Burners 101

[Mikey98118]

Two probable improvements with true venturi burners (AKA wasp-waist) will be shortened mixing tubes, and greater balance available with flame output to air input.

[G-son]

Out of the history of all cobbled together xxxx this might be the worst… but as it actually worked as a prototype and can be a useful method for someone else who perhaps want to do it a bit more seriously I'll show it here.

I got interested in the wasp waists the last few days, and threw together a very, VERY basic prototype. It's simply two sheet metal cones and a flame retention nozzle. Metal comes from old shaving foam spray cans - costs nothing and is very easy to work with, good enough for a quick n dirty prototype. I didn't bother trying to go for a specific length to diameter ratio anywhere, as my thinking is that I grind the small end off the cones shorter until the waist is large enough to produce a neutral flame - changing the diameter and length at the same time, so it's basically only good for finding the approximate waist diameter to go with the 0.6mm MIG tip I'm borrowing from my previous build (that has a 18mm mixing tube, yellow on the second picture, the prototype waist is 12mm for starters). The entry cone has the same angle as on the linear burner (that's 3x mix tube diameter att the entry and 4x diameter length, but in this case, as I said, I more or less ignored everything but the angle), and the exit cone opens up at 2 degrees from center line, 4 degrees included angle.

Testing conditions weren't ideal, couldn't see the flame well outside in daylight so I can't really say much about the waist size vs. tip size now - need to work on that in less light. But so far: It burns most stable with the gas turned up relatively high, and the gas jet needs to be rather far back from the waist, going closer causes fluttering. I held steel wire in the flame and it goes bright orange (same for the 18mm burner), but the hottest zone of the flame is closer to the burner.

It's far from a functional burner, but it's interesting that it burns as well as it does with the gas jet out of a burner with 50% larger mixing tube diameter = 2.25 times the size of the opening. My testing is probably shut down for winter soon, but this seems like a concept worth diving deeper into later. And anyone else can make the same kind of cones, thicker sheet metal just requires a round bar to work it around and a mallet to shape it with.

[Frosty]

A couple points. No need for a flame retention nozzle, that feature came about as an attempt to stabilize burners that didn't have a full length tapered mixing tube.

So long as the mixing tube taper does NOT exceed 1:12 the flow will remain smooth more taper and turbulence will break it up and it won't work.

The air intake cone is typically much more obtuse on commercial versions. 

Glad to see the experiment.

Frosty The Lucky.

[Mikey98118]

I see nothing wrong with "cobbled together" burners as a method of quicker experimentation, Frosty has used wooden multi-flames nozzles and others have used plastic air chambers, making temporary forms to experiment with; both tests methods having good results.

[G-son]

12 hours ago, Frosty said:

A couple points. No need for a flame retention nozzle, that feature came about as an attempt to stabilize burners that didn't have a full length tapered mixing tube.

So long as the mixing tube taper does NOT exceed 1:12 the flow will remain smooth more taper and turbulence will break it up and it won't work.

The air intake cone is typically much more obtuse on commercial versions. 

Glad to see the experiment.

Frosty The Lucky.

Thankyou.
I added the nozzle as some of the commercial wasp waist burners flare out into a short nozzle. Others just end without anything after the cone. Easy enough to remove and see what happens.
I should be good on the taper. Good to know a number to aim under, for future versions.
The intake cone on many commercial versions seem to often be a smooth radiused bell mouth, starting at a steep angle (often ~90 degrees), progressively going down to basically zero angle at the waist. As I have a "straight" cone, the steeper the angle, the more sudden change of direction at the waist. I wanted to keep the sudden change at the waist relatively small, especially as I have no "rounding off" between the cones = there's a sharp step there (and from what I know of aerodynamics big sharp steps are generally good to avoid). Could get both a steep angle entry and small angle change at the waist by making a multi-stage cone, two or three different angles from the start to the waist, but I think that belongs somewhere later in the testing procedures - when most of the pieces aren't held together with just wire and tape. 

[Another FrankenBurner]

The read somewhere that the measuring venturis typically have an inlet cone of 30° and an outlet cone of 5°(included angles) to reduce aerodynamic drag.  This would give you a throat angle of 200° which isn't very sharp.  That is provided the cones line up.  These venturis have a different intention and a lot of them are used in applications where the stream being measured is moving in relation to the venturi.

Page 17 of the 3D printing thread, member BriJasher links a couple of videos talking about the purpose of a trumpet/bell shaped inlet for turbine engines on a test stand.  It deals with efficiently inducing non moving air into an object with a static position.  

[Mikey98118]

he shape of a venturi inlet has impact on the burner, but so does the shape of its outlet side. I don't see a lot of attention being paid to this half of the equation. and feel that that is a BIG mistake!!!

[Frosty]

What appears to be an outlet nozzle in the pic I posted is a FTP coupler to screw onto male pipe thread. It is only a way to attach the burner, not involved in combustion.

Frosty The Lucky.

