Optimal tuyere dimensions for a given problem

[h4tt3n]

Hello, I'm hoping someone here might be able to shed some light on a prehistoric engineering problem that I am struggling with:

 

I'm working with the reconstruction of prehistoric iron smelting techniques. In short, what I do is build a clay furnace and fill it with various amounts of charcoal and iron ore while blowing air into it at the bottom. Coal and ore is transformed into solid iron that stays inside the furnace and liquid slag that runs out of the furnace. When the smelt is over, I pull out the iron and forge it into bars on a large rock. The furnaces are roughtly 30 cm in diameter and 70-100 cm high.

 

One of the major problems is getting the furnace content to burn evenly despite the fact that there is only blown air into the furnace through one small hole. Many different setups have been tried with various results.

 

Very recently a bit of interesting iron age furnace slag has been excavated in Denmark. It seems to be a exact cast of the hole through which air was forced into the furnace. Liquid slag simply ran out of the hole and solidified in situ, preserving the shape of the hole to this day.

 

The slag is a conic section with a length of 37 mm. the diameter in one end is 20 mm, and the diameter in the other end is 23 mm. Several other similar slags have been found, although less well preserved. The diameter of these varied between 20 and 30 mm. I am assuming that the bigger end of the hole was on the outside of the furnace and the smaller end was inside.

 

So, here comes the questions:

 

(1) Based on the above described hole size and shape, what would be the optimal tuyere size and shape if you want to force air all the way to the back of the furnace, 30 cm behind the hole?

 

(2) Based on the above, what approximate air pressure / volume per time / velocity is this air inlet designed to work with?

 

 

If my description of the problem seems unclear, then I can upload some photos or drawings of the setup.

 

Thanks in advance,

 

Mike

[michel123456]

Hello, I'm hoping someone here might be able to shed some light on a prehistoric engineering problem that I am struggling with:

 

I'm working with the reconstruction of prehistoric iron smelting techniques. In short, what I do is build a clay furnace and fill it with various amounts of charcoal and iron ore while blowing air into it at the bottom. Coal and ore is transformed into solid iron that stays inside the furnace and liquid slag that runs out of the furnace. When the smelt is over, I pull out the iron and forge it into bars on a large rock. The furnaces are roughtly 30 cm in diameter and 70-100 cm high.

 

One of the major problems is getting the furnace content to burn evenly despite the fact that there is only blown air into the furnace through one small hole. Many different setups have been tried with various results.

 

Very recently a bit of interesting iron age furnace slag has been excavated in Denmark. It seems to be a exact cast of the hole through which air was forced into the furnace. Liquid slag simply ran out of the hole and solidified in situ, preserving the shape of the hole to this day.

 

The slag is a conic section with a length of 37 mm. the diameter in one end is 20 mm, and the diameter in the other end is 23 mm. Several other similar slags have been found, although less well preserved. The diameter of these varied between 20 and 30 mm. I am assuming that the bigger end of the hole was on the outside of the furnace and the smaller end was inside.

 

So, here comes the questions:

 

(1) Based on the above described hole size and shape, what would be the optimal tuyere size and shape if you want to force air all the way to the back of the furnace, 30 cm behind the hole?

 

(2) Based on the above, what approximate air pressure / volume per time / velocity is this air inlet designed to work with?

 

 

If my description of the problem seems unclear, then I can upload some photos or drawings of the setup.

 

Thanks in advance,

 

Mike

 

First I cannot answer your question.

Second yes pictures & drawings would be great, I find that very interesting.

Third, as much as I know, prehistoric and even more recent technology worked without the use of sealant, so I suppose that the technique was to put something in the hole (a hollow bone for example) in order to blow. If my supposition is correct, the available diameter was smaller than the one you mentioned, and the 23mm dimension must correspond to the material used (the bone?). Maybe you can find some indication on the surface of the cast findings.

----------

(edit)

Maybe the conic shape was made to seal the tuyere when pushing from the outside. That means the smaller end was inside as you supposed.

Edited November 7, 2012 by michel123456

[Enthalpy]

My own guts feelings:

 

20mm diameter needs little blowing pressure, intuitively less than blowing through fine charcoal and ore, and far less than blowing through molten iron.

 

A few big stones at the pipe's outlet, followed by smaller ones (as roads are made), would spread the throughput and reduce the speed, adequate to blow through charcoal. Not so good to harvest molten iron.

 

I doubt about bones. They don't withstand molten iron's temperature and are known to make steel brittle through phosphorus.

 

You can take a few times (3 times?) 0.5*rho*V^2 pressure drop, in pascal, with rho=1.225 kg/m^3, and V in m/s, with throughput=S*V and S=0.0003 m^2 for D=20mm. - With our lungs, 1000 Pa is easy, 5000 Pa is an effort. 1000 Pa would then give 20 m/s or 7 dm^3/s, too much thoughput for our lungs (5 dm^3). Hence D=20mm isn't the limit. One can't blow strongly for half an hour anyway.

