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

Tue Jan 26 11:25:45 CST 2016

Dear Julien

First my compliments for demonstrating a series of constructions showing the
progression of a changes to a particular aspect of the burn. 

Second, without being a damper, I want to add some interpretations to your
already well-constructed set of experiments that will help those adopting
this approach to product development. They disagree in some ways with the
interpretations provided by Jock and you, so maybe it will stimulate
additional ideas.

The split between primary and secondary is uncontrolled, for all intents and
purposes, and should not be. In the same manner that you advocate at the end
that reducing the area of holes through which the primary air can enter,
this should be applied to the secondary air as well, arranged in such a way
that the incoming has has to go to one or the other. It should not be
controlling one and then the other is free to flow more or less depending on
the riser height. In short, don't use a macro structure as a fine tuning air
control mechanism.

There are several products that work in that 'controlled split' manner
though they are hard to find. The point is to decide in advance what the
pri/sec split should be, create it, test it to be sure it is correct, then
scale the draft accordingly. If you were to conduct this test on a SeTAR
type real time test bench, it would provide a very useful set of excess air
and CO/CO2 ratio charts the quickly identify the ideal conditions for the
pri/sec split. This will be independent of the riser height. In such a case
the riser height will affect the power, not the split, then the power can be
regulated by the gross control of the air entering. This is a different
approach taken be nearly all the TLUD designers so far which is to regulate
the primary and let the secondary run free according to flame temperature
and architecture.

If you have control over the entry and the correct split by construction,
the riser height is used for total available draft, and for directing that
energy (a risen has 'power') to the correct mixing of the air and gas, not
the flow of secondary air.

Something I noticed throughout is how random the mixing is. That gives just
about the lowest guarantee of good mixing at low power which worries me. If
you have at the bottom of the inner riser vee notches, or square notches
(bend a tab inwards) the secondary air would gain much more penetrating
power to reach the centre whether the mixing was particularly or not. 

Please see
http://www.newdawnengineering.com/website/library/Stoves/grasifier/ item 00
(the 100mm diameter Grasifier Stove). This is a fixed architecture version
similar to what you are showing in your series but not identical in terms of
the counter-flow. The point is to look at the videos: Item 23
<http://www.newdawnengineering.com/website/library/Stoves/grasifier/23%20Gra
sifier%20burning%20pistachio%20shells,%20high%20gas%20rate%20HRes.wmv>  (22
if you have a low speed connection) showing the high gasification rate
without enough penetration of the secondary air into the gas stream (= tall
diffusion flame). Then see Item 24
<http://www.newdawnengineering.com/website/library/Stoves/grasifier/24%20sta
ble%20flame%20needs%20a%20plate%20or%20pot%20on%20top.wmv>  which shows the
same device with a very low flow of primary air but the secondary air with a
short riser provides excellent mixing all the way to the centre. This sort
of contradicts your series of clips on low draft, but emphasizes what it
could do if the secondary air were shaped and not so free to flow.

The disturbance to the flame caused by removing the pseudo-pot is caused by
the entry into the flaming chamber of cold air from above. We discussed this
previously and I pointed to the same video. I suspect your large diameter
risers in the experiments were experiencing this effect. If the tubes has
been the same diameter as the chamber outlet the flaming appearance would
have changed, and changed even more if there was some sort of breaking up of
the secondary air into jets so they could reach the centre.

Yours is altogether an excellent piece and very useful for training about
pri/sec splits. Used together with the Grasifier clips they would support a
blackboard session on how to create good combustion across a range of
gasification rates and approaches to secondary air provision.

There are two stoves products both of which use a version of the two systems
(combining aspects of Julien's videos and mine) and both frustratingly not
yet launched so you can't see them. In both cases a good balance has been
achieved across a range of conditions. Fundamentally, the riser height sets
the maximum power level. A chimney does the same thing.  The rest of the air
controls allocate the distribution of air under different circumstances.

I will bring this up again in a few weeks when I am in Tajikistan where I
will continue to involve this Stoves group in the development of a couple of
simple stoves for space heating, and they will be based on these same
principles of construction, building on earlier conversations. 

Julien I will refer to your videos. Many thanks for your contributions to
the field. We need to find you (and others) some gas equipment. News in that
regard coming separately. Yixiang and I have been tossing the back cupboards
in China and Toronto and come up with some good sources.

Best regards

Crispin

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 Gill

P. O. Box 3

Peacham, VT 05862

<https://plus.google.com/_/notifications/emlink?emr=02164940119180120523&emi
d=CIn6jOiavboCFcgLTAodARYAAA&path=/102260924343967128597/op/u&dt=13830897461
65&ub=50> google.com/+JockGill

Extract CO2 from the atmosphere!

On Jan 25, 2016, at 7:10 PM, Julien Winter <winter.julien at gmail.com
<mailto: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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