[Stoves] Radiation Analysis for Gasifier

Sat Mar 17 14:00:30 CDT 2012

txs

From: stoves-bounces at lists.bioenergylists.org [mailto:stoves-bounces at lists.bioenergylists.org] On Behalf Of rongretlarson at comcast.net
Sent: Saturday, March 17, 2012 10:54 AM
To: Discussion of biomass cooking stoves; Marc Pare; Paul Olivier; mredmond3 at gatech.edu
Subject: Re: [Stoves] Radiation Analysis for Gasifier

Marc, Matt etal

    Two problems I see with your analysis.

   First is minor -  Your equation 1 show a linear variation with temperature, whereas it should show a 4th power.   But you were using the proper fourth power in your Excel spread sheet - so this was just a typo.

   More serious is your assumption that the metal portion of the mesh is 10%.  This is appropriate only for a very few mesh per inch and fine wire.  My guess is that Paul's mesh could be more like 30-40% - which will change your conclusion a great deal.  See pages like:

    http://www.twpinc.com/wire-mesh/TWPCAT_12/p_014X014S0170W48T

   So this is to ask Paul Olivier for a visual check on what he was using in his particular strainer.  A manufacturer and model number would be helpful, if available

   Conversely, I worry about assuming the mesh was as high as 750 degrees - based on the color in Paul's photo.  But I am used (vaguely - long time ago) to looking at solid materials through a peep hole in ceramic kilns.  The openness of the mesh must affect our visual color/temperature calibrations.  Anyone up on that?

   I'd like to know more about the maximum possible kiln power level - by knowing the amount of rice husk consumed per unit time  (same as question asked by Crispin, I think).  From this we can start to compute the convective heat transfer coefficient.  In other words, what part of the output energy was not getting into the cookpot?  I think we can assume a larger portion of the radiative energy was captured than of the convective.

   Also the amount of water evaporated should be easy to measure rather than guesstimate.   I also would feel better running longer and using the weight evaporated for these energy capture-power computations.

Ron

  _____  

From: "Marc Pare" <mpare at gatech.edu>
To: "Discussion of biomass cooking stoves" <stoves at lists.bioenergylists.org>
Sent: Saturday, March 17, 2012 6:30:50 AM
Subject: [Stoves] Radiation Analysis for Gasifier

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 <crispinpigott at gmail.com> 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 <crispinpigott at gmail.com> 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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