[Stoves] Radiation Analysis for Gasifier

[Paul Olivier]

Dear All,

Before Marc got involved in this discussion,
 I had already apologized for the confusion I had caused.
Marc is the expert in these matters.
All has been properly debunked.
As a gnarly pseudo-physic, I withdraw from this list.
So sorry.

Paul Olivier

On Sun, Mar 18, 2012 at 1:09 PM, Marc Pare <mpare at gatech.edu> wrote:

> *Alex:* I don't see an approach to modeling the convective heat transfer
> that would give us useful quantitative results. There are lots of messy
> quantities to deal with in that calculation (mass flow, heat transfer
> coefficient, ...)
>
> *Ron:* I really appreciate you spending the time chewing on the results.
>
> Crispin said most of what there is to say. I was aiming at a useful
> grounding in theory to guide the discussion. There were some pretty gnarly
> pseudo-physics used to support arguments over the last week. Debunking the
> doubling of performance was just an excuse to get started.
>
> # [RWL] "Since we so far have only one data point,  I think the question
> of radiation's importance is still open."
>
> You're right.
> The goal isn't to say "radiation is pointless", but to refine our tools to
> discover *how *it can be useful. Now we've got a spreadsheet we can
> twiddle around with testing out improvement scenarios!
>
> In the best of all possible worlds, I would have written something really
> eloquent about that at the end of my report, but I had already spent a week
> documenting a 30 minute calculation, and I wanted to get it out there.
>
> # [RWL]  The issue is still whether a mesh can  provide something that is
> worth the expense/bother.   There are many products on the market using
> radiation principles.
>
> I flipped through the chapters of radiative burners in the *Industrial
> Burners Handbook *and couldn't find an example of a radiative burner used
> to improve heat transfer efficiency. It appears the primary application is
> in drying operations that require even & responsive heat (at the sacrifice
> of some efficiency)
>
> Would love to see a counter-example.
>
> Marc Paré
> B.S. Mechanical Engineering
> Georgia Institute of Technology | Université de Technologie de Compiègne
>
> my cv, etc. | http://notwandering.com
>
>
> On Sun, Mar 18, 2012 at 6:39 AM, <rongretlarson at comcast.net> wrote:
>
>> Crispin etal
>>
>>    See few inserts below.
>>
>> ------------------------------
>> *From: *"Crispin Pemberton-Pigott" <crispinpigott at gmail.com>
>> *To: *rongretlarson at comcast.net, "Discussion of biomass cooking stoves" <
>> stoves at lists.bioenergylists.org>, mredmond3 at gatech.edu
>> *Sent: *Saturday, March 17, 2012 3:03:23 PM
>> *Subject: *RE: [Stoves] Radiation Analysis for Gasifier
>>
>> Dear Ron and Marc
>>
>>
>>
>> Before addressing the corrections or the exactitude of the analysis I
>> want to first recall the purpose of the calculation Marc has done. He is
>> looking to see if it is possible for the heat transfer efficiency to have
>> doubled by placing a dome of mesh that got very hot under a pot. The
>> postulations about *how* it might have accomplished the feat are
>> separate from the question of *if* it can be done.
>>
>>
>>
>> The figures Marc chose are reasonable, even favourable to the case. In
>> order for the dome to have doubled the cooking efficiency (reducing the
>> time to boil) it would have to be shown that from a low radiant baseline
>> the addition of a high radiant object could account for the change.
>>
>>
>>     *[RWL:  My message earlier today (in full below) was intended to
>> show that a different (higher) assumption on the metal area in a mesh would
>> allow one to think about the mesh doing what we at one time thought was
>> occurring.   But early on the 15th,  Paul Olivier wrote to us, saying in
>> part:*
>>
>>
>> **
>>
>>   * **"I did a second experiment with the dome in place.*
>> The boiling times were exactly the same with or without the dome.
>> I apologize to you all in leading you to think that thermal radiation was
>> making a big difference."
>>
>>
>>    RWL cont'd   -  You must have missed this message.   So I think the
>> issue is no longer to try to prove anything like a doubling in efficiency.
>> But (as you conclude also in last paragraph)  radiation can still possibly
>> be an important additional design parameter.  We can learn more as soon as
>> we hear from Paul on the characteristics of his particular strainer and put
