[Stoves] Strata combustor general principles

kgharris kgharris at sonic.net
Wed Oct 21 02:18:36 CDT 2015


>Frans,

 

Dear Kirk

 

Something is wrong in your concept op pressure .

>This is very possible since I am trying to work things out, but I am making progress.

 

One : you measure pressure- difference in and outside the stove ,an OPEN STOVE vessel!

Only in a closed vessel BOM- combuster you get a real pressure due solid/gas volume-expansion, versus container volume ;mind 12gram carbon gives 22,4 Liter =44gram CO2 or 22,4 liter CO=28gram !!! 

Depends the air you give it . Gas from pyrolyse is different .CO +H2+CO2 +organic gas methanol aceton acetic acid ao .

>It is possible for pressure variations to exist in an open vessel.  Consider an airplane wing, which is an open vessel and has pressure difference on top and bottom.  If there are no pressure variations in a wood stove then what makes the gasses move?  Some sources of pressure variation in a ND-TLUD are wood gas emergence from the wood, buoyancy, hot gas expansion from secondary burning, the cooling of the gasses as they give heat to the pot, and the Venturi effect from the gasses passing through a constriction.  I have not found any good descriptions of buoyancy as it operates in a ND-TLUD, so I am writing one.

 

Then you have GAS expansion to law PV/T=pv/t versus absolute temperature - 273°C in the calculation

In and out the stove you get the ARCHIMES law too ! Compare floating in water-pressure up ,depends displaced water or air with its density difference . Air MOL equivalent =27 CO=28 CO2=44 H2=2

>Displacement works well for static buoyancy, a boat sitting in the water or the kings crown under water, but the gasses in a wood stove are not static.  Column height works better as an indicator of the state of wood stove gasses.  Consider two chimneys that displace the same amount of atmosphere, one vertical and one horizontal.  The vertical one adds to the draft, the horizontal one does not.  Displacement does not necessarily affect draft.

 

A high chimney as mine is 8 meter ,it SUCKS till 3 mm water column PRESSURE-DIFFERENCE as the 10 Kw stove make the noise as a wolf ...!

>It sounds like your stove has great draft with that tall chimney.  It must work well.  

>The chimney does not suck to create the pressure drop.  The column of hot, light weight gasses in the chimney creates less pressure at the bottom than an equal column of the heavier atmosphere (see attachment).   The draft is actually buoyancy, which is a push from the bottom, not a pull from the top.

 

Your idea to measure gas escape surface- pressure must be done about ,with a cup shaped pressure sensor of 1cm² and 1mm long,, pressed on a flat peace of gas- steaming wood.of 450 °C.

But it is non-sense to try it!

>I do not need detailed measurements as much as to just watch how the gasses behave.  As I consider the stove and fire, it becomes clear that since the fuel chamber fills with wood gas, and the atmosphere is pushed out, the wood gas must be boiling out of the wood at above atmospheric pressure.  Yet at the same time the primary and secondary air are entering the stove, so the wood gas must be below atmospheric pressure to allow this air in.  How can the wood gas be both above and below atmospheric pressure at the same time?  My thought on the answer to this is that once the stove heats up, the light weight of the gasses in the upper stove produces a low pressure area in the lower stove where the wood is (see attachment).  The wood is in the low pressure region, and since the wood gas emerges at slightly above the surrounding pressure, it is now emerging from the wood at below atmospheric pressure.  It cannot hold the atmosphere out.  This allows the primary and secondary air to enter.  All of this can be explained if the wood gas emerges from the wood at just slightly above the ambient gas pressure surrounding the wood.  Which makes sense because the boiling wood volatiles are behaving like boiling water behaves.  At higher altitudes, and lower atmospheric pressure, the steam boils off at lower temperature and pressure.

 

You need the "FLOW" in a stove as most important related to stove dimensions .

Mind :Flow is related to your unmeasurable pressure difference .

Flow is measured with a PITOT tube of 1-2 mm and 20 cm long .

>The flow in a ND-TLUD is very complicated.  It might be a good doctoral thesis topic for a graduate student.  There are numerous flows inside a ND-TLUD.   There is the primary air flow, the wood gas flow through and above the char from the pyrolytic front to the secondary, the secondary air flow, the changing flow from the beginning of the secondary burn to its end (which is often complicated by flowing through a constriction and possibly hitting and giving off heat to a pot), the exhaust flow (which cools as it hits the pot, and may perhaps flow through a chimney), the plume above the stove, loss of energy if the wood gas cools into smoke, the flow resistance all along the way, and whatever else I cannot think of right now.  I don't want to get into all of that.  I just want to make my design work and understand something of the principles of why it works. 

 

You can make a flow indicator by an extra vessel around the stove with an unic total air input ,with a square tube input with a balancing hanging cat-door 1x5 cm ,metallised MILAR=polyester of microns thick.(Or thinnest Alu foil ) To get minimum air resistance! 

