adiel
Junior Member
Posts: 119
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Post by adiel on Nov 30, 2014 12:15:41 GMT -8
hey im looking for advice how to make the passage between the riser and the oven. here is a picture of the riser, www3.zippyshare.com/v/3020431/file.htmlouter barrel and the insulated plate that's above the riser. over that i'll put the oven. it is going to be a white oven so there will be two chambers and heat between them. at the bottom of the oven i'll put fire bricks. so - how wide should i make the gap between the plate and the barrel? should i make slots in the plate? actually - how does the heat goes up and down from the same gap like that? thanks |
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Post by satamax on Nov 30, 2014 15:52:07 GMT -8
Hi Adiel.
Well, your question is not very clear.
Tho, i've had ideas lately, which i would like to submit to your consideration.
We know that with a narrow gap, the usual barrel gets a hot spot. And with a bigger gap, the top doesn't get as hot.
We also know that we need a certain gap's CSA to maintain draft. But there's a trick.
In a previous life, i've built guitars. And there's two things from that world which gave me two ideas.
The tornavoz, which lowers the pitch of the hemoltz resonance of a guitar. Which resembles a lot the heat riser. It acts the same as thickening the rosette edge. And the hemoltz frequency itself, being lowered by bigger rosette oppenings.
Basicaly, if you want the top of a barrel to get as hot as possible, you want the smaller gap possible, without chocking the rocket. Immagine a small grease barrel, with the heat riser made inside. Most of the people make it kind of dome shaped on top! But i'd advise to do the contrary to avoid the hot spot and reduce the gap. The outer diameter of a 16 gal drum is about 38cm. If you shape the heat riser's end into a funnel or trumpet bell, you have an effective diameter for it's end of 38cm.
Let's do the calcs for an 8 incher, as for an oven, you need power.
20cm x 20cm x PI 1256,63 /4 = 314.159cm².
If you stay with a heat riser of constant diameter, the ring projection for the gap is
20 x PI = 62.83 cm circumference
314.159 / 62,83cm =5cm gap, x 1.5 to account for the drag. So you have a 7.5cm gap.
If you use that trumpet bell end for the heat riser, you still keep the system CSA at 314cm², except at the very end. Where gasses starts the trasition of direction, and slows down because of the increased volume theses have to expand into. Having more time to heat what's above
38 xPI = 119.38 circumference.
314.159 : 119.38 = 2.63cm. Times 1.5 again to account for friction, = 3.95cm. So that'a gap 3.55cm shorter than before. Ought to release more heat into the top of the barrel.
Plus, that widening of the restriction (we know that if the gap is too small it's a restriction) might also drop the pitch of the pulse often seen in rockets, and might even make it disapear.
Remember, the last part is just speculation. I haven't run the calcs! It's been a long time since i've played with hemoltz frequencies, and the rocket hasn't been put into equations.
And last, i would just put a big refractory slab on top of the barrel, and cut the metal top, except for a ring to keep the strengh.
Hth.
Max.
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adiel
Junior Member
Posts: 119
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Post by adiel on Nov 30, 2014 21:22:13 GMT -8
hey max thanks for your answer - truly interesting the comparison to sound - but it is not what i was asking  im looking for the gap between the barrel and the plate. i want to leave a gap so the heat will go up and around a chamber for baking. i want more heat for the oven the just from beneath. |
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Post by satamax on Nov 30, 2014 23:49:09 GMT -8
Radiating heat? Convective heat?
As i said, your question isn't very clear. You said white oven, so no smoke going in the oven.
So you need the most radiating possible on the top of the barrel. Hence my previous reply.
If you want a gap between the bottom plate of your oven and the barrel top, and a gap between that bottom plate and the sides of the oven; to take advantage of convection. Then anything smaller than 3cm wouldn't work too well. I'd say 5 to be sure.
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Post by Donkey on Dec 1, 2014 0:43:51 GMT -8
Max.. This is VERY interesting!
Have you ever "tuned" the barrel instead of doing the math, finding a height and setting it??
It would be interesting to do several "tuning" sessions and compare the resulting height with each other and a calculated height (or two).
By the way, I DO tend to build my heat risers with a flared top.. My thinking has always been to avoid ash piling up on the heat riser and eventually choking the system (which I've seen in other places before), so I taper the upper end to the outside and form a thin edge. I've never given a thought to any possible tuning issues with it, nor experimented with any differences that there might be.
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adiel
Junior Member
Posts: 119
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Post by adiel on Dec 1, 2014 3:15:56 GMT -8
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Post by satamax on Dec 1, 2014 3:50:10 GMT -8
Donkey, i've never tuned a barrel, it's all theoretical. Tho, ring projection wise, i'm prety sure i'm not talking gibberish! Most of what i see, is like this one from Thomas Rubino.  Or this one from Rick Edwards.  That's the contrary of what i think would be a good rocket with a narrow top gap. |
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Post by matthewwalker on Dec 1, 2014 9:44:17 GMT -8
Max, those shapes are a function of the materials. The little drums are only 28" high, so you make the internal mold longer and build around it. It may not be the ideal shape, but it's dead simple to build and works very well.
Adiel, I don't know the answer to your question. I think it's gonna get hot up there no matter what you do. The bigger gaps, the better. I would imagine if you make it 8cm everywhere that would be enough.
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