Marc,
Would you care to take the next step and calculate what happens to gas
temperature and convective heat transfer with the dome?
Just a thought........experiment.
Thanks for shedding light with your radiant contribution.
Alex
On 17/03/2012 8:30 AM, Marc Pare wrote:
Hi all,
First a short story, then a presentation of some analysis I've been
working on the last week (with the help of Crispin and Matt Redmond).
You can skip the story, if you like.
*the story*
My friend Isaac from Tech tells of his most traumatic experience
training to be an engineer: "We were giving a presentation on a design
of a hybrid vehicle. The professor asks us why we choose one
particular motor over another. 'Oh, it was bigger, so we figured it
would be better.' The professor stopped them there. Stood up, turned
to the class, and yelled: "ENGINEERS QUANTIFY!"
*the results*
In the spirit of Isaac's legendary professor, I sought to provide some
clarity to the speculation of radiative heat transfer and improved
cook stoves by doing some simple analysis.
The setup was simple: what is the theoretical upper bound on
contribution to heat transfer by the "dome-shaped emitter" described
over the last week.
It turns out that *the dome-shaped emitter has negligible impact on
the heat transfer*. Further testing supported this conclusion, but I
figured it was worth showing how we were able to show with theory that
the dome-shaped thing was not important.
The results place the upper bound on power from the emitter at *0.301
kW *vs. the power required to boil water at *1.65 kW*.
This means that even at 100% efficiency, the emitter will only improve
your thermal performance by about 20%. (not the reported 100% improvement)
I attached a pdf explaining the theory and a spreadsheet of the
calculation. You can also get them here:
http://notwandering.com/radiation.php
*thanks*
Matt Redmond for a first pass on the spreadsheet
Crispin for quadruple-checking the spreadsheet and adding improved
water boiling numbers
feedback is welcome. there is always the chance that there are mistakes!
Best,
Marc Paré
B.S. Mechanical Engineering
Georgia Institute of Technology | Université de Technologie de Compiègne
my cv, etc. | http://notwandering.com
On Sat, Mar 17, 2012 at 2:05 PM, Crispin Pemberton-Pigott
<[email protected] <mailto:[email protected]>> wrote:
Great!
Marc you are doing us a great service. Simply by collecting the
little spreadsheets circulated here, one can gain a good education
in stove design.
The volunteer's efforts are always rewarded.
Many thanks
Crispin
+++++++
# I appeal to Marc not to hide his light under a bushel. These
discussion also need numbers and methods so the reality of things
becomes widespread.
Write-up is coming :)
Putting in some effort so that it's a useful resource for folks in
the future.
Marc Paré
B.S. Mechanical Engineering
Georgia Institute of Technology | Université de Technologie de
Compiègne
my cv, etc. | http://notwandering.com
On Sat, Mar 17, 2012 at 1:55 PM, Crispin Pemberton-Pigott
<[email protected] <mailto:[email protected]>> wrote:
Dear Alexis and Paul
Thanks so much for the very open discussion and open design of the
burners. Alexis, when we met in Thailand you mentioned that you
have switched to a premixed flame and if you recall we did talk a
bit about the top end of the version of the stove you demonstrated.
Is it correct that you are now using a premixed flame? I have been
in extensive conversation with Paul O about the burner for some
time and it seems at present to be a blend of premixing with
secondary air and as Paul says, some tertiary air for the final
burnout.
The difference in performance that Paul mentioned and which got
some much discussion going about heat transfer from the hot dome
could have at least three quite different origins and there is a
lot of merit in tracking down the difference. The first might be
that the structure under the pot on Paul's present configuration
is much more closed than the one I saw in Thailand. That could
account for all the difference in the boiling time. A second
possibility is the reduction in excess air either through the
burner where flames are present or between the flames and the
final departure of the pot and stove structure, by which I mean
the outer ring. Until the hot gases leave the pot and vent into
the room, the air present in that gas stream is technically part
of the combustor. If there is a lot of cold air entering the
region under the pot, then it is counted as excess air in the heat
exchanger.
Paul, thanks for putting the pictures of the development work you
are doing. It is helpful for those who would like to work on
stoves with minimal equipment to see how things work and what has
been tried. It saves a lot of reinventing.
With regard to the heat transfer from a radiant dome, Marc has
been doing some calculations which I hope he will post here when
he is confident in the method. That should settle the question as
to whether or not a large increase in performance can be obtained
by changing hot, relatively non-IR radiant gas into IR emitted
from a wide gauze surface.
It is well worth remembering that because a flame is pale blue in
the visible range, that does not tell us what it is emitting in
the IR which is invisible to human eyes. Looking through a
translucent flame is not really a measure of emissions of heat. If
you point an IR gun at a flame it will register a high
temperature, even if it is as inaccurate as an unshielded
thermocouple.
I agree with the others that the discussion about heat transfer
has been a good exploration of the subject. I appeal to Marc not
to hide his light under a bushel. These discussion also need
numbers and methods so the reality of things becomes widespread.
Regards
Crispin
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