[Stoves] Highly recommended Re: Trials on TLUD gas burners

Crispin Pembert-Pigott crispinpigott at outlook.com
Sat May 3 19:06:43 CDT 2014


Dear Julien

 

I also appreciate the well written report.

 

Because it is being discussed here, I want to draw the attention of the
readers permanently to this point:

 

See photo three which has three burners operating at the same time and there
is a clear difference between the three combustor architectures:

 



 

My point is that NO COMBUSTOR operates with the pot off in the same manner
as with the pot on. You simply cannot tell how the air and gases will mix,
what the pattern is, what the penetration of air into the gas stream is,
with the pot off.  So please do not think that these flame patterns are 'how
these stoves work' when there is a pot present. Just about everything
changes.

 

This is not a slap on the hand for Julien who provides a very good picture
that contains some information of value. But please don't think that this is
'how the air flows and how the burner burn'. As soon as you put on a pot,
two very important changes are made to all three fires:

 

1.       The cold air outside the combustion zone can no longer travel into
the top of the burner and mix with the air that is coming through the walls,
and exit up the centre along with the flame (now a chilled flame).

2.       The pot adds substantial insulation/reflection/choking to the heat
and airflow which can dramatically change the primary and secondary air flow
rates and therefore (obviously) the quality of the burn.

 

The pot is part of the stove. To know what to do in developing a combustor,
the testing (observing) must be done in context - i.e. in a way that
reflects use. This is not a fire pit, it is a cooker.

 

The gap between the stove and the pot can be used as a total gas flow rate
limiter. The Philips stove is a good example of one that does not because
the gap is large enough to push fuel through into the combustion chamber
while it is running. That is a design choice. It subjects the combustor to
many influences such a light breeze, a periodic disturbance by people
walking by, or any big changes in the way the wood happens to fall inside
(which affects how the flames shoot up). That can cause the heat to
preferentially go to one side because it is so open.

 

Changing the Keren stove in Indonesia from a pot support height of 25mm to
7mm increased the heat transfer efficiency from 17% to >30% - about double.
It was able to do this because there was FAR too much air passing through
the stove messing with the combustion and heat transfer. The pot went from a
heat capture device to a gas flow regulator plus-heat-capturing-device. The
flame pattern changed dramatically. Primary air could be added from behind
the fire (like a Lion Stove) and the mixing much improved.

 

All the stoves shown in Julien's picture had clear effects of air entering
the top where the pot would be, descending into and cooling the fire. Part,
maybe all, of the flames being drawn together into a centre point is because
of this.

 

What to do to see how they really work?

 

I have tried two things, not counting making a small peep hole in a plate.

 

1.       Put a Pyrex casserole dish with water on it in place of a pot and
look through the glass.

2.       Put a metal plate on that represents a pot with lots of holes in it
- about 8 to10 mm. This will effectively block the air entry even though one
might think it would still allow air in. When 'retained' the flame or hot
gases fill the space and prevent cold air entering. You can look through the
holes.  I used a shower floor disk when the stove is small.

 

Figure 4 shows the result of the different architectures, which is helpful
because that is the result of the flame patterns you cannot see. Get the
point? The photos of the fire are not what you get so you can't use them to
determine the flame pattern, but the pot bottoms are a test of the reality.
You can of course mount a borosilicate glass window in the stove and look
through. A 50 x 50mm piece is easily large enough to look through if there
is a 20mm hole covered by it.

 

Conclusions

 

"The Rectangular burner was promising because it had a lower flame height."

 

That means the mixing was good, excess air had to be low (measure it at the
pot-stove boundary).

 

"However, when the secondary air enters as jets through air holes, a
concentrator disk above the secondary air intake may be counter productive."

 

There are sound reasons why, but another time.

 

There is a lot of combustion that takes place in the transparent non-visible
radiation zone away from and especially above the visible flame. Our eyes
are not particularly broad-band.  'Keeping the heat together' provides
burn-out conditions for CO in particular. Glowing particles make it easy to
know if there are particles present. CO is burning late in the flame and
above it without visibility (though more easily seen at night). In
principle, a single large flame can burn cleaner that a large number of
small flames in a large space.

 

The development of the FSP paraffin stove was almost entirely done at night
in order to be able to see the flame patterns that were invisible in the
daytime.  You have heard of 'golf widows'.  There are also 'stove widows'.

 

Regards

Crispin

 

From: Stoves [mailto:stoves-bounces at lists.bioenergylists.org] On Behalf Of
Paul Anderson



Julien,

Thank you for such a wonderful and well organized report (that is attached
to Julien's message yesterday).   We need such studies of, in your case, the
basic nature of the gas burners that are on the top of ND-TLUD stoves.

It is people like you and Kirk Harris and others who are doing some truly
important work about TLUD gasifiers.   
 
With your permission, I would like to have your report posted at the
drtlud.com   website, but I think you should first add your name, contact
info. and date of the report.   Then it can go to the website administrator
James Schoner    jss at bitmaxim.com <mailto:jss at bitmaxim.com>      for
posting.

Please discuss your next steps.    And I am sure that some of us would like
to know more about you and your circumstances that relate to stove work, if
you would please send such a message to the Listserv.

Paul



On 5/1/2014 10:22 PM, Julien Winter wrote:

Hello stovers;

 

I have just finished some tinkering on natural-draft, TLUD gas burners.  I
expect that much, if not all of what I have done, has been done before, but
it doesn't hurt to see some results to stimulate discussion.  The results of
my tinkerings are attached as a pdf.

 

I don't think my interpretations are complete, and I must revisit the paper
by Birzer et al. (2013).  However, it is past my bedtime.

 

Cheers,

Julien



-- 

Julien Winter
Cobourg, ON, CANADA

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