[Stoves] Studies of pressure variations in a TLUD

[Crispin Pemberton-Pigott]

Dear Julien

Could you please share a short video of the pulsing effect you are describing?  I think the cause may be hydrogen, and letting secondary air into the fuel too far above the top of the fuel pile.

Does it happen at any height of the pyrolysis, or at particular heights or range? (as measured from the top)

Thanks
Crispin


From: Stoves <stoves-bounces at lists.bioenergylists.org> On Behalf Of Julien Winter
Sent: Sunday, June 14, 2020 21:25
To: Discussion of biomass cooking stoves <stoves at lists.bioenergylists.org>
Subject: [Stoves] Studies of pressure variations in a TLUD

Hi Kirk;

I have been running an experiment on burners, including concentrator rings and nozzles.  I compare gasification rates as I turn down the TLUD by reducing the number of small holes that primary air to flows through.

I have changed the aperture of the concentrators and nozzles from very small to almost as large as the TLUD reactor.

With the small apertures, I get an interesting phenomenon; pulsing in the gas flame, whereby it oscillates 5 to 10 cm in height at a frequencies of about 1 to 4 hz (cycles per second).   The faster the gasification rate, or flame size, the higher the frequency.

My first hunch was that the oscillations were a result of pressure buildup below the burner.   However, when created an open space below one of the nozzles, the oscillations continued.  I now suspect that the oscillations are caused by an increase in buoyancy by an expanding flame above the throat of the concentrator or nozzle.  However, when a larger flame develops, it can't be sustained by the gas flow through the throat of the burner, because the throat is choking the flow; so the flame size contracts.

If I am right, then the constriction at the throat of these burners is limiting the size of the gas flame that can develop above it.

As the diameter of the throat or concentrator rings increase from 25% to 86% of the diameter of the TLUD reactor (7 inches), the gasification rate increases.  Oscillations were seen in concentrator rings and nozzles with a diameter <50% of the TLUD reactor diameter.  (Perhaps oscillations were also occurring in burners with a diameter >50%, but they were not apparent to the human eye and ear.)

These oscillations were not a good thing, because at the peak flame-size, they could be a puff of sooty smoke.

I will be presenting data on the turndown curves in a couple of weeks time, when I have completed the experiment.  At that time, what I am doing will become clearer.

The burner you presented last week is very interesting, because you limit the flame size, hence buoyancy, by the amount of air that enters the burner at various levels.  Thus, you are able to add air in stages as gasification rate and flame size increases.  In other words, you limit flame size by air supply rather than (or as well as?) by choking through a narrow aperture.

Cheers,
Julien.



--
Julien Winter
Cobourg, ON, CANADA
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