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Revelation: No heat stratification in burn chamber; my outdoor build  RSS feed

 
Philip Rothwell
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I have heard it asserted by someone of rank in the RMH community (I cannot remember who) that in the burn chamber, the hottest gasses will rise to the top. This made a lot of sense, and was concerning to me because I am (in my first build) planning to use a tall (9 inch) and narrow (5 inch) burn chamber. I envisioned great difficulty in getting a proper mixing of gasses with this tendency for the hot ones to float way up to the top.

The purpose for my narrow burn chamber is that it allows my 8 inch system to shoot gasses into the right side of its 10 inch octagonal heat riser, producing a vortex without any complicated steel contraptions like I have seen others using. I have just completed a successful weekend of outdoor testing, and discovered that the mix is excellent without any tinkering.

As the stove was getting up to temperature, a film of black soot developed on the interior of the insulative firebrick riser. By looking down the riser, I could see a uniform stream of flame coming out of the whole height of the burn chamber. As the stove continued to heat, the soot began to burn off, starting at the bottom of the feed tube, and gradually progressing to the top of the riser. When this clean portion emerged from the burn chamber into the riser, it did so first at the lower-most point-- meaning that the bottom, not the top, of the heat riser is the hottest!

In hindsight, this makes a lot of sense. Embers from the feed tube fall out into the floor of the burn chamber, heating it and streaming off hot gasses along the bottom. Also, air intake is sucked in primarily along the path of least resistance at the top. This all means that the insulation below, not above the burn chamber is most important, and that mixing is inherent in the design, as cool intake air tries to swap places with hot exhaust on the bottom.

-Phil
 
Satamax Antone
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Philip, i think you get that result, because the corner between the feed and burn tunnel push the stream of gases downwards. Your build must have quite a good suction speed.


Coud i ask you to take a mirror at 45° avove the heat riser, while burning, high enough so you don't burn yourself, and take a few photo of the insides of your heat riser, and flame vortexes? That would be cool.

Regarding the soot stage, how long does it last untill the soots start burning?

How long untill the heat riser is fully clean?

In this case you have a single vortex. Exept if there's some lip somewhere which perturbates it.

In a batch rocket, you have a double vortex, also called ram horns.



If your flame extends much more than this in the heat riser, you're overfueling your rocket i'd say.


A good read about cyclones http://billpentz.com/woodworking/cyclone/cyclone_plan.cfm#foreword

Especialy the neutral vane modification. And the words about cotton shakers.
 
Philip Rothwell
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I'll be happy to take some photos on my next weekend. I am interested by this double vortex... do you think it provides better mixing?

My idea with the single vortex is to establish a counter-clockwise rotation of gasses that continues into the barrel, with assistance from the manifold, which will suck gasses out one side (like the burn chamber shoots them in one side). I hope this will achieve three things; 1) higher velocity gasses in the barrel which lower pressure and increase draw strength, 2) gasses spending more time in the barrel (cooling) and in the riser (heating) increasing the thermosiphon effect, and 3) mono-directional flow through the manifold, reducing drag, or bottlenecking. I get the impression that the traditional manifold, even when it's made large enough, will force gasses from all directions to collide, reducing velocity.

The flame in the bottom of my burn chamber makes one complete rotation, directly impacting the flame coming out of the burn chamber.

Thanks for your input
-Phil
 
Philip Rothwell
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...Oh, and, in this mock-up made of only refractory fire brick and no additional insulation (and some small air leaks) the soot in the riser began to burn off after maybe 30 minutes of firing, and was completely burnt off an hour after that.
 
Peter van den Berg
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As far as I am aware of, there won't be any stratifying in the burn tunnel of a J-tube rocket heater. The gases are moving too fast for that, gases are mingling all the time. What I've found out years ago that at the ceiling of the tunnel there are relatively more unburned gases as compared to the bottom. Without the help of a gas analizer and thermometers to measure the temperature, it's impossible to say whether or not the top is hotter.

And yes, Satamax is right, mixing by means of the double vortex is far superior as compared to the single cyclone vortex. The double vortex will absorb less energy of the chimney draw and is running clean at an earlier stage. Not to say what you found is bad, but to tell by eye and ear how good the thing runs is utterly impossible, sorry.

By the way, is there clay between the bricks or is it just dry stacked? Without leaks it will run quite differently.
 
Satamax Antone
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Philip Rothwell wrote:The flame in the bottom of my burn chamber makes one complete rotation, directly impacting the flame coming out of the burn chamber.



Cotton shaker cyclone! May be you could add a neutral vane.
 
Philip Rothwell
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I have completed a second round of testing, this time with a neutral vane installed. I will post photos soon.

The neutral vane is an extension of the side of the burn tunnel that extends about an inch-and-a-half into the heat riser. It is cut at a 45 degree angle so the rotating gasses of the riser impact the incoming flame at 45 degrees instead of 90.

The change in performance is dramatic. At least 50% less soot is deposited while the stove is warming up (only a slight coloration at the top of the riser); This soot is burned off the first 10 inches of the riser almost immediately; Exhaust gasses are smokeless in something like 1/2 the time (2 or 3 minutes); The vortex is at least twice as strong at the top of the riser (an undeniable cyclone of fly ash); Heat output is nearly doubled (I can no longer hold my hand for a moment within a foot of the top of the riser), and the draft seems to be 50% stronger. These are not measured results-- merely my impressions-- but I'd consider it a big success.

It seems, however, that it may still take an hour of firing to completely burn off all the soot.

Thank you all for the input. Do these results seem more in line with your stoves? Perhaps double neutral vanes could be used to improve your double vortex.

-Phil
 
Satamax Antone
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Phil, the double vortex is more tested than your configuration, and is rather sensitive. It seems to work well on it's own. People have tried enhancing it with heart shaped risers and all. All failed.

When i sugested the neutral vane, it was all from theory. I've read a lot from life magazine's "scientific series of books" when very young, and it has stayed with me, those pics of fluid mechanics and all the diagrams etc! So when i read Bill Pentz site, i picked it up quite fast. And when building my first rocket, i made a cyclonic one. With good results, i melted the top of a gas bottle. To the point that it sagged under it's own weight. Well, mind you, i've tried the neutral vane in this one, but in tube form. I welded a L out of two pipes, and inserted that tangentialy into the gas bottle at the bottom, which was acting kind of an expansion chamber (for the second of the three T's, time!) then the dome was leading to a proper heat riser. I realy want to try a cone one day, a real cyclone heat riser. But lack time!




In this one, do you see any similarities with my video posted above?


Hth.

Max.
 
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