I have assembled my first J tube and made a burn with no insulation just to test it.
The burn went great, the idea was to harden the furnace cement I used to hold the fire bricks together until I encased them in a home made refractory.
The draught was so good that I had to use a metal plate to cut back on the sir flow into the chamber. I threw wood, cardboard, wet wood and even used engine oil in to see how well it would burn. The engine oil worked great, but had to cut the air way back or I had flames up into the stove pipe I was using as a stack.
The neat part is that I made my "J" tube more as a "U" tube, with the feed chamber the same height as the exhaust. If I took the stove pipe and moved it to the feed chamber, the draught reversed so I could cure the furnace cement on that side. If that hadn't worked, the feed chamber stayed so cool it wouldn't have cured.
The other interesting part was that since I didn't have it encased yet, I could feel the bricks and estimate the travel of the flame by the variations in temp. The top 2\3 of the feed chamber stayed cold (it was 1 Celsius outside), the bricks at the bottom of the feed chamber got hot. The bricks on the side of the burn chamber got hot, but it took a while. The exhaust end was very interesting. The wall farthest from the feed chamber is made of 3 bricks stacked on their sides. The centre brick heated fast and was hottest. The bottom brick got warm, but it took a long time. The wall above the exhaust end of the burn chamber stayed quite cool. The top of the burn chamber got hot. When I put in a splash of oil, it all got hot fast, probably because of the amount and intensity of the flame until I cut the air back.
My burn chamber is the length of the width of 2 bricks, so that would be 4.5 x 2, 9 inches. The height is 3.25 inches, the width of 1 brick minus the thickness of a brick. Feed and exhaust are the width of a brick square.
I was going to have a photo of the completed tube, but it kinda fell apart as I moved it back into the garage. LOL Guess I should have waited longer for the furnace cement to fully cure before moving it. At least I have my design figured out, so putting it back together should be a breeze.
I have attached a couple of pictures of the burn from the feed chamber. The rocket noise is strong enough, even with the air restricted, that I could hear it around the corner of the garage, about 20 feet away.
I will start building the mass and reassemble the tube in place where it will stay in my house. Hopefully I will have a working heater in a couple of weeks.