You measure the movement by an oblique laser rayon .. Movement multiplied by reflection longer distance ! For a TLUD stove without a fan..

>This will give the flow of air into the stove but does not take into account wood gas emergence and expansion from the secondary burn.  

 

With fan : a 15 cm PC fan you cannot use with a cone 12 to 3 cm ! All FLOW RETURNS due FLOW CONE RESISTANCE !!!

You need PRESSURE to OVERCOME the fuel compaction resistance !

An inversely used vacuum cleaner motor at 1/10 of its voltage and power is best !! Regulate FLOW, is airspeed to the best BURNSPEED but ...! (15-30 volt for 230V AC motor )

Give the gasses "REACTION TIME " !!!

And mind pyrolyse /combustion needs 1versus 9 air quantity !

>This is all good to know.  I do take into account flow resistance from fuel and constrictions.  I provide reaction time by spinning the flame with a stationary fan blade.  This makes the flame rise at an angle instead of straight up, giving it more time to burn.  

 

>Thank you for your challenging response.  It keeps me thinking.

 

>Kirk

 

Succes !       >Indeed..watch out propane.

P.F.

  ----- Original Message ----- 
  From: Frans Peeters 
  To: 'Discussion of biomass cooking stoves' 
  Sent: Saturday, October 10, 2015 5:07 PM
  Subject: Re: [Stoves] Strata combustor general principles


  Dear Kirk

   

        Something is wrong in  your  concept op pressure .

       One : you measure pressure- difference in and outside the stove   ,an OPEN STOVE  vessel!

  Only in a closed vessel BOM-  combuster  you get a real pressure due solid/gas volume-expansion, versus container volume ;mind 12gram carbon gives 22,4 Liter =44gram CO2 or 22,4 liter CO=28gram !!! 

  Depends the air you give it . Gas from pyrolyse is different .CO +H2+CO2 +organic gas methanol  aceton  acetic acid ao .

        Then you have GAS expansion to law PV/T=pv/t   versus  absolute temperature - 273°C in the calculation

  In and out the stove you get the ARCHIMES law too ! Compare floating in water-pressure up ,depends displaced water or air with its density difference .     Air MOL equivalent =27   CO=28 CO2=44    H2=2

   

        A high chimney as mine is 8 meter ,it  SUCKS till 3 mm water column PRESSURE-DIFFERENCE as the 10 Kw  stove make the noise as a wolf ...!

  Your idea to measure gas escape surface- pressure must be done about ,with a cup shaped pressure sensor of 1cm² and 1mm long,, pressed on a flat  peace of gas- steaming wood.of 450 °C.

  But it is non-sense to try it!

  You need the "FLOW" in a stove as most important related to stove dimensions .

  Mind :Flow is related to your unmeasurable pressure difference .

  Flow is measured with a  PITOT tube of 1-2 mm and 20 cm long .

  You can make a flow indicator by an extra vessel around the stove with an unic total air input ,with a square tube input  with  a balancing hanging cat-door 1x5 cm ,metallised MILAR=polyester of microns thick.(Or  thinnest Alu foil ) To get minimum  air resistance! 

  You measure the movement by an  oblique laser rayon .. Movement multiplied by reflection longer distance  !    For a TLUD stove  without a fan..

   

          With fan : a   15 cm PC fan you cannot use  with a cone 12 to 3 cm !  All FLOW RETURNS due FLOW CONE RESISTANCE !!!

  You need PRESSURE to OVERCOME the  fuel compaction resistance !

  An inversely used vacuum cleaner  motor at 1/10 of its voltage and power  is best !!    Regulate FLOW, is airspeed to the best BURNSPEED but ...! (15-30 volt for 230V  AC motor )

  Give the gasses "REACTION TIME " !!!

  And mind pyrolyse /combustion needs 1versus 9  air quantity !

   

  Succes !

  P.F.

   

   

  Van: Stoves [mailto:stoves-bounces at lists.bioenergylists.org] Namens kgharris
  Verzonden: vrijdag 18 september 2015 4:21

  Onderwerp: Re: [Stoves] Strata combustor general principles

   

  Frans,

   

  It is exciting to have you bring up these concerns.  There is a question I have had for a long time, and your contribution reminds me about it.  It has to do with the pressures within the stove.  Perhaps you and the list can help answer this question.

   

  First to address your concerns.  There is a sweet spot as to how much blockage the stove can manage.  It lies between to much, which blocks the flow and creates the attenuation you describe, and to little so that the combustor doesn't work.  I did not know that the venturi created a back pressure, I assumed that the back pressure was created by the obstruction of the tubes and fan.  I found the sweet spot by emperical experimentation.  I now know that this balance includes back pressure from the Venturi, if I understand you correctly.  So the equation now includes the resistance of three things: tubes, stationary fan blade, and Venturi back pressure.  I included your concern into the design without my even knowing it.  Its all a balance.  Thank you for your input, it is very good to help me understand more about the stove.  The combustor has been tested at Aprovecho and does work, so the idea is good.

