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

Tue Jan 26 10:01:28 CST 2016

Very interesting Julian.

Have you registered (or noticed) any mixing improvement with this prototype over the non counter-current ones? I suppose better mixing shoud be noticed as higher temperatures registered. Have you measured anything like this?

Marc

From: jock at jockgill.com
Date: Mon, 25 Jan 2016 20:04:58 -0500
To: stoves at lists.bioenergylists.org
Subject: Re: [Stoves] Riser Height and a 'Counter-Current' Woodgas Burner -	YouTube Vid

Julian,
Interesting video. 
1. I would try this with a flame retention disc rather than a central hole.  I view central holes above fuel bed as a fundamental error.  I know central holes are the given wisdom, but I view them as the source of problems that can not be overcome.  They are a barrier to better achievements.
2. I would strive for the maximum amount of blue flame. I did not see any in your video.  People want to cook with blue flames. Aspirational.
3. Perhaps an hour glass type device might work?  Could be asymmetrical?
Cheers,
Jock

Jock GillP. O. Box 3Peacham, VT 05862
google.com/+JockGill
Extract CO2 from the atmosphere!
On Jan 25, 2016, at 7:10 PM, Julien Winter <winter.julien at gmail.com> wrote:

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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