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Ice will melt faster which case -- over or below hot block?

  1. May 18, 2016 #1
    Natural convection always occurs opposite of gravity due to buoyancy; but I am confused as ice is solid so how could possibly convection even contribute to the heat transfer. I doubt about air movements nearby block that may contribute to greater heat transfer rate in case of hot block placed below the ice.

    Pls help
     

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  2. jcsd
  3. May 18, 2016 #2

    CWatters

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    Your drawing shows the ice and hot metal blocks are in contact. So most heat will be transported by conduction not convection. It will not make much difference which one is on top.

    Convection only occurs in liquids and gasses, NOT in solids like ice.

    If you look what happens at the edges...

    If the metal block is below the ice it will heat up the air causing the air to rise past the edges of the ice.
    If the metal is on the top that energy is carried away into the room without going past the edges of the ice.

    Convection.jpg
     
  4. May 20, 2016 #3
    I am talking about even the negligible difference between the timing of melting of ice.
    I think ice on top of metal block will win the race.
     
  5. May 20, 2016 #4

    CWatters

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    I agree.
     
  6. May 21, 2016 #5

    Nidum

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    Heat transfer is almost entirely determined by flow of melt water and steam . Simple air current convection will probably only be be a minor contributor .

    Try thinking about the actual physics of what is happening .
     
  7. May 21, 2016 #6
    I think steam would not be there as metal block is at 50 degree Celsius only.
     
  8. May 21, 2016 #7

    Nidum

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    In between the ice block and the metal block there must be a layer of melt water .

    What happens to this layer of water as the melting of the ice block progresses ?
     
    Last edited: May 21, 2016
  9. May 21, 2016 #8
    I think water will flow down between the edges of block
     
  10. May 21, 2016 #9

    Nidum

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    Yes - water will flow out continuously while the melting process is active .

    So :

    (a) What is the main mechanism of heat transfer between the blocks ?

    (b) Is there any additional mechanism of heat transfer involving the external flow of the melt water ?

    (c) Back to the original question - does it make any difference which block is on top ?
     
  11. May 22, 2016 #10
    Yeah that water would be at just more than zero degree Celsius. That means we have a gradient between atmosphere and water flowing. So somehow it will drop the temperature nearby air. If Ice would be on top; it will decrease the metal block temperature side wise also. In the meantime the gradient between metal block and ice will reduce consequently it would reduce the heat transfer. But if Ice would be below the metal block, then water will just fall to the ground.
     
  12. May 22, 2016 #11

    Nidum

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    What is the answer to (a) ?
     
  13. May 22, 2016 #12
    Conduction only
     
  14. May 22, 2016 #13

    Nidum

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    Is that correct ?

    What do other PF members think ?
     
  15. May 22, 2016 #14
    I must say conduction is predominant. Convection would be there due to liquid water flowing and air movement also. Magnitude of both will decide, in which case ICE will melt fast?
     
  16. May 22, 2016 #15

    Nidum

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    Seems about right for the problem you originally described . At higher temperature differences convection would probably become more active .

    That's it for now .
     
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