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Measuring the h conv from velocity not the same as that he gave

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  1. Jan 21, 2017 #1
    1. The problem statement, all variables and given/known data
    Determine the heat loss per square meter of skin surface due to convection at -40degreeC in moderate wind (0.5 m/s, Kc=10 Kcal/m2-hr-degreeC), assuming that the skin temperature is 26degreeC.

    2. Relevant equations
    hcov= 10.45 - v + 10√v
    ΔQ/Δt = A hconv (Tskin - Tair)

    3. The attempt at a solution
    ΔQ/Δt/A= 10 x (26+40)
    =660 kcal/hr*m^2

    NOTE: i think it is wrong because when i measure hconv from the velocity =17≠10
     
  2. jcsd
  3. Jan 21, 2017 #2
    Well, there are a couple of ways of looking at this. First, convective heat transfer coefficients are dependent on so many variables that in this complex situation you can't expect a simple calculation to be very good. His made up number is as likely to be right as yours. Second, made up problems often have made up numbers. You've undoubtedly calculated the acceleration of boxes sliding down ramps with completely made up coefficients of friction, and a hundred other similar examples. That didn't bother you then, why should this bother you now? Third, his number is in the ballpark and it is 10 which makes the calculation simple. Since he is just making this up and he can't say very accurately what the true number might be, give him credit for making a convenient choice. He could have made up a number with eighteen digits of precision. That wouldn't have illustrated the concept any better.
     
  4. Jan 21, 2017 #3
    hey, thank you for the response, I found after a long period of searching that if the velocity between 2 to 20 m/s we can calculate the hconv from this hcov= 10.45 - v + 10√v
    and from this question I found when I can use the equation above that will help in other questions , always simple questions helps

    Thank you very much
     
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