Convective heat transfer coefficient

  1. Ok it's my first time here and I was hoping to get some help on some questions I have been given. I am a first year chem eng and I'm finding the work pretty hard so any help at all will be useful, thanks.

    I need to find the convective heat transfer coefficient, h for petrol using this equation:

    hD/Lamdaf = 0.37 Re^0.6

    so using information I am given:

    D = 3mm = 0.003m
    Lamdaf (thermal conductivity) = 0.145 w/m k
    (M)=viscosity = 0.0006 Pa s
    u=Velocity of petrol = 19.2 m/s
    P=density of petrol = 737.22 kg/m^3

    Ok so to calculate Re I am using: Dup/(M) = (0.003*19.2*737.22)/0.0006 = 70773.12

    Is this correct so far?

    Then I put this back into the equation and rearrange for h which I get to be =14533.475! w/m^2 k

    Is this correct because it seems very big to me! if not please can someone point me into the right direction.... thank you very much!
     
  2. jcsd
  3. Mech_Engineer

    Mech_Engineer 2,299
    Science Advisor
    Gold Member

    I'm assuming this is flow through a pipe? You seem to have run the numbers correctly, and that convective coefficient doesn't seem out of the realm of possibility to me.

    Your Reynold's number does seem a bit low, I caluclated 126,500 but I might have used some fuzzy numbers in there.
     
  4. It says the temperature of petrol is monitored by a thermocouple in the flow, so i'm guessing pipes?

    How did you get your Re at that value? Have i used the wrong values to calculate it?

    Thank you for your help
     
  5. Mech_Engineer

    Mech_Engineer 2,299
    Science Advisor
    Gold Member

    I calculated the Reynold's number using the equation:

    [tex]Re_{D}=\frac{\rho*u_{m}*D}{\mu}[/tex]

    where
    [tex]\rho=719\frac{kg}{m^{3}}[/tex]
    [tex]u_{m}=19.2\frac{m}{s}[/tex]
    [tex]D=3mm[/tex]
    [tex]\mu=3.3*10^{-4}Pa*s[/tex]

    With these inputs the Reynold's number works out to 125,200.
     
  6. Mech_Engineer

    Mech_Engineer 2,299
    Science Advisor
    Gold Member

    So since I was a third of the way there anyway, I went ahead and tried calculating the convective heat transfer coefficient. The equations I used are out of my heat transfer text book, "Introduction to Heat Transfer" by Incropera and DeWitt.

    The answer I got was h= 30,950 W/m^2*K

    I attached the MathCAD sheet I used to calculate it rather than trying to type it out in Latex.
     

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