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Homework Help: Question combined mode radiation and convection ?

  1. Nov 14, 2013 #1

    Here we have question with answer
    Could please explain to me the answer of this question ?

    and can explain to me why we don't use area 2pi . r^2 because we have pipe ?

    and how we can know this combined modes ? I mean conduction and radiation ?

    In addtion , As I know s = the stephan-Boltzmann constant 5.6697 x 10-8

    why here we used that value for emissivy ?
    Last edited: Nov 14, 2013
  2. jcsd
  3. Nov 14, 2013 #2


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    Staff: Mentor

    The solution is adding the heat losses from each mode (convection and radiation).

    You want the surface area of the pipe, not the cross-sectional area.

    There are three possible modes of heat transfer: conduction, convection, and radiation. Heat transfer by conduction doesn't work so well for a gas, it's main method of transport is convection.
  4. Nov 14, 2013 #3
    thanks but:

    1 ) this value 5.67 X 10 ^ -8 for what ? becuse some time we used this for emissivy and some stme for stefan boltoman constant

    2 ) Is alwyes in this formula we used surface area of the pipe ?

    3 ) what is surface area for plate and cycle ?

    4 ) How we know here in this question there is radiation of heat transfer ?
  5. Nov 14, 2013 #4


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    Staff: Mentor

    It is Boltzmann's constant. See: Stefan-Boltzmann Law
    The surface area of a cylinder is given by the circumference multiplied by the length. The circumference is given by ##2 \pi r## or ##\pi D##, where D is the diameter of the cylinder.
    Surface for a plate is length x width. That gives the area of one side of the plate.
    A circle is the usual ##\pi r^2##

    The obvious hint is that they gave you a value for the emissivity, ε.
  6. Nov 14, 2013 #5
    thanks so much but you said that is for Boltzmann's constant but here in this quetions we used 5.67 X 10 ^ -8 fro
    emissivy ? why ?
  7. Nov 14, 2013 #6
    yes now I got it ..

    thanks so so muck gneill
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