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Post by peterberg on Feb 9, 2012 1:21:31 GMT -8
Andy made a rectangular exit in the barrel. This is 12" wide x 9" high (approx 4" from bottom lip of barrel) and runs into a 6" spigot. Connected a 90 deg bend and a 1 metre uninsulated stack. Excellent! In order to achieve even a stronger draw, an insulated stack is what you need. The bare stove pipes are dissipating much heat so there's a limit to its length, as you discovered yourself.
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berko
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Post by berko on Feb 9, 2012 5:36:50 GMT -8
Gases will run faster only if the overall flow through the stove is maintained. Otherwise by reducing the gap, resistance will increase so the flow through the stove will be slower. The flow through the gap may be faster, but because more of the gas will have contact with the surfaces, the heat transfer will be greater. I think, it's false. We are talking about heat energy and not kinetic energy. The heat transfer depend on : Heat Transfer Coefficient,temperature difference, time. The flow depend on pressure difference. Therefore when you burn woods, fire increase pressure in firebox and gases flow through the stove (tunnel). When decrease tunnel cross section gases run faster, but flow smaller volume of gases in the tunnel in a time. I hope it's clear, my knowledge of the English is limited.
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Deleted
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Post by Deleted on Feb 9, 2012 7:12:50 GMT -8
The fire lowers the pressure as heat lowers the weight per volume and the gas streaming out causes additional under pressure and thus the colder havier air in the room will fill the void. The gas passing in a time remains about equal. With smaller cross-sectional area and higher speed the pressure to the walls will become lower and with more cross-sectional area and lesser speed the pressure to the walls will raise. en.wikipedia.org/wiki/Venturi_effectThe heat exchange is not only influenced by cross-sectional area, the resulting surface and speed but also by pressure to the walls.
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Post by grizbach on Feb 9, 2012 12:15:36 GMT -8
Donkey, It brings up a good point, when does an area become a bell? It does depend on turbulence. So if you have an operating bell system, and you slow down the flow enough to become laminar, it wouldn't be a bell anymore?? You would still get a lot of heat extraction because the volume of the flow is so slow. Grizbach, definition for a bell system would be like this: confined space with feed and exit openings at low level, or the feed slightly higher. The top of this confined space is completely closed, no hole at all. For example see the sketch below, a large part of the lowest barrel wouldn't qualify as a bell, but the top one does. So, the barrel as used in a rocket mass heater can't be a bell because the feed opening is quite near the top instead of in the lowest region. Hope this is clear, sometimes my knowledge of the English language is too limited. Thanks Peter, Any size can be a bell I suppose. I thought there was a turbulence issue also.
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Post by Donkey on Feb 9, 2012 15:47:58 GMT -8
The barrel is reversed, in actuality the bigger you make the barrel, the more heat you'll get from it (with practical limits). In THIS case, heated gasses are trying to move up and are being pumped (in effect) down and out. A wider gap will slow the pumping action, giving the heat more time to hang out in the barrel area. I know it's not technically a bell, but it will act a bit like one. As a general rule, if you want more heat from the barrel, make the gap larger. I have found this to be true in a couple of stoves that I've built in the last couple of years. It's counter-intuitive, but seems to be true none the less. A tighter gap up top will create a hot spot, as the book says. A tighter gap around the sides just seems to run the gasses through faster, so they end up in the bench earlier, leaving less heat behind in the process. So far, barrels seem to heat a room faster if the gap is larger. I could be wrong, but that appears to be what I've been seeing.
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Deleted
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Post by Deleted on Feb 10, 2012 1:47:01 GMT -8
That is as it has to be expected, as it corresponds exactly to the rules of physics.
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berko
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Post by berko on Feb 12, 2012 0:54:35 GMT -8
The fire lowers the pressure as heat lowers the weight per volume and the gas streaming out causes additional under pressure and thus the colder havier air in the room will fill the void. You are in right. I was too slubberer and i have studied this a long time ago You are in right again But we usually calculate simpler formula. I wanted to reveal that rule : when the gap is larger, you get more heat.