>> those numbers into the Ga Tech computations.  Since we so far have only one
>> data point,  I think the question of radiation's importance is still open.
>>
>>
>>
>>
>> If the absorbed power with the dome was at a rate of 1.65 kW, as Marc has
>> calculated, then the baseline case is half of that, viz 0.83 kW. While
>> there is certainly a radiative element in the baseline case, we do not know
>> what it is because we do not have a photo of the stove taken in the IR
>> band. Let us suppose it was 20% radiant and 80% convective, giving some
>> credit for hot cases such as water vapour and CO2 being emissive in the IR.
>>
>>
>>
>> Next, assume a baseline thermal efficiency for the whole system of 30%.
>> That means the pot was absorbing 0.825 kW from a 2.75 kW fire if I do the
>> sum correctly to an additional decimal place.
>>
>>
>>
>> Of that 0.825 kW, 20% is radiant and 80% convective. That means 165 Watts
>> of radiant heat and 660 Watts of convective heat.
>>
>>
>>
>>     [RWL  _ Repeat that the 20% value is based on an assumed strainer
>> metal ratio of  0.1.  Might be considerably more (as calculated below).]
>>
>>
>> In order for the net power to double, the ‘wasted’ heat would have to be
>> converted to IR and emitted to the pot. As is shown in the drawing, there
>> is quite a substantial area of the pot+housing that is ‘not pot’. Disregard
>> the low incident angle of the IR, even though an experiment reported here
>> this week showed a significant change in the efficiency in the reception of
>> low angle IR.
>>
>>
>>
>> If an additional 0.83 kW was to be obtained from the 2.75 kW fire,
>> assuming no change in the fire power or reduction in the excess air, the
>> radiant contribution would have to rise from 165 Watts to 825+165 = 990
>> Watts, plus retain the 660 Watts of convective heat. The system efficiency
>> will have to rise to 60% from 30% to achieve this. That seems unlikely but
>> let’s not draw conclusions just yet.
>>
>>
>>     [RWL:  We can do some more computations on the convective heat
>> transfer efficiency if we could agree on the theoretical convective heat
>> transfer coefficient for this geometry.    I think we will find that 30%
>> efficency is going to be hard to achieve  (based on some material I have
>> read from Dr.  Dale Andreatta - who has a nice report on trying to maximize
>> the convective heat  phenomena.
>>
>>
>>
>> What Marc is showing is whether this is possible at all. Using the
>> generous temperature of 750 C and a wire mesh of 10% coverage and an
>> unbelievably generous emissivity of ε = 1.0 for the wire the emitted
>> power is 301 Watts, 1/3 of the needed IR power, and not yet discounting for
>> the fact that only about 60% of the radiant energy is hitting the pot. If
>> you consider that the mesh radiates downwards as well as out and away, the
>> % is probably even less.
>>
>>
>>
>> Let’s be generous and add more surface wire: 40% wire coverage. The power
>> emitted jumps to 1205 Watts which is above the needed 990. Add the more
>> realistic emissivity for stainless steel wire of ε = 0.6 and it drops to
>> 723 watts. Then reduce the temperature to a more realistic 650 C and it
>> drops to 477 W. Factor in the losses to the local environment that is ‘not
>> pot’ and it drops to 346 W. As the pickup of heat is not 100% efficient,
>> the IR heat available is even less. My guess is closer to 250 W (about 70%).
>>
>>
>>
>> Even if there was zero heat transfer from IR in the baseline, an increase
>> of 250-350 Watts is not enough to cut the cooking time in half – it is
>> still 3-fold short of making this happen. And that still has to be factored
>> for the mesh area which is probably less than 40%, and we must consider the
>> round wires emitting in all directions.
>>
>>
>>
>> Conclusion: there is absolutely no way for a radiant dome to double the
>> cooking efficiency of this particular stove.
>>
>>     [RWL:  Agreed - but no one is now claiming that.  The issue is still
>> whether a mesh can  provide something that is worth the expense/bother.
>> There are many products on the market using radiation principles.]
>>
>>
>> Whatever the differences are between the two burners, the improvement in
>> IR is at the most no more than a few % because the radiant heat from the
>> baseline is not zero and a realistic calculation of what it could be gives
>> about 250 Watts absorbed IR energy from the dome, or 16% of the total heat
>> getting into the pot.
>>
