[Stoves] Rapid mixing

Thu Sep 29 20:49:23 CDT 2016

Alex, 

Thank you for your excellent responses.  You pose some very good ideas and concerns.

On the question of obstruction, I have to disagree with you.  The Venturi mixer I am working on reduces the open area to .5 that of the fuel reactor chamber.  Your design reduces the open area from 12 inch dia to 2 inch dia which is 113 sq in to 3.14 sq in.  That is .028 the open area of the fuel reactor chamber, which is much more restriction than I am using.  You must be getting an incredible velocity through the 2 in pipe, which would be giving you a substantial drop in pressure via the Venturi effect.  It would be very interesting to get pressure measurements throughout your stove (with the secondary air entrance open/closed, and compared to a straight wall stove of the same height for a control).  Your placement of the secondary air entrance is perfect to use the Venturi pressure reduction for mixing.  I have not found tangential secondary air mixing to work very well, but I am guessing that the increased pressure difference makes it work for you.  

I very much like the way you have found to pre-mix the wood gas with the secondary air.  I had thought it would be much more difficult to achieve this.  You have cleverly increased the wood gas velocity so it is faster than the flame can burn back, and thus give the gasses time to pre-mix before they burn.  When the mixture ignites in the mixing area, the high gas velocity just pushes the flame out.  Or perhaps there is always a small amount of flame in the mixing zone, but the gasses are moving so fast that the majority of gas doesn't burn until it reaches the burning area.  Or perhaps the gasses mix and begin combusting, but because of the gas velocity they don't have time to burn completely in the mixer area.  Even well mixed gasses take time to burn.  The only way I can see that there would be no flame in the mixing area is if the mixture is below ignition temperature.

It appears to be a rapid mixing technique, though you have designed for narrow and tall whereas I designed for wide and short.  Both will find their place.  The height of the cone that accelerates the wood gas plus the height of the mixer plus the height of the cone to decelerate the mixture and hold the flame adds up to some height.  That is not a negative if it works for the application.  

How do you use the flame for cooking?  A grill over the top?  A sunken pot?

What is the fire power?  The constriction must be creating a back pressure that is slowing pyrolysis, and thus fire power.  This might be countered by the large diameter fuel reactor chamber providing a large pyrolysis front and lots of wood gas.  It is natural draft and with how much turn-down?

I agree with you on the concern for material life.  Your design allows the materials some cooling by giving off heat to the outside.  My design holds the material in the heat.  The only cooling I get is from the secondary air entering and picking up some of the heat.  Things often glow red hot.  Material life will be a serious concern for my design, but this is the twenty-first century, and materials science has progressed.  Possible solutions include making the mixers from ceramics or other heat resisting material, and designing for cheap and easy replacement of the heat stressed parts.

Very nice concept!  Thank you for sharing.

Kirk H.
  ----- Original Message ----- 
  From: alex english 
  To: Discussion of biomass cooking stoves 
  Sent: Thursday, September 29, 2016 4:22 AM
  Subject: Re: [Stoves] Rapid mixing

  Kirk, 
  Its quite wonderful what you have demonstrated. A holy grail of sorts. The durability and complexity, and ultimately this is the cost, of the structure you have placed in the flame/gas path will determine its future usefulness. Interesting solution none the less.

  Here is a recent video of my premixed blue tlud-gas flame. Its all geometry, no obstruction.
  https://www.youtube.com/watch?v=jnV1e60NTss&feature=youtu.be

  Alex

  On Wed, Sep 28, 2016 at 12:05 PM, kgharris <kgharris at sonic.net> wrote:

    Crispin and All,

    Some clarification:  By radial mixing I assume you mean the method in the lower photo where the air travels inward, radially.  By helical mixing I assume you mean swirling gasses by using fan blades like in the top photo.  Neither of these represents a conclusion on my part.  Your opening sentence put me on guard immediately because it defines your techniques as my conclusion.  A more accurate starting sentence for your response would have been 'Here are two techniques to look at that have worked for me'.  My thinking is not limited to these two ideas.  

    Your techniques are what I was looking for, ideas for rapid mixing.

    Kirk H.

      ----- Original Message ----- 
      From: Crispin Pemberton-Pigott 
      To: 'Discussion of biomass cooking stoves' 
      Sent: Tuesday, September 27, 2016 8:09 PM
      Subject: Re: [Stoves] Rapid mixing

      Dear Kirk

      I think your conclusion is correct – that you can shorted the height of the combustion zone with radial or helical mixing.

      I provide two photos with this in mind, though accomplished in completely different ways. The first is (for a wood burner, not pellets) is to put the mixed below the fuel, to continuously create a series of ‘induced vortices’ a few of which pass through the fuel and provide mixing by spinning. A vortex tends to perpetuate itself so all that is needed is to get a few going all the time.

      The material is a low chrome stainless steel. The production is in three stages: blanking, punching the cuts, then forming.

      The second method of shortening the flame is to use the secondary air entering the fire as a method of driving the flames horizontally. This should be done in a ‘divided’ manner, not a ring of air, so that it breaks up the rising gases into manageable jets. This kind of short clean burn can only be done with preheated secondary air – 300-500˚C works well.

      The fire above is burning pellet gas (as a TLUD) though it also works for wood if the gas generation is consistent. Consistency requires very good air control. The amount of is required to achieve this is so low that it pretty much rules out having an open bottom with fuel sticking out. 

      Readers may recall Julien in Coburg combined the injection of air and placement of the air feed in the gas stream. A problem commonly reported with that approach is the cooling of the metal by the air can interfere with the combustion.

      Readers may also recall Dr Nurhuda in Indonesia achieved a downward turning flame consistently in his recent stoves reducing the flame height basically to zero. All of these approaches can assist the provision of lower height stoves – a major challenge for typical Indian rural households where cooks won’t compromise on the cooking height. 

      Everyone should try these approaches and report what works well. 

      Regards

      Crispin

       ……

      Not to be distracted from the purpose of this list, I have a thought about wood stoves.  A small natural draft wood burning cook stove that mixes the wood gas with secondary air in 2 cm has a considerable advantage over a stove that may take 20 cm or more.  It's a little like transistors, which gave advantage over vacuum tubes in electronics.  Rapid mixing can enable a small wood stove to burn quick and clean at both high and low power levels.  Witness the excellent numbers for the Wonderwerk TLUD-ND stove which uses a rapid Venturi mixing technique.  Combined with the user friendly power level controls, the principles used in the Wonderwerk stove hold considerable potential.  Rapid mixing seems to me to be a very good area for research.  I wonder how many rapid mixing techniques will be found over the next few years?  I intend to contribute.

      List, keep writing about wood stoves, don't be distracted.

      Kirk H.

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