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Post by ytksgmt on Feb 13, 2012 9:26:10 GMT -8
The fire lowers the pressure as heat lowers the weight per volume and the gas streaming out causes additional under pressure and thus the colder havier air in the room will fill the void. I don't think so. Fire causes a mass of air with lower weight and with higher temperature but with the same pressure as the surrounding air. Therefore, the mass of air can keep its volume. In the atmosphere,say, at 1 meter high, the pressure decreases slightly from the pressure at level 0. In the same way ,in the riser with 1 meter height, the pressure at the top decreases slightly from the pressure at the bottom. But, the value of decrease of pressure in the riser is smaller than that in the atmosphere. That difference causes negative pressure at the bottom and positive pressure at the top of the riser.That is the tunnel effect. Well, I think the Donkey's effect was caused by the higher speed of the fluid. In case of forced fluid flow, when fluid speed increases, heat transfer through boundary surface increases. In the Donkey's case, with larger cross-sectional area, mass conductivity at downward pass increased. That caused the increase of mass conductivity of total system. And that increased the value of mass flow and increased the speed of fluid at the barrel,that is, at the downward pass. And, the increase of fluid speed effected the increase of heat transfer through the barrel surface. I guessed in that way. As for Venturi effect, comparison should be taken on the same fluid lines. Yes. The speed of fluid at the barrel with larger cross-sectional area is lowered THAN the speed at the riser. But, the speed at the riser in the system with the barrel of larger cross-sectional area is not always the same as the speed at the riser in the system with the barrel of the same cross-sectional area. I have once calculated the functions of mass flow of a system with different cross-sectional areas. According to the result, in some cases, the increase of cross-sectional area at downward pass causes the increase of mass flow. But, I can't verify it by experiments. Building of the experimental system started last autumn, but not yet finished, mostly because of my laziness. So, I can't be confident enough to speak of the details of the calculation.
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Deleted
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Post by Deleted on Feb 13, 2012 10:27:36 GMT -8
If the mass keeps it's volume then it keeps also it's weight per Volume.
That difference causes nothing, if the mass keeps it's volume.
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morticcio
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Post by morticcio on Feb 14, 2012 0:51:48 GMT -8
Added 3m of horizontal uninsulated flue and 2m of insulated vertical flue. A few findings... - With the gap between the sides of barrels at 4" the draw was minimal.
- With the gap at 1.5" the draw was excellent.
- With the gap at 1.5" the temperature of the barrel (top and sides) was hotter than with the gap at 1".
- You can bake cakes on top of the barrel (with a lid over the top).
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morticcio
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Post by morticcio on Feb 19, 2012 4:42:43 GMT -8
Installed in the workshop. After some issues levelling the stove and lining up the exhaust flue Andy welded three legs with bolts to adjust the height if required. This raised it up 11" to line up with the horizontal flue on the ledge behind. The bricks are stacked around the frame with the vermiculite poured in starting at the 4th course. The bench still needs to be cobbed and some cosmetic finishes such as filling in around the feed tube with some bricks. Added more photos to the Picasa album.
Many thanks for everyone's help and advice so far.
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morticcio
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Post by morticcio on Feb 20, 2012 10:11:20 GMT -8
Looking down onto the feed tube / burn tunnel with the vermiculite removed... 15 minutes before we had added 3 courses of bricks around the sides and back of the barrel, filled with vermiculite and also added a 6" sleeve on to the feed tube. The whole stove ran hotter - the draw improved and the barrel radiated more heat. I noticed a red glow between the bricks on top of the burn tube. The top of the burn tunnel (10mm or 3/8" plate) was glowing red. The top of the barrel now reached 315*C (600*F).
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morticcio
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Post by morticcio on Feb 20, 2012 10:13:33 GMT -8
Different angle...
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