- With two bags (peritoneum? make the valves of the same stuff) pressed alternately by the hands: press 1kg on 2dm^2 down 0.3 m in 1s, it makes 500 Pa and 6 dm^3/s. That's more or less what fits the nozzle alone, but I expect charcoal and ore to drop more pressure.

- Or operate the bags with the feet.

 

20mm against 23mm up or down won't make a difference for air pressure. The direction of the cone can result from the end where the boring tool was introduced, or where it was removed if the pipe material was clay. Or maybe, if pieces of solid dirt passed the small end, they could remove them from the bigger end.

 

I have on TV seen similar furnaces in operation in remote parts of Africa (as a tourist attraction?). Maybe people could tell you from direct experience?

[h4tt3n]

Thanks for your replies, both of you. I can see that I haven't quite explained the process well enough. I've attached an image showing the furnace.

 

First, bone is a clear no-go, partly because it's much easier to make a tube of other materials like clay or metals like iron and copper. Secondly because of the phosphorus content, as Enthalpy mentions. P is a huge problem because it is also in the ore and results in large-grained brittle iron.

 

Second, the iron is not liquid. It never melts but consolidates into a lump of solid iron just below the air inlet. The liquid slag flows into the bottom of the furnace, so you don't have to "blow through" either. The only obstruction for the air beam is charcoal sized like small rock or gravel and powdered ore as fine as sand.

 

Third, the air is not blown into the furnace by blowing with your mouth into the tube. I am using hand operated double bellows or (mostly) a lab-grade electric blower.

 

I am trying to understand what correlation there is between the shape and size of a tuyere and the air beam produced by it at a given air velocity / pressure. For instance if we have a conic tube that is 35 mm at one end and 20 mm at the other, you would still get very different air beams if the tuyere was 600 mm long or just 100 mm long. What is the optimal angle at which the tuyere narrows in? What is the optimal diameter in each end? What is the optimal length?

 

I need some mathematical tools to work with this problem. I know about the Venturi effect and Bernoulli's law, but i feel I'm still missing a few pieces to complete the puzzle.

 

Cheers,

Mike

Edited November 9, 2012 by h4tt3n

[Essay]

I need some mathematical tools to work with this problem. I know about the Venturi effect and Bernoulli's law, but i feel I'm still missing a few pieces to complete the puzzle.

...Cool!

 

 

You might enjoy this, where they show the process, as well as find some new ideas....

 

http://www.pbs.org/w...king-sword.html

Produced only from about 800 to 1,000 A.D., this Viking sword was made from a pure steel, not seen again in Europe for nearly 1,000 years.

 

Medieval blacksmiths in Europe didn't make slag-free steel, because their fires weren't hot enough to fully liquefy the iron. In modern times, metals are melted at temperatures over 3,000 degrees. This separates out the slag and allows more carbon to be mixed in evenly. But in the Viking era, carbon could only be introduced incidentally, mainly through the coal in the fire, and the only way to remove the slag from the metal was to try to hammer out the impurities with each strike.

 

Of the thousands of European swords from the Middle Ages that have been found, all were thought to have been made from this inferior steel, until Williams analyzed the Ulfberht.

 

ALAN WILLIAMS: One or two swords I looked at seemed to be different. They were made of steel, which I'd never seen, before or since, in a medieval object. This seems to be a completely different material.

 

The first thing that strikes you is that there are none of these long, grey slag inclusions, which make the metal brittle. The uniformity is more like a modern steel than it is a medieval one. And it has got a carbon content of about three times as much as the medieval steel we looked at a moment ago. I thought it was very odd. I couldn't think of a reason for it.

 

NARRATOR: The only swords Williams found that were made of this clean, high-carbon steel were those marked as Ulfberht. The metal, known as "crucible steel," gave the swords capabilities far ahead of their time. But it could only be made by melting iron at high temperatures. And no one in Europe would know how to do this for centuries.

....

 

 

NARRATOR: Starting with raw iron, Ric begins the process of making crucible steel.

 

It's the first of many steps on the way to a finished sword. A mistake at any point could lead to failure.

 

RICHARD FURRER: This charcoal will be the carbon source for the steel, so the charcoal here will get absorbed into the steel. And we don't need much.

 

NARRATOR: If all goes well, carbon from the charcoal will harden the raw iron into steel.

 

RICHARD FURRER: This is sand, and this is our bottle glass. They will melt and chemically bond with the waste material, the other slags, and will help float them away and leave clean metal so the carbon can be absorbed.]

 

NARRATOR: As part of their mystical practices, some medieval smiths might have used a different carbon source to strengthen their swords.

 

GUNNAR ANDERSSON: You can also use bone—burnt bone—together with coal in hardening the steel right. And assume, now, that you're using burnt bones even from your ancestor or from a bear or something like that. And you hammer in the power of the animal or your ancestor into the weapon, in itself, together with charcoal, and you make a perfect steel blade, a very powerful steel blade, probably, as well. [?]

 

RICHARD FURRER: We are putting the top onto the crucible. This clay will completely seal this crucible from the furnace environment. Mostly it's to keep more charcoal from getting in. So, we want a set amount of carbon, and if the charcoal were to enter, we would have too much. And there it is.

