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

[Julien Winter]

Hello all;

Thanks, Jaakko for pointing me in the direction of Joao Carvalho' research.
I have searched out some of his papers, and others under the key words
'pulsating combustion.' There is a lot of interesting research there to
consider, and it will take a bit of digesting.

Presently, my thinking is that the phenomenon in TLUDs is based on the
choking at the concentrator.  If we look at the video (
https://youtu.be/0HBCHGQk1AU), the flame expands and fills the entire
aperture of the concentrator.  I suspect that turbulence and increased
viscosity of heated gases causes the flame to become choked, increasing
resistance of gas flow pulled by buoyancy.  The draw of primary and
secondary air below the concentrator drops, and the flame below the
concentrator shrinks.  However, once the flame has retracted, it can expand
again, drawing more primary and secondary air.  A cycle ensues.

I have been testing a range of concentrator ring apertures ranging from
25% 37% 45% 53% 62% 70% 86% of the TLUD reactor diameter in "Champion" or
"Peko Pe" style TLUD with preheated secondary air.  Pulsing occurs with
25% 37% 45% apertures, in which the flame swells to occupy the full width
of the concentrator.   With 62% 70% 86% apertures, the width of the
incandescent flame is 2/3 or less of the aperture of the concentrator, so
the flame is unimpeded and the choking phenomenon doesn't develop.

In the video I posted a concentrator cap has been turned upside down, and
supported on metal bracket to create a 1.5 cm gap for secondary air.  Aside
from allowing us to see the flame develop underneath the concentrator, what
this little experiment does is separate the TLUD reactor from the
concentrator gas burner, so we see that the phenomenon is likely caused at
the burner, rather than by pressure changes below the burner (because the
burner is no longer sealed to the top of the TLUD).

My take-home message from this is that "if you are designing a concentrator
type TLUD, and you have a flame that pulses (with a frequency of 1-4 hz),
then the aperture of your concentrator is too small."  (Unless it can be
shown that pulsing increases the efficiency of heat transfer to the cooking
pot.)


Cheers,
Julien.

-- 
Julien Winter
Cobourg, ON, CANADA
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