i will be looking at the differences. everything with my obviously "magic-stubby-nozzle".

(1) same with shorter pipe: 0/+62°

(2) same setup with cat-liter intake (last picture): +15/+22: not much gain. this is not surprising, because all my burners can be leaned out until they go out in free air. but consider this: inspite of the minimal opening of the choke, once in my small (experimental) forge it can be completely opened up without the flame going out. thats why im trying to get more air in there.

(3) not relevant for what im doing here, but just for the hell of it (compare with 2165/2206):

- machined fitting: -27/-18 (dont mess with the fittings, they do exactly what the are "supposed" to do, for some reason.)

- expansion pipe: -10/+94 (maybe im onto something here)

- welding reduction: -14/-10

then the cast expansion pipes:

(4) big (16° included, length 3.15"): +81/?

(5) small (12°, same lenght): +182/+216

(4), (5) with cat-intake, all numbers referring to "2165/2206°f.

 

 

 

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Dian:  We really need to work out a common terminology so I don't have to keep asking you what you mean. 

Until then:)

I don't know why you made your mixing tube from multiple pipe nipples and couplers. In my early experimenting days I found anything but a single length of pipe inhibited performance.

I discovered that machining the threads out of either the T fitting's air intake ports or the thread protectors I use for the flame nozzle inhibited performance. I have a hypotheses and empiric results but no measurements to back it. maybe later if you're interested.

I really like the flame nozzle in your 3rd. picture, it's much closer to a commercial fame nozzle in shape. 

I much prefer to build burners that operate with a SLIGHTLY richer than neutral flame across it's operating range. I can't think of a good reason to design one to run so lean it goes out, then install a choke to correct a designed problem. I did similar things when I first started to explore the "envelope" so to speak. Envelope refers to the outer boundaries of what will or won't work, it's not a measure of performance.

 There is back pressure when you fire a burner in a forge, this is why I keep telling people not to tune burners in free air and expect them to work as expected in the forge. 

Carburetors or air intakes for injected engines have true "Venturis" but designed for a different purpose than what we need for a NA burner intake. We need one that makes it easier for the propane jet to induce combustion air intake. A side and highly valuable side benefit is inducing a strong spin to the intake air. 

The venturi in an engine is designed to maximize the strength of the boundary low pressure zone to assist in introducing fuel and atomizing it. The motivating force drawing air in is generated by the pistons making ease of flow insignificant to the root purpose of an engine intake venturi. The same principles but entirely different purposes. Make sense?

An engine type venturi might be an improvement in blown (Gun) burners but maybe probably not enough improvement to be worth the effort to develop and make. 

I really don't understand the thinking behind the spherical attachment around your gas jet. What is it for? All my experience tells me is, it severely restricts air intake. I suppose it could act as a choke but adjusting it would also move the gas jet closer to or farther from the mixing tube which alters induction rates. 

I'd make more comments and ask questions but I'm having too much trouble staying focused. It's not on you, I'm a Traumatic Brain Injury (TBI) survivor and poor focus is a common issue. I'll try to take your posts one thing at a time and see if that works better.

Frosty The Lucky.

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