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Homework Help: Radiation and convection

  1. May 10, 2010 #1
    Hey ive got this question and the answer that is giving is 219.12 but i keep getting 319 could someone please help me out heres the question.

    Consider a person standing in a room. The room air temperature is
    maintained at 22OC at all times. The inner surface of the walls, floors and
    the ceiling of the room are observed to be at an average temperature of
    10OC.
    Determine the total rate of heat transfer from this person if the exposed
    surface area and the average outer surface temperature of the person
    are 1.4 m2 and 30OC. Take the convective heat transfer coefficient to be
    6 W/m2 OC and the emissivity of the human skin to be 0.95

    any help would be great
     
  2. jcsd
  3. May 10, 2010 #2

    Mapes

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    Can you please show your calculations? I get 219.13 W without any problems. Make sure to check your units.
     
  4. May 10, 2010 #3
    cheers for replying here what i done:

    Q radiation - 0.95 * 5.67*10-8 * 1.4 * ((273+30)^4 -(273+10)^4) = 151.9

    Q convection - 6 * 1.4 * (30-10) = 168

    Qtotal = 319.9

    god knows what im doing wrong probably something really stupid, how did you do it

    cheers
     
  5. May 10, 2010 #4

    Mapes

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    Is convective heat transferred from the person directly to the walls?
     
  6. May 10, 2010 #5
    i'm not really sure to be honest, how did you get 219
     
  7. May 10, 2010 #6

    Mapes

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    Convection is defined as heat transfer via the movement of a fluid (like air). So I'm not sure why you'd have the temperature difference that drives convection be (30-10)°C when 10°C is the temperature of the walls. Know what I mean?
     
  8. May 10, 2010 #7
    yeah i know what you mean i actually used the 220C in the equation but the final answer was coming smaller than the actual answer so i tried the 100C
     
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