[Mikey98118]

This kind of reminds me of a news headline back in the early sixties about lasers; it read something like "If LASER is the answer, what is the question?" Then as now, it is only a matter of application. The entrance side of a wasp-waist burner's applications are well established here, right? But what happens in the outlet side? The principles of flow get reversed; during that reversal more fuel air mixing is forced to happen, so that is to the good, but what else? If you don't reverse flow completely, than you can consider that side to be a process of subtraction. Once again you gain more control of the output flame. The fiercer the flame the longer, all things being equal. Once you have done everything you can to make a very hot flame, then you start working to keep that flame around longer, yes? So the next improvement is to slow that flame down, without losing its heat. If we were to write a description of the perfect air/fuel combustion, it would be a hot laminar flame; which would seem to be something of an oxymoron.

But we can still improve on air/fuel flames by playing with flame shape;  but all of that happens in the burner's various chambers, before combustion (the flame) begins.

[D.Rotblatt]

On 9/21/2019 at 3:05 PM, Mikey98118 said:

But what happens in the outlet side?

I tried a few wasp waisted 3/4” inlet with a 1:12  to 1” outlet the other day. Full write up is in the AFB’s 3D printed burner section. The burners required I use a larger mig tip .056 vs .046 with same inlet and straight pipe), and the flame was stable at pressures above 3lbs, was unstable at lower pressures and huffed or back burned. My thought is that the gas velocity slowed and couldn’t keep up with FAM. Not good for forge, great for furnace  

DanR

[Mikey98118]

More on wasp-waist burners'

Every naturally aspirated burner depends on Bernoulli's Principle, which has several aspects--some but not necessarily all of which--apply. Wasp-waist burners use all of the principle because they have a divergent (expending) taper in their mixing chamber. Frosty points out the importance of a one in twelve (1:12) expansion in thiss part in order to keep laminar flow going there. How much flow pressure/velocity is reduced depends of mixing chamber length. This is important because pressure is best kept low  in this area in order to keep the flame from blowing out, and also to increase hang time of the hot gases in equipment, but not too low, because that would invite back-firing. 

[Mikey98118]

The water stream test

When trying to figure out why a burner isn't working "as advertised," some people hook it up to a water hose. If you don't want to end up soaked, that requires a little plumbing job (to marry a gas fitting to a water fitting), but it is well worth while. I first found this idea being suggested on a casting group site about eighteen years back, and tried it out. While not the only way to check out burners, it remains one of the best testing instrument for identifying problems, and well worth the time to pursue. 

The water stream will instantly show whether or not the burner's gas stream is actually centered down the length of the burner's axis, and if there is a nick or other partial obstruction diverting the gas stream or mall forming it.

[Mikey98118]

Overheating

After twenty years helping to develop ever hotter forge burners it seems odd to run into a problem with burners getting too hot, but it shouldn't be. All that keeps burners from overheating is cool incoming air and cold incoming fuel gas. If a burner is too large for the space it is intended to heat, than it must be turned down, which means that there will be far less cold fuel gas and cool air to keep its internal temperature from overheating. Thus, it will need to be changed out for a smaller burner, which can be turned up higher to achieve the needed forge heat, and therefore will maintain cooler internal temperatures in its mixing tube.

[G-son]

The same thing goes for oxygen/acetylene welding torches. Turn the flame down too far and reflected heat can overheat the tip, causing backfiring inside it (and getting you sprayed with molten metal in the process). 

[671jungle]

On 4/19/2017 at 3:19 PM, Mikey98118 said:

Frosty,

I want you to go right on thinking these things! If two people agree about everything, one of them isn't needed.

I am at the point of diminishing returns--mechanically; or rather at the point where the hill gets a lot harder to climb. But if you're talking about usefulness, than not with the 3/8" burner, which  can be turned down enough to run a coffee-can forge, or up enough to rock in a two gallon forge; they also make awesome hand torches. I don't see my audience as traditional blacksmiths. Is there a traditional anything left anymore; amd should there be?

From the first, I was dealing with beginners. My book was dedicated to "the innovative tool makers and multimedia artists still to come." It had in mind the poor suckers our school system has turned out with bachelors degrees in fine art, who were never given any tools skills to use it with. This system is especially pernicious for those who want to do multimedia.

Getting back to blacksmiths, I see tomorrow's workman as a smith of all trades; or at least of several trades. When you start thinking of other metals, let alone glass and wood--all of which go wondrous well with iron-- the blacksmith who wants to make a buck needs to handle a larger variety of tools.

Sixty years ago, I started learning ornamental ironwork in my father's shop. The first thing I learned was how to handle tools, but the next thing was how to make them.  Clear back them, most of the  tools we needed had to be made; they weren't available any other way. The most important lesson I learned was that, for a craftsman, the tools make the man. The extent of an artist's reach isn't his his hand, but what he holds in it. So, viva la jewelers, silversmiths, casters, and, wood carvers; you all have a place in my vision of blacksmiths

This speaks to me on levels. It is because of teachers like you who understand change and are willing to pass on knowledge and lessons to a non-traditional audience that people like me (jungleboy moved to the city) have some kind of primal outlet. I believe the skills of the smith and other builders are something that must be preserved for when the rebuilding is needed. I also wonder on Traditional vs New Age. After all wasnt everything "new Age" at one point or another?

