[Stoves] Studies of pressure variations in a TLUD - Pulsing Flame with Concentrator Rings

Thu Jun 18 20:34:18 CDT 2020

Dear Julien

I forgot to add that the reason the gas gets to the tub without igniting is that the secondary air is provided as “sheet” of air.  If you cut the top edge with a pair of tin snips to make castellations the air would be entering as a set of inward facing jets.  This would promote excellent mixing and the flame would be able to burn far more towards completion.

Generally speaking you have a flame coming out of the tube that should be 100 or 150mm shorter, but due to poor mixing it cannot burn.

In the past we discussed a few times that the air entering should be through holes that ensure the jet reaches the centre of the diameter. It is clear in the video that is not the case.  It doesn’t even reach the centre hole, or barely.

The diameter that will work best will have to be determined experimentally because the draft is unknown.  More draft, smaller holes will still reach the centre.

How about this: create some 16mm holes around the periphery perhaps 30 or 40mm below the current top lip. Close the upper air entry ring-gap. Leave everything else the same.  Look down the tube to see if the air jets reach the centre.

We discussed also that “blurting” the holes to create a slightly rounded edge to the hole and a very short pipe for each.  The limitation for doing this has to do with the tensile elongation of the material when it breaks.  An example is that a 7.5mm hole can be stretched to 9mm without cracking the lip.  Make the radius 3, so the outside diameter of the beginning of the radius is 9+3+3 = 15.  There is a description in the archive here about how to do this and what simple tools are used.

The reason for all this is the velocity of the air will be increased considerably and by inertia, reach the centre of the diameter more easily.  Getting it “right” means you can reduce the total secondary air volume to maintain an oxygen level under 10% in the exhaust.  If you are lucky, maybe 8%.  This will reduce NO to N2 and O2, as well as increase the heat transfer efficiency.

By adding even three holes will probably cure the pulsing. Paul Olivier in Vietnam had a similar problem (not pulsing, the mixing) a few years ago.  He chose in the end to create a large number of flamelets burning above a perforated plate with the exit velocity being higher than the flame speed of the gas. That is how a regular gas burner works, including the one on my own stove.

Kirk’s recent effort tries to get containment them mixing using a circle of flame.  Everyone who has tried it will notice those times when the flame is uneven from left to right.  He added a “bluff body” to create the ring.  It can work.  For example this kerosene stove which has a high gas velocity emerging from a jet:

[cid:image001.jpg at 01D645B7.F37C4E30]
Obviously the TLUD’s don’t have that advantage so the bluff body (unless it is really small in diameter) tends to produce and uneven flame.

In the example above the flame is pushing to one side because the jet is not centered under the bluff body.  You can see the dark spot in the off-centre which is the center of the kerosene gas jet spray.   When the bluff body is moved to centre, the flame is centered and very stable.

This flame has a CO/CO2 ratio near zero. It has pre-heated primary air.

Here is a two burner version showing “the other one which is correctly centered.  This stove costs about $6 plus the pot stand. It has two fuel control valves.

[cid:image002.jpg at 01D645B7.F37C4E30]
The streaks are burning fuel droplets that did not get vaporized.

If the bluff body is removed, for example if the stove fall over, the flame immediately disappears as the gas jet velocity is much higher than the flame speed.

Regards
Crispin

From: Stoves <stoves-bounces at lists.bioenergylists.org> On Behalf Of Crispin Pemberton-Pigott
Sent: Thursday, June 18, 2020 20:58
To: Discussion of biomass cooking stoves <stoves at lists.bioenergylists.org>
Subject: Re: [Stoves] Studies of pressure variations in a TLUD - Pulsing Flame with Concentrator Rings

Perfect demo.

The cause is the level where the air enters.

It got into people’s stoves sometime during the work of Paal Wendelbo that the secondary air could enter at the top of the chamber.

The gas accumulates there and burns back from above (very slightly) and once started, continues.

If you admit at least some of the air at the level of the fuel it should stop because the gas will burn as soon as it leaves the fuel.

In the example stove there are two flames actually, one below the tube and one in and above the tube.  The lower one is not burning all the gas and the upper one is.

If you greatly extend the system vertically you should be able to get a machine gun. The blast burn above will push gas backwards and extinguish the flame (probably) and then it will fill up again. This effect was noticed in Ulaanbaatar on the Turkish TLUD coal stoves that were (against recommendation and an advertising campaign) refuelled while hot. The “pop” was sufficient to blow the top off the stove and hurl flaming coals around the room.

The ignition source as a flamelet creeping up one side of the combustion chamber.

Regards
Crispin

From: Stoves <stoves-bounces at lists.bioenergylists.org> On Behalf Of Julien Winter
Sent: Thursday, June 18, 2020 19:27
To: Discussion of biomass cooking stoves <stoves at lists.bioenergylists.org>
Subject: [Stoves] Studies of pressure variations in a TLUD - Pulsing Flame with Concentrator Rings

Hi Folks;
I made another video so that you can see the relationship between above and below concentrator flames
https://youtu.be/0HBCHGQk1AU<https://nam10.safelinks.protection.outlook.com/?url=https%3A%2F%2Fyoutu.be%2F0HBCHGQk1AU&data=02%7C01%7C%7C2a5fab370301458edde508d813ec0fe7%7C84df9e7fe9f640afb435aaaaaaaaaaaa%7C1%7C0%7C637281251877241272&sdata=6sMHbblsGdCowh%2B3BugiAaNWsMTD67ZIKgSj46x5pQM%3D&reserved=0>
Cheers,
Julien

--
Julien Winter
Cobourg, ON, CANADA
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