1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Question combined mode radiation and convection ?

  1. Nov 14, 2013 #1
    274335830.jpg


    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

    gneill

    User Avatar

    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

    gneill

    User Avatar

    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.
    (Circle?)
    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
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Question combined mode radiation and convection ?
  1. Convection or Radiation (Replies: 12)

Loading...