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

[Crispin Pemberton-Pigott]

Dear Julien

On the last point,

>… there is no (perceivable) pulsing with concentrator rings greater than 50% TLUD diameter.

This is because the poorly mixing air entering as a ‘sheet” gets to move vertically “soon enough”.  If you were to make the top a saw tooth shape instead of an annular gap that alone would improve the mixing of air with burning gas.

But it is still too high (above the fuel).  The top of the fuel effectively acts as a muffler, a stall point, for the flame.  I find that putting a couple of holes just below the top of the fuel keeps an ignition flame(s) going under all conditions.  It will burn a small part of the char and it is really worth it.

Regarding the video:

What you are achieving is a limitation on the secondary air flow rate, which could be done by making the channel smaller.  Bringing the secondary air (better preheated of course) down to the fuel bed is doing what I am suggesting which is to limit the distance between the top of the fuel and the air entry point.

Having a “sheet” of air is still causing problems unnecessarily.  The reason for adding secondary air in the first place is because there isn’t enough coming through the fuel.  So, it also has to be distributed throughout the gas, not just on the edge leaving the centre to fend for itself.  That always results in a tall diffusion flame desperately seeking oxygen.

So again, if you were to let that air into the gas steam through slots, holes, saw-tooth-shape it would improve mixing. The visible result would be a shortening of the flame, overall.  The temperature would rise in the central column, and the O2 level (potentially) could be reduced.  Increasing the temperature alone reduces CO and PM.

You mention that the cylinder height “creates turbulence” but letting the air in as a circular sheet already causes the problem of not having enough turbulence.  In other words raising the tube creates turbulence that should already be there by getting the air better mixed at the place of first contact, but isn’t.

The gasification rate is not governed by the secondary air flow rate, it is only the primary air flow which does that.  Increasing the “chimney effect” pulls more primary air through the fuel, so naturally the gasification rate increases. The “turbulence” noted at 2 minutes is there, but could easily be a set of air jets instead to guarantee good missing without all the “shaking around”.

Have a look at the flames created using fuel-level secondary air divided into holes that are the correct diameter to get the air to the centre of the gas stream.  Notice the greatly reduced turbulence speed (if there is such a term) achieving very good mixing without expending much draft power at all.

The intention at this point in the combustion is to get the air distributed, so how can you achieve that in the least vertical space, with the least excess air, and with the shortest vertical height (least material).

On top of that, the result is a better cooking experience.

Regards
Crispin

From: Stoves <stoves-bounces at lists.bioenergylists.org> On Behalf Of Julien Winter
Sent: Thursday, June 18, 2020 21:50
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 Chrispin;

Thanks for your posts.  Your theory will be put to the test shortly, because it is in my schedule of experiments to test a combination of holes in the sidewall of the TLUD reactor with concentrators and counter-current burners.

The counter-current burner (seen here: https://youtu.be/KzYUzJPM3eQ<https://nam03.safelinks.protection.outlook.com/?url=https%3A%2F%2Fyoutu.be%2FKzYUzJPM3eQ&data=02%7C01%7C%7C9df5778159a3490ddc8408d813f3597a%7C84df9e7fe9f640afb435aaaaaaaaaaaa%7C1%7C0%7C637281283182799876&sdata=O66P4G70PimO5W1ZZTIRYp5EeqZIAaf5cHpkNnoRuqo%3D&reserved=0>) works well at high gasification rates, but not at low gasification rates.  Thus holes in the sidewall of the TLUD reactor will be used to try and improve the turndown.  At the same time, if your theory is correct, it should solve the pulsing problem with the concentrator rings.

Note that there is no (perceivable) pulsing with concentrator rings greater than 50% TLUD diameter.

Cheers,
Julien


--
Julien Winter
Cobourg, ON, CANADA
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.bioenergylists.org/pipermail/stoves_lists.bioenergylists.org/attachments/20200621/8fd76542/attachment.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: Secondary air.mp4
Type: application/octet-stream
Size: 716854 bytes
Desc: Secondary air.mp4
URL: <http://lists.bioenergylists.org/pipermail/stoves_lists.bioenergylists.org/attachments/20200621/8fd76542/attachment.obj>