[Stoves] Increasing ND-TLUD riser height accelerates gasification and increases bed temperature

[Julien Winter]

Hi Stovers;

Paul Anderson proposed the following

"About height, whether of riser or of chimney:  I recommend
some simple experiments when your unit is running.   Have some different
lengths of extensions that can be easily placed (not even attached) on top
of the current unit.   The additional draft will pull more secondary air
and make the flame be LOWER as long as there is control over the entry of
primary air.   But if the primary air is free flowing, the primary air will
eventually "catch up" with the increased draft, and the flame will become
higher than before.   If that is not what you observe, please let me know
so I can adjust my thinking."


According to the formula for draft found on Wikipedia,
https://en.wikipedia.org/wiki/Chimney#Chimney_draught_or_draft

increasing the riser from 10 to 20 cm could result in a potential 1.41
times increase in the volumetric flow rate of air, regardless of flame
temperature. (According to the formula, increasing flame temperate will
increase the flow rate, but not the ratio of flows at 20cm / 10cm)

Since gasification rate increased 1.16 in my experiment rather than 1.41
times, that suggests that doubling the chimney height should result in a
'drawing' a greater proportion of secondary air.

Therefore, the lower flame that Paul hypothesizes is quit possible.

I ran the test, using the 2.7% aperture grate.  It was hard to say if the
flame was lower or not immediately after placing the 10 cm riser extension
on top of the built-in 10 riser.  Basically, NO CHANGE.

One possibility is that the 1.41 potential increase in flow is not being
met, because to resistance to air flow through the secondary air holes.
Thus the fuel/air ratio of the flame doesn't change that much.  An gap,
rather than holes for secondary air may be more responsive to an increase
in draft (but I have found problems with gaps, so I prefer holes).

I am running these tests with the TLUD inside a 0.4 m diameter x 0.6 m tall
garbage can with the bottom cut out.  That reduces air turbulence.  If this
TLUD was used in the open, it would have to include shielding around air
intakes to reduce the effect of wind.  All the same, the burner doesn't
blow out easily.

All the best,
Julien.



-- 
Julien Winter
Cobourg, ON, CANADA
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.bioenergylists.org/pipermail/stoves_lists.bioenergylists.org/attachments/20141213/67389e62/attachment.html>