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Thread: A problem i'm having trouble with

  1. #1 A problem i'm having trouble with 
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    Hey guys i am currently in Uni studying civil engineering and i'm having trouble with one of the question and would really appreciate some help.

    This is the question;

    If the net radiation in a day is 20 MJ/m^2 and the latent heat lost with evaporation is 15MJ/m^2, a well mixed 2m deep lake will heat up in one day by (choose the closest value):
    a) 0.6 degrees Celsius
    b) 2.39 degrees Celsius
    c) 1.20 degrees Celsius
    d) 0.006 degrees Celsius
    e) 0.0005 degrees Celsius
    (assume conductive heat fulx (Q)=0; Cp = 4.18 KJ/kg/degrees Celsius and pw=1000kg/m^3)


    These are the formula i know;

    A.Rn =dmv/dt*(hlg) +dT/dt*Cp m Q

    and

    dmv/dt = B ρw (Pvap,sat − Pvap )A


    I can't seem to figure it out since they haven't given you the Temperature.

    Any help is appreciated. Cheers


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  3. #2 Re: A problem i'm having trouble with 
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    Quote Originally Posted by rushil01
    These are the formula i know;

    A.Rn =dmv/dt*(hlg) +dT/dt*Cp m Q
    This is the one you need.

    Q=0 (given)
    dmv/dt*(hlg)=0 (I suppose, I'm not entirely sure what this is, but you don't need it)
    Rn= incoming power per area - power going to evaporation per area (again, assuming this is what the symbol means)

    rewrite:

    divide both parts by surface A

    with d being the depth of 2 m
    you know all variable except , just fill them in.


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  4. #3  
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    Cheers for the reply. I was making such a small error however it was totally throwing me off. I got the answer to be B which i'm hoping is correct. Also just so you know dmv/dt is the evaporation rate and Hlg is the latent heat lost with evaporation.

    I'm guessing that dmv/dt is zero because i'm assuming that all the energy goes into increasing the temperature of the water.
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  5. #4  
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    Ah, in that case, my assumptions about dmv/dt*(hlg) and Rn where wrong. dmv/dt*(hlg) isn't zero at all, since question clearly states that the evaporation energy of one day is 15 MJ/m^2. I put all of it in Rn, but that's not the case.

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  6. #5  
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    hmmm in that case the answer would be a) which is what my mate got. Cheers for your help Bender
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