[Stoves] Studies of pressure variations in a TLUD

[Crispin Pemberton-Pigott]

Dear Julien

Are you familiar with this type of device:

https://www.youtube.com/watch?v=bTa_GicWBuM

It is a machine gun simulator, in that case operating on propane and oxygen.  They used to be made with acetylene which I expect makes a louder noise.  It is used on model aircraft and vintage aircraft to mimic the sound of a machine gun.

The principle of operation is I think what you have accidentally replicated in the stove.  You are producing a combustible gas with at least some oxygen in it, and you maintain above a cavity a flame that is able to light that mixture.  I suspect you have a very low but non-zero oxygen content in the gas below the flame.

The principle of operation of the gun simulator is to fill a pipe with a combustible mixture (say, 15% acetylene and 85% oxygen) from the “back” and to have a spark plug connected to a Model T spark box (which sparks continuously) at the open end of the pipe.  As the gas reaches the spark, it ignites and burns back into the tube, at a very high speed in this example. The gas exhausts explosively out of the tube leaving it full of incombustible gases, which are then displaced from the back by a new batch of mixture. The flow rate and the pipe diameter determine the firing frequency.

Translated into a gasifier stove, it would appear as a pulsation.  You can overcome it by introducing air through at least one hole into the gas near the source (assumed to be the top of the char bed) so that after one pulse, it remains alight.

People have used pulsed gasifier combustion deliberately:
https://www.sciencedirect.com/science/article/pii/S0956053X03000382

I think on a small unit you will not want to because of the enhanced probability of a flame-out.

Regards
Crispin


From: Stoves <stoves-bounces at lists.bioenergylists.org> On Behalf Of Julien Winter
Sent: Monday, June 15, 2020 10:45
To: Discussion of biomass cooking stoves <stoves at lists.bioenergylists.org>
Subject: [Stoves] Studies of pressure variations in a TLUD

Hi Chrispin;

The pulsing I observed is with concentrator rings on a 7-inch diameter TLUD.  The concentrators are about 6 cm above the top level of the pellet fuel.   The fuel bed is 10 cm deep.   The pulsing occurs with the narrowest aperture concentrators (25-50% TLUD reactor diameter), from moderately low to highest primary air, and during the entire gasification until the ignition front hits the grate, and we start to approach flame-out.

At first I thought that the oscillations may be pressure build-up, below the concentrator (or nozzle), because the pathways for preheated secondary air and primary air will have buoyancy and momentum.  The pathways also have resistance: the secondary air is up a narrow 1/2 inch gap between concentric cylinders of the TLUD, and the primary air flow faces resistance at the regulator and in the fuel bed.

I thought that perhaps I was getting some pressure build-up below the concentrator or nozzle throats, like the inner chamber of a rocket engine.  I was even thinking we should rename TLUDs to 'rocket stoves.'

Then, I lifted the nozzle off the TLUD to create a gap that would allow for pressure release.  The pulsing was unchanged.

It will be a couple of weeks before I can get a video of the pulsing, because my gear is doing other duties at the moment.  I will also summarize my observations, because I recorded the frequency of pulsing for the different treatments (following Ron Larson's advice).

Cheers,
Julien


--
Julien Winter
Cobourg, ON, CANADA
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