[Stoves] iCan w/ Deflectors @ 20 minutes

[Crispin Pemberton-Pigott]

Dear Jock

 

>...you will see that they have quite different performance characteristics.

 

That is an interesting comment. 

 

>1. Increase turbulence, yielding cleaner combustion, as in a TDI diesel; 

Turbulence is good.

 

>2. Keep the combustion mainly contained in the combustion zone; 

I favour this approach, other do not.

 

>3. Prevent the establishment of stable central spire of flame; 

That may shorten the stove, vertically.

 

4. Create a very clean combustion with very clear stack gases.

For that I have to see numbers :) 

 

>As for persistent central spires of flame, I have observe that such spires
often eject considerable streams of soot particles.  I get cleaner results
when there is no persistent central flame spire.

 

While this is your experience, I caution that there are other explanations.
The environment in which that thin spire burns has a huge effect on the
final result. I have been working on a stove that has 18 inches of
deliberately created 'spire' that achieves very good combustion at a low
excess air level (30%). The PM you see are emerging from the cooling effect.
The existence of it in a very small stove is usually cause by the way the
secondary air is introduced. As you are using a ring-gap, you do not get the
advantage provided by multiple hole jets (which create a lot of turbulence.
Left alone, the ring-gap creates a disturbance-free fire which are
compensating for by adding parts. When a stove like the POCA used a
ring-gap, the whole combustion process takes place to completion in the
space between the fuel and the bottom of the pot in a very hot local
environment. This can overcome quite effectively the need for turbulence in
the whole chamber. In the case of the POCA the flame is conical with a
CO-free zone at the top of the cone, in the centre of the chamber. 

 

A different approach is to create the spire and spin it.

 

>.I achieve the same central disk effect when I put pots inside the flue
pipe with sufficient space around the pot for flue games to escape.  

 

Because the pot serves as a bluff body on nearly all stoves, the selection
of the pot often changes the measured performance. Pot size can change
emissions. It also point to the problem of people testing stove without any
pot at all 'testing the combustion'. IT gives completely different answers.

 

>.I find the fixed gap I am using works quite well on average.  

 

We need to do creative things to have it automate by ducted flow! Most
people try to get that average you are talking about. Running the combustion
zone hotter corrects many minor design problems, however.

 

>Hugh has suggested that there is almost never enough secondary air in many
TLUD configurations.

 

If you see a diffuse flame wandering around looking for oxygen, that is a
starving flame.

 

>Please tell me more about what the eyes tell you.  I have noticed that some
configurations I have used make my eyes water.  The current one does not.
Also some stack gases really aggravate the nose and throat.

 

I can't say the approach is attached to any metrics, but as I demonstrated
to AD Karve at ETHOS once, burning damp wood in a Vesto, that it is
unexpectedly clean, and that it did not make one's eyes water. I encouraged
him to remove his glasses and stare into the exhaust about 3-4 feet above
the flame where one can feel the heated emissions rushing by. He has been
using it ever since as a way to test for 'big differences' between stoves
and recommends it to all the people he interacts who do not have any
instrumentation. While it is obvious that particles will cause irritation
there are chemicals we have been discussing recently as well. Anything that
irritates the eyes is bound to be bad for us. If it is not detectable, then
it is probably better than the other conditions. CO is not detectable of
course, but there is often (I can't say 'usually') a correlation between PM
and CO.

 

In short, a stove that makes your eyes sting is not as clean-burning as one
that does not. As more testing gets done we will have a better understanding
of the value of the 'sniff test' and the 'eye-blink test'. Sometimes that is
all the equipment we have.

 

Regards

Crispin

 

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