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Firebrick wrapped with aluminum foil

  1. Oct 12, 2012 #1
    I two firebricks, one is wrapped with 4 layers of aluminum foil and the other one is not. If both are placed 10 " above a gas burner (not in contact with the flames), which one will have a higher temperature? What equations need to be used to calculate heat transfer between aluminum foil and firebrick?

    Assuming no air gap between aluminium foil and firebrick.

    Thanks
     
  2. jcsd
  3. Oct 12, 2012 #2
    Is the burner hot enough to melt or burn the foil?
     
  4. Oct 12, 2012 #3
    The burner Max BTU is 25000
     
  5. Oct 12, 2012 #4
    At what temperature?
     
  6. Oct 12, 2012 #5
    I am not sure what is the temperature a 25000 btu burner can generate.... lets assume 1000 deg F
     
  7. Oct 12, 2012 #6
    Any one of several things can happen since we are getting close to the melting point of the aluminum. At the very least it will deform and maybe fall off. It will certainly oxidize rapidly, which will result either in it turning to powder and falling off, or it could catch fire and burn away. The rapid oxidation may produce enough hydrogen to burn, and that may cause it to reach its melting point. So it really is not possible to know for sure what will happen.

    But let’s assume we pick a lower number so that the aluminum foil retains its original physical configuration. Let’s also assume that the hot gas flow is the same and we will ignore any burning hydrogen shed by the aluminum.

    Now the convective heat transfer in the two cases will depend upon the convective heat transfer coefficient. Without testing it, I don’t know the numbers for sure. But most likely the coefficient is higher without the foil, so more heat will be transferred to the brick in that case and it will get hotter.

    We also have radiant heat transfer from the flame. That will be higher without the foil, so again the brick will get hotter.

    Any heat that gets transferred to the aluminum covered brick will have to conduct through the aluminum, which will provide a small amount of thermal resistance, slowing down heat transfer to the brick. So again, the brick gets hotter without the foil.

    So for this temperature assumption, all modes of heat transfer result in a hotter brick without the foil.

    Edit: I'm used to working with flames that are between 2300 and 5000 degrees F. So that is why I was asking the question. But after thinking about it, a common burner is sucking combustion air in at a low rate via natural convection, so the flame temperature will be much less than my flames that are fed by huge air compressors. Also, in the ten inches above the flame, much ambient air will be drawn in, further cooling the flame exhaust gas. I'm guessing that actual gas flow temperature at the brick would be in the neighborhood of 500 degrees F, unless you did something to prevent exhaust gas dilution.
     
    Last edited: Oct 12, 2012
  8. Oct 12, 2012 #7
    Thank you for the great response. You'r right, the flame temp is more likely to be around 500 than 1000.

    What about if I just cover the top of one stone with foil?

    I am trying to get a delta of at least 100 deg F between the two stones. Any other material you recommend to use as an insulator (can withstand at least 1000 F)
     
  9. Oct 12, 2012 #8

    AlephZero

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  10. Oct 12, 2012 #9
    Thanks for the link, is it sufficient to insulate just top of the stone to get a 100 deg f delta. I am looking for formula to calculate temperature
     
  11. Oct 14, 2012 #10

    Integral

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    You will not find it.

    The temperature depends upon too many factors to be easily calculatable. The best you can do is analyse all the energy exchanges to set up a differnetial equation which you will then need to solve.
     
  12. Oct 14, 2012 #11

    AlephZero

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    It might help if you explained the bigger picture of what you are trying to do here. "Getting a temperature difference between the two bricks" doesn't sound like the whole story.
     
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