feeding tube too hot on a J rocket stove

[christophe]

Hello,

I am building a J rocket stove without mass. i made it fully in metal because it needs to be transportable.
It works very well the first hour, but when it starts to be very hot, the feeding tube heats up and the wood inside it start to pyrolyse inside the feeding tube. There is no smoke because the draft leads the smoke inside the rocket stove and burns, but this is very troublesome.
This is not my first rocket stove, but this has never happen before.
Do you have any idea why?

Here is some photos and video and the dimensions.
The front hatch is only to start the fire, afterward I close it.

Thank you for your help

Dimentions

Photo inside the rocket

Photo inside rocket next

Photo insulation heat riser

Photo rocket with its bell

Photo burn 1

Photo burn 2

[christophe]

Usually, it is the draft that cool down the feeding tube. but because the feeding tube is welded with the combustion chamber, the heat from the combustion chamber goes to the feeding tube. I removed the feeding tube and will make a new one with a mix of vermiculite and refractory cement (ciment fondue). I hope, it will fix the issue.
If you have any other idea, please, let me know.

[Sprocket]

First off, instead insulate the burning chamber from inside so the heat won't transfer to the pipe so much. It will also help combustion and hopefully make for a cleaner and even hotter burn, using the wood more efficiently. I think it's a good idea to keep the feeding tube in metal to avoid damage by the wood. Vermiculite is quite soft. I guess you have to use ceramic fibres for this, but I have basically no clue of what materials are suitable or available as I have just started to look into building stoves. Maybe there is ceramic boards of some kind that kan be cut and slided in?

I think this would be enough, but if not maybe you could attach the feeding tube to the combustion chamber using flanges and fire rope in between, bolting it together. The fire rope does not conduct heat very well. The flanges will also act as cooling fins.

[satamax]

Well, looking at this



There is not enough draft. Even more if you have insulation around the riser. The bell is too small in diameter. Let's guess, feed tube is 120mm in diameter. 113 square centimeters of cross section. 

Gas container is 290mm inside. that's 660cm² minus 113 that's 547cm². Because the exit is at 90° on one side, you can count just half of that in real effective surface area. even worse when you think ring projection. 

Been there, done that. 


Metal rockets are doomed!