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

[Hugh McLaughlin]

Paul,
I am unconvinced. One of the comments (the first - Arthur Noll) provides an explanation - it is the air being drawn into the base (primary air inlets) that stimulate the burning at the bottom.
Hugh

Noll's comment is copied below:

That is interesting, but I'm not convinced that pyrolysis products are coming out of the bottom.  You don't see any smoke coming out the bottom until you put the flaming stick in there.  The stick could be producing the smoke that hits the bottom of the can, turns sideways and joins the flow of air, much of which is rising up the sides between the containers, while smaller amounts are going in to the wood.  If it were correct that products of  pyrolysis  were going down and then up, I would expect to see a significant amount of smoke coming out the bottom and up the sides all the time, not just when the stick was put in.  And I would expect to see soot and tar precipitating out on the surfaces between the containers. Pyrolysis produces a combination of gases, smoke, soot and tar. It is messy.  I have built these stoves and this area is always clean, even after many burns,  just like what you have is clean.  I have always felt that the smoke, tar and gas from the pyrolosis was rising up, and the preheated air coming out the secondary air holes,  going into this mixture of flammable gas and vapor, made the jets of flame.  I don't think it makes any difference whether you have a jet of air going into a mass of flammable gas or if you have a jet of flammable gas going into a mass of air, both can give you a jet of flame. 

    On Thursday, March 22, 2018 9:05 AM, Paul Anderson <psanders at ilstu.edu> wrote:
 

   Explanation of downdraft in the fuel chamber of TLUD (UP draft) stoves. Paul S. Anderson, PhD     21 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   Skype:   paultlud    Phone: +1-309-452-7072   Website:  www.drtlud.com    
     

   
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