[Stoves] Riser Height and a 'Counter-Current' Woodgas Burner - YouTube Vid

[Julien Winter]

Hi all;


I have posted a video on YouTube on Riser Height and a 'Counter-Current'
burner that continues on the theme of discussions earlier this month about
premixed flames.



https://youtu.be/KzYUzJPM3eQ



The purpose of this video is to demonstrate the importance of riser height
on the function of a natural draft, top-lit updraft gasifier using the
“Counter-Current Woodgas Burner” (CCWB).  Riser height affects the flow
rate of both primary and secondary air, and the degree of turbulent mixing
of woodgas with secondary air.  Increasing riser height creates greater
buoyancy forces that can improve the efficiency of stove combustion, and
stove responsiveness for the operator, but it can lead to excessive
secondary air, and excessive turbulence under some conditions.



With a true counter-current burner, the fuel gas and oxidizer gas collide
head-on.  This is more-or-less what happens with the CCWB, because a
downward laminar flow of secondary air is sent against the upward flow of
woodgas.  However, it is not a perfect counter-current burner, because a
lot of the secondary air is pulled sideways by the buoyancy force created
in the gas flame.



The main objective of the CCWB is to get a much turbulent mixing of
secondary air and woodgas at the base of the gas flame as possible.  We are
trying to approach an ideal of a pre-mixed flame at somewhat above (to be
determined) the stoichiometric requirement for oxygen.



The second objective of the CCWB is to locate the base of the flame over
the fuel bed char.  Some secondary air supports glowing char, and sustains
a higher temperature in the fuel bed, from the ignition front up to the top
of the char, than would be obtained if the secondary air and the gas flame
were located at some distance above the fuel bed.   Glowing char can help
with cracking of tars, and provide heat to assist in the piloting of gas
ignition.  This helps maintain woodgas flame stability at low gasification
rates.



Some modifications of the CCWB have been tried, but did not show much
benefit.  (1) A ring of small air holes (of various numbers and diameters)
in the sidewall of the reactor were positioned just below the inner riser.  It
was thought that these holes would provide small flamelets that would keep
the gas flame from extinguishing at very low gasification rates.  It was
found that these holes didn’t help, and functioned more to diminish the
role of the counter-current air flow. (2)  Various fins were made at the
bottom of the internal riser to see if they would increase turbulence.  Fins
were found to be unnecessary.



The stoves in this video are prototypes.  They are working hypotheses that
need scientific stove testing to optimize their geometry, and validate the
counter-current approach. To become working stoves, they need a regulator
for primary air, an insulated external riser, and mechanism to transfer the
heat to the pot or other object of work.


If the CCWB can be shown to improve combustion efficiency on diverse
biomass fuels, and over a wide range of gasification rates, that would be
good, because it is a very easy burner to build.


Cheers,

Julien.


-- 
Julien Winter
Cobourg, ON, CANADA
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