>>    [RWL:   Might be - but this is an estimate - based neither on
>> computation nor measurement.
>>
>>
>>
>> All of the above does not say that a radiant mesh dome can’t increase the
>> efficiency of the stove. It just shows it can’t double the it. Because
>> there is a real possibility it will help, this spreadsheet can be used to
>> optimise the effect, and to calculate what effect a radiant structure might
>> have. Designers, rejoice.
>>
>>     [RWL:   I agree, mostly.  But I can imagine situations (with
>> reflectors) where more than a doubling is possible.    We need more
>> experimental results.  One data point is not enough.]
>>
>>
>>     I hope other Excel computation include the actual as attachments.
>>
>>
>>     For Paul Olivier -  Thanks for the full descriptions given today for
>> the secondary air path - which I now think I understand.  I think there is
>> a significant pre-heating of you secondary air - and it might be that the
>> improvement in boiling times is likely due to much less excess air  (The
>> vertical flow is in a channel of 15 mm width , but the final short
>> horizontal path only has 6 mm height.  Might be optimum, but as above, we
>> only have one data point - further improvement might still be possible.
>> Anyone with computational fluid dynamics capabilities might be able to give
>> some guidance.  Changes in these two dimensions will not make much
>> difference in costs.
>>
>>     For others - I have had off-list conversations with Paul on the
>> double row of "Belonio" holes.  They may be optimum - but I would like to
>> see other designs tested as well.   I think secondary air could more easily
>> reach the interior if the circumferential hole placement was replaced by
>> radial slits.    I must add that the array of small holes has led several
>> to think there was pre-mixing.  So it is clear that the array of small
>> holes (or slits?) seems to have considerable value.  We are at the
>> beginning of knowing how to optimize hole placement and szing - much less
>> achieve pre-mixing - in small cheap stoves.
>>
>>
>> Ron
>>
>>
>>
>> Regards
>>
>> Crispin
>>
>>
>>
>> Final numbers used:
>>
>>
>>
>> Prepared 3/10/2012 by Marc Pare
>>
>> Reviewed and Revised by Crispin Pemberton-Pigott 2012/3/17
>>
>> Re-released 2012/3/17
>>
>> *Dimensions*
>>
>> radius_pot
>>
>> mm
>>
>> 125
>>
>> radius_dome
>>
>> mm
>>
>> 125
>>
>> gap height
>>
>> mm
>>
>> 40
>>
>> *Area of mesh*
>>
>> percent metal
>>
>>
>>
>> 0.3
>>
>> A
>>
>> mm^2
>>
>> 49087.38521
>>
>> A_mesh
>>
>> mm^2
>>
>> 14726.21556
>>
>>
>>
>> m^2
>>
>> 0.014726216
>>
>> *Radiation*
>>
>> stefan boltzmann (σ)
>>
>> W/(m^2-K^4)
>>
>> 5.67E-08
>>
>> emissivity (ε)
>>
>>
>>
>> 0.6
>>
>> T_mesh
>>
>> K
>>
>> 923
>>
>> T_pot
>>
>> K
>>
>> 333
>>
>>
>>
>>
>>
>>
>>
>> q
>>
>> [W/m^2]
>>
>> 24272.81586
>>
>> Q
>>
>> W
>>
>> 357.4467187
>>
>>
>>
>> kW
>>
>> 0.357446719
>>
>> *Power to boil water in 1L, 222s scenario*
>>
>> Spec Heat Water
>>
>> kcal/kg-C
>>
>> 1
>>
>> Temp Difference
>>
>> C
>>
>> 74
>>
>> Density Water
>>
>> kg/m^3
>>
>> 1000
>>
>> Volume Water
>>
>> L
>>
>> 1
>>
>>
>>
>> m^3
>>
>> 0.001
>>
>> Energy required
>>
>> kcal
>>
>> 74
>>
>>
>>
>> kJ
>>
>> 310.06
>>
>> Water evaporated
>>
>> g
>>
>> 25
>>
>> Latent heat of Evap
>>
>> J
>>
>>                  2,257
>>
>> Heat absorbed by pot
>>
>> J
>>
>>              366,485
>>
>> Time
>>
>> s
>>
>> 222
>>
>> Power
>>
>> W
>>
>>                  1,651
>>
>>
>>
>> kW absorbed
>>
>>                    1.65
>>
>> Percent of heat that might be contributed to cooking by Radiation from a
>> red hot mesh dome under the centre.
>>
>>
>>
>> *21.65%*
>>
>> *Bonus View Factor Calculation*
>>
>> *View Factor*
>>
>> r_1
>>
>> mm
>>
>> 125
>>
>> r_2
>>
>> mm
>>
>> 125
>>
>> a
>>
>> mm
>>
>> 40
>>
>> R_1
>>
>>
>>
>> 3.125
>>
>> R_2
>>
>>
>>
>> 3.125
>>
>> X
>>
>>
>>
>> 2.1024
>>
>> F_1-2
>>
>>
>>
>> 0.727
>>
>> Percent of heat actually contributed to cooking by Radiation from a red
>> hot mesh dome under the centre.
>>
>>
>>
>> *15.74%*
>>
>>
>>
>>
>>
>> *From:* rongretlarson at comcast.net [mailto:rongretlarson at comcast.net]
>>
>>
>>
>> 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
>>
>
>
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-- 
Paul A. Olivier PhD
27C Pham Hong Thai Street
Dalat
Vietnam

Louisiana telephone: 1-337-447-4124 (rings Vietnam)
Mobile: 090-694-1573 (in Vietnam)
Skype address: Xpolivier
http://www.esrla.com/
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