I tend to agree with Jon: most of any extra heat comes from the BS.
I will also make the bald flat statement that wood does not burn. Yes, you read that right!
I will quote the Encyc Brittanica here:
Pyrolysis, the chemical decomposition of organic (carbon-based) materials through the application of heat. Pyrolysis, which is also the first step in gasification and combustion, occurs in the absence or near absence of oxygen, and it is thus distinct from combustion (burning), which can take place only if sufficient oxygen is present. The rate of pyrolysis increases with temperature. In industrial applications the temperatures used are often 430 °C (about 800 °F) or higher, whereas in smaller-scale operations the temperature may be much lower. Two well-known products created by pyrolysis are a form of charcoal called biochar, created by heating wood, and coke (which is used as an industrial fuel and a heat shield), created by heating coal. Pyrolysis also produces condensable liquids (or tar) and noncondensable gases.
Throw a cut log on a wood fire and watch. First, it does not go up in an explosion of flame. (If it does, you are not handling wood!)
It heats up until pyrolysis happens inside the log. Volatiles come out the end of the log as the grain is more open there, and those volatiles further decompose and burn. Eventually, when all possible volatile gases have been boiled off, you are left with charcoal, glowing red.
So – wood does not burn; the pyrolysis products and the remaining charcoal are what burns.
In an open fire this is all obvious. In a tin can … exactly the same pyrolysis happens. There are flames at the top as the volatiles mix with air and burn. Not enough air – and combustion is quenched and you get soot and smoke.
Does the exact air flow pattern matter, or the temperature of the incoming air? Most unlikely. Mix fuel and air and apply a match. Have enough air and the fuel will burn pretty much to completion. Ah – chill the flame too much and the combustion will fail. This does not mean however that heating the incoming air from ambient to 100 C is going to do very much. After all, with a combustion (flame) temperature of 1200 – 1400 C, who cares about the bottom end?
Can you recirculate the combustion products (basically, CO2 and H2O) and burn them again? Don’t be daft! Is there all that much unburnt fuel available for secondary recirculation and combustion? Not if you have enough air flow at the start.
So what is with all the double walls, wicks and secondary air holes? Well, most of it (in my own personal opinion) is good marketing spin. After all, if a soup can with some holes punched in it is just as good, why pay the extra?
My apologies to all those whose products I have insulted. Eric asked, and I answered.
I remember a story Will Rietveld once told. He had the alcohol, but had forgotten his alky stove. His wife found an empty Coke can at the camp site (shame on someone else!). A few minutes later with his SAK and he had a fully functional alky stove and was cooking dinner. Moral: you don’t need fancy.
Footnote: the TD cone is another matter. It is also a very good windshield, which can be essential. Other arrangements may be similar.
Cheers