 

NARRATOR: Ric will try to melt the metal in an oven he's building. It's based on an ancient furnace made of clay and brick that was recently uncovered 2,500 miles from Scandinavia, in Central Asia.

[...my bold/ital.]

 

I work on making charcoal from different materials, and creating a vortex (or any broad or less direct circulation) seems to affect the quality of the burn. So....

 

I wonder if angling your nozzle to direct air around the perimeter, or toward some deflector, might improve some dimension of your process.

===

 

Search: Tuyere CFD, or Nozzle CFD, to find some info and new ideas.

 

e.g.

...tangentially-fired furnace and the effect of changing φ in the near-burner region of the developing jets was again investigated. Experiments were carried out on an isothermal physical-burner model to obtain mean velocity and turbulent statistics for different nozzle geometries and a range of φ. A computational fluid dynamics investigation of these same jets was also performed to gain further insights into the complexities of flow field with experimental results used to validate CFD predictions. The primary jet substantially deviated from the geometric axis of the burner towards the furnace wall and became very unstable for higher φ. The causes of unfavourable aerodynamics were discussed and suggestions were made on possible remedies for such behaviour. Conventional lignite combustion in a full-scale tangentially-fired furnace was modelled.

 

http://researchbank....zle+CFD&y=0&x=0

 

Computational fluid dynamics (CFD) is your new friend....

 

~

 

p.s. Cool pics at:

http://www.sciencedi...305440305000099

"Computational simulation of air flows through a Sri Lankan wind-driven furnace"

Edited November 9, 2012 by Essay

[Enthalpy]

Fluid dynamics is seriously complicated, alas. The difference you notice between 100mm and 600mm length results from viscosity and turbulent motion, so no simple law like Bernouilli will tell you.

 

CFD need a computer, a software, detailed information about surface smoothness etc - and a strong faith in the output of a software...

 

Fortunately, scientists have studied fluid dynamics, and found empirical laws. Still not easy, and need some learning, but usable for hand calculations, with reasonable results.

If you read German (I haven't seen any translation), the best usable book I know is:

Sigloch - Technische Fluidmechanik

Typically, you have to compute dimensionless numbers like Reynolds and get relations with experimental powers of these numbers.

 

The optimum diameter is "as big as possible at both ends" if you just wanted to minimize the pressure drop, and the optimum length nearly zero, so I suspect an other effect is sought, like the faster and hotter combustion of coal obtained by turbulence or by a a fast jet, as you can observe in a barbecue. In this case, the smaller end should be at the coal, and the angle not very steep - 100mm suffice between 35mm and 20mm, and (35/20)^2 suffice to lose little pressure at the D=35 inlet. You would gain more by rounding nicely the inlet.

 

Looks reasonable to me: because of heat losses, you get a hotter combustion if air is brought faster in a small volume, so this must be the desired effect. Now we're full in a nozzle, and the cone does make sense, though it's not the only possible shape.

[Enthalpy]

Thought further about this puzzling question...

 

I'm convinced now that the aim of the nozzle is to bring a big throughput or air in a small volume to counter the heat losses and achieve a hotter zone.

 

For that, a convergent taper is better than a narrow cylinder, because slower air at and near the inlet loses less pressure. It is also better than a short small hole, which reduces the throughput exceedingly because the air jet continues to contract after the hole's smallest section.

 

The slowly convergent taper, as one benefit more, produces a flow with little added turbulence and losses, thanks to the pressure gradient. The optimum is between a wide section kept as long as possible and a slow taper. Not having my Sigloch at reach, I suggest to begin with existing cone angles

http://en.wikipedia.org/wiki/Bellows

http://en.wikipedia.org/wiki/File:English_Bellows.jpg

and experiment 2-3 angles around this value, which can't be very critical.

 

A sharp end at the outlet is better than a round one, if ancient technology permits it. This separates the free jet neatly.

[h4tt3n]

Enthalpy, thank you very much for your reply. You have given me some very tangible answers that I can work with.

 

I'm not quite sure what you mean by "slowly convergent taper". Are you suggesting a tube that narrows in a lot at the far end and gets closer to beeing cylindrical at the near end?

 

As you mention, the purpose of the tuyere is getting a relatively small volume of air / time moving as fast as absolutely possible. Slower air means more uneven furnace combustion, which again means worse result. In my experience, faster moving air also always implicitely means a more even air distribution across the furnace bottom.

 

So, in that respect my question might be reformulated into: "How do you make a given volume of air / time move as fast as possible through a tapering hole approx. 20-30 mm in diameter?"

 

When running at its best the furnace consumes 0.30-0.40 g charcoal / minute / cm2 furnace area. With a diameter of 25 cm the furnace thus consumes 150-170 g of charcoal per minute. This may give you an idea of the air volume / time needed.

 

And yes, among other languages I read german. Will look for the Sigloch book.

 

I've attached an image where you can see the size and shape of the original air inlets.

 

Cheers,

Mike

Edited November 17, 2012 by h4tt3n