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Does a flame mantle get "wetted"?

  1. Nov 6, 2015 #1

    DaveC426913

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    So, when I light up my outdoor heater, the blue flame starts off outside the mantle (the inner glowey bit in the pic), and after about ten seconds of warming up, the flame jumps inside the mantle where it starts to turn the mantle red hot.

    Presumably this is the same thing that happens in a camper lantern.

    Nothing I do will cause the flame to jump inside until it's ready. If I blow on the flame, it will momentarily blow out outside the mantle, burning inside for a second, but then it jumps back outside on it own.

    Only when it's good and ready will the flame jump inside the mantle.

    Is it the relative coolness of the mantle that's stopping ignition from occurring inside the mantle until it's the right temp?

    51R3NR355YL._SY355_.jpg
     
  2. jcsd
  3. Nov 6, 2015 #2

    sophiecentaur

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    Interesting observation. I think it must be that the mantle gradually warms up and becomes hot enough to ignite the gas/air mixture inside it.
     
  4. Nov 6, 2015 #3
    Is it premixed before you burn it? If so, then:
    Premixed flames have a property called the burning velocity. If you have a tube filled with premixed gas and ignite it, it is the traveling velocity of the flame. If the velocity of the incoming mixture is as high as the burning velocity, the flame will stand still in the tube. If the incoming velocity is higher, the flame will eventually move downstream and out of the tube. If you place some kind of mesh at the exit of the tube, the flame will stabilize on the mesh. If you blow the flames inside, it will get blown out again because the mixture velocity is higher than the burning velocity.

    I think sophiecentaur is correct and the mixture will auto-ignite on the upstream side of the mesh once the mesh gets hot enough. "Red Hot" starts already at 500 Celcius and most gases ignite below that temperature (butane=400, propane=470). Also, the burning velocity increases when you preheat the unburnt mixture, so the flames will move closer to the mesh, which will in turn increase the temperature of the mesh...
    Is there also a second flame holder/mesh inside of the mantle?
     
  5. Nov 6, 2015 #4

    DaveC426913

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    No. The next feature is the ring of nozzles.

    Nozzle. Great word, that. Nozzle.
     
  6. Nov 9, 2015 #5

    DaveC426913

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    Like this (only less blurry):



    I light it at 0:07s. You can see how it's outside the mantle.

    Then I blow on it 4 times between 0:09 and 0:12. It will not jump inside the mantle - until at 0:14 it does so spontaneously.
     
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