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Post by flybyjohn on Jan 19, 2018 20:29:33 GMT -8
I was just starting to build my mold for the lower riser section that will include the port and was remembering back to my fluid dynamics class and wondered if a slight radius on the leading edges of the port would allow gasses to flow through the port with more velocity.
If a radius was put on it, then it would allow the floor port secondary air vertical section to be a little closer to the back of the fire box.
Has this been tried before. Is it a do not do this kind of thing. I just have not seen any questions or comments on it.
I also was was wondering if the floor port vertical portion gets in the way of loading firewood. The box is already pretty short and the vertical tube would take up another 2-3 inches of length.
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Post by satamax on Jan 19, 2018 23:52:17 GMT -8
donkey32.proboards.com/post/17953/threadPeter has tried it, IIRC. And it kills the turbulence. You need the boundary layer in the port to create the turbulence. Boundary layer builds up a radius at the leading edge, with gases coming from a perpendicular direction to the port, the boundary layer under a slight underpresure, or else; builds up thicker with gases sort of sticking to it. Then due to flames velocity and all the factors involved, the layers of gases which have "stuck" to the boundary layer, wash off. Then conditions changing it comes back and off again,and so on.
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Post by peterberg on Jan 20, 2018 0:35:07 GMT -8
I was just starting to build my mold for the lower riser section that will include the port and was remembering back to my fluid dynamics class and wondered if a slight radius on the leading edges of the port would allow gasses to flow through the port with more velocity. If a radius was put on it, then it would allow the floor port secondary air vertical section to be a little closer to the back of the fire box. Has this been tried before. Is it a do not do this kind of thing. I just have not seen any questions or comments on it. Yes, it has been tried before although not by me. It turns out speed is not all, the good people of Dragon Heaters built molds for a batch box in 2013 or 2014. They rounded off all the sharp corners because computer simulation showed a dramatic speed increase. The resulting heater smoked like mad and was very reluctant to do otherwise. Last year Pablo Kulbaba in Argentina built one with generously rounded off leading corners of the port and that one also didn't want to burn without much smoke. I recommended to change back to sharp corners and after that was done the thing behaved itself as if it was touched by the magic wand. I discussed this with Larry Winiarsky in Warsaw, back in 2015 and the conclusion was that the port with sharp corners acted as being much more narrow, by forming cushions of turbulent air at the sides. There are more effects but this seemed to be the most important. Look at pictures and videos of the riser from above: the emerging stream is much more narrow than the port itself, down to about half of it in some instances. Those air cushions slow down the sides of the stream and initiate strong small eddies at the edge of the main stream. By keeping this in mind it's logical that a port that's much deeper won't work in the same way as its swallower cousin. I also was wondering if the floor port vertical portion gets in the way of loading firewood. The box is already pretty short and the vertical tube would take up another 2-3 inches of length. Two inches in a 6" system, to be precise. Keep in mind that in order to get the most out of this combustion core, there need to be a space between fuel and back wall. By arranging the fuel like this, the first part of the burn is much cleaner (and efficient) especially when there's also a space of two inches above and in front of the fuel and the fire is lit on top. In case a short firebox is giving you trouble, just lengthen it by a couple of inches. In short: no, the vertical part of the floor channel won't be in the way. On the contrary, it prevents you sticking fuel into the port which is a definitely do not do this, because it will disturb our precious turbulence. I know, the whole of the thing looks just too simple but all what you see need to be there in order to deliver the best combustion properties.
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Post by flybyjohn on Jan 20, 2018 6:49:51 GMT -8
Thank you guys for keeping me from making a mistake. It makes perfect sense. Sometimes fixing something to work better does quite the opposite.
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Post by Deleted on Jan 20, 2018 8:06:47 GMT -8
I discussed this with Larry Winiarsky in Warsaw, back in 2015 and the conclusion was that the port with sharp corners acted as being much more narrow, by forming cushions of turbulent air at the sides. There are more effects but this seemed to be the most important. Look at pictures and videos of the riser from above: the emerging stream is much more narrow than the port itself, down to about half of it in some instances. Those air cushions slow down the sides of the stream and initiate strong small eddies at the edge of the main stream. By keeping this in mind it's logical that a port that's much deeper won't work in the same way as its swallower cousin. Obviously with rounded edges the port could be made much more narrow.
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Post by satamax on Jan 20, 2018 9:41:41 GMT -8
I discussed this with Larry Winiarsky in Warsaw, back in 2015 and the conclusion was that the port with sharp corners acted as being much more narrow, by forming cushions of turbulent air at the sides. There are more effects but this seemed to be the most important. Look at pictures and videos of the riser from above: the emerging stream is much more narrow than the port itself, down to about half of it in some instances. Those air cushions slow down the sides of the stream and initiate strong small eddies at the edge of the main stream. By keeping this in mind it's logical that a port that's much deeper won't work in the same way as its swallower cousin. Obviously with rounded edges the port could be made much more narrow. But might not have a turbulent behavior.
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