[Stoves] Oversized stoves / thermal insulation

[ajheggie at gmail.com]

On Sunday 07 November 2010 06:53:00 Crispin Pemberton-Pigott wrote:
> I have made very short paraffin flames from a fairly heavy oil
> (compared with Butane of Methane) but still, getting a really clean
> burn with solid fuels or solid fuels with heavy oil seems to have a
> clear relationship with length of the flamed region.

For a given mass flow and similar products of combustion the flame length 
is directly related to combustion time, the shorter the flame the shorter 
the time for completion of any reactions. An oxyacetylene flame 
progressively shortens as oxygen is added and it changes from a pure (and 
sooty) diffuse flame to a short premixed blue flame. So the length of a 
clean flame determines the amount of time necessary to complete 
combustion, if combustion cannot complete withing this time then those 
bits of the chemical mixture  that haven't finished reacting are emitted 
as pollutants. We can see this in a candle flame by lowering a cold spoon 
over it, above the flame the spoon remains clean but as it enters the 
flame and quenches the reaction there is a build up of soot. Similarly if 
we increase the fuel massflow of an oil lamp by increasing the wick 
length then the flame gets bigger but there is insufficient surface area 
of flame for oxygen to diffuse into the fuel vapour and sooty particles 
aren't able to burn out within the length of flame. My supposition is 
that the bonding in the hydrocarbon vapours is such that it favours 
splitting a hydrogen atom away and oxidising this leaving a higher carbon 
compound behind and it is this that glows in the diffuse flame until it 
is oxidised within the flame, if it doesn't have sufficient time and 
energy available in the flame to dissociate and find an oxygen atom them 
it is emitted as a Product of Incomplete Combustion.

The flame is radiating heat away from itself constantly so it is losing 
temperature and high temperature is one thing that leads to good 
combustion. If the combustion chamber is better insulated and can re 
radiate heat back to the flame then I would expect  the flame length to 
increase as these PICs contribute their heat and continue burning.

I'm guessing there's a bit of a corollary with the lamp test that Tom Reed 
told us about for establishing the detonation point of a hydrocarbon by 
the maximum flame length that could be achieved in a standard oil lamp 
before the flame went sooty  i.e. maintain the flame temperature higher 
and you increase the rate at which oxidation takes place so you can 
increase fuel feed within the same flame length and still have a clean 
flame.

AJH