   

  The question I have had for so long is this:  When the wood gas emerges from the wood what is its pressure and how long does it take for the wood gas to drop to near atmospheric pressure?  When I measured the pressure in a stove with a manometer which measured down to 1/100th of an inch of water column, it measured at atmospheric pressure throughout the stove.  The water column did not move at all (I understand from Crispin that I must measure down to 1/1000th of an inch to find pressure differences).  Since the water column did not move when I measured the pressure at the pyrolytic front, that implies that the wood gas either reaches atmospheric pressure quickly or comes out of the wood at near atmospheric pressure.  Whatever the case, the wood gas comes out of the wood with enough force to push the atmosphere, which weighs twice as much as the wood gas, out of the stove.  That is a considerable amount of force.  I wonder how much pressure pyrolysis could build up in a confined container?  A lot I would guess, enough to cause an explosion.  Is the pressure of the emerging wood gas dependent on the pressure of the gas it emerges into?

   

  Respectfully,

   

  Kirk

   

   

    ----- Original Message ----- 

    From: Frans Peeters 

    To: 'Discussion of biomass cooking stoves' 

    Sent: Thursday, September 17, 2015 3:06 PM

    Subject: Re: [Stoves] Strata combustor general principles

     

    Dear,Kirk

     

           You don't see :it is not a closed compressed system !

    But any  reaction vessel (V1 ) with a  venturi  attenuation at the output gives  a  back reaction in all directions ,so flow  attenuation  under the grate air input .

    By over total power the air input BECOMES SMOKE OUTPUT TOO .! So invers to your idea ;

    A V1  rocket has an input COMPRESSOR powered by the traveling speed plus a constant fuel input so a nice VENTURY output .

    1and 2 you don't have it in a stove of half info .

     

           Yours

    Rocket scientist

     

     

    Van: Stoves [mailto:stoves-bounces at lists.bioenergylists.org] Namens kgharris
    Verzonden: donderdag 17 september 2015 4:58
    Aan: Discussion of biomass cooking stoves
    Onderwerp: Re: [Stoves] Strata combustor general principles

     

    Dr. Boll and Crispin,

     

    Thank you for your well considered responses.  I especially like the idea of balancing the reactor fuel chamber volumn with the combustor size.  I will keep this in mind as I progress, and seek that balance.  

     

    I don't quite understand Crispins flame tube.  The stove appears to be loaded with charcoal, so there should not be any smoke.  If the clay is insulating clay than the tube would provide a hot environment for the CO to burn, if there is a source of secondary air.  If the stove burns wood, by what principles does the tube work?  Is air injected inside the tube?  This would make use of the Venturi effect like Roberto's stove.  The flame gasses accellerating into the tube would drop in pressure and the pressure difference would help the atmosphere push secondary air into the flame to equalize the pressure, providing good mixing.  This seems to me like it might work well if that is how it works.  Is there something else that I don't see?

     

    Kirk  

     

     

      ----- Original Message ----- 

      From: Boll, Martin Dr. 

      To: stoves at lists.bioenergylists.org 

      Sent: Wednesday, September 16, 2015 6:35 AM

      Subject: Re: [Stoves] Strata combustor general principles

       

      Kirk, 

       

      first of all, your stoves and thoughts about TLUD's are very interesting. And in my opinion

       

       the venturi-concept, you are talking about, -how ever managed- will be generally the next important step for better burning; - and for less blacked pots!!! 

       

      A presumption I made a year or two ago: 

      The (nominal-) power rate of a good working stove has a more or less narrow relation to the grate-surface, and/or the burning-chamber-volume.

       

       I calculated the given numbers of some different commercial stoves.  My calculations confirmed my guess.

      I dont remember the numbers, but it is easy to calculate the numbers out of available commercial data.

       

      Additional I remind to Crispin's postings, some time ago:

      He reported, that he made the burning-chamber of some coal stoves in Ulanbaator smaller, by just filling some space of the burning-chamber-space with bricks. And some surface of the grate. In combination with special incending-procedure, that gave better burning; even by transforming the may of stoves there, there was an enormous reduction (more than 90%) of air-pollution. 

      So I suspect, if you would use the right-balanced amount of burning-chamber-volume, you could get a step forward.  The volume within the venturi-riser-burner-tube should be included in the calculation; but in the tube there will be, -hopefully- a burning some similar to a LPG or NG flame, which will afford less volume. 

      You could take the power-rate which you achieve with low emission, for extrapolating the stove dimensions for your desired 5 kW output.

       

       

      Regards

       

      Martin


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