[Stoves] A question on air control

[AJH]

On Tue, 29 May 2007 13:06:50 -0700, frank wrote:

>
>I have packed toilet paper rolls with all sorts of potential 'small' 
>fuel and am making arrangements to control the air in my lab set-up. 
>Having one air source enter the stove with it split into primary and 
>secondary - all the air goes to secondary due to the resistance from the 
>fuel.

Yes this will happen until the fuel is ignited, after that the flue
gases from the fire are more buoyant and cause a (very slight)
pressure differential, sucking in primary air, they also then entrain
secondary air.


>   It looks like I need to make all the air go to the 
>primary then open a port to adjust the primary to optimum depending on 
>the fuel type and packing (void space?). That leaves the secondary 
>taking the excess air without any control. 

I'm not sure that I understand you but generally you use a control on
the primary air to set the power of the fire, then you supply the
balance (including the excess air) to get the right mixture for the
secondary flame. Intuitively this makes a premixed flame need less
excess air.


>What is this optimum flow rate for the primary?

Difficult one to answer for a tlud but for a traditional updraught
fire aiming for complete gasification of the char you'd need about 1
part by mass air to one part char (the heat from this drives off
pyrolysis products which requires no primary air), then the balance of
about 5 parts air and a further 4 excess will be needed to burn out
the secondary flame. This full gasification depends on keeping the
primary air speed high enough and in practice doesn't happen on the
small scale, so a higher proportion of primary air becomes necessary.

>How can these TLUD stoves work without having this adjustment and using 
>different fuels?

Well there's a fairly wide range over which the tlud will work,
especially as the fuel gets more moist until it just won't sustain a
flame. The tlud is an off gas generator, so the intent is to supply
the minimum amount of heat to sustain the descending pyrolysis front.
This heat is provided by burning a small portion of char in primary
air, as soon as this region has conducted enough heat to char an
adjacent particle (remembering that these particles have a finite size
and hence the outside chars and then the inside continues to do so
*above* the combustion zone) it is then snuffed by rising combustion
products and offgas.

So the amount of primary air for a tlud is well under that required
for full gasification unless you burn out all the char.

AJH