[Stoves] Inner color of combustion chamber?

Sun Apr 26 03:07:52 CDT 2015

Dean,

This is a clever idea.  The inner dark radiating surface of the chamber liner would return heat to the flame while it's outer shiny surface, being a less efficient radiator of heat, would radiate less heat outward.  Below are some thoughts which I hope are not too far off base. 

My first inclination is that your rocket stoves are so well insulated that it would make very little difference since all that heat is being retained anyway.  A re-radiating surface would be little more effective than a reflective surface since they both do the same thing, return the heat to the flame. However, if a radiant surface could radiate fast enough to keep the wall temperature lower than it would be otherwise (conservation of energy, if the surface is radiating heat then there is less heat in the surface so the temperature is lower), then the stove could be operated with cooler walls as well as a hotter flame. This would be helpful by allowing less heat loss to the outside and by keeping the wall cooler for better lifespan.  A hotter flame would not be more efficient unless there is enough preheated air, mixing, and dwell time to take advantage of the extra heat.  Back to the basics required for a fire; fuel, oxygen, and heat.  If there is not enough air then increasing the heat won’t help.

If I were working on improving the efficiency of the natural draft rocket stove, I would want to first design an experiment that would tell me what the gas flow is inside the stove. Where is any excess air, flame and any unburned wood gas, how are they moving, and how much of each is present?  I understand from Dr. Winiarski that the flow up through the chimney is not symetrical and contains excess unmixed air.  A 3D computer model of flowing gasses through the stove could allow ideas to form for improved mixing of air with wood gas and flame. This could allow a secondary burn using the excess preheated air already in the stove, and perhaps some method of concentrating the flame for increased dwell time. A clean burn is not necessarily hotter, but rather more complete. I agree with what you said in an earlier e-mail, that a clean low soot flame requires excess air to make sure all the fuel and soot is burned. The excess air cannot be allowed to cool the flame, so using the excess preheated air that is already in the stove is the way to go.  With improved mixing and dwell time, increasing flame temperature by radiating heat from the wall could help.

Kirk

  ----- Original Message ----- 
  From: Dean Still 
  To: Discussion of biomass cooking stoves 
  Sent: Saturday, April 25, 2015 3:36 PM
  Subject: Re: [Stoves] Inner color of combustion chamber?

  Hi All, 

  We were wondering if an inner refractory metal black wall would absorb the IR (get very hot) and then emit back into
  the fire. The outside of the metal wall would be shiny and then not emit very much into the stove body?

  Best,

  Dean

  On Fri, Apr 24, 2015 at 3:12 PM, Bamboo Science Group <greg at bamboosciencegroup.com> wrote:

    Dean
    Great topic
    When I was designing solar thermal panels for INCO in 1979, we used a coating (selective) process (chrome oxide spinel that had a alpha ( absorptivity) of 0.9 and and epsilon ( emissivity) of 0.04) on stainless steel that allowed it to perform at parity with a black oxide on copper ( fin and tube).  The parity was achieved despite the overwhelmingly superior conductance of copper versus s/s.
    The high alpha allows absorption of the short wave solar radiation and the low epsilon slows the longer wave radiation emission in this flat plate solar collector application. These "selective" surfaces have cavernous surface area structure, utilizing the peaks and valleys of the coating to slow down the emissivity of the longer wave radiation.

    So you want an low emissivity coating for your refractory material to reduce ( inhibit) the black body radiation. SERI could certainly help out in this area as they are acquainted with high-temperature selective coatings for their work on concentrating collectors.

    Sent from my iPad

    > On Apr 24, 2015, at 5:07 PM, Dean Still <deankstill at gmail.com> wrote:
    >
    > Hi All,
    >
    > At the Aprovecho weekly meeting we were discussing how to improve performance in a Rocket stove. We were wondering about radiation.
    >
    > Would temperatures rise if the inside of the refractory metal combustion chamber was black and the outside shiny?
    >
    > Best,
    >
    > Dean
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