Dear Marc and Ron and All interested in air flows
This is a response to questions about air and Marc’s tube.
Here is an old photo of secondary air entering the combustion chamber of a Vesto pushing the flame to the centre. This accomplishes the following:
Keeps the fire away from the wall, reducing the temperature it has to survive (a lot)
Keeps the flame going
Not allowing it to spread to one side away from the smoke on the other side that might otherwise ‘get away’.
Provides turbulent mixing of flame, hot secondary air and smoke
Allows for preheating to a significant degree (250-500 C)
The fire is circular because it is spinning rapidly, though pushed to the side by the way the fuel happened to be sitting. The spin adds turbulence without a fan and assists in keeping the flame away from the combustion chamber wall.
Here is a Vesto burning switchgrass pellets operating as TLUD, showing that there is nothing special about a TLUD in the sense of it having to operate in a particular fashion. The air flow through the fuel is reduced by the fuel and it operates as a TLUD. The secondary air is send across the surface to keep a deck of flame going at the height of the holes. This obviates the need for adding a circular disk at the top to ’keep the flame going’. Adding a ‘concentrator’ as Paul calls it takes more material and moves the fire too far away from the heat of the pyrolysis bed leading to unwanted flame-outs from time to time. A major issue with all pyrolysing TLUD’s. It is simply not necessary. Just keep the fire near the fuel. This also provides additional vertical space for the flame to finish burning before getting to a cold pot surface.
Finally, here is a photo of a Vesto cutaway showing the inside parts in their correct positons.
The primary air controller is the ring with holes in it. When the handle is moved to the side the holes are closed.