1. Limited time only! Sign up for a free 30min personal 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!

Homework Help: Radiation/convection heat transfer problem. (Thermodynamics)

  1. Sep 11, 2011 #1
    1. The problem statement, all variables and given/known data

    A 1.5-m^2 black surface at 120 degrees C is losing heat to the surrounding air at 30 degrees C by convection with a convection heat transfer coefficient of 18 W/(m^2 degree C), and by radiation to the surrounding surfaces at 10 degrees C. The total rate of heat loss from the surface is : ?

    2. Relevant equations

    Q (convection) = h * A *( delta T)

    Q (radiation) = [itex]\epsilon[/itex] * [itex]\sigma[/itex] * A * (T^4 (surface) - T^4 (surroundings))

    3. The attempt at a solution

    I calculate the heat lost by convection to be 2430 Watts. However, I need to find the total heat loss from the surface, which includes the loss from radiation.

    I have the surface Area, the sigma value (Stefan-Boltzmann constant) , and the temperatures (393 K for the emitting surface and 283 K for the surrounding surface) but no epsilon value is given. (emissivity). All it says is "black surface" and not the material. Should I assume the surface is ideal (blackbody radiation) and put a value of 1? Should I disregard the term altogether?

    I wanted to post this here before I emailed my instructor, because I unless I'm doing something wrong I feel I lack enough information to complete the problem.
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Sep 12, 2011 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    "Black" implies an emissivity of 1.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook