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

Paul Anderson psanders at ilstu.edu
Fri Mar 23 20:05:01 CDT 2018


Hugh,

Thanks for the reply.  I am forwarding it to the Stoves Listserv.

The impact of the air being drawn in (creating a draft onto the ignited 
stick) should be able to be checked by shielding the flame from the 
direct draft.

To all:   How can we get copies of these messages to Heath Putnam for 
his input?

Paul

Doc  /  Dr TLUD  /  Prof. Paul S. Anderson, PhD
Email:  psanders at ilstu.edu
Skype:   paultlud    Phone: +1-309-452-7072
Website:  www.drtlud.com

On 3/22/2018 5:41 PM, Hugh McLaughlin wrote:
> 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, 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__Website: www.drtlud.com 
> <http://www.drtlud.com/>
>
>
>

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