[Stoves] Trials on TLUD Gas Burners - Counter Current Flow

[Julien Winter]

Hi Kirk and Stovers;

Thanks for your comments, Kirk.  They are will be very useful as I, and
anyone else continues to tinker with counter current flow for secondary
air, especially when the primary air is turned down and fuel is moistened.

For readers who may not have see it, Kirk contributed an insightful study
on TLUD operation at low primary air.  His posting was on May 15, 2014 and
can be found here:
http://lists.bioenergylists.org/pipermail/stoves_lists.bioenergylists.org/2014-May/008751.html

Kirk's "Method 1" and "Method 4" have a component of counter current air.
Kirk's insight was that getting secondary air down to the fuel bed can play
an important role in keeping the gas flame alight when the primary air has
been turned down to as low as possible.  I think this is also important for
fuel that is a bit damp, which would be common in humid climates.

The main difference between what Kirk did, and the burner I have mooted
("the cone") is the velocity of secondary air.  The cone attempts to create
a straight path, and the full strength of draft to propel the secondary on
downward trajectory.

If a cowling was added about the cone burner, it could be possible to
preheat secondary air.

There are few assumptions (not necessarily correct) that are guiding my
design choices.  Basically, this is the "Add Secondary Air and Let Her Rip"
school of thought.

1) A broad horizontal flame is better than a tall narrow one (such as the
tall conical flames that you can get with narrow aperture concentrator
disks),

2) so a wide burner allows horizontal space for the gas flame to develop.

3) It is better not to put objects in the path of the flame, such as
concentrator rings.  Instead, try to manipulate mixing of reactants by how
the secondary and pyrogas are introduced into the burner.  Once reactants
have been introduced, let the flame (i.e. the reaction) go where it wants
to go.

4) Secondary air should be added only once (except for small pilot holes)
so you have only one chance to get it right.  Trying to add substantive
amounts of secondary air higher up the burner may not incorporate properly
into the flame.  It may also arrive too late for burning larger
hydrocarbons if the exothermal heat from oxidizing more reactive gases was
released lower in burner.  (I have noticed that an air leak below a
secondary air hole can cause incomplete combustion at that hole.)  It is
best to supply stoichiometric oxygen so that reactive and less reactive
gases burn together.

I have some experimental results that I have not yet shared.  I found that
how the secondary air was introduced (size and shape of holes or
slots) affected the rate of gasification in the TLUD.

5) Starting the gas fire low over the fuel bet is good.  Radiant heat keeps
the char pyrogas hot, and not much cooler than the flaming pyrolysis.  As
Kirk as pointed out, some burning char at the top of a turned-down TLUD
maintains both gasification and the gas flame.


As I said above, these are assumptions.  They are speculations that are
underdetermined by available evidence.  They shouldn't go unchallenged.
Some people have gone a completely different way to insert objects into the
burner to create turbulence --- that is also a worthwhile inquiry.   Point
#4 is worth challenging, because you may want one level of secondary air
for a turned-down TLUD, and a large stream of secondary air for a TLUD
running full blast, which is what works for Kirk.

The next problem will be: what would a TLUD look like from a 3-D printer?
TLUD builders are going to be forced to start "thinking out of the can".

Keep tinkering.

Cheers,
Julien.


-- 
Julien Winter
Cobourg, ON, CANADA
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