So here it is, blocked up on the front of the hearth to get the vent into the woodstove, still without the barrel.
It was getting late, so I just did something sort of okay with a Workmate and a couple of wood blocks to get the necessary elevation. I'll do something neater and more level later. With a taller barrel stand cut at the second ridge, I would just have to level the support, and I could have a heat riser a foot taller. This heat riser is 30", previous have been 36" because the wire mesh barrel stand held the barrel 6" higher.
So here it is, fired up for the first small test with some paper, a pitch stick and a handful of bamboo kindling some time around 10:15 PM. Done in a day, sort of.
It started easily and ran up to about 450˚ on this minimal but fast-burning fuel load. However, there was considerable smokeback from this large and shallow fuel inlet without some sort of cap. There was also a bit of smoke leaking from the barrel-barrel stand connection during startup. I'm sure the leakage persists all through the burn, but it stopped smoking in a couple minutes.
This morning we ran it again with some more bamboo, smallish sticks such as you see in the background, and a couple of larger sticks, all in the comfortable working range for my lopper. Which is good, because it really is best to cut this stuff in half before putting it into the fuel port. The system heated up very quickly, on the order of 100˚/min, up to about 750˚.Then it drifted down and plateaued at 650˚± 50˚. I suspect the relatively fast heatup is attributable to the much reduced air infiltration around the burn channel-riser connection and perhaps to the foil. On the other hand, it didn't seem as inclined to run up to 900˚+ as previous systems. This may be due to the shorter riser reducing the draw along with more restriction on the exaust—that 8" vent isn't actually connected to a chimney, so there is no draw to compensate for the increased restriction. Another limitation to the system heating up may be the very considerable heat loss from the fuel channel/air inlet. However, that isn't much changed from earlier systems.
The force available to draw air through of air the system is fundamentally limited by the dimensions of the heat riser, and the air supply fundamentally limits the heat production. I think this is the main thing here. The initial overshoot fits with this theory, as the draw is also affected by the cooling of air in the barrel. Initially, the barrel is cold so the exhaust in it cools rapidly, becomes denser offers less counterforce to the riser.
I wonder if the rule of thumb about equal cross section through the system is really valid with this setup. I suppose I could make a 10" riser and see how that actually works. With the blanket walls, I've got plenty of space inside the barrel to work with.
On the other hand, as a practical matter, without a better fuel inlet, I'm not sure I actually want the system running that hot. So far, my experience indicates that the hottest part of the system is right where the flames are max, which is very close to the inlet. So there's that much more of a smokeback/firecreep potential to deal with.