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  • Directing heat one direction with asymmetrical insulation on a heating panel?

    Discussion in 'The main mechanical design forum' started by slightreturn, May 28, 2013.

    1. slightreturn

      slightreturn New Member

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      Hi all,

      I'm new here and I sure hope this is the right place. :) I'm building a heater that uses carbon fiber floor style panels and stands on-end. I need to direct the heat one direction and have played a bit with different types of insulators (mostly cotton- or fabric-based) to limit the heat moving backward. The front face of the panel is glass. The back in aluminium. I need to limit the heat from radiating backward but I'm also hoping to keep the panel really thin. So far the best I have managed is about a 4 deg C difference, front-to-back (the back is only 4 deg less than the front glass panel, despite the insulator...~ 65deg front, ~60deg back). I need to get the thermomotive activity happening better in one direction and wonder if anyone has any advice on materials that meet both "reasonable" cost-sensitivity requirements (I have some wiggle-room here, but need to be practical), and are sufficiently thin.

      I am going to try some thermal paste now but I suspect even with it, the heat is getting trapped up in the insulator in the back.

      Thanks in advance~!

      Kind Regards,

      M
       
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    3. Michael Ross

      Michael Ross Well-Known Member

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      Some salient principles...

      Heat ALWAYS transfers in the direction of cold to hot.
      heat never transfers if there is no difference from one location to another.

      Your ideas about an asymmetry of heat movement have to follow these principles. In a bulk material with no active means, these are that only principles in play.

      So you need to introduce asymmetry by some means - you can use gravity, which is unidirectional, and trap heat up high by convection (using the buoyant forces from differing density).

      You can introduce processes with hysteresis - such as phase changes - you can look up phase change materials in insulation.

      You can sequester, or dissipate heat, moving it with conductive materials through less conductive surroundings (like the copper heat busses in laptop computers).

      You can introduce fully active means like heat pumps (thermodynamic), or harness effects like Peltier, Seebeck, Johnson (thermo electric).

      I saw a description of a Sterling cycle used to create a hot container and a cold container. The difference could be used in reverse by the same means that created it very efficiently. Not sure how this idea scales. The description I saw was for large scale energy storage, but I wondered if you could miniaturize it somehow, for some purpose. Anyway it creates an asymmetry.

      The problem is that nature abhors this, so you will always have some trouble.
       
    4. mikewjansen

      mikewjansen New Member

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      From what I understand about CF heaters they radiate in the long wave IR region and are by design/nature cooler heating elements.

      So if you are strictly trying to analyze performance based on surface temperature, you aren't going to see much. Also, simply by the thermal conductivity of the materials on the "front" and "back", sensible heat will more readily flow through the aluminum panel - which will keep the element surface temperature cooler (possibly sapping some of the energy going into the element). However, the radiation from the carbon element will transmit through the glass, thus heating the surfaces in view of the heater (the real purpose of IR heaters).

      Maybe more information regarding your intended application would help identify the problem.
       
    5. hollandsd

      hollandsd New Member

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      If you are having issues with the thermal transfer into the aluminium case therefore the heat loss with it then something like Aspen Aerogel sheets (http://www.aerogel.com) would be one of the better insulators.

      As Michael Ross mentioned above, heat will always move across a gradient. If you can stop heat loss through your aluminium housing then the thermal gradient will be higher and you'll get better thermal transfer.


      Dan
       

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