Fuel burner tube enlargement for the kiss stove
An experimental with fuel burner tube enlargement without compromising the cooking surface.
Background to fuel burner tube enlargement
My KISS Stove is tiny compared with regular tent stoves so it only has a tiny cooktop. I was relieved to at last have a simple, light and compact design that worked without any fuss. The diameter of the stove pipe and fuel/burner ports were both about 40mm so that they would not encroach on the already small cooktop.
During a recent ski/camping trip, I found that the stove burnt thin sticks quickly. I thought that it would have been nice to be able to load slightly thicker fuel sticks to make the fuel preparation easier.
Fuel burner tube enlargement considerations
Simply increasing the fuel/burner port diameter was not an option as it would further encroach on the cooktop area. Consequently, I thought that elongation of the port, keeping as close as possible to the fire dome wall, would be one way of achieving this enlargement. I also thought that such enlargement could be done in small stages in order to find the optimum aperture.
During the many trials and errors involved in the development of the KISS Stove, I became aware of some critical design features that make my downdraft stoves burn reliably. I have described them in a post; Micro Tent Stove Design. Three critical features depend upon the maintenance of a suitably velocity of the incoming airflow down into the inverted burner tube.
The stove pipe draft largely determines the steady rate of air entry into the burner, once the wood gas flame is ignited. This means that as the fuel/burner tube aperture increases, the velocity of the incoming air will reduce in inverse proportion to the aperture area.
Firstly, if the velocity becomes too low, the stove will develop a dangerous tendency to reverse burn. This means that the flames and combustion gases flow in reverse up the fuel/burner tube and into the tent!
Secondly, the fast downward flow of the air attacks the charcoal to make it burn more quickly than it normally would ( by simple air diffusion processes). “This is the heart of the KISS stove design.” This in turn eliminates the unwanted accumulation of charcoal and the phenomenon that I call ‘charcoal choking’ that can prevent the steady entry of fresh fuel, the supply of wood gas and the flame that drives the stove draft. This series of events will cause unwanted and dangerous reverse burning. Consequently, adequate air velocity must be maintained.
Lastly, increased airflow into the stove, beyond that required for complete wood gas combustion will cool the combustion gases and will reduce the stoves heating efficiency.
The enlargment
The original effective aperture was 11.6 cm² (based on an effective diameter of 38mm inside the burner tube. The new aperture that I cut by hand with a Dremel tool is a bit more tricky to estimate, so I used a simple ‘cut and weigh method’ that I learned for the laboratory when computers were mysterious big things that were locked up in massive rooms and we used giant slide rules for calculations instead.
The new fuel/burner tube aperture shape weighed 0.13g when cut out of a sheet of photocopier paper and the whole sheet 298*211mm weighed 5.00g so, by proportion the new aperture is approximately 16.4 cm² that is 44% larger than the original one.
This 44% increase in the area should theoretically reduce the input air velocity by 44% if there were no fuel sticks in the port. However, when the port is fully loaded with round sticks it will return the velocity to a similar value to that with a loaded smaller port. It will be important to keep this enlarged port fully loaded in order to maintain optimum air velocity and prevent reverse burning.
Stove evaluation with an enlarged fuel burner tube
I found that the stove burnt well with the enlarged fuel burner tube. Once the stove was up to operating temperature and had developed a glowing charcoal bed. The long axis of the port ended up 49mm long and this simply could accommodate more sticks and larger ones.
The burning of large sticks is difficult when they are wet or damp as the inner core is slow to dry and this delays the collapse of the charcoal stem. So they will still need to be split to burn effectively. On the other hand, large dry sticks will burn more easily as the inner core does not need to be dried and the charcoal stem will collapse more willingly. Whether wet or dry, the larger port makes the fueling easier. It does not appear to the burn rate or increase the stove temperature and probably slows combustion a little as it allows thicker slower burning sticks to be used as fuel.
I saw no evidence of reverse burning but I think more testing would be prudent before moving on to make the aperture even bigger in order to find the optimum size that still does not allow reverse burning.
Tim
