[Stoves] Explaination of downdraft in TLUD updraft stoves --- was --Re: Mis-information

[Paul Anderson]

*Explanation of downdraft in the fuel chamber of TLUD (UP draft) stoves.*

Paul S. Anderson, PhD21 March 2018

**Stovers,Previously I wrote:
*******************
This link takes you to  [what I am calling Video A.)
https://www.youtube.com/watch?annotation_id=annotation_1962734105&feature=iv&src_vid=wzN-cYR84_Y&v=b0vM9aD78XY
Same fellow.   and showing clearly UPdraft.   Side by side comparisons.  
Well worth watching.
That is dated 2015.  I hope that somebody will delve into this further.

*****************************

  First, we all should thank Heath Putnam for his research and for 
reporting it publicly.   He also provided an earlier video that lh cheng 
saw and called to our attention:
https://www.youtube.com/watch?v=wzN-cYR84_Y

This I am calling Video B.  Although dated earlier, it is better to 
watch Video A first.

After sleeping on this question last night, I think I have an 
explanation.   And it also would explain what Nate Mulcahey presented as 
the "Everything Nice Stove" which he claimed was not a TLUD stove and 
claimed to be a downdraft flow of the pyrolytic gases (or Opposite 
draft).   Putnam's work shows (but does not explain) the answers about 
downdraft in updraft TLUD stoves.

The big clue (revealed in video (A) above) is the difference between the 
two trial units.  The difference is a sealed bottom that enables a 
"co-mingled air supply zone" for somewhat restricted primary and 
secondary air versus abundant secondary air that arrives separately from 
the supply of primary air.

If the primary air entry is direct or very nearly direct and sufficient 
even with a small, restricted flow, it will sustain the migratory 
pyrolytic front (MPF), and all the air and gases will flow upward.   
This is the CLASSIC description of TLUD operation.

But consider the case of a TLUD-design stove that has a closed bottom 
(or is sitting reasonably tightly on a flat surface that prevents entry 
of abundant air)  AND also has somewhat limited entry (via 4 holes in 
Putnam's glass outer cylinder) of air for *_BOTH _*primary and secondary 
air into a space (a "co-mingled air supply zone") from which BOTH types 
of air must be drawn.  Therefore, the only exit is upward.  The only 
DRAFT for the stove is powered by the flame of the burning gases at the 
top.

Consider the case of a functioning TLUD stove when the MPF is below a 
layer of charcoal about 3 to 6 cm down from the top, with another 7 to 
12 cm of raw fuel below the MPF.  The pyrolysis occurs, and the hot 
gases tend to rise upward through the layer of char and into the zone of 
the cooking flame.   But the flame requires secondary air, which can 
only come up in the ring (annulus) between the two cylinders, and it 
does come up. This is the vast majority of the total supply of air 
(about 5 units for secondary to 1 unit of primary air).   In fact, that 
natural draft by the flame is pulling the air from the "co-mingled air 
supply zone" (that one place of air supply which is also feeding the 
primary air). There is therefore a reduction of air pressure below the 
MPF, and that means less movement of the primary air upwards.

The result is that there is sufficient lower pressure that SOME of the 
pyrolytic gases move downward.   Probably some swirling also, or some 
channels of gases going down but with SOME (at least some) primary air 
(the O2 is the important part) moving to the MPF.
With a little bit of time, some of the pyrolytic gases reach the entry 
holes of the primary air and leak outward into the "co-mingled air 
supply zone" where there is fresh air entering and where those gases can 
be combusted (as shown in the Putnam demonstration in Video B). 
Impressive.  And if there is no flame down there, those pyrolytic gases 
can be pulled upward to become part of the upward flowing secondary air 
THAT IS NOW PRE-MIXED (-but rather diluted to some unknown amount 
-) WITH COMBUSTIBLE GASES.   Nice trick, and you can see Putnam's 
demonstration of a taller, stronger flame (Video A).  This is 
important.   Pre-mixing is to be encouraged.   But it should be 
understood and done intentionally to attain consistent results.

BUT in the described simple setup, production of the pyrolytic gases is 
suffering.   There is a somewhat deficiency of primary air. That could 
be forgiven (or overlooked or ignored) except for one very important factor:

When the downdraft is occurring, the stove user loses some control over 
the fire.   The draft from the burning gases is now regulating (in part) 
the operations of the TLUD stove.  The normal control of a TLUD fire is 
by closing off some primary air, or using a small fan, but these are no 
longer as effective because of the co-mingled air.   As the flame at the 
top changes when there is downward flow (shown by Putnam), there is a 
ripple effect to the air flows.   Adjust, then adjust again, and then 
adjust again.

You can look at the Champion TLUD (only one hole for primary air entry) 
or the Quad or the Troika (by Awamu) with only one entry for primary 
air, or some of the other more established true TLUD stoves.   The Peko 
Pe by Wendelbo also keeps the two air sources separate. Then look at 
Putnam's variation and at the Everything Nice stove and see how the 
primary and secondary air are comingled and subjected to the draft 
created at the top of the stove.

Also consider what would happen if there actually was sufficient 
downward draft for the FULL reversal of the air flow in the fuel 
chamber.   At the top there is flame.   And how is there any O2 
surviving in that flame so that it could go downward enough to go 
through the 3 to 6 (and deeper) layer of hot charcoal in order for O2 to 
reach FROM ABOVE the top side of the MPF and to sustain that MPF for 
sending gases further downward?  And then those pyrolytic gases would 
need to go out through what were the primary air inlet holes, and then 
be mixed with secondary air (but never catching fire because somehow 
there was not a spark there, even at the end of the batch with 
red-glowing coals????), and then rising in the annulus between the two 
cylinders, and only when entering the area of the main flame would those 
gases combust.  But this supposition of FULL reversal of the air flow is 
impossible because there are no pyrolytic gases moving upward from the MPF.

Conclusion: In a TLUD stove, there can be PARTIAL downward drafting of 
the created pyrolytic gases when caused by natural draft of secondary 
air to counteract the flow of primary air.  This is educational, but 
what is possible is not necessarily desirable or practical.
-- 
Doc  /  Dr TLUD  /  Prof. Paul S. Anderson, PhD
Email: psanders at ilstu.edu <mailto:psanders at ilstu.edu>__Skype:   paultlud 
   Phone: +1-309-452-7072 <tel:%2B1-309-452-7072>__Website: 
www.drtlud.com <http://www.drtlud.com>


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