[Stoves] stoves 101

[Jock Gill]

Crispin,

A few notes:

FYI: I reduced my secondary air slot by 50% and got better performance.  It is now about ¼ inch.  The larger slot, however, is required for photography ;-)

Here is a thought test.

1. If there is zero secondary air, clearly that is a fail.

2. In my systems, if the draft can is removed, 100% secondary air, the pyrolysis fails -- insufficient draft.

3. So the challenge is to find the "just right" secondary air gap for a given TLUD of a given diameter with a given fuel type.  Adjusting the air gap, is, perhaps, another way to vary the output of the stove.  The more secondary air, the cooler the heat delivered to the target.  This suggests a draft can that can be set at a range of heights?

As for testing, If you put a pot over the TLUD, or, better, in the exhaust gas flow ala the Swiss Volcano Stove, you will rapidly discover if the pot bottom gets covered in soot.  Not good.  Generally, I get a slight varnish of tars if everything is well balanced, but no soot.  These should wash off with ease.  When I see stoves in the field with soot covered pots I know that the combined stove+pot system is sub-optimal.

Otherwise,  I concur with your points.

BTW:  If you would like to see the video, just ask.  It is a 5 meg file I will send you via email -- OFF list.  I have not posted the video to the list.

Cheers,

Jock

Jock Gill
P.O. Box 3
Peacham,  VT 05862

Cell: (617) 449-8111

google.com/+JockGill

:> Extract CO2 from the atmosphere! <:

Via iPad

> On Mar 10, 2014, at 12:12 PM, Crispin Pembert-Pigott <crispinpigott at outlook.com> wrote:
> 
> Dear Juanito
>  
> >Would you point me to introductory posts re:
>  
> >>"...to give the flames a change (sic) to burn out before getting to the pot"
>  
> Thanks for the question.
>  
> Carbon monoxide and particulate matter (partially burned fuel or carbon particles that grow from free elemental carbon liberated during combustion are both 'products of incomplete combustion'.  When the combustion conditions are not perfect, there are inevitably components of the fuel which do not burn to completion. This can be quantified as how inefficient (2%) or now efficient (98%) the combustion is. Usually the number relates to the heat lost that 'could have been' or it is a ratio of how well something like Carbon or Sulphur was burned.
>  
> When a flame touches something cooler than itself, it fails to burn the fuel completely. A pot is always cold because what is inside (usually) is liquid. If no space is left between the fire and the pot, it is inevitable that the pot interferes with the combustion. This can easily be shown in a couple of ways. The first is that some stoves make a heck of a lot more smoke with a pot on that off - that is the first sign. The second is that elevating the pot can reduce substantially the PM produced (try it and see - don't work about the heat transfer efficiency). This is most evident in ethanol stoves where one can get a very clean fire quite easily, or a dirty one by preventing proper combustion.
>  
> A third is to pass a saucer (without the tea cup) across the top of a candle flame. Make several passes starting well above the flame tip and descending slowly with each subsequent pass. Below a certain clearance, the saucer will starting blackened as it crosses over the flame. That is the point at which it the interference happens. It is not in a visible region of the combustion, but carbon is still burning there. Below that point, the saucer will get very dirty with a single quick pass because it massively interferes with the combustion.
>  
> Creating a TLUD is quite easy. Creating one with no flame space means having dirty pots and a lot of smoke. Nearly every charcoal stove in the world has insufficient space to burn the charcoal properly. People even put the pot directly on top of the burning charcoal thinking it 'helps heat transfer'. It is in fact creating a cloud of CO from unburned carbon. The absolute minimum space between the fuel and the pot (on a charcoal stove) is 50mm. You will see in that space the blue flame of burning CO. Lower the pot and it disappears.
>  
> You will see from the video Jock provided, the flame burning above the secondary air gap is completed well below the pot. The amount of air entering is far more than is needed to burn the gases indicating that there is a high excess air condition and this a cool flame (sorry, Jock). The best heating efficiency will be achieved when the secondary air quantity is 'just right' and the pot is immediately above the flame. The difference in fuel consumption can be large, a facto of three, between an ideal system and one with too much air and too close a pot.
>  
> When you see flames running along the bottom of a pot and up the sides, you know that is an inefficient, smoky device.  To get a clean, short burn, the secondary air should be introduced hot enough, low enough and fast enough to get good mixing. That does not require a fan, just right-sizing.
>  
> Regards
> Crispin
>  
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