[ThomasPowers]

You know I once had a conversation with S.M.Stirling, (wrote "Dies the Fire", etc), at a con about smithing in a "post apocalyptic" scenario; my contention is that scrounging would be a much more valuable skill than smithing. Almost all such scenarios posit a massive die off---like 90%, which would leave so much stuff around that making new would be a waste of time and effort---and the ability of a hobby smith to be able to do something like a sword properly would be low until they got a lot of practice. Meanwhile I could find *real* swords even in this small rural town to equip a useful guard. Especially Cavalry Sabers as this is also a well horsed  area.

Now when things start coming back together; someone who can repair/rebuild/duplicate horse drawn farm equipment, (you know where the local museum or farmer with a collection is?), without modern electrical powered tools---then it becomes a life skill!  (Boy I'd hope to have a lifetime supply of nuts and bolts, drillbits and hacksaw blades to hand!)

Of course when I became an insulin dependent diabetic I got to stop worrying about this type of stuff...

[671jungle]

8 minutes ago, ThomasPowers said:

scrounging would be a much more valuable skill than smithing.

Strong point. In the beginning competition will be fierce. Good thing most of us here (IFI) know how to scrounge. My father was/is a scrounger and used to take me to the junkyard and turn me loose with a couple wrenches while he ripped out whatever he was looking for. The ability to identify an object in a sea of mangled clutter is learned in places like this.

 

[ThomasPowers]

Also designing for what you can find rather than trying to find what you designed to make something from.

[Mikey98118]

I don't see making versus scrounging as viable, as whichever preference you start with, it goes best in PARTNERSHIP with the other

[Frosty]

On 9/23/2019 at 8:36 PM, D.Rotblatt said:

I tried a few wasp waisted 3/4” inlet with a 1:12  to 1” outlet

Please everybody disregard my this post, it's as wrong as can be. I had ratios backwards in my head. See the post below.

 

I've qualified the 1:12 diverging taper rule of thumb often but I need to say it more strongly. The 1:12 taper is the MAX increase or decrease that can be placed on an fluid flow and still maintain a smooth stream. Next time I see Mark I'll have to remember to measure the burner on his forge and see what it is.

Dan: next time try less, say 1 :10 or just pick something more gradual. We know that 1:12 is too fast an increase. Yes? That's what your burner performance sounds like to me. It's that old empirical evidence vs. theory thing again. 

Were I making a concerted effort to find the best taper for a particular intake structure I'd bracket and split the difference. It's pretty darned fast. So, knowing 1:12 is too much and we know zero works but not as well as tapered. Split the difference is 1:6 and test. Still to much? 1:3 and test. Not enough? 1:9 and test. Lather rinse repeat till it's silky smooth and manageable with that special glow. 

I'm not suggesting anyone go to this hassle but if you do PLEASE take pictures, notes and keep us in the loop! 

Frosty The Lucky.

[D.Rotblatt]

4 hours ago, Frosty said:

Lather rinse repeat till it's silky smooth and manageable with that special glow. 

I'll put it on the list!

[timgunn1962]

21 hours ago, Frosty said:

I've qualified the 1:12 diverging taper rule of thumb often but I need to say it more strongly. The 1:12 taper is the MAX increase or decrease that can be placed on an fluid flow and still maintain a smooth stream.

If 1:12 is the "fast" limit, it seems like longer tapers would be "slower", so you'd be looking at 1:13 and onward.

 

[Frosty]

 

14 hours ago, timgunn1962 said:

If 1:12 is the "fast" limit, it seems like longer tapers would be "slower", so you'd be looking at 1:13 and onward.

YES, that's exactly right Tim!  ARGHHH!!!!! I got that completely backwards! Please everybody disregard my last post, it's as wrong as can be. 

I had ratios backwards in my head. I never thought of tapers as fast / slow but it's a much better way to visualize what's happening. Heck, a straight pipe would be a ratio of 1: infinity. 

I think I'm going to be quiet for a while, that's as fundamental an error as can be.

Thank you for pointing my mistake out Tim, seriously I appreciate it.

Frosty The Embarrassed. 

[G-son]

Would perhaps make more sense to talk about angles than diameter to length ratios. 

[timgunn1962]

Stuff happens. I think we all knew what you meant. Occasionally making a mistake is normal: it certainly doesn't mean you should stop thinking.

I only pointed it out because it's the internet and someone encountering this thread at some point in the future might otherwise pursue a path of extreme frustration.

On 10/12/2019 at 4:10 PM, Frosty said:

YES, that's exactly right Tim!  ARGHHH!!!!! I got that completely backwards! Please everybody disregard my last post, it's as wrong as can be.