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Homework Help: Rate of combustion?

  1. Aug 10, 2008 #1
    Rate of combustion??

    1. The problem statement, all variables and given/known data

    1. The broader side of a rectangular fiberglass of dimensions 3m x 5m is brought near the flame. The flame is produced upon combustion of gasoline. If the rate of combustion is 0.002 g/s, determine the minimum thickness of the fiberglass so that the temperature difference between hot and cold sides of the fiberglass is 40 degree Celsius. The latent heat of combustion of gasoline is 46,000 J/g.

    k = 0.04 W/m.C

    2. Relevant equations

    Fourier's Law of Conduction

    H = kA(Th-Tc)/L
    H - Heat if conduction (Watts of J/s)
    k - Thermal Conductivity
    A - Area (meter squared)
    Th - Hot side temperature in Celsius
    Tc - Cold side temperature in Celsius
    L - Thickness

    Heat of Combustion
    Qc = mLc
    m - mass in kg
    Lc - Latent heat of combustion

    3. The attempt at a solution
    I really don't know where to start. The rate of conduction that is given is supposed to be H right? But the unit is g/s instead of J/s. And also, there aren't any Th and Tc given. Using the heat of combustion formula, i supposed you can get the mass of gasoline in the given 0.002 g/s. This is the last problem I'm solving. (I solved the other 4 but can't solve this one)
  2. jcsd
  3. Aug 11, 2008 #2


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    Science Advisor
    Homework Helper

    Welcome to PF!

    Hi daimlerpogi ! Welcome to PF! :smile:

    You don't need either Th or Tc separately … the formula only needs the combination (Th - Tc), which is 40C.

    If you're bothered about the units, just put them side-by-side until you see how to fit them like a jigsaw :biggrin:

    In this case, they give you a g/s and a J/g, but you want a J/s. :frown:

    ok … just multiply … (J/g)(g/s) = J/s ! :smile:
  4. Aug 11, 2008 #3
    Re: Rate of combustion??

    My bad, didn't notice that! Thanks tiny-tim!

    I got it now!

    (46000 J/g)(0.002 g/s) = 92 W

    Thanks again! :D
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