[Stoves] TLUD pellets stove with a venturi burner. Pictures of a cooking cycle.

Roberto Poehlmann roberto.poehlmann at gmail.com
Tue Oct 13 18:31:18 MDT 2015

Dear Alex, thanks for your comments. Below some commentaries.

I like what you have done. A stable blue flame is interesting. Do you think
you are getting a bit of premix happening with you lowest secondary holes?
I have two theories about the colour. One is a combination of premix and
excess air. The other is that at low turn down, low superficial velocities,
you get peak bed temperatures down around 500C or even lower at the sides
of the chamber or maybe even inside the pellet. I think there might be a
different mix of gasses at these temperatures that make it easier to burn
without forming soot that turns the flame yellow/orange. Perhaps a lower
C/H ratio.
When I add a logs to a bed of coals in my stove, with the primary air shut,
I see dancing blue flames for quite a while before they turn yellow."

RP> Alex, i have tried to premix the gases before burning it, but without
much success. With this burner, you see flames emerging from every holes,
so, i think there is not a premix flame. At the first row of holes, the
flames forms a plane (completly blue) with vertices starting at every 8

About excess air, maybe it is the cause of the blue flame generation, I
don't know. But, to contradict that, at the first rows of holes, you can
see blue flames starting at every holes, where there is excess of smoke.
Maybe, when the flame disappears at the top of the burner, there is an
exccess air. I cannot determine if i had or not excess air. I only can see
the flames, smell the gases and put the eyes at the exhaust gases to
determine if i have or not a good flame.

My next step, as Crispin says once, is to measure the exhaust gases with
the adequate equipment, to optimize the stove.

Another thing i discover, is that a certain conditions, the flame start at
the base of the cone and not inside the tube. The reason i thing, is that
the velocity of the "premix" gases at the tube is greater than the flame
velocity, and when the premix gases reach the cone, velocity decreases,
sufficiently to stabilize the flame. I see this phenomenon two or three
times, and i have no idea how to reproduce it. Maybe is a way to have a
premix flame.

About the idea of the low superficial velocity, maybe is one of the reasons
to produce a blue flame, i don't know (again). If you suddently close the
primary air control at normal operation in a TLUD stove, the flames turns
more yellow/orange, and if you open the primary air control again after a
few of seconds, the flames turns completly blue, before it stabilizes
again. ¿Why?

"I had a choked exit on some of my TLUD burners, cone in to secondary air
and cone out. I called them venturi burners because of the look but I
abandoned the name because I felt it was all chimney-effect.  I'd like to
see you have control of the secondary air on the cylinder portion.

If you get a combustion test done, don't be surprised if the best result is
with a flame with yellow tips.

Nice work.

RP> Thanks Alex. Maybe the mos important factor is the chimney effect as
you say, and the venturi effect is not as important. What i see is that the
cone helps to widen the flame. Without the cone, the flames transforms it
in a tall flame, so the shape of the burner (tube and cone combination)
helps to maintain the flames near the surface of the cone (venturi effect?,
sudden expansion effect?).

Here is a link to a series of photos and videos of a cooking cycle executed
yesterday. Every picture have the time printed in it, to see the evolution
of the cycle and flames behavior.


I have burned 3 kg of pellet.

At 13:51, i started the fire with the 2 primary air control fully open and
the 3 secondary air controls fully close.

At 13:59, i closed the primary air controls, and fully open the 3 secondary
air controls.

At 14:02, i remove the chimney, to lower the gasification rates to see what

At 14:04 i put the chimney

At 14 29, i remove the chimney, to lower the power or gasification rates

At 15:17 i put the chimney again.

At 16:02, i fully open the primary air controls, and close two of the
secondary air controls

Between 16:03 and 16:08, started the char gasification stage.

At 16:26 , i close more the only open secondary air control.

At 17:06, the flame extinguish

At 17:07 i can ignite again the stove.

At 17:13 the flame extinguish again.

The char and ash left weight 230 grams.

The pyrolisis stage last 2 hours and 10 minutes aprox. The gasification
stage last 1 hour more aprox., before the first flameout.

If i put a more taller chimney, maybe i can have 30 to 45 minutes more with

With this prototype, i am very satisfied with the results, and fullfill my
requirements of an outdoors cookstove and patio heater with natural draft
with pellets.

I can see that to lower the power with a chimney, i need to have a more
sealed primary air control. My goal is to reach the 0,5 kg/hour rates. That
is, 6 hours with 3 kg of pellets.


Roberto Poehlmann

Valdivia, Chile
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