thomas rubino wrote:Jacob;
White smoke is steam , your wood is very dry so no moisture from it. I can tell you it takes a long while to dry out your cob. The condition and temp. of the home's chimney will have an effect as well.
You are an experienced rmh builder. You built to proper specifications. You did no "experimenting" I think your dragon just needs to warm up and dry out, I think she will roar just fine when all is warm.
Satamax Antone wrote:Jacob, two details.
First, 250C° is not that low a temp. Your fire, in a 8 inch is small. Compared to a box stove. And a barrel is 1.86m² of ISA (internal surface area) This sheds a lot of heat. Plus your heater being wet. I don't think you should worry too much. What is interesting in a rocket. Is the amount of heat recovery. Compared to a normal box stove. When you have the mass installed.
Then, about your top gap.
You have a CSA (cross sectional area) of the heat riser of about 64 sqin. If i understood well, you made a square heat riser. Then your top gap is 4x8" X2" which is 64 sqin again. Or, due to the boundary layer, your heat riser must be closer to 50 sqin. But still, you are under the recommended 1.5x csa usually accepted. You would be better off "freeing" your draft, i think. by raising your barrel a bit more. At 2.5 or 3 inches.
Satamax Antone wrote:Jacob, trust me, your top gap is too small.
So it's round, then your CSA is 50.26 sqin.
8 inch round circumference, 25.13 inches. X2 for your top gap. 50.26 sqin again; So, considering the direction change, the turbulence created by the direction change, and the friction resulting of this, your top gap is too small.
Remember, the figure is 1.5 times the CSA for the top gap, at least. So that's 75.39. /25.13 = 3, you need three inches top gap.
Satamax Antone wrote:Hi Jacob.
Well, that number has been devised using guesswork and experience.
I don't know if you are familiar with fluid dynamics.
But what happens when your gases are exiting your heat riser, is pretty much what happens against one half of this cube.
Plus a sharp direction change, squashing the gases.
You can see the boundary layer here
Here is the turbulence behind a port.
What happens at the top of the heat riser is as turbulent as this. Plus the top plate of the barrel squashes everything.
Well, i thought, i have a kind of real life example. Tho, with a 20cm gap.
See the radius of the gases, when these hit the barrel top? That's the reason you need a bigger top gap. If you are really set on keeping it at two inches. There is a trick, forming the top of the riser, like a trumpet endbell. So your circumference is larger than the one of the heat riser, allowing your top gap to be smaller. But there isn't many people who have tried this. Two or three i am aware of.
I have searched a bit on permies and donkey's forums. I can't find the first instances of the 1.5 figure.
thomas rubino wrote:Jacob; I run mine at 2.5", there is flex with the removable lid so gap can be 2.25" to 2.75"