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Heat Transfer Rates (help)

  1. Apr 3, 2009 #1
    what is the measurement for describing heat transfer rate if i know the specific heat of the objects? as example, i want to know if the heat transfer rate of a hot copper block attached to a block of room temp aluminum is different than same hot copper attached to a block of gold at room temp, etc. and more specifically i am interested in knowing what the relationship is between specific heat and transfer rate. more or less a type of heat sink question "what makes a good heat sink?", a metal with higher or lower specific heat property?



    thanks
    PK
     
    Last edited: Apr 3, 2009
  2. jcsd
  3. Apr 3, 2009 #2
    Keep in mind that heat energy is not the same thing as temperature.

    Specific heat is the amount of heat energy it takes to increase a material's temperature.

    Specific heat does not affect the rate at which heat energy transfers, but it does affect the rate at which temperature increases.

    As far as heat-sinks go, keep in mind that things with high specific heat often have a higher heat capacity (water, volcanic rock), which is a good thing in a heat sink.
     
  4. Apr 3, 2009 #3
    ok, let me complicate things some. i am studying the cooling ability of motor oil with and without viscosity improver additives (aka VI's), "what oil cools better". the VI's are long chained polymers that affect how the oil cools. i found some scientific testing data that shows oil w/o the VI's had a slightly lower specific heat. so this is really a problem of perhaps three steps of heat transfers:

    1. from engine block to the oil
    2. from the oil to the bottom of the engine (oil pan, filter casing, etc).
    3. pan to air, filter casing to air, or oil cooler to air, etc

    so if the oil had very very high specific heat it could in essence absorb a large amount of energy from the engine block but not increase its temp much, and if the temp between oil and pan are small then i get little heat transfer between oil and pan, so cant i conclude that i want the oil to have a bigger temp diff between oil and pan so that more energy is removed from the oil during the oil-to-pan heat transfer period (call it watts if you want, etc)?? if the temps are exactly the same between two objects, even though one is holding much more energy than the other, there will be no heat transfer (zero watts).

    and to note about thermal conductivity, from the charts i have found at the Engineering Toolbox site the thermal conductivity goes up as specific heat goes up. so perhaps i am wrong and the better "heat sink" is that which has (in general) greater specific heat. but then again, specific heat does not always describe thermal conductivity this way, as example:

    copper Cp=0.39kJ/kgK k=223btu/hrFft
    cobalt Cp=0.42kJ/kgK k=40btu/hrFft

    so here copper makes a better heat sink but cobalt can absorb more energy for same increase in temp.
     
    Last edited: Apr 3, 2009
  5. Apr 3, 2009 #4
    i guess i am really looking for thermal conductivity of the liquid, motor oil in this case. how does one measure thermal conductivity of a liquid when thermal convection is the main mode of heat transfer through the liquid? is it acceptable to ignore convection and just fix the container size being used for the test?
     
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